Understanding interlayer interactions in layered superconducting materials

Candidate: Eugenio Paris

Supervisor: Prof. Naurang L. Saini(Dated: March 31, 2015)

Background and motivations

The physics of layered functional materials is one ofthe most intriguing problems in solid state physics. Dueto the low dimensionality, the presence of correlationsand strong interplay between different degrees of freedomthis class of materials show many different functionalproperties from multiferroics to Mott insulators and in-tercalating materials for Li-ion batteries. Among these,high temperature superconductivity, discovered in 1986by Bendnorz and Muller1, is one of the most notablefindings and it has been puzzling physicist for almost 30years. However, unconventional superconductivity is notonly present in copper-oxide based systems. The ironbased superconductors (FBS), discovered in 2008 byHosono and coworkers2 are layered multiband systems inwhich the active layer (responsible for superconductiv-ity) is composed by iron atoms tetrahedrally coordinatedwith pnictogen (As) or chalcogen (Se,Te) anions. Theactive layers are arranged in a stacked fashion separatedby alkali, rare-earth, oxifluoride or alkaline-earth layers.These “spacer layers” have fundamental importanceeven if they are not directly involved in superconduc-tivity. Similarly, the newly discovered Bismuth-basedsuperconducting family14 is composed by a stackingof BiS2 (or BiSe2) active layers and rare earth oxidespacer layers. In all layered superconducting materialsthe parent compounds do not show superconductivity,they become superconducting after electron (or hole)

FIG. 1: Cristal structure of the skutterudite-type iron pnic-tides, the 10-4-18 (left panel) and the Bismuth-based systems(right panel). Both structures show alternating stacking ofsuperconducting active layers and spacer layers.

FIG. 2: In iron based superconductors, the atomic arrange-ment in the active layers controls the transport and electronicproperties and superconductivity as well. The figure showsthe dependence of the superconducting transition tempera-ture on the As-Fe-As bond angle measured in different ironpnictides. The maximum Tc is showed by compound in whichα is closer to the regular tetrahedron value of 109.47◦.

doping via chemical substitution. In addition, someof the fundamental properties (Fermi surface topology,transport and magnetism, superconductivity) can betuned by means of chemical substitution, mechanicalpressure and/or chemical pressure exploiting the extremesusceptibility to the external conditions3.It has been pointed out that, due to multiband natureof the Fermi surface and its tendency to instabilities,the local structure of the FeAs layers controls both theantiferromagnetic fluctuations and superconductivity4.In fact, structural quantities like bond angles (see Fig.2)and bond distances in the active layers show a clearcorrelation with Tc. Chemical pressure via isovalentsubstitution, doping and external pressure can tune thebasic properties via optimization of the local structure4.The same holds for Bi-based superconductors as well,in which the Bi-S bond correlation and the localstructure of the active layer is predicted to determinethe electronic properties. It is well known also, thatthe unconventional superconductive properties dependon structural defects and on their arrangement, andknowing the average crystal structure is not sufficient.Therefore, an experimental technique which is sensitiveto the local atomic arrangement in an element-selectiveway is strongly desired.Even if the spacer layers are not directly involvedin the superconducting mechanism, different features

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showed by compounds which share the same active layerunderline the importance of the composition of spacerlayers in tuning the basic properties3. For this reason,understanding the interlayer coupling can provide a newcontrol parameter to obtain desired properties. It iswidely accepted that the local lattice structure in theactive layers is related to superconductivity. However,the role played by spacer layers and interlayer interactionhas not been investigated in depth.

My PhD project will be devoted to the study oflocal lattice distortions and disorder in the atomicstructure of layered multiband superconductors. I willalso provide indication about the role of interlayerinteraction in controlling the above properties. Themain experimental technique will be X-ray absorptionspectroscopy (XAS) which is element-selective andprovide local structural sensitivity. Space and angleresolved photoemission spectroscopy will be exploitedto study possible inhomogeneous electronic states dueto local disorder. The focus of my PhD project is ontwo representative systems: the skutterudite-type ironpnictides and the Bi-based superconductors.

Iron pnictides with skutterudite spacer layers

Generally, in the FBS family the spacer layers are in-sulating and this is considered a key feature for super-conductivity. However, recent theoretical investigationhave pointed out the promising potential of metallic in-termediary layers as a novel route for superconductivityin iron pnictides8. Discovery of superconductivity in lay-ered Ca10Pt4As8(Fe2As2)5 (so called 10-4-18 type, Tcof 40 K) and Ca10Pt3As8(Fe2As2)5 (10-3-18 type, Tc13K) compounds with complex spacer layers5,6 has fur-ther underlined importance of interlayer coupling as akey parameter for controlling physical properties of lay-ered systems to find new superconductors with higherTc. Indeed, a new system Ca10Ir4As8(Fe2As2)5 has beensuccessfully synthesized with a Tc of 17K7. Unlike otheriron-based superconductors, the new families have verypeculiar spacer layers with M (Ir or Pt) atoms in squareplanar geometry7. The structure consists of alternatingstacking of (Fe2As2)5 and M4As8(or M3As8) layers withfive Ca ions between them resulting in a relatively bigunit cell (see Fig.1). Similar to other Fe-based super-conductors, dominant contribution to the electronic DOSat the Fermi energy is driven from the iron 3d-electronorbitals. However, unlike most of the FBS having non-metallic spacer layers, the new quaternary systems pos-sess a fundamental difference of having metallic spacerlayers with the non-negligible weight from M 5d states atthe Fermi level suggesting larger interlayer coupling5,7,8.The temperature dependence of the in-plane resistivityof Ir-based system, Ca10Ir4As8(Fe2As2)5 shows a kink at100K that has been assigned to some kind of structuralphase transition (SPT)7,9 involving the Ir-As bond ac-

FIG. 3: Temperature vs doping phase diagram forCa10Ir4As8(Fe2As2)5 (right hand side) in comparison witha typical iron based (the 122 system, left hand side).The letter “T” stands for tetragonal while “O” is for or-thorombic symmetry. T∗ is the low temperature region ofCa10Ir4As8(Fe2As2)5, which keeps the tetragonal symmetry.The superconducting region is labeled as “SC”.

companied by an electronic orbital crossover transition.The Tc versus doping phase diagram clearly shows thatSC takes place even without any doping due to inter-layer carrier injection(the phase diagram is sketched inFig.3). These very interesting properties come from theunusual interlayer interaction and one can expect to con-trol it by means of certain tuning parameters such aspressure/doping. Generally speaking, low doping regionof the phase diagram in Fe-based superconductors is char-acterized by the presence of a structural phase transition(SPT) from a high temperature phase to a low-T phaseshowing some kind of order (charge or magnetic). Asdoping level increase the SC appears with suppression ofthe low-T phase3, e.g. the typical phase diagram in FBSfamily is presented in Fig.3 (left hand side). Interestingly,in Ca10Ir4As8(Fe2As2)5 SC and SPT coexist and getssuppressed in the same manner as a function of electrondoping (see sketch in Fig. 3) making it the only exampleof FBS in which superconductivity appears only in thelow-T phase. Recently, Kitagawa et al10 found the samebehavior of Tc and SPT as a function of external pres-sure. Nevertheless, no magnetic order has been found inthe whole phase diagram making this system a suitablematerial to investigate the relationship between structureand SC in FBS disregarding magnetic effects.

PhD project: Early results on Ca10Ir4As8(Fe2As2)5

In the system Ca10Ir4As8(Fe2As2)5 with skutteruditeintermadiary layers strong interlayer interaction is ex-pected. In particular i am interested in observe the locallattice distortions in the FeAs layers reulting from this in-teraction. In fact, this kind of local distortions are knownto affect the electronic and superconducting properties4.Moreover, from ARPES measurements high degree ofdisorder has been found with a so-called “glassy” non-dispersing electron band coming from Ir 5d orbitals11.Electronic disorder may be linked to structural disorder,

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FIG. 4: X-ray absorption spectroscopy on Ir 10-4-18 system.Panel a) shows the absorption coefficient at Ir L3 and L2

edge respectively at 11.215 KeV and 12.824 KeV. Inset showsthe temperature dependence of the R2,3 intensity ratio whichis proportional to the 〈L ·S〉 spin-orbital operator. Fe K-edge XANES is presented in panel b) in comparison to typical1111 type iron pnictide. Panel c) shows the Fourier transform(magnitude) of Fe K-edge and Ir L3-edge EXAFS presentedalong with model fit. Temperature dependence of the cor-related Debye Waller factors obtained from EXAFS fits arepresented in panel d).

for this reason i am interested in quantify the degree ofdisorder and to compare it with other FBS showing differ-ent phase diagrams. The above issues can be addressedby means of X-ray absorption spectroscopy (XAS). Inaddition, the presence of 5d levels at the Fermi energyhas triggered a natural question about the role of spinorbit coupling in the Ir-related structural phase transi-tion at 100 K, even this last topic can be clarified bymeans of XAS. From Ir L3-edge EXAFS measurements(see Fig. 4c,d) we found highly disordered spacer layers.From Fe K-edge XAS we observed the effect of interlayerinteraction i.e. an enhancement of local disorder in theactive layer causing distortions in the FeAs4 tetrahedraand relaxation of the Fe-Fe network. Nevertheless, bydirect comparison of Fe K-edge XANES with typical Febased material such as SmFeAsO (see Fig. 4b) we foundrather broad features, a clear indication of higher dis-order. By cooling the sample down to 20 K with a Hecryostat we successfully obtained the Einstein frequenciesfor Fe-As, Fe-Fe and Ir-As bonds disentangling dynam-ical disorder from statical disorder. We also monitored

the temperature evolution of Ir L2 and Ir L3 edge XASspectra (see Fig. 4 a). By integrating the main peak(reated to 2p→ 5d electronic transitions) and taking theratio R2,3 = I(L3)/I(L2) it is possible to estimate thespin orbit operator12 〈L ·S〉. We found a rather low ra-tio of about 3 for every temperature investigated whichpoint out the marginal role of spin orbit coupling in theelectronic properties. In addition, we didn’t observe anyanomaly or renormalization in the 〈L ·S〉 operator so weconcluded that the spin orbit interaction is not drivingthe SPT at 100 K.This experiment has been published on Physical ReviewB13.

Bismuth based superconductors

Very recently a new class of SCs with a layered struc-ture has been discovered14. The active layer is com-posed by Bi atoms coordinated with S (or Se) atomsto form a square lattice while the spacer layer is com-posed by rare earth (RE) oxide (the structure is sketchedin Fig.1). In REOBiS2 superconductivity takes placeupon substitution of oxygen with optimal amount of flu-orine. Despite a rather low Tc (maximum at about10.6 K) these systems are considered valid candidatesas a new family of non-conventional superconductors.In fact, evidence of multiband character26 and indica-tion of Fermi surface nesting16,21 in a quasi 2D lay-ered structure are reminiscent of Fe-based supercon-ductors. Several studies indicate usual BCS-like su-perconductivity coming from electron-phonon coupling.Strong electron-phonon coupling has been predicted16

and found experimentally17 along with the finding oflow electron-electron correlations27,28. On the otherhand, no anomaly in the phonon spectrum has beenobserved across Tc from neutron scattering29 and ananomalous value of 2D/Tc = 16.8 has been observedfrom STS30 indicating a pairing mechanism which is be-yond the usual BCS-like. In addition, from specific heatmeasurement31 indication of a quantum critical point hasbeen found. As a result, debate between a BCS and non-conventional superconducting scenario in bismuth basedmaterials is still far to be settled. As in the Fe-basedfamily, superconducting dome can be reached by elec-tron doping (O1−xFx)15 in the spacer layers. One ofthe most notable experimental results is the large en-hancement of Tc (from 3K to 11K) by applying externalpressure18,19 and by high pressure annealing techniquesin sample preparation20. Along with external pressureeffects, chemical pressure can directly affect the super-conducting properties22. The above peculiar features areshared with Fe-based superconductors and underline theinterplay between local structure and the electronic andsuperconducting properties.

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FIG. 5: In panel a) the Ce L3-edge XAS spectrum ofCeO1−xFxBiS2 at different F-doping level is presented, theinset shows the background subtracted XAS. The peaks la-beled 4f1 and 4f0 are related, respectively to Ce 3+ and4+ components. Panel b) shows the behavior of the relativeweight of 4f0 peak as a function of doping level (x). Theside panel show the crossover between coupling and decou-pling between the active and spacer layers above and belowthe critical F-concentration of x = 0.4.

PhD project: Early results on REO1−xFxBiCh2

Among different Bi-based superconductors, theCeO1−xFxBiS2 system shows very peculiar propertiesas a function of F-doping. In fact, for x < 0.4 itis non-magnetic and non-superconducting while forx > 0.4 it show coexistence of superconductivity andferromagnetism. We have exploited XAS spectroscopyin order to answer the following questions: (i) What isthe origin of the coexistence of FM and SC? (ii) How isit related to the valence of Ce atoms? (iii) What is therole played by the local structure?From Ce L3-edge XAS we studied the evolution ofelectronic structure and valence fluctuation upon F-substitution and we provided understanding of thecoexistence of ferromagnetism and superconductivity23.We found that the Ce 4+ component disappears atx > 0.4 where the system start to show the coexistenceof superconductivity and ferromagnetism (see Fig.5)and the effect of F-doping is to break the Ce-S-Biinterlayer coupling channel. This system is known to besusceptible to external pressure18, namely Tc increasesfrom 2 to 10 K under high pressure or with high pressuresynthesis (HP) in sample preparation. From combinedCe L3 and Bi L3-edge EXAFS spectroscopy we observedthe evolution of disorder and local lattice distortionsin CeO1−xFxBiS2 upon F-doping24 for both ambientand HP prepared samples. Local structure of bothactive and spacer layers showed clear doping evolution(e.g. local bond distances are presented in Fig.6a), the

FIG. 6: Left panel shows the F-doping dependence of thebond distances obtained from combined Bi L3 and Ce L3-edge EXAFS. The right panel shows a sketch of the two-layercrystal structure, the BiS2 layer on bottom and the REO layeron top.

effect of HP annealing is prominent in the BiS2 layerwhich shows structural disorder. The local bucklingof the BiS2 layer decreases with increasing F-contentconfirming the scenario of structural instability. In termsof electronic structure there is disagreement betweendifferent ARPES studies25,26 due to microscale phaseseparation and band reconstruction at the surface. Toaddress these issues we decided to investigate the elec-tronic structure of LaO1−xFxBiSe2 using space resolvedARPES with sub-micrometer spatial resolution. Ourmeasurement28 revealed microscale inhomogeneity al-lowing us to measure the actual bulk electronic structureavoiding inhomogeneity effects. Our findings are in goodagreement with band structure calculations suggestingthat electron/polaron correlations are irrelevant. More-over, we found rather broad features indicating disorderin the BiSe2 plane. The results on REO1−xFxBiCh2 hasbeen reported in three publications23,24,28.

PhD project: Summary and perspectives

It is well known in the field of layered superconductorsthat the superconducting properties depend on defectsand on their arrangement and knowing the average orideal structure is not enough. In addition, in the ironbased family local distortions in the active layers stronglyaffects Fermi surface shape and hence superconductivity.In this context, the knowledge of interaction betweendifferent layers and its effects on the electronic transportmechanism is crucial. The goal of this research field is toshed light on basic physical mechanisms for SC and alsoprovide indication for fabrication of new SC materialswith desired properties. The focus of my PhD project ison two representative and recently discovered systems,in particular:

Disorder and local lattice distortions in the iron

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pnictide Ca10M4As8(Fe2As2)5 will be studied as afunction of control parameters like temperature andpressure. By changing stoichiometry, going from M =Ir to M = Pt, the system Ca10M3As8(Fe2As2)5 showdifferent properties due to effect of 5d-metal vacanciesin the spacer layer.

In REO1−xFxBiCh2, disorder and local latticedistortions as a function of control parameters liketemperature, pressure and chemical pressure in thespacer (RE = La, Ce, Nd, Sm) and in the active (Ch =S, Se) layers will be investigated.

A natural question which arise is What is the ef-fect of the local structure in the electronic properties?From X ray absorption spectroscopy we can extractinformation on the unoccupied electronic states. Infact, XANES spectroscopy is capable to probe bothatomic and electronic structure, being sensitive to both.In order to disentagle the structural and electroniccontribution to the XANES features I will exploit thecomparison with theoretical calculations using multiplescattering approach. In order to obtain a completeview I will study the effects of nanoscale distortions inthe functional properties. In this class of materials thenanoscale distortions can be polarized in space givingrise to non-homogeneous distributions and intrinsicphase separation even in the electronic structure. Theseeffects will be observed by means of µ-ARPES which isa space resolved photoemission probe, capable to give amicroscopic view in the electronic structure.

Experiments and collaborations

This work will be mainly done using the local andelement sensitivity of EXAFS spectroscopy to unveil

the local lattice distortions. XANES spectroscopy willbe performed and compared to multiple scatteringcalculations, in order to observe the local unoccupiedelectronic states and band filling as well as the localatomic arrangement. The extreme susceptibility of thisclass of materials from external conditions allows tuningof some of the fundamental properties by applyingpressure, chemical or mechanical. Along with chemicalpressure (obtained by element substitution), hydrostaticpressure effects will be investigated by means of diamondanvil cells (DAC). In order to look for non-homogeneitiesand intrinsic phase separation in the structural andelectronic features we will use space resolved probeslike micro-XAS and micro-ARPES. Our results willbe integrated with other techniques like synchrotronX-ray diffraction and Raman spectroscopy. The hardX-ray absorption measurements require high intensityand energy-tunable sources, for this reason this kindof measurements will be carried out in internationalsynchrotron facilities like ESRF in Grenoble (France)and ELETTRA in Trieste (Italy). In our laboratoryi will do preliminary studies for morphological andphotoemission characterization of the samples.

To carry out the proposed work a long standingcollaboration with the following groups will be exploited:Space resolved ARPES (Prof. Takashi Mizokawa’sgroup from the University of Tokyo) Raman and highpressure measurements (Profs. Paolo Dore andPaolo Postorino from the HPS group @ La Sapienza)X-ray diffraction (Dr. Boby Joseph from ELETTRAsynchrotron) Sample preparation of 10-4-18 family(Prof. Minoru Nohara’s group from Okayama Univer-sity) Sample preparation of Bi-based family (Profs.Yoshihiko Takano from NIMS institute and YoshikazuMizuguchi from Tokyo Metropolitan University) Sam-ple preparation of Fe-based family (Prof. MarinaPutti’s group from Genova University).

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