Introduction Since 2001 the Condensed Matter Physics Department (DPMC) in Geneva is the leading house of the National Center of Competence in Research (NCCR) MaNEP "Materials with Novel Electronic Properties". This center, with the University of Geneva as host institution and a broad Swiss network, focuses its activities on research and development of novel materials with promising electronic properties. The NCCR allows a long term reinforcement of the research activities at the DPMC in the field of correlated oxide systems and related materials.

In 2003, one important event in our Department was the nomination of Prof. D. van der Marel, a world specialist in optical spectroscopies of correlated electron systems.

The DPMC has a long tradition in the studies of metallic materials with unconventional properties. In the last decade the interest of the department has been focused mostly on the understanding and development of high temperature superconductors and more recently superconducting MgB2 as well as several classes of key novel materials, including colossal magnetoresistance compounds and ferro/dielectric oxides. All these materials have low electronic densities and a physics dominated by electronic interactions. The DPMC in Geneva has long standing experience in producing high quality materials in the form of single crystals, thin films and heterostructures, and tapes. The department also has a large expertise in the study of the physical properties of such materials. Several top class high quality experimental set-ups allowing high magnetic field magnetotransport, transport under pressure, specific heat measurements, and recently optical spectrometry are available. The department has also developed a large expertise in local probe systems. These include Scanning Tunneling Microscopy and Spectroscopy, STM/STS, and Atomic Force Microscopy, AFM, that allow the characterization and study of materials on a nanoscopic scale. This report represents the research carried out at the DPMC, regardless of the source of funds. Part of this research was financed by the NCCR MaNEP and by the Swiss National Science Foundation project No 2000-67963/1. Organization of the department Director: Jean-Marc Triscone

Faculty Øystein Fischer Alain Junod Tel. (022) 379 6270 Tel. (022) 379 6204 e-mail: Oystein.Fischer@physics.unige.ch e-mail: Alain.Junod@physics.unige.ch René Flükiger Dirk van der Marel Tel. (022) 379 6240 Tel. (022) 379 6234 e-mail: Rene.Flukiger@physics.unige.ch e-mail: Dirk.VanDerMarel@physics.unige.ch Thierry Giamarchi Jean-Marc Triscone Tel. (022) 379 6363 Tel. (022) 379 6827 e-mail: Thierry.Giamarchi@physics.unige.ch e-mail: Jean-Marc.Triscone@physics.unige.ch

Secretariat Elisabeth Jeantin, Joséphine Mirabile1, Nicole Nguyen, Fabienne Piguet2

(1until 31.10.03; 2from 01.11.03)

Department facilities Computing Service: Ivan Maggio-Aprile Helium: Gregory Manfrini and Spiros Zanos Electronics: André Dupanloup1, Patrick Magnin and Edouard Perréard (1until 31.03.03) Teaching support: Charly Burgisser, Gérard Drocco2, Vincenzo Fontana3, Yves Joly4, Lionel Windels5

(2until 30.06.03; 3from 01.11.03; 4until 31.12.03; 5from 01.04.03) University of Geneva Department of Condensed Matter Physics 24 quai Ernest-Ansermet 1211 Geneva 4 Switzerland Phone: +41 22 379 6511 / 6224 / 6264 Fax: +41 22 379 6869 Web site: http://dpmc.unige.ch

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The Triscone group

The Fischer group

The Giamarchi group

The van der Marel group

The Junod group

Secretariat Technical staff

The Flükiger group

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Research groups

Nanoscopic studies of superconductors and other interacting electron systems Professor Øystein FISCHER

MER Michel DECROUX, Alfred MANUEL Postdocs Louis ANTOGNAZZA, Morten ESKILDSEN1, Isabelle JOUMARD2,

Edmond KOLLER, Olivier KUFFER, Martin KUGLER, Ivan MAGGIO-APRILE, Serge REYMOND3, Sivaperumal UTHAYAKUMAR4;

PhD students Cédric DUBOIS, Pierre LEGENDRE, Giorgio LEVY DI CASTRO, Alexander PETROVIC5, Alexandre PIRIOU6, Silvia SEIRO, Matthieu THERASSE, Emmanuel TREBOUX Visiting Research Fellow Christian HESS Diploma students Cédric GOETSCHMANN7, Nathan JENKINS Electronics Engineer Laurent STARK Technicians Paul-Emile BISSON, Jean-Gabriel BOSCH, Géraldine CRAVOTTO8 1until 31.08.02;

2until 30.09.03; 3until 31.08.03;4from 01.09.02; 5from 01.08.02; 6from 01.11.02;7until ; 8from 01.06.03

Applications of superconductivity Professor René FLÜKIGER

Postdocs Nicholas CLAYTON, Vincent GARNIER1, Enrico GIANNINI, Hongli SUO

Visiting Research Fellow Hiroki FUJII2, GLADYSHEVSKII Roman3, Carmine SENATORE4 PhD students Nicolas MUSOLINO, Michael SCHINDL, Grégoire WITZ Technicians Patrick CERUTTI, Simon HUGI

1until 31.08.03; 2 until 30.09.03; 3until 28.02.03, 4until 30.06.03

Theory of Condensed Matter Professor Thierry GIAMARCHI

MER Thomas JARLBORG Postdocs Christophe BERTHOD, Carlos BOLECH GRET, Alejandro KOLTON1 Alberto ROSSO Visiting Research Fellow Claudia PEÇA2

1from 01.11.03; 2from 01.07.03

Specific heat and magnetocaloric effect of metals and superconductors Professor Alain JUNOD

Postdocs Satoko ABE1, Frédéric BOUQUET2, Tomasz PLACKOWSKI3, Rolf LORTZ4, Ulrich TUTSCH5

PhD student Yuxing WANG DEA student Violeta GURITANU6 Technician Aldo NAULA

1from 01.06.03, 2until 31.08.03; 3from 01.10.03;4from 01.04.03; 5from 01.11.03 6until 30.09.03

Fundamental excitations of correlated matter Professor Dirk van der MAREL1

Postdocs Alexey KUZMENKO2,

PhD students Fabrizio CARBONE3, Violeta GURITANU

4, Erik van HEUMEN

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Hajo MOLEGRAAF6, Riccardo TEDIOSI

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Technician Cornelis BOS8

1from 01.07.03; 2from 01.07.03; 4from 01.10.03; 5from 01.12.03; 6from 01.09.03; 7from 01.09.03; 8from 01.07.03

Growth and electronic properties of unconventional metals and oxides Professor Jean-Marc TRISCONE

MER Didier JACCARD Postdocs Matthew DAWBER1, Albin DE MUER2, Stefano GARIGLIO, Sarin KUMAR, Kei TAKAHASHI PhD students Alexandre GUILLER, Alexander HOLMES, Céline LICHTENSTEIGER, Daniel MATTHEY, Patrycja PARUCH, Nicolas STUCKI3 Technicians Renald CARTONI, Daniel CHABLAIX Diploma students Nicolas REYREN4, Anna-Sabina RÜETSCHI5, Nicolas STUCKI6

1from 01.10.03;2until 31.10.03; 3from 01.07.03;4from 01.09.03;5from 01.09.03; 6until 30.06.03

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Nanoscopic studies of superconductors and other interacting electron systems

Research summary

The group investigate materials with novel electronic properties using two experimental tools : Scanning tunneling microscopy/spectroscopy on one hand and the fabrication and study of thin film multilayers on the other. The scanning tunneling microscope (STM) is a powerful tool which offers the opportunity to perform imaging as well as spectroscopy with atomic resolution on conducting and semiconducting surfaces. Our group has a large experience in scanning tunnelling spectroscopy (STS) studies of high temperature superconductors, whereby we have uncovered some of their unusual electronic properties, such as gap values, pseudogaps features, and localised states in the vortex cores. This year we have in particular continued our investigations of MgB2 with a study of tunnelling onto the ac plane, uncovering the two gaps in this compound. We have started the low temperature Potentiomentry studies of manganites and we have made an important effort on developing new instrumentation. In our study of thin film fault current limiters, we have sucessfully demonstrated a 5kW 2 inch wafer, and we have developed a new tool for local measurements of the abrupt high current instability observed in thin superconducting films. Finally we have carried out a detailed study of strain on ultra thin films of NdBCO.

Magnesium diboride under different tunneling directions

MgB2 is a recently discovered superconductor that has undergone wide study over the past few years mostly because of its interesting two-band nature: The isotropic π-band can be seen via tunneling measurements in any direction, while the highly anisotropic σ-band contributes very little to tunneling measurements parallel to the crystalline c axis. Previous measurements in this group showed that tunneling parallel to the c axis revealed a π-band superconducting gap of ∆π ~ 2.3 meV and a coherence length of ξ ~ 50 nm. Figure 1 clearly shows that the π-band makes the major contribution to the tunneling spectra along this direction.

Motivated by the success of these experiments and the desire to investigate properties of both bands simultaneously, measurements made in 2003 using scanning tunneling spectroscopy with the tunnel direction perpendicular to the c axis. These experiments allowed us to measure a σ-band superconducting gap of ∆σ ~ 7 meV. Furthermore, vortex imaging allowed to determine the anisotropy of the vortex flux line

lattice yielding γ = 1.2 (see Fig. 2). This value is much lower than the upper-critical-field anisotropy, γH = 6, but in good agreement with theoretical calculations for the penetration depth anisotropy.

For more details, see M. R. Eskildsen et al., Phys. Rev. B 68, 100508 (2003); Physica C 385, 169-176 (2003); Phys. Rev. Lett. 89, 187003 (2002).

Homogeneous samples of high-Tc super-conductors

Scanning tunnelling microscopy is a unique tool to investigate the local density of states (LDOS) at the atomic scale. Since the discovery of high temperature superconductors great efforts have been done to synthesize homogeneous high quality single crystals. However, looking at the atomic scale, generally reveals superconducting gap features (peak position and height, gap shape) that strongly vary in space over distances of a few nanometres, due to impurities, defects or oxygen inhomogeneities, thus making the study of intrinsic properties more difficult. With the objective to obtain truly homogeneous

Øystein FISCHER

Fig. 2. Tunneling perpendicular to the c axis, both the π and σ-band gaps are clearly visible at zero field. Application of amagnetic field also perpendicular to the c axis quicklysuppresses the π-band. The vortex lattice is found to have an anisotropy of roughly 1.2 in an applied field of 0.3T.

Fig. 1. Tunneling parallel to the c axis, at zero field oneclearly sees the smaller π-band while only weak shouldersdue to the larger σ-band are visible. The hexagonal VortexLattice (VL), imaged at 0.2 T, is isotropic.

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samples, we concentrate our efforts on the synthesis and annealing conditions of Bi2Sr2CuCa2O8+δ, as well as Bi2Sr2Cu2Ca3O10+δ. The fact that the gap varies with doping indicates that the often observed inhomogeneous LDOS, is due to a inhomogeneous oxygen distribution in the sample (Fig. 3c). An adequate oxygen treatment (annealing) should thus yield homogenous samples. As shown in Fig. 3b, by annealing at 500°C and 15bar oxygen pressure for 7 days, it was possible to obtain a very homogeneous LDOS, indicating that disorder or inhomogeneity is not inherent to high-Tc superconductivity, at least not to overdoped samples. Our study also suggests that a prerequisite for long-range homogeneity is a sharp superconducting transition width of less than 1% of Tc (Fig 3a).

Further investigation for obtaining homogeneous samples with different oxygen compositions, treatments and substitutions on Bi or Ca sites are under way.

For more details and further reading, see B.W. Hoogenboom et al., Physica C 391, 376 (2003).

New Instrumentation for Nanolithography As we have demonstrated previously, the tip of a scanning tunneling microscope can also be used to write and read nanoscale structures on surfaces. With the aim to study mesoscopic effects on the local density of states of superconductors at low temperatures we designed and developed a novel STM device: a

miniature xy-stage that allows to access a larger scan area and to track nanoscale structures when cooling down the sample. Based on our home-built STM head, we added an inertially

driven carriage which allows for an extended motion of 6 mm along one horizontal axis. A differential capacitive detector permits the tip to be positioned on the sample surface with micron precision. Our next aim is the construction of a carriage with two degrees of freedom in order to access the whole surface.

Low-Temperature Potentiometry on Manganite Thin Films

The remarkable magnetotransport properties of manganites have not been completely understood, in spite of a huge experimental and theoretical effort. The question whether the colossal magnetoresistance is related to a mesoscopic phase separation and if it is of intrinsic character is being addressed by several groups by local probe techniques, showing inhomogeneities from nm to µm scale depending on the technique and the sample.

Our scanning tunneling potentiometry (STP) measurements, which probe the local electric potential distribution under current flow, together with a careful sample preparation, have not provided any clear sign of phase separation at room temperature for thin films of La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3, two canonic manganites. During 2003 some low temperature STP measurements have been performed on

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Fig. 4. Left: Home-built STM with original XY stage fixed to the scanner and able to move the tip over 6mm. Right: Newlaboratory with 4 low-temperature STM setups.

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La0.7Ca0.3MnO3, giving highly homogeneous potential drops, even at Tc (Fig. 5). This throws a doubt on the universality of the phase separation hypothesis. Experimental difficulties encountered at low temperatures have encouraged the construction of a new 4-300K controllable temperature STM/STP probe suited for studies on manganites, which is to be set in operation during 2004.

Fault current limiter

We are currently working on superconducting thin films based Fault Current Limiter (FCL). These devices limit the current in an electrical network during a short circuit. The FCL is a meander of Au/YBCO/CeO heterostructure on a 2” wafer. In the previous report we have presented a new and patterned design wich allows us to control and to minimize the dissipated power during a short circuit, both at short time and at longer time (~few ms). In this design constrictions are regularly distributed along the meander in order to homogeneously distributed the initial dissipated power and a thicker gold layer is grown on top of them in order to decrease the initial dissipated power. Using this design we have successfully tested a 5kW FCL with DC voltage pulses. Even if in the future DC electrical network will probably be used the FCL’s are currently working in an AC network. We have then focalised our studies on the behavior of our FCL submitted to an AC short circuit. A new electronic set-up has been built which allows us to precisely choose the phase angle at which the short circuit starts and ends. During this year we have measured the complete behaviour of several wafers by recording the voltage on each line of the meander; all these wafers were designed to support a power of 5 kW (300V-16A). These tests have been performed in the ABB laboratory in Dättwill. Fig. 1 presents the voltage and the current in the FCL during a short circuit started at a phase angle close to zero, which is one of the most constraining condition since only a small fraction of the constrictions is switching at the beginning of the short circuit. This result shows that our FCL can sustain this short circuit, under an AC voltage of 240rms (340V peak), for a period of 65 ms and that after this period, the current is as low as 25% of the nominal current or Ic. It is also important to know behavior of the FCL in a remote fault current regime; in this case the prospective fault current is only 2-3 times the critical current. Our measurement show that our FCL can also sustain this regime for a period of 65ms.

A new versatile tool for local measurements We are developing a very versatile tool, based on the classical STM’s architecture developed in the group, for local measurements at low temperature. By changing some elements of the scanning head we have the possibility to perform Scanning Contact Potentiometry (SCP), STM,

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STS, or SPS measurements. One of these important innovations, realized during this year, is a sample-holder which can move along one axis over a large length (few mm). One of our purpose is to study the appearance and the propagation of Highly Dissipative Domains (HDD) in HTS thin films, which are at the origin of the current induced transition into the normal state in FCL. For these measurements we will use the SCP mode to acquire an instantaneous potential profile of a sample carrying a current and in particular to directly image the formation of HDD in the film. Fig. 2.: Scanning Contact Potentiometry measurement along Pt bridge on part of which a superconducting Nb film was deposited

To illustrate the possibilities given by this system (in SCP mode), we have probed the potential profile across a normal (Pt)-low temperature

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superconducting (Nb) junction. The figure 2 clearly evidence the accuracy of the potential’s variation along the scanning direction (about 10 nm). Higher noise on the Nb side is due to the native oxide layer on top, resulting in a large (few 10 kΩ) contact resistance. An other goal is to make STM and STS measurements on YBa2Cu3Ox thin films and related materials. Such measurements are however very difficult in these compounds due to the last layers which are insulating over large areas. The origin of this degradation is usually attributed to air exposure, but the mechanisms are not so clear. The possibility of motion with the sample-holder over few millimetre will be very useful by allowing us a lot of attempts without removing the sample from the system. If STS measurements can be achieved a wide panel of new experiments will be open. In order to realize all these experiments new technical developments and work are needed.

Effects of strain on superconducting thin films We have pursued our study on thin films of Nd1+xBa2-xCu3O7+δ (NBCO), grown onto SrTiO3 (STO) substrates. Previous work showed that the effect of thickness is clearly different from that of oxygen doping, and we have continued this investigation to better understand the influence of strain. In this approach, we have made more precise X-Ray diffraction measurements, which confirmed the presence of strain in thinner films. Also, Hall effect measurements showed that underdoped films and thin films have different behaviour (see fig. 3).

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If we normalise the inverse Hall number, it appears clearly that underdoped films show a

non-linear variation of Hall constant with temperature, a feature that has already been observed on underdoped samples and that can be related to a pseudo-gap. But this feature does not appear at all in thinner films, even for those with low Tc. This proves once again that the Tc loss can not be only due to an underdoping effect. Then, to isolate the effect of interfacial strain, we have grown a set of bi/tri-layers of superconducting NBCO and insulating PrBa2Cu3O7+δ (PBCO). The thickness of NBCO is fixed at 6 nm and the PBCO total thickness at 55 nm. We measured the resistivity of these samples and observed a dramatic change, depending on the structure (see fig. 4). Actually, the Tc of 6 nm of NBCO embedded in PBCO is 20 K higher than that of 6 nm of NBCO alone, and corresponds to the Tc of 25 nm of NBCO alone. But this preliminary study needs to be continued with more samples to clarify this question and check the possibility of a proximity effect in PBCO.

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This study can also be compared to the work done in Lausanne by Abrecht et al. (Phys.Rev.Lett. 95-5, Aug. 2003) on La2-

xSrxCuO4. The authors reported that strained films had a higher Tc, and also that their Fermi surface was different from that of unstrained samples (ARPES measurements). This can be related to the different electronic properties observed in our films.

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Applications of superconductivity

Research summary: The research in our group is centred on the formation conditions of superconductors, on crystal growth and on the analysis of metallurgical and physical properties, in particular the critical current density. Based on the results obtained on our prototype High Pressure Differential Thermal Analysis device, several fundamental thermal parameters have been determined, for the elements Au, Al, Au as well as for the compound Bi,Pb(2223). Large Bi(2223) and Bi,Pb(2223) single crystals have been grown, the latter using a new “Vapour-Assisted Travelling Solvent Zone” technique, which compensates for Pb losses. Their transition width was ∆Tc ≤ 1K. The refinement of the Bi(2223) crystal structure confirmed an orthorhombic space group, with a commensurate 5-fold superlattice. For Bi(2223) an anisotropy of γ = 50 was found, i.e. much smaller than in Bi(2212), which is correlated to an

observed enhancement of the irreversibility field. The vortex phase diagram has been established for the phase Bi(2223). We found that the enhanced irreversibility fields in Bi,Pb(2212) with respect to undoped Bi(2212) has its origin in a new, modulation-free crystal structure, of the rocksalt type. The effect of pressure on the formation of the Bi,Pb(2223) phase inside Ag sheathed tapes has been studied: pressure was found to enhance Jc above 40%, provided that the amount of transient liquid was reduced by a first reaction heat treatment. The effect of pressure was also studied on Fe sheathed MgB2 wires. It was found that the observed Jc enhancement after HIP treatments is higher for fields parallel to the wide surface, thus suggesting texturing effects. Finally, the effect of applied strain on Fe/MgB2 tapes was studied.

Thermodynamic Studies High-Pressure thermodynamic studies on Au, Ag, Al and (Bi,Pb)2Sr2Ca2Cu3O10+d

The new High-Pressure Differential Thermal Analysis device (HP-DTA) was used for studying thermal properties of pure metals (Al, Ag and Au). A very good agreement was found with the existing literature and the HP-DTA device proved to be an efficient and reliable tool for thermodynamic investigations. By describing the dependence of the melting point on the pressure by means of the Clausius-Clapeyron equation, several fundamental thermal parameters of Al, Ag, and Au were calculated.

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From our experimental data we obtained the values of ∆V (volume variation at the melting point Tm), ρsm (density of the solid phase at Tm) and αm (linear expansion coefficient at Tm), which were either lacking or not reliable in the literature. The effect of pressure on Bi,Pb(2223) phase formation has been investigated by HP-DTA up to P = 2000 bar. A decrease of the amount of transient liquid was observed with increasing pressure. The decomposition temperature of the precursor phases Bi,Pb(2212) and Ca2PbO4, and therefore the formation of Bi,Pb(2223), was found to increase by 49°C at P = 2000 bar.

Crystal Growth and Characterisation Pb-free and Pb-doped Bi2Sr2Ca2Cu3O10 crystals Large and high-quality single crystals of both Pb-free and Pb-doped HTSC compounds (Bi1-xPbx)2Sr2Ca2Cu3O10-y (x = 0 and 0.3) were grown by means of a newly developed "Vapour-Assisted Travelling Floating Zone" technique (VA-TSFZ). This modified zone-melting technique was realised in an image furnace and allowed for the first time to grow Pb-doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm3 were successfully grown. Post-annealing under high O2 pressure (up to 10 MPa at T = 500°C) was undertaken to enhance Tc and improve the homogeneity of the crystals. By using single-crystal X-ray diffraction (XRD), the structure of the 3-layer Bi-based superconducting compound was refined for the

René Flükiger

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first time. A commensurate 5-fold superlattice was found in the Pb-free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(3) Å, b = 5.4133(6) Å and c = 37.009(6) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kind of crystals (∆Tc ≤ 1 K). In optimally doped Pb-free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2·105 A/cm2 were measured at T = 30 K and µ0H = 0 T.

Figure 2: ZFC susceptibility of an as-grown Pb-doped Bi-2223 single crystal (shown in the insert). The crystal is ~3x1 mm2 large. Transition temperature is 106 K with a transition width of 3 K.

Vortex Phase Diagram of Bi-2223 Single Crystal The recent growth of Bi-2223 crystals in our group has allowed us to study the magnetic properties and vortex phase diagram of this technologically important superconductor. A key parameter is the superconducting anisotropy, which we have deduced from measurements of the lower critical field. A value of γ = 50 is found for Bi-2223, which is significantly reduced compared to that of Bi-2212 (γ = 165). Using a SQUID magnetometer, we have mapped out the vortex phase diagram of Bi-2223, as shown in Fig 3. The open circles represent the irreversibility line, and the triangles (squares) are the onset (peak) of the second magnetisation peak. We find that the irreversibility line of Bi-2223 occurs at higher fields compared to Bi-2212, and is comparable to that of heavily Pb-doped Bi-2212.

Figure 3: Vortex phase diagram of the Bi2Sr2Ca2Cu3O10 superconducting phase

Crystal structure of modulation-free Bi,Pb-2212 Systematic investigations were performed with the aim to improve the properties of Bi2Sr2CaCu2O8 (Bi-2212), with and without doping elements. The structure of Bi-2212 is modulated, the atom arrangement in the BiO layers changing from square meshes (rocksalt-type) to chains with a given periodicity (Fig. 4a). Due to the insertion of additional O atoms, the actual composition of these layers is Bi9O10 (Bi2O2.22 per formula unit, leading to a total oxygen content per formula unit 8.22). This deviation from electroneutrality, which is partially compensated for by the presence of Bi3+ in the Ca layers, is a crucial parameter for the existence of the superconducting phase. By replacing part of the cations in Bi-2212 by chemically similar cations in a lower oxidation state it is possible to keep the same electron concentration, while removing the additional O atoms, thus suppressing the structural modulation. This was achieved by substituting ~22% of Bi3+ by Pb2+: Bi3+

2Sr2CaCu2O8.22 → Bi3+

1.56Pb2+0.44Sr2CaCu2O8. The arrangement of

atoms in the BiO layers in modulation-free Bi,Pb-2212 can be considered as distorted rocksalt-type (Fig. 1b). The unit cell parameter along the stacking direction is slightly larger for Bi,Pb-2212 than for the Pb-free phase. However, the distance between the two Bi layers is decreased by 0.13 Å, whereas all other interlayer distances are increased by 0.02-0.04 Å. The decrease of the distance between the Bi layers is in agreement with the fact that these layers are no longer corrugated but planar. The anisotropy of the Pb-doped crystals was found to be reduced with respect to undoped ones. Consequently, modulation-free Bi,Pb-2212 has an enhanced irreversibility field and a lower relaxation rate than in modulated Bi-2212.

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Fig. 4. Projections of two BiO layers along the stacking direction for (a) modulated Bi2.09Sr1.90Ca1.00Cu2O8.22 and (b) modulation-free Bi1.70Pb0.4Sr2Ca0.90Cu2O8.

Processing and characterisation of super-conducting tapes and wires for practical applications High-Pressure synthesis of Bi,Pb(2223)-Ag superconducting tapes. High-pressure (HP) processing of Ag-sheathed Bi,Pb(2223) tapes has been investigated both by HIP thermal treatment (up to 200 MPa) and in-situ uniaxial pressing (up to 10 MPa), in order to increase the density of the ceramic core thus improving the critical current. In order to prevent the pressurising gas in the HIP furnace from penetrating into the tape, the tape ends were immersed in liquid silver. Extended comparisons between tapes treated with or without overpressure have been performed, and the effect of applying pressure at different stages of thermal processing has been investigated. The effect of the pressure (up to 1000 bar) on the formation kinetics, microstructure and critical current density was studied. A pressure of ~100 bar proved to be effective in improving the filament density and promoting the Bi,Pb(2223) phase formation. However, annealing without pressure and a cold deformation step were found to be more suitable for improving the grain connectivity and the critical current. The work is still in progress in order to find the appropriate combination of low pressure and high pressure processing steps. High pressure annealing steps are found to be more effective if applied on already partially reacted tapes than at the early stages of the reaction thermal treatment. As an alternative processing route, uniaxial hot pressing was applied during either the first or second stage of the heat treatment in air. In both cases, an increase of the oxide filament density was observed. If the pressure is applied during the first stage, when a large amount of liquid is involved in the reaction, the results are not well reproducible and the critical current can be

strongly degraded by the pressure. On the contrary, applying pressure only during the second stage, when the amount of liquid is lower, caused a high densification and a reproducible enhancement of Ic and Jc by about 40 and 100% respectively.

Fe-sheathed MgB2 tapes and wires The development of ex-situ Fe-sheathed MgB2 conductors is undertaken to investigate the potential and the limits of this material in view of possible industrial applications. We used “ex-situ” Powder In Tube (P.I.T.) technique to fabricate monofilamentary tapes, including a series of deformation (drawing and rolling) steps and a final heat treatment at 920°C for 30 min. Controlling the conditions of the precursor powder preparation allowed to reach a good homogeneity of the tapes over the whole length. The highest values of Jc of our tapes were 1 104 A/cm2 at 7 T. The PIT technique has the disadvantage of leaving some porosity in the conductor core. Hot Isostatic Pressure (HIP) has been applied during reaction in order to enhance the density of the superconducting filaments inside the iron sheath, thus improving the transport properties. The critical current density was found to double at high fields after HIP treatment, as shown in Fig 6. Stainless Steel sheathed samples are expected to favour the compaction of the filament also during the early deformation steps.

Fig 6. Critical current density Jc of a MgB2 tape treated in vacuum and at 500 bar. Critical current is enhanced by a factor 2 at high applied fields.

Pinning properties of Fe-sheathed MgB2 tapes The pinning properties of the Fe-sheathed MgB2 tapes fabricated at the DPMC have been investigated by means of AC susceptibility measurements. In particular, the third harmonic response has been measured as a function of the DC magnetic field up to 9 T, at various

11

temperatures, frequencies and amplitudes of the AC field. The irreversibility line has been determined using the third harmonic onset criterion for samples made with different ball-milled MgB2 precursor powders. The study of the third harmonic has allowed us to probe the nature of the vortex pinning and to prove the effect of the precursor particle size on the irreversibility field. The onset of the third harmonic, that is ascribed to a pinning enhancement, was found to be shifted at higher field in samples made with a strongly ball-milled precursor powder having sub-micron particle size.

Fig. 5: ( )H3χ dependence measured at T=20 K, f=1607 Hz

and µ0H=4·10-4 T for tapes prepared using precursor powders from various treatments. The shift of the onset to higher field is due to the enhancement of vortex pinning. IC versus strain measurements in superconducting wires and tapes In collaboration with B. Seeber and D. Uglietti (GAP-Supra), the dependence of the critical current under strain has been measured on Nb3Sn, Nb3Al, Bi2223 and MgB2 wires and tapes. The testing device (WASP) can feed up to 1000 A in fields up to 17T. The Ic(ε) values of MgB2 tapes manufactured at the DPMC have been measured as a function of field and temperature Fig 7. The critical current under tensile strain was measured on tapes of 1 m. The critical current increases under strain, the phenomenon being more relevant at high field. The occurrence of a maximum around 0.31% applied strain confirms the "pre-stress" model. The application of further strain causes a sudden decrease of the critical current, which become zero at 0.36 % tensile strain corresponding to the breakage of the filament.

Fig 7: Critical current density Jc versus tensile strain at various applied fields and temperatures .

The longer length of the wire compared to other devices (where L<1cm) allowed to apply a more sensitive criterion for Ic, 0.01µV/cm (instead of 1µV/cm). The relation between the strain behaviour and the shape of the V-I curve has been studied. As shown in Fig.8, the beginning of filaments fracture at ε > 0.7% corresponds to a non linearity of the V-I curve.

Fig 8. V-I curves of a Nb3Sn wire at 13T from 0% up to 0.9% tensile strain.

Selected references V. Garnier, R. Passerini, E. Giannini, and R. Flükiger, Supercond. Sci. Technol. 16 (2003) 820. E.Giannini, V. Garnier, I. Savysyuk, R. Passerini, G. Witz, X.D. Su, B. Seeber, L. Hua and R. Flükiger, IEEE Trans. Appl. Supercond. 13(2) (2003) 3265. E. Giannini, V. Garnier, R. Gladyshevskii and R. Flükiger, Supercond. Sci. Technol. 17 (2004) 220. N. Musolino, S. Bals, G. van Tendeloo, N. Clayton, E. Walker and R. Flükiger, Phys. C 399 (2003) 1. R. Flükiger, H.L. Suo, N. Musolino, C. Beneduce, P. Toulemonde and P. Lezza, Phys. C 385 (1-2) (2003) 286, and Phys. C 387 (3-4) (2003) 419. D. Uglietti, B. Seeber, V. Abächerli, A. Pollini, D. Eckert, and R. Flükiger, Supercond. Sci. Technol. 16 (2003) 1000.

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Thierry GIAMARCHI

Theory of condensed matter Research summary: The group works on the theory of condensed matter, in connection with various experimental groups. The research activities include the study of strongly correlated electronic systems, ab initio calculations, and disordered elastic systems. For strongly correlated systems the main focus has been to understand out of equilibrium phenomena, such as tunnelling and transport, and to study the dimensional crossover in quasi-one dimensional structures, such as organic superconductors and ultra-cold atoms in optical lattices. For disordered elastic systems, the X-ray diffraction properties of pinned charge density waves have been analysed. Given the resolution of diffraction experiments possible in these systems, they are prime candidate to test the new phases proposed for disordered elastic systems (such as the Bragg glass). Ab initio calculations have been used to study the metal insulator transition in Ge doped FeSi, to analyse the Fermi surface in systems such as Nb3Sn and to study the spin-phonon coupling in High Tc systems. In High-Tc the structure and the tunnelling properties of d-wave vortices have been investigated numerically.

Tunneling in organic superconductors

It was observed during recent years that, at low temperatures, certain superconducting organic compounds (like (TMTSF)2 -PF6, -ClO4, etc.) show upper critical fields in excess of the Pauli-Clogston paramagnetic limit; usually depending on the angular orientation of the field with respect to the crystalline axes. High critical fields constitute an indication of possible triplet-pairing superconducting phases. An unambiguous interpretation of this effect is of course difficult and independent confirmations of the unconventional paring scenario are therefore desirable. A large number of independent experimental checks were both proposed and some carried out. We were concerned with the relevant signatures of triplet superconductivity to be found in proposed point-contact tunneling experiments.

Fig.1. Three different sets of I-V characteristics vertically displaced for clarity. From top to bottom: (a) for an N-S junction, (b) N-T junction, (c) S-T junction. The red (blue) lines correspond to the characteristics in the absence (presence) of an oriented magnetic field. In all three sets the curve for the N-N case (green) is plotted as reference and in the third one the curve for an S-S junction (purple) is also given for comparison.

We developped a method to compute the tunneling in superconducting junctions based on a non-equilibrium Green function formalism that allows to treat the case of spin-triplet pairing. We have shown that distinctive features are present in the I-V characteristics of different kinds of junctions, in particular when the Zeeman effect of a magnetic field is taken into account (see Fig.1). The observation of such features would constitute a sensitive experiment that permits to identify the type of pairing. For more details and further reading, see C. J. Bolech and T. Giamarchi, cond-mat/0309195 (to be published in PRL).

Deconfinement in quasi-1d systems

The physical properties of systems vary enormously with their dimension. In particular in one dimension, interactions and quantum fluctuations play such an important role that a one dimensional quantum system has very specific properties. In particular a one dimensional system is very sensitive to the presence of a lattice and leads, for densities commensurate to the lattice to an insulating state known as a Mott insulator. Since in one dimension a Mott insualtor state is much more easy to obtain than in higher dimension, this prompts for the question of what happens for a quasi-one dimensional geometry. For coupled 1D chains the competition between the Mott insulating physics and interchain tunneling leads to a deconfinement transition where the system goes from a 1D insulator towards a 3D metal or superfluid. For fermions this transition remains a theoretical challenge and has been observed in coupled chain systems such as the organic conductors. We developped a mean field approach to tackle this question, by replacing the problem of coupled chains by a single chain coupled to an effective bath. This method allows us to describe the deconfinement transition for coupled Hubbard chains and to analyse the lifetime and

13

excitations of the system. A strong variation of the lifetime has been observed close the deconfinement transition. Extention of this analysis to quarter-filled Mott insulators is under way. Another particularly interesting situation is provided by coupled bosonic chains. In condensed matter, the bosonic situation applies in the context of spin chains, but recently an extremely clean and tunable realization of such a system has been provided by ultracold atoms trapped in optical lattices. We investigated a system of a two dimensional lattice of long one dimensional tubes. Such a system has a deconfinement transition between a Mott insulator of bosons and a three dimensional superfluid (see Fig. 2). We computed the phase diagram and the various phyiscal quantities (condensate fraction, modes etc.). Due to the interactions there is a large depletion of the condensate which makes the phyiscs quite different from the usual case of a simple anisotropic three dimensional bose condensate.

Fig.2. Zero-temperature phase diagram of a 2D lattice ofcoupled 1D boson systems. K is the Luttinger parameter.and J the interchain tunnelling for the bosons and µ the chemical potential. For infinite 1D systems only two phases exist: a 1D Mott insulator (1D MI) and a 3D (but anisotropic) superfluid (SF). For finite tubes, a 2D Mott insulator can exist below the horizontal dashed curve. The insert is the phase diagram of a 2D optical lattice of finite 1D systems.

For more details, references and further reading, see T. Giamarchi et al., cond-mat/0401268 (to be published in J. Physique IV); A. F. Ho et al., cond-mat/0310382.

X-ray diffraction of pinned charge density waves

The statics and dynamics of disordered elastic objects govern the physics of a wide range of systems, either periodic, such as vortex flux lines and charge density waves (CDW), or involving propagating interfaces, such as domain walls in magnetic or ferroelectric systems. It was recently shown that periodic systems have unique properties, quite different from the ones of interfaces. If topological defects (i.e. dislocations, etc.) in the crystal are excluded, displacements grow only logarithmically instead of the power-law growth for interfaces. The positional order is only algebraically destroyed leading to divergent Bragg peaks and a nearly perfect crystal state. Evidence of this phase, nicknamed Bragg glass, has been provided recently in vortex systems by neutron diffraction experiments.

Fig.3. Intensities of different contributions to satellite peaks. The more divergent term is symmetric. The other two are antisymmetric and generate the asymmetry observed in the experiments.

Other periodic systems in which one can expect a Bragg glass to occur are charge density waves, where the electronic density is spatially modulated. Disorder leads to the pinning of the CDW. In such systems very high resolution X-ray experiments can be performed. The resolution is in principle much higher than the one that can be achieved by neutrons for vortex lattices, consequently CDW systems should be prime candidates to check for the existence of a Bragg glass state. However, compared to the case of vortex lattices the interpretation of the spectrum is much more complicated for two main reasons: (i) the phase of the CDW is the object described by an elastic energy, whereas the X-rays probe the displacements of the atoms in the crystal lattice (essentially a cosine of the phase); (ii) since the impurities substitute some atoms of the crystal, the very presence of the

14

impurities changes the X-ray spectrum. This generates non-trivial terms of interference between disorder and atomic displacements. It is thus necessary for the interpretation of the diffraction due to a pinned CDW, to carry out a detailed theoretical analysis. We have shown that the diffraction spectrum consists of two asymmetric and power-law divergent peaks with an anisotropic shape. This form is consistent with the Bragg glass behavior. For more details and further reading, see A. Rosso and T. Giamarchi, Phys. Rev. B 68, 140201(R) (2003). Non linear transport in Mott insulators and Luttinger liquids

Luttinger liquids are very sensitive to perturbations that can localize the charge. This is the case of a perdiodic potential (due for example to the presence the lattice) or to impurities. The first case leads to a Mott insulating state, the second one to an Anderson insulator where the electrons are localized by the impurities. These two states are insulators, and thus their linear conductivity vanishes at zero temperature, and these systems present a non linear I-V characteristics. We have computed this non linear response for the case of very small electric field E. In that case the transport is determined by tunnelling events between pinned states (either on the periodic potential or the impurities) and is of the form

ν)/( EAeI −∝ where ν=1 for the Mott insulator and ν=1/2 for the disordered case. This is the quantum analog of the creep phenomenon for classical systems where the system overcomes barriers though thermal activation. Here quantum effects allow to pass the barriers. At finite temperature the system can overcome the pinning thanks to thermal activation and the response becomes linear with the current proportional to the electric field. The conductivity has the same exponential form than above but with E replaced by the temperature T. For the disordered case this is a law similar to the variable range hopping of Mott. It is interesting to note that one recovers here this result directly from the bosonized formalism. The interactions are thus completely taken into account. As a consequence the energy scales (the constant A) are determined by the kinetic energy and the electron-electron interaction, which considerably changes the value of this constant (simply the density of states for the standard VRH law).

For more details and further reading, see T. Nattermann et al., PRL 91 056603 (2003).

Metal-Insulator transition in Ge doped FeSi

The family of FeSi related alloys include the alloy with Ge-substitutions for Si. Recent mesurements show that FeSi1-xGex becomes metallic and magnetic for x larger than about 0.3 (Yeo et al, PRL 91, 046401, 2003). We have shown previously that thermal disorder makes FeSi metallic and exchange enhanced at elevated temperature. We pursued this study by consideration of substitutional disorder in FeSi1-

xGex by studies of the electronic structure in 64-site supercells as function of x. The gap is closed for x larger than 0.3 and weak Stoner magnetism occurs because of the large DOS near the gap region. The magnetic moment increases with x until x=1 when disorder is absent. The effective density of states (DOS) at the Fermi energy is large for intermediate x when the moment is small, while at larger x when the moments are larger, it is lower. This qualitative behaviour can explain the observed electronic specific heat as function of x. For pure FeGe it is shown that the gap is closing when the volume is increased, and the existence of relatively large magnetic moments in FeGe is an effect of the increased lattice constant. These results suggest that the use of pressure in FeGe is better than Si-Ge substitutions for studies of the metal insulator transition, because of the absense of substitutional disorder. The role of enhancement effects near the metallic transition has been studied for disordered FeSi in order to search for explanation of reports of non-conservation of spectral weights in optical measurements. The electron phonon coupling is not as important as spin fluctuations for the increased effective mass. Spin fluctuations are less important when a magnetic state becomes stable on the metallic side of the transition.

Fig.4. Magnetic moment as function of x for the FeSiGe alloy system. Alloys with x smaller than 0.3 has a gap (1-6 mRy) at EF.

15

Spin-phonon coupling in high Tc copper oxides

Band calculations for Hg-based high-Tc oxide including staggered magnetic fields to model spin fluctuations show that so called half-breathing phonons within the Cu planes are favorable to spin fluctuations. These excitations look as stripes in real space and they generate pseudo-gaps in the density of states. The wave lengths of the excitations are short for large doping, while at zero doping the wave length becomes infinite. However, when the local density approximation (LDA) is used one needs a large magnetic field to stabilize the anti-ferromagnetic (AFM) insulating state. By using the same potential, but with unusually low linearization energies in the LMTO band calculation one obtains narrower, more localized (O-p) states. The hybridization energy caused by the Cu-O orbital overlap will decrease in this case, and the band results show that an AFM state with a pseudogap can be stabilized without a supporting magnetic field. The same type of calculations for the doped system show that the tendency for spin fluctuations are increased. The coupling for spin fluctuations λsf increases from 0.25 in the normal LDA to about 0.5. No calculation of the superconducting Tc for a spin-phonon coupled system is attempted, but it is expected that the results contain both isotope shifts and magnetic excitations within the superconducting regime. Internal distortions and strain increase the spin-phonon coupling in the Hg-based system. Various reports of enhanced Tc in strained La-based systems led us to make calculations also for this system. The first results appears to be similar to those for the Hg-based case, but a difference is that the pseudogap caused by the phonon alone (without spin wave) is more pronounced in the La-system than in the Hg-system. Fermiology and momentum densities

A study of the effect of tetragonal distortion and dimerization on the Fermi surface (FS) of Nb3Sn has been inspired by the search for two band superconductivity made by the group of A. Junod. The structural deformation leads to a drop of the DOS, which moves the system away from a Stoner instability, but at least 4 FS pieces remain with one piece dominating the FS properties. The situation in V3Si is similar, but with larger DOS and even larger Stoner enhancements.

The group in Bristol is active in theoretical and experimental determinations of momentum densities, both via Compton profiles and positron annihilation. The collaboration has continued through studies of many systems. For instance, the comparison between measured and calculated momentum density in the newly discovered superconductor ZrZn2 have shown that the Fermi surface and magnetic moment is essentially correct when calculated within LDA. The study does not give an immediate reason for superconducting pairing, but two of the FS pieces show nesting, which is important for some theories of magnetic fluctuations and superconductivity.

Vortex-core states

The local density of states (LDOS) of vortex cores in high-Tc materials (HTS), as measured by STM, continues to challenge the theories of high-temperature superconductivity. Even in the BCS framework, no complete analytical solution for the d-wave vortex has been found so far, and our understanding of the key parameters which determine the electronic structure in the core remains poor. Based on extensive numerical simulations, we have found that the suppression of the superfluid density in the core region plays no significant role for the core states, while the vortex supercurrent is the main actor in their formation. More precisely, it turns out that the energy spectrum is determined primarily by the vorticity in the region occupied by the core states. In particular, the calculated LDOS for a "vortex" without supercurrent (zero vorticity) exhibits no core states. Some theories of the HTS's assume that a phase of fluctuating unbound vortex-antivortex pairs exist in the normal state. Whether the spectrum of these pairs does or does not resemble the spectrum of vortices in the superconducting state is a question which has not been investigated yet. We considered a variety of vortex-antivortex configurations and reached the conclusion that the core states should be absent in a BCS vortex-antivortex soup. Indeed, in this situation the vorticity vanishes on average, and one can expect that the core LDOS is similar to the LDOS of an isolated vortex without supercurrent. For more details and further reading, see T. Jarlborg, Phys. Rev. B 68, 172501 (2003); C. Berthod, cond-mat/0401417.

16

Specific heat and magnetocaloric effect of metals and superconductors

Research summary: Our group specialises in thermodynamic studies of superconductors and new materials. In 2003, part of the effort was turned to the renewal of experimental techniques (high pressure and thin-film calorimetry). We showed how heat capacity measurements can determine the bulk distribution of superconducting critical temperatures in multifilamentary wires. More traditional techniques were used to study superconducting borides, high-temperature super-conductors, and a technologically important A15 compound. The study of irradiation effects on the double gap of MgB2 was completed. We found that also Nb3Sn exhibits the characteristic features of a two-gap superconductor. An unusual crossover between type-I and type-II superconductivity was discovered in ZrB12. For the first time, the magnetocaloric effect was used to study extensively the irreversibility and the melting line of the vortex lattice in NdBa2Cu3O7 and YBa2Cu3O7 crystals. Bulk signatures of d-wave superconductivity were searched for in YBa2Cu3O7 and Bi2Sr2Ca2Cu3O10. The isotope effect on the pseudogap temperature T* was investigated in HoBa2Cu4O8.

MgB2: irradiation effects

The model of two-gap superconductivity is now well accepted for the ~39-K superconductor MgB2. After having investigated the thermo-dynamic and anisotropic properties of clean samples, we focussed on the crossover to single-band behavior which is expected to occur when scattering exceeds some threshhold. In a collaboration with the group of H. W. Weber (Atominstitut der Österreichischen Universitäten, Vienna), a polycrystalline sample was irradiated by neutrons up to a fluence of 6×1022 m-2. The critical temperature Tc decreased to 26 K, suggesting that interband scattering was large enough to reach the single-gap limit. Although some convergence of the partial gaps was noted, we did not observe the expected single-gap BCS behavior. The specific heat data could be fitted either by a two-gap model, or by an anisotropic gap ∆0(k) with minimum and maximum values of 0.6kBTc and 2.2kBTc, respectively. Such an ambiguity did not occur for smaller fluences. It is concluded that if the final gap is unique, then it must be strongly anisotropic.

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Fig.1. Electronic specific heat of MgB2 after irradiation. Fits using a two-gap model and an anisotropic single gap model are shown to be nearly equivalent in this case.

ZrB12: thermodynamic properties

The superconducting critical temperature of ZrB12, ~6 K, is the second highest of all borides. Uncertainties remained as to the shape of the vibrational spectrum. We performed specific-heat (C) and magnetization (M) measurements on a single crystal synthesized by Y. Paderno (IPMS Kiev, Ukraine; collaboration with V. Gasparov, ISSP Chernogolova, Russia). The results could be interpreted in terms of single-band, medium-coupling superconductivity with 2∆0/kBTc = 3.7. A model of the phonon DOS obtained by inversion of the specific heat up to 300 K helped to understand the anomalous shape of the resistivity versus temperature curve. An uncommon feature was observed in the mixed-state specific heat. ZrB12 appears to be one of the few superconductors that changes from type I to type II as the temperature is lowered, developing 1st-order transitions with a latent heat at low fields, and 2nd-order transitions in higher fields.

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1.2 0 G 4 G 44 G 76 G 94 G 144 G 244 G 344 G 444 G 464 G 5000 G

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)

T (K)

Fig.2. Specific heat of ZrB12 in different magnetic fields.

Magnetization curves could also be interpreted in terms of marginal type-II superconductivity with a Ginzburg-Landau parameter κ(T) close to 2−½. A similar situation occurs in tantalum [J.F. Sporna et al., J. Low Temp. Phys. 37, 639 (1979)].

Alain JUNOD

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Fig.3. M versus H curves for ZrB12 at different temperatures.

Nb3Sn: specific heat of multifilamentary wires

Superconducting wires made of Nb3Sn filaments embedded in a copper matrix are widely used. The bronze technique produces a material that is characterized by a range of compositions Nb1-xSnx with 0.18 ≤ x ≤ 0.25. The corresponding distribution of Tc is not easily measured by conventional techniques because of percolation and/or magnetic shielding effects. In a collaboration with R. Flükiger, the specific heat of a 30 mg Nb3Sn multifilamentary wire was measured with its bronze matrix in H = 0 and 14 T. We fitted a thermodynamic model which, together with the use of a reference material, allowed the distribution of Tc to be obtained in the volume of the sample. This new method of characterization is of primary importance as it can guide and quantitatively assess technological improvements.

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C (

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.g)

T (K)

Fig.4. Specific heat of a bronze-coated multifilamentary Nb3Sn wire at 0 and 14 T. Inset: distribution of Tc obtained from these curves. The minor peak is due to unreacted Nb.

Nb3Sn: a second gap

The specific heat of bulk Nb3Sn, a super-conductor of great technological interest with a critical temperature Tc ≅ 18 K, was reinvestigated in the temperature range up to 200 K, and in fields up to 16 T. A dense and homogeneous polycrystalline sample was

synthesized by W. Goldacker (Forschungs-zentrum Karlsruhe) and characterized in detail with respect to its microscopic and superconducting parameters (e.g. Hc2(T)). We particularly investigated an anomaly, already noticed in the past at low temperature in zero field, and found that it can be ascribed to a second superconducting gap ∆S ≅ 0.4 kBTc, in addition to the main one ∆L ≅ 2.4 kBTc. The signature of this minor gap, which affects 7.5% of the electronic density of states, vanishes in fields above ~7 T. Based on this result, and on tentative spectroscopic data (G. Goll, Univ. Karlsruhe, private communication), it is concluded that Nb3Sn belongs to the intriguing family of multi-gap superconductors. Further experiments are in progress.

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Fig.5. Specific heat Nb3Sn (main panel) compared to that of the two-gap superconductor MgB2 (inset). The excess at low temperature with respect to the BCS curves (shown in blue) is characteristic of a second gap.

NdBa2Cu3O7: magnetocaloric effect

The H-T phase diagram of a single crystal of the high temperature superconductor NdBa2Cu3O7 synthesized by T. Wolf (Forschungszentrum Karlsruhe) was studied by specific heat C = (δQ/δT)H, isothermal magnetocaloric effect MT = (δQ/δH)T, and magnetization M. Figure 6 summarizes some results and shows the complementarity and wealth of information given by these different techniques. Circles denote the melting line of vortex matter (Tm): the red ones are points determined by specific heat, the green ones by magnetocaloric effect. Diamonds denote the onset of irreversibility (Tirr): the green ones are determined by magnetocaloric effect, the yellow ones by magnetization loops. The color-scale contour plot based on the magnetocaloric irreversibility (∆MT = MT

+ − MT−) represents the

distribution of critical current densities. The gray-scale contour plot shows the electronic contribution to the specific heat Ce/T. It describes the progressive evolution from the

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sharp second-order superconducting phase transition in zero field toward a wide crossover for higher magnetic fields.

Fig. 6. Phase diagram of NdBa2Cu3O7 in the H-T plane (see text) The dash-dotted line shows a hypothetical line connecting both parts of the Tirr line.

YBa2Cu3O7: magnetocaloric effect

The isothermal magnetocaloric effect MT = (δQ/δH)T was studied for a 60 mg detwinned single crystal of YBa2Cu3O7 synthesized in the group of S. Tajima (ISTEC, Tokyo). The sample was overdoped (Tc ≈ 88.5 K).

B [ T ]

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84 K82 K

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Fig.7. Magnetocaloric effect in YBa2Cu3O7 for H//c (see text).

Figure 7 presents different regimes of MT(H) depending on temperature. The black curve (89 K) shows essentially zero magnetocaloric effect just above Tc. The red curve (88 K) taken just below Tc shows a negative reversible signal due to condensation energy that diverges for H → 0, and asymptotically vanishes for H → Hc2. The green curve (84 K) shows irreversibility setting in, and vanishing abruptly at a given value of the field (here Birr = 1.9 T). The blue curve (82 K) shows an intermediate behavior where the hysteresis ends into a weak 1st-order peak. The magenta curve (77 K) shows a well-developed melting peak associated with a 1st-order transition between the ordered and disordered phases of vortex matter. The position of the peak shifts towards higher fields with decreasing temperature. At 60 K (cyan curve), melting occurs beyond the maximum experimental field;

|MT| decreases and hysteresis reappears at low fields. Irreversibility vanishes smoothly; no "fishtail" effect is present below 13 T, showing low pinning. This subset of data demonstrates how detailed information can be obtained from the magnetocaloric effect up to high fields. Contrary to specific heat, phonons do not contribute, and irreversibility can be investigated. This method gives better access to thermo-dynamic contributions than magnetization, and melting anomalies are better defined. YBa2Cu3O7: d-wave and ab-plane anisotropy

Although surface-sensitive experiments support d-wave superconductivity, bulk evidence is still scarce in superconducting oxides. One such evidence would be given by the observation of a π/2 angular periodicity in the mixed-state specific heat γ(H,φ)T at T << Tc in the basal plane. A variation up to 30% peak to peak is predicted, with minima for the field H along the nodal direction [110]. The specific heat of a detwinned crystal of YBa2Cu3O7 (see previous section) was measured with H // [100], [110], and [010]. The exceptionnally small contribution of para-magnetic centers, equivalent to 0.02% of the Cu atoms having a spin S = ½, still tends to mask the anisotropy of the electronic contribution γ(H,φ)T. The latter surprisingly shows a π periodicity, 6% peak-to-peak, without any extremum on the nodal direction. This means that the angular dependence of C/T is dominated by the anisotropy of Hc2(θ,φ) in the basal plane rather than the effect of the nodes. This experiment does not support simple views on d-wave pairing symmetry in YBa2Cu3O7. More detailed measurements are in progress.

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H=0 [100]

H=0 [110]

H=4 T [100]

H=8 T [100]

fit H=8 T [100]

H=8 T [110]

fit H=8 T [110]

H=8 T [010]

fit H=8 T [010]

Fig.8. Specific heat C/T of YBa2Cu3O7 for the field along different crystallographic directions in the basal plane.

Bi2Sr2Ca2Cu3O10: d-wave contribution

Single crystals of the Bi-2223 superconductor with Tc = 111 K have been prepared by E. Giannini at the DPMC. We looked at the low temperature specific heat in magnetic fields 0≤H≤14 T to see if the characteristic contribution

19

of gap nodes could be identified. The analysis was made difficult by the small mass (300 µg), the large lattice background, and by a parasitic magnetic Schottky contribution (both about 6 times larger than for Y-123). It is concluded that if a term C/T ∝ H½ does exist, its amplitude amounts to about ¾ of that present in Y-123.

0

1

2

3

4

0 5 10 15

T² (K²)

C/T

(m

J/K

²ga

t)

H=0

fit H=0

H=4 T

fit 4 T

H=8 T

fit 8 T

H=12 T

fit 12 T

H=14 T

fit 14 T

Fig.9. Specific heat C/T of Bi2Sr2Ca2Cu3O10 for the field along the c-axis.

Cr2FeSe4: giant diamagnetism

Measurements of the DC magnetization and AC susceptibility of polycrystalline Cr2FeSe4 powder samples synthesized by G. Lamarche (University of Ottawa, Canada) have been performed, showing an anomalously large diamagnetic signal appearing below room temperature in small fields of several Gauss, somewhat reminiscent of superconductivity at unusualy high temperature. Based on magnetic relaxation experiments and the analysis of the slightly hysteretic behavior, we showed that the signal may be assigned to a metastable inversed weakly ferromagnetic state, in analogy to a phenomenon known in the literature as the ‘inverse Wohlleben effect’. This effect is known to appear in some small ferromagnetic samples with a surface having a slightly higher transition temperature than the bulk, so that the surface magnetization polarizes the bulk antiparallel (‘diamagnetic’) to the applied field.

0 50 100 150 200 250 300-0.3

-0.2

-0.1

0.0

FC

ZFC

H=77 Oe

χ V (

S.I

.)

T (K)

Fig.10. Susceptibility in a field of 7.7 mT of a polycrystalline Cr2FeSe4 sample in ZFC-mode (black data), upon cooling in a constant field (blue) and the corresponding FC-data (green), showing the anomalous large diamagnetism below room temperature.

HoBa2Cu4O8: pseudogap

Is the opening of a pseudogap at T* > Tc a true phase transition in high-temperature super-conductors? Sharp features at T* and a large isotope effect were reported in HoBa2Cu4O8 (Ho-124) [Rubio Temprano et al., Eur. Phys. J. B19, 5 (2001)]. We searched for anomalies at T* in the specific heat of the same samples. Our results based on three isotopic substitutions exclude any sharp signature in thermodynamics to within the resolution of the experiment (≈0.02%). It is concluded that the opening of the pseudogap is either a smooth crossover, or a transition only in the dynamics of the system (collaboration with A. Furrer, PSI/ETHZ).

-1%

0%

1%

2%

3%

4%

5%

6%

7%

50 100 150 200

T (K)

(C -

Cfit)

/ C

HoBa263Cu4O8, Tc = 78.6 K

HoBa2Cu418O8, Tc = 77.6 K

HoBa265Cu4O8, Tc = 79.1 K

HoBa2Cu416O8, Tc = 78.1 K

Fig.11. Specific heat of isotopes of Ho-124 relative to that of natural Ho-124 (11 independent runs). Circles indicate the pseudogap temperatures T* reported by Rubio Temprano et al. The anomaly on the left is due to superconductivity, that at 212 K to CuO (<3 wt%).

Experimental projects: status

A 6 T split coil magnet system was successfully commissioned, allowing field-dependent effects in anisotropic materials to be studied at low temperature. It will be fitted with a variable temperature insert in 2004. A pressure cell using liquid and solid pressure transmitting media, especially designed for the use in high magnetic fields, in the temperature range from 1 to 300 K, and at pressures up to ~20 GPa, was successfully tested (collaboration with D. Jaccard). It was possible to compensate for the thermal expansion of the cell over the whole temperature range, thus maintaining the pressure constant. Calorimetric experiments on high-temperature superconductors are currently in preparation. Tests of calorimetry based on the 3ω technique have been conducted in view of applications to thin films. Further reading: F. Bouquet et al., Physica C 385, 192-204 (2003); DEA work of V. Guritanu (2003) and Ph.D. thesis of Y. Wang (2004), University of Geneva.

20

Fundamental excitations of correlated matter

Research summary: The group works on the properties of interacting electrons in solids. Prominent examples of unanticipated new phenomena are the fractional quantum Hall effect, high temperature superconductivity, colossal magneto resistance, and quantum phase transitions. The project focuses on physical situations where changes of state of matter take place. Invariably a transition of one state of matter to another is accompanied by a characteristic change of the kinetic energy of the electrons, which can be monitored by measuring the phase stiffness as revealed by the low energy optical spectral weight. Phase transitions are moreover often accompanied by an abrupt change of symmetry at the critical point, and this is revealed optically by the appearance or disappearance of vibrational modes in the infrared range, and by changes of the visible part of the optical spectrum. Ordering phenomena are usually accompanied by the appearance of new collective modes, which in some cases can be detected optically. With this approach we investigate the electronic structure of

Dirk van der Marel superconductors in the cuprate and heavy fermion families, Kondo insulators, itinerant ferromagnetic materials and (in)organic one-dimensional materials. Scale invariant optical response in optimally doped Bi2212

Fig.1. Universal powerlaw of the optical conductivity and the phase angle spectra of optimally doped Bi2Sr2Ca0.92Y0.08Cu2O8+δ. In a the phase function of the optical conductivity, Arg(σ(ω)) is presented. In b the absolute value of the optical conductivity is plotted on a double logarithmic scale. The open symbols correspond to the powerlaw |σ(ω)|=Cω–0.65[1].

The possibility that a quantum critical state of matter can be realized in correlated electron systems near a quantum phase transition (QPT) occurring at zero temperature, is at the focus of current research in condensed matter physics, because the response of such a system is

expected to follow universal patterns defined by the quantum mechanical nature of the fluctuations. This universality manifests itself through power-law behaviours of the response functions. Candidates are found both in heavy fermion systems and in the cuprate high Tc superconductors. Although there are indications for the occurrence of a quantum critical state in optimally doped cuprate superconductors, the reality of a QPT in these materials and the physical nature of the phases are still under debate. The most important piece of evidence comes from the measured phase of the optical conductivity, which turns out to be independent of the wavelength and frequency of the light (see Fig. 1). We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of power law dependence of the optical conductivity. Further proof was presented by demonstrating that temperature and (radiation-) frequency are, up to a point, interchangable in the quantum critical region. This points towards a QPT in the cuprates close to optimal doping, although of an unconventional kind.

Novel collective modes in cuprate superconductors In the bi-layer high Tc materials materials the coupling within the bi-layers may provide an additional source of frustrated interlayer kinetic energy, which can in principle be released when the material enters the superconductng state. This can in principle be monitored with infrared spectroscopy, because quite generally a stack of Josephson coupled layers with two different types of weak links alternating (in the present context corresponding to inter-bilayer and intra-bilayer) should exhibit three Josephson collective modes instead of one: Two of those are longitudinal ones, which show up as peaks in

21

the energy loss function, Im(-1/ε(ω)). In between the two longitudinal resonances the theory predicts the presence of a transverse optical plasma resonance, which is revealed by a peak in the optical conductivity, Re σ(ω). In essence the extra two modes are out-of-phase oscillations of the two types of junctions. The existence of two longitudinal modes and one associated transverse plasmon mode at finite frequencies has been confirmed experimentally for SmLaySr1-yCuO4-x at various chemical compositions (see Fig. 2)[2].

Fig.2. (a) Real part of the c-axis dielectric function of SmLa0:8Sr0:2CuO4±x for 4 K (closed symbols), and 20 K (open symbols) (b) The c-axis loss function(c) Real part of the c-axis optical conductivity.

The most commonly used experimental method for studying optical properties of solids is by measuring the reflectivity spectrum, and to use Kramers-Kronig analyses to obtain the optical conductivity. This method fails if the spectrum contains large frequency regions where the conductivity is small, due to the non-local progression of experimental errors which are intrinsic to Kramers-Kronig analyses. One encounters this situation for the c-axis optical conductivity of most cuprate superconductors in the frequency range between 0.1 and 3 eV. In these cases one can try to measure both the reflectivity and transmission through a slab of suitable dimensions. Because the electric

polarization of the infrared waves has to be parallel to the c-axis, it is necessary to have the c-axis parallel to the surface of the slab. The thickness of slab should be of the order of 10 to 20 micrometer, in order to have a reasonable throughput of infrared light, and the lateral dimensions should be on the order of a millimeter. The fact that most cuprates are weakly bonded along the c-direction poses a serious experimental challenge. In Figs 4 and 5 we illustrate two examples of such slabs, for a La214 single crystal and for a Bi2212 single crystal.

Fig.3. 20 µm thick slab La1.85Sr0.15CuO4 which was cut perpendicular to the planes. The light gray area in the middle is the ac-face of the sample. The dark circular shape is a conical aperture placed in front of the sample. It serves both as a sample support and as an optical aperture defining the diameter of the infrared beam passing through the sample [3].

In all different cuprate superconducting investigated upto date (underdoped and optimally doped La214, optimally doped and overdoped Y123, optimally doped Bi2212) we observe a weak absorption peak at approximately 0.1 eV. This peak therefore appears to be a common feature of the cuprate family. The intensity of the 0.1 eV resonance tracks the superfluid density, hence it appears to have some connection to the superconducting order.

0

5

10

15 300 K 100 K 29 K (T

c)

10 K

x=0.12

100 1000 100000

5

10

15 300 K 100 K 37 K (T

c)

25 K

x=0.15

σ 1 (

Ω-1cm

-1)

Wavenumber (cm-1)

Fig.4. C-axis optical conductivity of La2-xSrxCuO for two different doping concentrations [3].

22

The nature of this new resonance mode is not clear. In the case of the single layer materials La214 bi-layer plasmons do not exist, making the assignment to internal Josephson modes unlikely. Several alternatives should now be considered, for example amplitude modes of the superconducting order parameter, amplitude or phase modes of competing types of order such as charge-, spin-, or current ordering.

Fig.5. 20 µm thick slab of Bi2Sr2Ca0.92Y0.08Cu2O8 which was cut perpendicular to the copper-oxygen planes. The gray area surrounding the slab is stycast, which serves to support this brittle material. The sample with stycast was mounted on a conical sample holder of the kind shown in Fig. 4.

Bi-2212 (Y)Tc = 97 K

M.Grueninger et al,PRL 84, 1575 (2000)

1000 1500 2000

-2

0

2

σ(10

K)

- σ(

100K

) (

S/c

m)

Wavenumber (cm-1)

A.Kuzmenko et al,PRL, 91 037004 (2003)

La1.85Sr0.15CuO4

O Tc = 29 KO Tc = 37 K

1000 1500 2000

-20

0

20

σ(4K

) -

σ(10

0K

)

Wavenumber (cm-1)

Y-123Tc = 91 K

M.Grueninger et al,PRL 84, 1575 (2000)

1000 1500 2000-20

-10

0

10

σ(4K

) -

σ(100K

)

Wavenumber (cm-1)

Y-123, ODTc = 87 K

1000 1500 2000

-0.2

0.0

0.2

0.4

Wavenumber (cm-1)

σsc

(ω)-

σ n(ω)

Fig. 6 Optical conductivity along the c-axis of four different high Tc superconductor measured at temperatures far below the superconducting phase transition. The optical conductivity of the normal state has been subtracted. For all four materials a weak absorption peak is observed at approximately 0.1 eV. [3]

Heavy Carriers and Non-Drude Optical Conductivity in MnSi

Recent neutron and transport data of MnSi suggest that at pressures above 18 GPa this material has a non-Fermi liquid state, coexisting with a novel form of short range magnetic order (see C. Pfleiderer et al, Nature 427 (2004) 227 and references therein).

10 100 1000 10000

10

10 Kelvin 20 30 40 50 75 100 200

MnSi

σ(ω

) (

kΩ-1cm

-1)

Wavenumber (cm-1)Fig.7. Optical conductivity of MnSi for a number of different temperatures

Our optical conductivity experiments of this material indicate screening properties intermediate between a metal and a superconductor[4]. MnSi has anomalous optical reflectivity properties at low frequencies, which deviate considerably from the Hagen-Rubens behaviour observed in common metals [4]. The magnetism of MnSi and Co-doped FeSi is commonly attributed to itinerant charge carriers. For MnSi this is confirmed by the observation of a narrow Drude-like peak in the optical spectra at low temperatures (see Fig. 7). Yet the spectral weight in this peak is small, indicating that the charge carriers are rather heavy.

0 200 400 600 800 10000

2

4

6

8

10

12

0 50 100 150

0

5

10

15

2

4

6

0 50 100 150

0

1

2

3

4

(b)

1/τ

(ω)

(1

04 m-1)

Wavenumber [cm-1]

100K75K

30K20K10K

(a)

m*(

ω)/m

Fig.8. Effective mass and (upper panel) and frequency-dependent-scattering rate (lower panel) of MnSi at different temperatures. Insets: Same quantities below 200 cm-1.

23

When MnSi is cooled below 100 K the effective mass develops a strong frequency dependence at low frequencies (see Fig. 8), while the scattering rate develops a sub linear frequency dependence. The complex optical conductivity is significantly different from conventional Drude behaviour, which can be attributed to a strong coupling of the itinirant charge carriers to spin-fluctuations or other collective modes.

0.00 0.01 0.02 0.03 0.040

10

20

1/T (K-1)

1/ρ

(k

Ω-1/c

m)

Fig.9. The temperature dependence of the DC resistivity of MnSi, plotted as 1/ ρ(T) versus 1/T [4]. The upward curvature at 1/30 K is where the phase transition into the helimagnetic phase takes place.

The DC resistivity obeys the relation ρ(T)=1/(σIR+αT-1) with a remarkable accuracy, where σIR =3.500 S/cm and α=620.000 S/Kcm. This so-called parallel resistor formula has been known since 1977, in particular for the A15 compounds. MnSi presents the cleanest example of this behaviour reported in the literature, where the temperature dependent component seems to follow a perfect linear temperature dependence over the entire temperature range in the paramagnetic state (i.e. for T>30 K), with no apparent negative zero temperature offset. The formula suggests, that the approach of the Ioffe-Regel limit at high temperature effectively follows a two-fluid approach.

X-ray absorption spectroscopy of transition metal silices

The question whether the transition metal silicides should be regarded as itinerant or localized magnetic materials has been long debated. Most transport properties of MnSi, Co-doped FeSi, and FeGe1-xSix indicate that these materials belong to the class of itinerant ferromagnets. The issue has been dormant for several decades, but recently the discussion has

been rekindled, due to the Kondo-like characteristics of of the semiconductor gap of FeSi, the discovery of Altshuler-Ahronov scaling in Fe1-xCoxSi, and the discovery of short-range helical order in the paramagnetic phase of MnSi. In collaboration with prof. Fulvio Parmigiani (Elettra, Trieste) we have carried out XAS experiments on a number of transition metal silicides. The XAS spectra of MnSi (see Fig. 10) strongly suggest that the Mn-atoms in this compound retain their quasi-atomic character in this compound, with five 3d electrons grouped in the Hunds-rule ground state, with a total spin of S=5/2. This is somewhat surprising, since the saturated moment of Mn measured in the magnetically ordered (helical) phase is on 0.4 Bohr magnetons. Taken together these observations suggest that the possibility must be taken seriously, that Mn has a localized high spin state in these compounds, but that this large value of the spin is very effectively screened by other electrons in the system.

ExperimentalX-ray Absorption Spectrumof MnSi

Opt

ical

Con

duct

iviy

(arb

units

)

Theoretical simulation for a Mn2+ ion (3d5, S=5/2)

640 650 660

Photon Energy (eV)

2p3/2

2p1/2

1S

1D

1G3P 3F

1F1D1S

3D1I1D3P 3F3H

5D

2p1/2

2p3/2

hω ~650 eV

12 eV

3d intra-shellexcited states

Fig.10 Righthand panel: A photon with an energy of 640 to 660 eV can excite a core electron of the Mn atoms to an unoccupied 3d state. Lefthand panel: Experimental and theoretical XAS spectra of MnSi.The relative intensities of the 3d intra-shell excited states form a sensitive fingerprint of the spin and orbital quantum numbers of the Mn atom in the ground state. In the simulation it was assumed that in the ground state the Mn 3d-states are occupied with 5 electrons, with their spins parallel.

For more details and further reading, see: [1] D. van der Marel, H. J. A. Molegraaf, J. Zaanen, Z. Nussinov, F. Carbone, A. Damascelli, H. Eisaki, M. Greven, P. H. Kes, and M. Li,, Nature 425 (2003) 271-274. [2] D. van der Marel in "Strong interactions in low dimensions", Edited by D. Baeriswyl and L. Degiorgi, in press, Kluwer (2004); cond-mat/0301506. [3] A. B. Kuzmenko, N. Tombros, H. J. A. Molegraaf, M. Grueninger, D. van der Marel, and S. Uchida, Phys. Rev. Lett. 91 (2003) 037004. [4] F. P. Mena, D. van der Marel, M. Fäth, A. A. Menovsky, J. A. Mydosh, Phys. Rev. B 67 (2003) R241101.

24

Growth and electronic properties of unconventional metals and oxides

Research summary: Unconventional metals and oxides, including dielectric and ferroelectric insulators, ferromagnetic metals and superconductors, are studied using standard transport, local probes, and high pressure techniques. The materials investigated include single crystals, epitaxial thin films and heterostructures. Epitaxial ferroelectric films are used to study finite size effects. Ferroelectric domain manipulations and domain wall motion, as well as novel devices are studied and realized using atomic force microscopy. Heterostructures of ferroelectric perovskites with superconducting or metallic oxides allow the realization of local field effect experiments using the nonvolatile ferroelectric polarization field, while ferroelectric / dielectric structures are used to design novel materials with tailored properties. High pressures are used to tune the electronic properties of heavy fermion systems, in particular close to phase transitions. The effect of strain on epitaxial oxide thin films is studied by applying very high hydrostatic pressures.

Jean-Marc Triscone

Ferroelectricity and tetragonality in ultrathin PbTiO3 films

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 (PTO) films with thicknesses ranging from 500 Å down to 24Å. High resolution x-ray analyses showed a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200Å as illustrated in Fig. 1.

Fig.1. Evolution of the c/a ratio with the film thickness. Top: experimental results, samples with different growth rates (circles and squares) and samples with a gold top electrode (crosses). Bottom: theoretical prediction from the model Hamiltonian calculations. Inset: thickness dependence of the corresponding spontaneous polarization from the model Hamiltonian.

Using a first-principles model Hamiltonian approach, the decrease in tetragonality can be related to the reduction of polarization in thin films. The experimentally observed continuous decrease of c/a confirms that the films remain ferroelectric down to a thickness much smaller than 100Å but also indicates a significant reduction of the polarization at small

thicknesses. The data also suggest that the reduction of ferroelectricity occurs throughout the film and is not merely a surface effect. This is confirmed theoretically. Both the range of thicknesses over which the c/a ratio is observed to decrease substantially and the shape of the c/a versus thickness curve suggest that the depolarizing field is the main driving force for the global reduction of the polarization in perovskite ultrathin films (as predicted in Ref.1). For films thinner than about 100Å, however, the theoretical model predicts a decrease of tetragonality larger than the one observed experimentally. [1] J. Junquera and P. Ghosez, Nature 422, 506 (2003).

Domain wall creep and macroscopic defects

Using atomic force microscopy (AFM) on epitaxial Pb(Zr0.2Ti0.8)O3 thin films, we have investigated the effect of macroscopic defects on domain wall motion. We had previously shown that this domain wall motion is a disorder controlled creep process, driven by the applied electric field E, with the domain wall speed v ∝ exp[-(1/E)µ]. The characteristic dynamical exponent µ, related to the dimensionality of the film and the nature of the disorder potential, was found to be between 0.7 and 1. To introduce defects in the pure c-axis films, heavy ion irradiation over half of the film surface was used, allowing domain dynamics in the irradiated and as-grown regions to be directly compared. Thick films, containing a-axis inclusions, were also investigated. In all cases, 25-domain arrays were written in uniformly pre-polarized areas by applying voltage pulses via the AFM tip, and imaged using their local piezoresponse. The writing time dependence of the average domain radius was measured, allowing the field dependence of the radial domain wall velocity to be extracted [1]. We find that the presence of

0 100 200 300 400 500

1.045

1.050

1.055

1.060

1.065

1.070

Thickness (Å)

Te

trag

ona

lity

c/a

0 100 200 300 400 500

1.031.041.051.061.071.081.09

0 100 200 3000

20406080

Po

lariz

atio

n (

µC/c

m2 )

25

defects decreases the value of µ to 0.3-0.4 in the case of the columnar irradiation defects, and even more significantly to 0.1-0.2 in the presence of the planar a-axis inclusions[2]. The variation does not appear to be thickness dependent. Since µ controls the divergence of the energetic barriers to domain wall motion as the applied field goes to zero, this decrease may have an effect on the stability of domain structures in low electric fields. [1] Tybell et al., Phys. Rev. Lett. 89, 097601 (2002) [2] Paruch et al., Ann. der Phys. 13, 95 (2004)

Variable temperature atomic force microscopy

In order to study the nanoscale ferroelectric properties of thin films, we have implemented a new variable temperature atomic force microscope. This instrument, an Omicron AFM, is designed to measure over a broad temperature range, from ~30 K up to ~750 K, in ultrahigh vacuum. Due to the complexity of the system, an important effort has been necessary to exploit the available electronics and fine tune the vacuum system, the heating and the cooling facilities. Preliminary results at room temperature in air demonstrate the possibility of writing ferroelectric domains in a Pb(Zr0.2Ti0.8)O3 thin film and imaging them, using the piezoresponse technique, as shown in figure 2.

Fig.2. In the left panel a topography map of the surface of the PZT film over an area of 1.5µmx1.5 µm is shown; the middle and right panels show the amplitude and phase contrast of the piezoresponse signal of a previously written 1µm size ferroelectric domain.

Working in ultrahigh vacuum (~10-10 Torr), we have scanned in contact and non-contact mode up to a sample temperature of 430°C, observing a lateral and vertical drift during the temperature changes. Once the temperature is stabilized, however, stable topographic maps can be acquired. These first results will allow us to design future experiments on the temperature dependence of the ferroelectric domain dynamics at the nanoscale level. A high frequency surface acoustic wave filter based on ferroelectric domain manipulation

Surface acoustic wave (SAW) devices are used as filters in mobile communication networks and tiny signal processing devices such as delay

lines, and resonators. In classical SAW devices, two metallic interdigital transducers (IDTs) deposited on a uniformly polarized piezoelectric material, crystal or film, act as electric input and output ports. Application of an appropriate RF voltage to the input IDT results in the launching of SAW which travel along the piezoelectric surface and are detected by the receiving IDT and converted back to an electric signal. These surface waves travel with the sound velocity of the material. The centre frequency (fC) of the SAW device is determined by the wavelength (λ), which is the distance between the IDT fingers of the same electrode, and the sound velocity (vS) of the material, fC = vS/λ. Using nanoscale domain inversion to write piezoelectric IDTs on ferroelectric materials by atomic force microscopy, we have developed a novel type of SAW filters. The novelty in the approach is to replace the conventional metallic IDTs by a piezoelectric one. The piezoelectric input and output IDTs were realized by writing line shaped domains with alternating polarization on Pb(Zr0.2Ti0.8)O3 films. The distance between the IDTs of the same polarization defines the wavelength of the device which can be very short. S11 reflection measurements on two devices with different wavelengths show clear minima related to the launching of the SAW at frequencies (2 and 3.4 GHz), in good agreement with theoretical analysis. These results are very encouraging and transmission measurements will be carried out soon on a suitable device. Electrostatic modulation of super-conductivity in Nb-doped SrTiO3 thin films

We have performed ferroelectric field effect experiments using epitaxial heterostructures composed of ferroelectric Pb(Zr0.2Ti0.8)O3 and superconducting Nb-doped SrTiO3 (Nb-STO). The films were prepared on (001) SrTiO3 substrates by off-axis radio-frequency magnetron sputtering and pulsed-laser deposition. To switch the direction of the ferroelectric polarization (parallel or anti-parallel to the c-axis), the metallic tip of an AFM was used as a mobile gate electrode, scanning at room temperature over the area of the conducting path. In the whole temperature range investigated, large differences in the resistivities (≈30 %) between the P+ and P- states were observed. The P+ state corresponds to the polarization direction which removes electrons from the Nb-STO layer. Figure 3 shows the normalized resistivity at 0.5 K as a function of temperature for the P+ and P- states. Under polarization reversal a clear shift of the critical temperature Tc was observed. With Tc defined as the temperature at which the resistivity decreases to 50 % of its value at 0.5 K, the Tc

26

value for the P- state is 0.30 K and that of the P+ state is 0.25 K. The inset of Fig. 3 shows the carrier concentration dependence of Tc for the P+ state and the P- state and that for reduced and Nb doped STO bulk crystals. Even if there are slight deviations from the bulk crystal data, the average electrostatic doping can be rather well mapped onto the chemical doping phase diagram. This model system may allow us to realize nanoscale superconducting switches, artificial Josephson junctions arrays, or re-writable pinning sites.

¨

0.4

0.2

0

T c [

K]

1019

1020

n [cm-3

]

1.0

0.8

0.6

0.4

0.2

0

ρ(T

)/ρ(

0.5

K)

0.50.40.30.20.10

Temperature [K]

Field effect studies of vortex activation energies in thin Nd1Ba2Cu3Ox films

The electrostatic modulation of activation energies for vortex motion was studied in a thin Nd1Ba2Cu3Ox (NBCO) film using a "single crystal" field effect device. The thin NBCO layer was epitaxially grown on top of a (001) SrTiO3 single crystal. In the linear regime of the "liquid" vortex phase, the thermally activated flux motion was probed by measuring the tails of the resistive transitions as a function of temperature and magnetic field while applying an electric field across the 150µm thick SrTiO3 dielectric single crystal. The sample was patterned using standard lithography, and measurements were performed using a 4 points technique. The activation energy was determined from the slope of Arrhenius plots of the resistivity, ln(ρ) versus 1/T. Figure 4 shows the resistivity as a function of 1/T for a 36Å NBCO layer, an applied

magnetic field of 0.03 T, and for three different voltages: –200 V, 0V and 200 V applied across

0.025 0.030 0.035 0.040

0.1

1

10

100

1000

B=0.03 T

ρ (µ

Ω*c

m)

1/T (1/K)

E=-200V E=0V E=200V

Fig.4. Arrhenius plot of the resistivity at 0.03 T for a thin Nd1Ba2Cu3Ox layer as a function of an electric field applied across the SrTiO3 dielectric layer.

the SrTiO3 dielectric layer. The relative change in the critical temperature ∆Tc/Tc, where Tc is defined in zero magnetic field as the temperature at which R=0.1 Ω, is 3.5%. The relative change in the activation energies ∆U/U is almost constant as a function of the applied magnetic field and is about 7%. We also characterized the dielectric properties of the gate insulator to be able to quantify the field effect. The mean values for ∆U/U and ∆Tc/Tc of 7% and 3.5 % are obtained for voltages of–200 V corresponding to polarizations between –4 and -4.5 µC/cm2.

For more details and further reading, see D. Matthey et al., Appl. Phy. Lett. 83, 3758 (2003). Signatures of valence fluctuations in CeCu2Si2 In CeCu2Si2 the superconducting state at ambient pressure is thought to be related to an antiferromagnetic quantum critical point (QCP). Under pressure Tc rises sharply from 0.7 K to over 2 K, for reasons so far poorly understood. We have identified a second QCP at a pressure Pv around 4.5 GPa, where there is a sudden change of Ce 4f valence. A variety of properties show anomalies at this point, and we believe that the enhancement of Tc under pressure is due to a new mechanism of superconductivity, where the pairing interaction is due to the exchange of critical valence fluctuations. A theoretical model proposed by our collaborator Kazumasa Miyake, (Osaka, Japan) predicts the enhancement of Tc due to valence fluctuations, along with several other anomalies found around Pv. These include an enhancement of the residual resistivity, a peak in the electronic specific heat, and linear resistivity above Tc.

Fig.3. Temperature dependence of the resistivitynormalized at 0.5 K for the P+ state () and the P- state (). The inset shows the dependence of the criticaltemperature on carrier concentration for each polarizationstate. The behavior of reduced STO bulk crystals (dottedcurve) and Nb-STO bulk crystals (dotted bar) are alsoincluded.

27

T1max

T2max

TN

AF

SC I SC II

Pc

Pv

log

T

CeCu2Ge2 P(GPa)

CeCu2Si2 P(GPa)

0 10 20

0 105

ρ ~ T

Fig. 5 Schematic P-T phase diagram for CeCu2(Si/Ge)2 showing the two critical pressures Pc and Pv. Around Pc, where the antiferromagnetic ordering temperature TN

vanishes, superconductivity in region SC I is mediated by antiferromagnetic spin fluctuations; around Pv, in the region SCII, valence fluctuations provide the pairing mechanism and the resistivity is linear in temperature. The temperatures T1

max and T2max, which correspond to the crystal field split

resistivity maxima, merge at a pressure coinciding with Pv. The dotted line represents a first order valence transition whose critical endpoint (yet to be seen) is at low enough temperature that the associated fluctuations can induce superconductivity.

The model introduces into the Anderson lattice Hamiltonian a Coulomb repulsion term between conduction and localized f electrons. This leads to an abrupt change of occupation number nf as the pressure is increased. [cond-mat/0306054, to be published in Phys. Rev. B] Recent experiments have investigated the role of anisotropic stress on the superconducting properties of CeCu2Si2. This was motivated in part by previous results on CePd2Si2 (see 2001 report) which indicated that the superconducting pressure range and transition temperature were extremely sensitive to small non-hydrostatic components in the pressure, with different crystal orientations relative to the stress showing very different phase diagrams. In CeCu2Si2, we find a similar effect, with the superconducting pressure range highly sensitive to the crystalline orientation relative to the stress axis. This can be interpreted in terms of a local pairing scenario discussed by Monthoux and Lonzarich [cond-mat/0309551], where the symmetry of the pairing potential (either due to spin or charge fluctuations) is matched with that of the lattice, leading to extreme sensitivity to dimensionality or anisotropy.

Unconventional superconductivity in ε-iron We have recently carried out resistivity and ac specific heat measurements under pressure on

0 1 2 3 4 5 60.0

0.5

1.0

1.5

2.0

2.5P(GPa) =

22.2

22.5

23.1

# 1

# 3

# 0

Fe

ρ(µΩ

cm

)

T(K)

Fig. 6 Effect of disorder in different samples on the superconducting transition of iron. The dashed line suggests the Tc variation vs. residual resistivity.

single crystal iron whiskers, whose residual resistivities at ambient pressure were extremely low. The resistivity results confirmed previous observations, with the value of Tc slightly higher. No anomaly was found in the specific heat signal.The sensitivity of Tc to disorder, along with the T5/3 resistivity, which indicates the presence of ferromagnetic spin fluctuations, strongly points to the possibility that ε-iron is a spin-triplet magnetically mediated superconductor. [cond-mat/0310519, to be published in J. Phys.: Condens. Matter]

Hall effect in SrRuO3 thin film under pressure We performed new magnetotransport measurements on a high quality SrRuO3 thin film pressurized up to 21.4 GPa. The linear decrease of the Curie temperature TC, with pressure and its saturation above 14-15 GPa at about 75 K already observed in resistivity measurements are confirmed by Hall effect measurements. The observed hysteretic loop associated with the extraordinary Hall effect, demonstrates the persistence of ferromagnetic order at 21.4 GPa up to at least 100K. Selected references:

Field effect in correlated oxides. C.H. Ahn, J.-M. Triscone, and J. Mannhart. Nature 424, 1015 (2003). Field effect experiments in NdBa2Cu3O7-δ ultrathin films using a SrTiO3 single crystal gate insulator. D. Matthey, S. Gariglio, and J.-M. Triscone. Applied Physics Letters 83, 3758 (2003). Signatures of valence fluctuations in CeCu2Si2 under high pressure. A.T. Holmes, D. Jaccard, K. Myake. Phys.Rev. B69 024508 (2004) Ferroelectricity at the nanoscale: Local polarization in thin films and heterostructures. C.H. Ahn, K. Rabe, and J.-M. Triscone. Science 303, 488 (2004).

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Collaborations Prof. P. Aebi, Prof. P. Martinoli University of Neuchâtel, Switzerland Dr. G. Aeppli, Department of Physics and Astronomy, University College London, United Kingdom Prof. C.H. Ahn Yale University, USA Dr. N. Andrei Rutgers University, USA Prof. A. Baldereschi Institut Romand de Recherche Numérique en Physique des Matériaux (IRRMA), Lausanne Prof. L. Balents University of Santa Barbara, USA Dr. S. Ballandras LPMO, Besançon, France Dr. B. Barbiellini, Northeastern University, Boston, USA Dr. F. Becca SISSA, Trieste, Italy Prof. H. Beck University of Neuchâtel, Switzerland Dr. G. Behr Institute for Solid State and Materials Research, Dresden, Germany Dr. C. Beneduce Bruker Biospin SA, Fällanden, Switzerland Dr. H. Berger IPMC, EPF Lausanne, Switzerland Dr. S. Biermann LPS, Université Paris Sud, Orsay, France Dr. N. Binggeli International Center for Theoretical Physics (ICTP) INFM DEMOCRITOS National Center, Trieste, Italy Dr. Brenig Institut für Theoretische Physik, TU-Braunschweig, Germany Dr. B. Büchner, Dr. M. Knupfer Institute for Solid State Research, IFW Dresden, Germany Prof. P. Buffat EPF, Lausanne, Switzerland Prof. D. Caplin Imperial College, London, GB Dr. M.A. Cazalilla Donostia International Physics Center (DIPC), Spain

Dr. M. Chen, Dr. M. Abplanalp, Dr. W. Paul ABB Research Center, Baden-Dättwil, Switzerland Dr. S.-W. Cheong Department of Physics and Astronomy Rutgers University, NJ, USA Dr. R. Chitra LPTL, Université Paris 6, Paris, France Dr. R. Citro University of Salerno, Italy Dr. M. Dhallé Technische NatuurWetenschappen University of Twente Enschede, The Netherlands Dr. D. Dominguez Centro Atomico Bariloche, Argentina Dr. S.B. Dugdale University of Bristol, GB J. Duron, F. Grilli EPF, Lausanne, Switzerland Dr. B. Dutoit EPF, Lausanne, Switzerland Dr. D. Eckert Bruker Biospin SA, Fällanden, Switzerland Dr. H. Eisaki, Low-Temperature Physics Group, National Institute of Advanced Industrial Science and Technology, Umezono, Tsukuba, Japan Prof. C.B. Eom University of Wisconsin-Madison, USA Dr. D. Eshchenko, Dr. R. Khasanov, Dr. E. Morenzoni, Prof. H. Keller LMU, PSI, Villigen, Switzerland Dr. M.R. Eskildsen Notre Dame University, IN, USA Dr. D. Feinberg LEPES Grenoble, France Prof. J. Flouquet CENG, Grenoble, France Dr. Ph. Fluckiger, Dr. G.-A. Racine CMI, EPFL, Lausanne, Switzerland Dr. J. Fompeyrine, Dr. J.-P. Locquet IBM Rüschlikon, Switzerland Prof. L. Forro EPF, Lausanne, Switzerland Prof. A. Furrer, Dr. K. Conder Laboratory for neutron scattering, ETH Zürich and PSI Villigen, Switzerland

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Prof. V. Gasparov Institute of Solid State Physics, Russian Academy of Sciences, Chernogolova, Russia Dr. C. Geibel MPI, Dresden, Germany Dr. A. Georges CNRS-LPTENS, Paris, France Prof. R.E. Gladyshevskii Dept. of Inorganic Chemestry Ivan Franko National University, L’viv, Ukraine Dr. G. Goll Physikalisches Institut, Universität Karlsruhe, Germany Dr. G. Grasso INFM Genoa, Italy Dr. D. Grempel SPCSI, CEA-Saclay, France Dr. J.-C. Grivel Risø National Laboratory, Roskilde, Denmark Dr. B. ten Haken Technical University Twente, Enschede, The Netherlands Dr. A. Ho University of Birmingham, GB Dr. B.W. Hoogenboom Université de Bâle, Switzerland Dr. Y.-B. Huang American Superconductor, Westborough, MA, USA Prof. H. Hwang Tokyo Institute of Technology, Tokyo, Japan Prof. J.L. Jorda and Dr. Ph. Galez Laboratoire de Structure de la Matière, Université de Savoie, Annecy, France Dr. H. Johannesson Chalmers University, Sweden Dr. I. Joumard ESRF, Grenoble, France Dr. S. Kancharla Sherbrook University, Canada Dr. J. Karpinski High Pressure Synthesis Group, ETH Zürich, Switzerland Prof. H. ten Kate CERN, Geneva, Switzerland Prof. M. Kawasaki Tohoku University, Japan

Prof. T. Klein LEPES/CNRS et Université Joseph Fourier, Grenoble, France Prof. P. Komarek and Dr. W. Goldacker, Forschungzentrum Karlsruhe, Germany Dr. A. A. Kordyuk Institute for Solid State Research, IFW Dresden, Germany Institute of Metal Physics of The National Academy of Sciences of Ukraine, Kyiv, Ukraine Dr. A. Koshelev Argonne National Laboratory, USA Dr. G. Kotliar Rutgers University, USA Dr. W. Krauth LPS-ENS, Paris, France Dr. K. Kwasnitza PSI, Villigen, Switzerland Prof. G. Lamarche University of Ottawa, Canada Prof. D.C. Larbalestier University of Wisconsin, Madison, USA Dr. P. Le Doussal CNRS-LPTENS, Paris, France Dr. A. Lichtenstein University of Nijmegen, The Netherlands Prof. M. Lippmaa University of Tokyo, Japan Dr. C. Marcenat DRFMC/SPSMS/LCP, Commissariat à l'énergie atomique, Grenoble, France Dr. C. Meingast, Dr. K. Grube, P. Popovich, Dr. T. Wolf Institut für Festkörperphysik, Forschungszentrum Karlsruhe, Germany Dr. V. Marconi ICTP, Trieste, Italy Dr. D. Marré University of Genoa, Italy Prof. K. Miyake University of Osaka, Japan Prof. F. Mila EPF, Lausanne, Switzerland Dr. S. Moulinet LPS-ENS, Paris, France Dr. T. Nattermann Köln University, Germany

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Dr. E. Olive University F. Rabelais, Tours, France Prof. Y. Onuki University of Osaka, Japan Dr. E. Orignac CNRS-LPTENS, Paris, France Dr. Y. Paderno Institute for Problems of Materials Science, Academy of Sciences of Ukraine, Kiev, Ukraine Dr. F. Parmigiani Istituto Nazionale per la Fisica della Materia Laboratorio Elettra, Trieste, Italy Dr. M. Potel, Dr. R. Chevrel Université de Rennes, France Prof. M. Rappaz EPF, Lausanne, Switzerland Dr. J. W. Reiner Yale University, USA Dr. A. Revcolevschi Laboratoire de Physico-Chimie des Solides Université Paris-Sud, Orsay, France Dr. E. Rolley LPS-ENS, Paris, France Dr. G. Santi University of Aarhus, Denmark Dr. J. Sarrao Los Alamos National Laboratory, Los Alamos, USA Dr. G. Schehr University of Saarbrucken, Germany Prof. J. S. Schilling Washington University, St Louis, USA Prof. W.-D. Schneider EPF, Lausanne, Switzerland Dr. Schuppler Forschungszentrum Karlsruhe, Germany Dr. B. Seeber Groupe de Physique Appliquée, Université de Genève, Switzerland Dr. S. Sharapov ISI Foundation, Torino, Italy Dr. D. Sheptyakov PSI, Villigen, Switzerland Dr. C. Simon and Dr. A. Ruyter CRISMAT Caen and LEMA Tours, France Prof. A.S. Siri University of Genoa, Italy

Dr. J.-C. Soret Université F. Rabelais, Tours, France Prof. S. Tajima, Dr. S. Lee, Dr. T. Masui Superconductivity Research Laboratory, ISTEC, Tokyo, Japan Dr. P. Tixador and Dr. L. Porcar, Dr. C. Villard CNRS, CRETA, CRTBT, Grenoble, France Dr. P. Toulemonde LPMCN, Université Claude Bernard Lyon-I, Villeurbanne, France Prof. G. Triscone École d'ingénieurs de Genève HES Genève, Switzerland Prof. T. Tybell NTNU, Trondheim, Norway Dr. S. Uchida, Department of Superconductivity, University of Tokyo, Japan Dr. J. Vannimenus LPS-ENS, Paris, France Prof. H. W. Weber, Dr. M. Eisterer Atominstitut der Österreichischen Universitäten, Vienna, Austria Dr. F. Weiss, C. Gimenez LMPG, CNRS, Grenoble, France Dr. Kay J. Wiese CNRS-LPTENS, Paris, France Dr. H. Wilhelm MPI, Dresden, Germany Dr. J. Zaanen Leiden Institute of Physics, Leiden University, The Netherlands

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Publications Groupe Fischer Decroux M., Antognazza L., Reymond S., Paul W., Chen M., and Fischer Ø. Studies of YBCO strip lines under voltage pulses: optimisation of the design of fault current limiters. IEEE Trans. On Applied Superconductivity, Vol.13, 1988 (2003). M. R. Eskildsen M.R. , N. Jenkins, G. Levy, M. Kugler and Ø. Fischer, Vortex imaging in magnesium diboride with H ⊥ c, Phys. Rev. B 68, 100508 (2003). Eskildsen M.R., Kugler M., Levy G., Tanaka S., Jun J., Kazakov S.M., Karpinski J. and Fischer Ø. Vortex lattice imaging in single crystal MgB2 by scanning tunneling spectroscopy, Physica C 388-389, 143 (2003). EskildsenM.R., Kugler M., Levy G., Tanaka S., Jun J., Kazakov S.M., Karpinski J. and Fischer Ø. Scanning tunneling spectroscopy on single crystal MgB2, Physica C 385, 169 (2003). Hoogenboom B.W., Kadowaki K., Revaz B., Fischer Ø. Homogeneous samples of Bi2Sr2CaCu2O8+d , Physica C 391, 376 (2003). Hoogenboom B.W., Berthod C., Peter M. and Fischer Ø. Modeling scanning tunneling spectra of Bi2Sr2CaCu2O8+d, Phys. Rev. B 67, 224502 (2003). Hess C., Büchner B., Ammerahl U., Colonescu L., Heidrich-Meisner F., Brenig W. and Revcolevschi A. Magnon Heat Transport in Doped La2CuO4, Phys. Rev. Lett. 90, 197002 (2003). Hess C., Büchner B., Ammerahl U. and Revcolevschi A. Phonon thermal conductivity in doped La2CuO4: Relevant scattering mechanisms, Phys. Rev. B 68, 184517 (2003). Reymond S., Antognazza L., Decroux M. and Fischer.Ø. Simulation of the initial response of a high Tc Superconducting film submitted to a voltage source. Superconducting Science and Technology 17, 522-526 (2004). Reymond S. and Fischer Ø. Low temperature Scanning Contact Potentiometry. Review of Scientific Instrument 75, 694-698 (2004).

Groupe Flükiger Bellingeri E. B3.2.3 – Processing of high Tc conductors: the compound Tl(1223). “Handbook of Superconducting Materials”, Volume 1, Superconductivity, Materials and Processes, edited by D. Cardwell and D. Ginley, IOP Publishing Ltd, Bristol and Philadelphia, 539-564 (2003). Bellingeri E. and Flükiger R. C3 – TIBCCO. “Handbook of Superconducting Materials”, Volume 1, Superconductivity, Materials and Processes, edited by

D. Cardwell and D. Ginley, IOP Publishing Ltd, Bristol and Philadelphia, 993-1027 (2003). Flükiger R., Lezza. P., Beneduce C., Musolino N. and Suo H.L. Improved transport critical current and irreversibility fields in mono- and multifilamentary Fe/MgB2 tapes and wires using fine powders. Supercond. Sci. Technol. 16 (2), 264-270 (2003). Flükiger R, Suo H.L., Musolino N., Beneduce C., Toulemonde P. and Lezza P. Superconducting properties of MgB2 tapes and wires. Physica C, 385 (1-2), 286-305 (2003) and Physica C 387 (3-4), 419 (2003). Flükiger R. B2.5 – Growth of A15 type single crystals and polycrystals and their physical properties. “Handbook of Superconducting Materials”, Volume 1, Superconductivity, Materials and Processes, edited by D. Cardwell and D. Ginley, IOP Publishing Ltd, Bristol and Philadelphia, 391-406 (2003). Flükiger R. B3.3.1 – Overview of low Tc materials for conductor applications. “Handbook of Superconducting Materials”, Volume 1, Superconductivity, Materials and Processes, edited by D. Cardwell and D. Ginley, IOP Publishing Ltd, Bristol and Philadelphia, 585-602 (2003). Garnier V., Passerini, R., Giannini E. and Flükiger R. New understanding of the role of Ag at underformed Ag/Bi,Pb (2223) interface: texturing and outgrowths. Supercond. Sci. Technol 16, 820-826 (2003). Giannini E., Garnier V., Savysyuk I., Passerini R., Witz G., Su X.D., Seeber B., Hua L. and Flükiger R. Partial melting and HIP processing of Bi(2223): bulk and tapes". IEEE Transactions on Applied Superconductivity 13 (2), 3008-3013 (2003). Murakami M. and Flükiger R. B1 - Introduction to processing methods. “Handbook of Superconducting Materials”, Volume 1, Superconductivity, Materials and Processes, edited by D. Cardwell and D. Ginley, IOP Publishing Ltd, Bristol and Philadelphia, 209-219 (2003). Musolino N. Bals S. van Tendeloo V., Clayton N., Walker E. and Flükiger R. Modulation-free phase in heavily Pb-doped (Bi,Pb) 2212 crystals. Physica C 399 1-7 (2003). Passerini R., Dhallé M, Giannini E., Witz G., Seeber B. and Flükiger R. The influence of thermal precompression on the mechanical behaviour of Ag-sheathed (Bi,Pb)223 tapes with different matrices. Physica C 37, 173-184 (2002). Seeber B. B3.3.5 – Processing of low Tc conductors: the compounds PbMo6S8 and SnMo6S8. “Handbook of Superconducting Materials”, Volume 1, Superconductivity, Materials and Processes, edited by D. Cardwell and D. Ginley, IOP Publishing Ltd, Bristol and Philadelphia, 685-706 (2003).

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Suo H.L., Toulemonde P. and Flükiger R. B3.3.6 – Processing of low Tc conductors: the compound MgB2. “Handbook of Superconducting Materials”, Volume 1, Superconductivity, Materials and Processes, edited by D. Cardwell and D. Ginley, IOP Publishing Ltd, Bristol and Philadelphia, 707-719 (2003). Suo H.L., Lezza P., Uglietti D., Beneduce C., Abächerli V. and Flükiger R. High transport critical current densities and n factors in mono-and multi-filamentary MgB2/Fe tapes and wires using fine powder. IEEE Transactions on Applied Superconductivity 13 (2), 3265-3268 (2003). Toulemonde P., Musolino N., Suo H.L. and Flükiger R. Superconductivity in high-pressure synthesized pure and doped MgB2 Compounds. Journal of Supercond.: Incorporating Novel Magnetism, Vol. 15, No. 6 (2002). Toulemonde P., Musolino N. and Flükiger R. High pressure synthesis of pure and doped superconducting MgB2 compounds. Supercond. Sci. Technol. 16, 231-236 (2003). Uglietti D., Seeber B., Abächerli V., Pollini A., Eckert D. and Flükiger R. A device for critical current versus strain measurements up to 1000 A and 17 T on 80 cm long HTS and LTS technical superconductors. Supercond. Sci. Technol. 16, 1000-1004 (2003).

Groupe Giamarchi Andrei N. et Bolech C.J. On the Multichannel-Channel Anderson Impurity Model of Uranium compounds. In: "Concepts in Electron Correlation", Proceedings of NATO Advanced Research Workshop, Hvar, Croatia, 2002). Eds. A.C. Hewson et V. Zlatiac. Kluwer Academic Publishes (2003). Barbiellini B., Dugdale S.B. and Jarlborg T. The EPMD-LMTO program for electron-positron momentum density calculations in solids. Computational Material Science 18, 287-301 (2003). Berthod C., Binggeli N. and Baldereschi A. Schottky barrier heights at polar metal/semiconductor interfaces. Phys. Rev. B 68, 085323 (2003). Bolech C.J., Kancharla S.S. et Kotliar G. Cellular dynamical mean-field theory for the one-dimensional extended Hubbard model. Phys. Rev. B 67, 075110 (2003). Hoogenboom B.W., Berthod C., Peter M., Fischer Ø., and Kordyuk A.A. Modeling scanning tunneling spectra of Bi2Sr5CaCu5O8+δ. Phys. Rev. B 67, 224502 (2003). Jarlborg T. Temperature dependence of magnetism near defects in SrB6. J. Phys.: Cond. Matter 15, L249-L252 (2003). Jarlborg T. Electronic structure and magnetism in

( )ReVNbXOWX xx ,,31 =− from supercell calculations.

J. of Magn. Magn. Materials 267, 261-266 (2003).

Jarlborg T. Spin-phonon interaction and band effects in the high-Tc superconductor HgBa2CuO4. Phys. Rev. B 68, 172501-1-4 (2003). Jarlborg T. Spin fluctuations, electron-phonon coupling and superconductivity in near-magnetic elementary metals-Fe, Co, Ni and Pd. Physica C 385, 513-524 (2003). Johannesson H., Andrei N. et Bolech C.J. Critical theory of the two-channel Anderson impurity model. Phys. Rev. B 68, 075112 (2003). Johannesson H., Andrei N. et Bolech C.J. Conformal Field Theory Approach to the Two-Channel Anderson Model. Proceedings of ICM conference, Rome, 2003. Elsevier Science (2003). Major Zs., Dugdale S.B., Jarlborg T., Bruno E., Ginatempo B., Staunton J.B. and Poulter J. Electronic structure of ordered and disordered Fe3Pt. J. Phys.: Cond. Matter 15, 3619-3629 (2003). Nattermann T., Giamarchi T., Le Doussal P. Variable-range hopping and quantum creep in one dimension. Phys. Rev. Lett. 91, 056603 (2003). Olive E., Soret J.C., Le Doussal P., et al. Numerical simulation evidence of dynamical transverse Meissner effect and moving Bose glass phase. Phys. Rev. Lett 91, 037005 (2003). Rosso A., Giamarchi T. X-ray diffraction of a disordered charge density wave. Phys. Rev. B 68, 140201(R) (2003). Rosso A., Krauth W., Le Doussal P., Vannimenus J., and Wiese K.J. Interface width distributions at the depinning threshold. Phys. Rev. E 68, 036128 (2003). Schehr G., Giamarchi T., Le Doussal P. Specific heat of classical disordered elastic systems. Phys. Rev. Lett. 91, 117002 (2003).

Book Giamarchi T. Quantum Physics in one Dimension. International Series of Monographs on Physics 121, Oxford University Press, Dec. 2003.

Groupe Junod Bouquet F., Wang Y., Sheikin I., Toulemonde P., Eisterer M., Weber H.W., Lee S., Tajima S., Junod A. Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2. Physica C 385, 192-204 Junod A., Wang Y., Bouquet F., Sheikin I., Toulemonde P., Eskildsen M. R., Eisterer M., Weber H. W., Lee S., Tajima S. Specific heat of ceramic and single crystal MgB2. Physica C 388-389, 107-108 (2003).

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Plackowski T., Junod A., Bouquet F., Sheikin I., Wang Y., Jezowski A., Mattenberger K. Specific heat and isothermal magnetocaloric effect for single-crystal UAs. Phys. Rev. B 67, 184406 1-13 (2003). Wang Y., Bouquet F., Sheikin I., Toulemonde P., Revat B., Eisterer M., Weber H. W., Hinderer J., Junod A. Specific heat of MgB2 after irradiation. J. Phys.: Condens. Matter 15, 883-893 (2003).

Groupe van der Marel Kuntscher C.A., van der Marel D., Dressel M., Lichtenberg F., and Mannhart J. Signatures of polaronic excitations in quasi-one-dimensional LaTiO3.41. Phys. Rev. B 67, 035105 (2003). Kuzmenko A.B., Tombros N., Molegraaf H.J.A., Grüninger M., van der Marel D., and Uchida S. c-Axis Optical Sum Rule and a Possible New Collective Mode in La2-xSrxCuO4. Phys. Rev. Lett. 91, 037004 (2003). van der Marel D. van der, Molegraaf H.J.A., Zaanen J., Nussinov Z., Carbone F., Damascelli A., Eisaki H., Greven M., Kes P.H. and Li M. Quantum critical behaviour in a high-Tc superconductor. Nature 425, 271-274 (2003). van der Marel D., Molegraaf H.J.A.., Presura C., and Santoso I. Superconductivity by kinetic energy saving ? Chapter in "Concepts in electron correlation", NATO Science Series, II. Mathematics, Physics and Chemistry – Vol. 110, Edited by A. Hewson and V. Zlatic, 7-16 Kluwer (2003). van der Marel D., Molegraaf H.J.A., Presura C. and Santoso I. Superconductivity by kinetic energy saving? Chapter in "Concepts in electron correlation", Edited by A. Hewson and V. Zlatic, Kluwer (2003); page 7-16. Mena F.P., van der Marel D., Fäth M., Menovsky A.A., Mydosh J.A. Heavy Carriers and Non-Drude Optical Conductivity in MnSi. Phys. Rev. B Rapid Communications 67, 241101 (2003). Presura C., Popinciuc M., van Loosdrecht P.H.M., van der Marel D., Maris G., Palstra T.T.M., Yamada H., and Ueda Y. Charge ordering signatures in the optical properties of b-Na0.33V2O5. Phys. Rev. Lett. 90, 026402 (2003). Tajima S., Uchida S., van der Marel D., Basov D.N. Comment on "Phase Diagram of La2-xSrxCuO4 Probed in the Infrared: Imprints of Charge Sripe Excitations. Phys. Rev. Lett. 91, 129701 (2003). Tsvetkov A.A., Mena F.P., van Loosdrecht P.H.M., van der Marel D., Ren Y., Nugroho A.A., Menovsky A.A., Elfimov I.S. and Sawatzky G.A. Structural, electronic, and magneto-optical properties of YVO3. Phys. Rev. B 69, 075110 (2004).

Groupe Triscone Ahn C.H., Triscone J.-M. and Mannhart J. Field effect in correlated oxides. Nature 424, 1015 (2003). Ahn C.H., Rabe R. and Triscone J.-M.. Ferroelectricity at the nanoscale: Local polarization in thin films and heterostructures. Science 303, 488 (2004). Demuer A., Jaccard D., Reiner J., Ahn C.H. and Triscone J.-M. Magnetism of SrRuO3 thin films under high hydrostatic pressures. Annalen der Physik 13, 72 (2004). Demuer A., Holmes A.T., Jaccard D. Effect of anisotropic strain on the electronic properties of the pressure incuced superconductor CePd2Si2. Acta Phys. Pol. B 34, 459-462 (2003). Holmes A., Demuer A, Jaccard D. Resistivity and AC-calorimetric measurements of the superconducting transition in CeCu2Si2 under very high hydrostatic pressure in a helium-filled diamond anvil cell. Acta Phys. Pol. B 34, 567-570 (2003). Matthey D., Gariglio S. and Triscone J.-M.. Field effect experiments in NdBa2Cu3O7-δ ultrathin films using a SrTiO3 single crystal gate insulator. Applied Physics Letters 83, 3758 (2003). Paruch P., Giamarchi T. and Triscone J.-M. Domain wall creep in mixed c-a axis Pb(Zr0.2Ti0.8)O3 thin films. Annalen der Physik 13, 95 (2004). Sheikin I., Gröger A., Raymond S., Jaccard D., Aoki D., Harima H., Flouquet J. High magnetic field study of CePd2Si2. Phys. Rev. B 67, 094420-094431 (2003). Takahashi K., Matthey D., Jaccard D. and Triscone J.-M. Transport properties of reduced SrTiO3 single crystal "thin films". Annalen der Physik 13, 68 (2004)

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Work performed by members of the DPMC before they joined it, but published while they were in Geneva Abe S., Itoh T., Shinoda T., Seimiya Y. Superconducting Transition Temperature and Lattice Parameter of A15-type Nb3(Al,Ge). J. Jpn Inst. Metals 67, 193-196 (2003). Abe S., Kadokura T., Yamaguchi T., Shinoda T., Oya-Seimiya Y. Dependence of Superconducting Critical Temperature of A15-type V3Ga on Composition. J. Low Temp. Phys. 130, 477-492 (2003). Bouquet F., Fisher R. A., Phillips N. E., Hinks D. G., Jorgensen J. D. Specific heat of Mg11B2: evidence for two energy gaps. Physica C 388-389, 109-110 (2003). Fisher R. A., Guangtao Li, Lashley J. C., Bouquet F., Phillips N. E., Hinks D. G., Jorgensen J. D., Crabtree G. W. Specific heat of Mg11B2. Physica C 385, 180-191 (2003). Fisher R. A., Bouquet F., Phillips N. E., Hundley M. F., Pagliuso P. G., Sarrao J. L., Fisk Z., Thompson J. D. Specific heat of CeRhIn5: pressure-driven transition from antiferromagnetism to heavy-fermion superconductivity. Physica C 388-389, 547-548 (2003). Lortz R., Meingast C., Ernst D., Renker B., Lawrie D.D., Franck J.P. High-Resolution Thermal Expansion of MgB2. J. Low Temp. Phys. 131, 1101-1104 (2003). Lortz R., Meingast C., Welp U., Kwok W. K., Crabtree G. W. Crystal-lattice coupling to the vortex-melting transition in YBa2Cu3O7−δ . Phys. Rev. Lett. 90, 237002 1-4 (2003). Lortz R., Meingast C., Rykov A. I., Tajima S. Magnetic-field-induced finite-size effect in the high-temperature superconductor YBa2Cu3O7−δ : a comparison to rotating superfluid 4He. Phys. Rev. Lett. 91, 207001 1-4 (2003). Peça C.S., Balents L., and Wiese K.J. Fabry-Perot interference and spin filtering in carbon nanotubes. Phys. Rev. B 68, 205423 (2003). Phillips N. E., Fisher R. A., Bouquet F., Hundley M. F., Pagliuso P. G., Sarrao J. L., Fisk Z., Thompson J. D. Specific heat of CeRhIn5: the pressure-driven transition from antiferromagnetism to heavy-fermion superconductivity. J. Phys.: Condens. Matter 15, S2095-2099 (2003). Rosso A., Hartmann A.K. and Krauth W. Depinning of elastic manifold. Phys. Rev. E 67, 021602 (2003). Weber C., Capriotti L., Misguich G., Becca F., Elhajal M., and Mila F. Ising Transition Driven by Frustration in a 2D Classical Model with Continuous Symmetry. Phys. Rev. Lett. 91, 177202 (2003).

Ph.D. Theses GARIGLIO Stefano Transport Properties of LaTiO3+δ and REBa2Cu3O7-δ Thin Films: a Study of Correlation Effects January 2003 WITZ Grégoire Préparation et caractérisation de conducteurs à base de Bi,Pb(2223) ayant des pertes AC réduites December 2003 DEA GURITANU Violeta September 2003 Diploma GÖTSCHMANN Cedric Etude par microscopie à effet tunnel de films minces de C60 déposés sur différents substrats Juillet 2003 STUCKI Nicolas Réseaux artificiels de domaines ferroélectriques : recherche d'une interaciton domaine-domaine et haute densité Juin 2003