quasiparticles survive—for now -...

/ www.sciencexpress.org / 26 April 2001 / Page 1/ 10.1126/science.1061205

The concept of quasi-electrons is central to understandingthe physical behavior of solids, but the behavior of somematerials, especially copper oxide superconductors, seemsto be incompatible with this concept. Manganese oxides,the more conventional cousins of the cuprates, have beenstudied intensively to test the quasiparticle concept. AsKeimer explains in his Perspective, the manganitesseemed to behave even more strangely than the cuprates,but a report by Chuang (Science Express) now comes tothe rescue of the quasiparticles. The case remains open forthe cuprates.

A typical centimeter-sized piece of solid contains on the orderof 1023 electrons, each of which interacts with (1023−1) otherelectrons and a comparable number of positively chargednuclei through long-range Coulomb forces. An exact solutionof the resulting many body problem is hopeless even with themost advanced computing techniques. Nonetheless, thanks toingenious concepts developed by solid–state physicists duringthe past century, our understanding of ordinary solids hasreached a remarkable degree of sophistication. As long assome very general conditions are satisfied, the complexsystem of interacting electrons can be mapped one-to-oneonto a system of noninteracting fictitious particles, termed"quasi-electrons." This reduces the initial 1023-body problemto 1023 much more tractable one-body problems.

Like free electrons, quasi-electrons carry a spin of 1/2 andan elementary charge e. However, because the quasiparticlesdrag along the polarization cloud of displaced electrons andnuclei as they move through the solid, their "effective mass"can be much larger than the free electron mass. Furthermore,quasi-electrons away from the Fermi level (the highestoccupied energy level in the solid) are not infinitely long-lived but decay after a finite period of time.

The quasiparticle concept has been extremely successful asa basis for describing the bulk properties of solids. Forinstance, the electrical and thermal conductivities of simplemetals calculated with this concept are in near-perfectagreement with measurements (1). At the microscopic level,theory and experiment can be compared with the help ofangle-resolved photoemission spectroscopy (ARPES), whichis in essence a highly accurate measurement of thephotoelectric effect (see the figure). For layered compoundswhose electronic states near the Fermi level are effectivelytwo-dimensional (2D), ARPES allows one to obtain the fullenergy spectrum of quasielectrons at a fixed wave vector k.The energy, E, and lifetime, τ, of the quasi-electron can bedirectly extracted from the position and width of thephotoemission peak, respectively.

ARPES data on simple 2D systems have revealed well-defined E-versus-k relations in quantitative agreement withquasiparticle models (2). In contrast, the photoemissionspectra of layered copper oxides—quasi-2D materials that

exhibit high-temperature superconductivity—are generallyvery broad in the normal state. Moreover, in many of thesematerials, a gap (termed ''pseudogap'' because of its soft edge)is present on some portions of the Fermi surface (3, 4). Takenat face value, these observations are incompatible with thequasiparticle concept, which is only valid as long as theinverse lifetime τ−1 of the quasi-electron is much smaller thanits energy E. They have thus inspired theories of ''spin-chargeseparation,'' according to which the true ''elementaryparticles'' of the solid are characterized by spin and chargequantum numbers that differ from those of the electron (5).Do we have to abandon the quasi-electron concept, one of thepillars of our understanding of solids, in order to describe thecopper oxides? And if so, is the breakup of the quasi-particlea necessary or sufficient condition for high–temperaturesuperconductivity? These questions have been vigorouslydebated for the past 15 years.

Some manganese oxide compounds—which are notsuperconductors but are famous for the colossalmagnetoresistance effect—are well suited to test these ideasbecause their chemical composition and layered crystalstructure are similar to the copper oxides. The physicalproperties of these materials are in many ways moreconventional than those of their copper-based analogs, buttheir ARPES spectra have proved even more mysterious. Forexample, the spectra for the metallic manganiteLa1.2Sr1.8Mn2O7 are very broad in energy, and a ''pseudogap''extends isotropically over the entire Fermi surface (6). Manyphysicists have been skeptical because ARPES is a surface-sensitive technique that probes only the top few atomiclayers, and there is no guarantee that these are identical to thebulk. Among the possible explanations that have to beconsidered for the observed spectral broadening and thepseudogap in the manganites are an insulating surface layer, asurface reconstruction, and other extrinsic effects (7).

In one stroke, Chuang et al. (8) now demonstrate thepower of ARPES to reveal the intrinsic electronic structure ofthe layered manganites, and go a long way toward solving themystery of the pseudogap. In a report published in ScienceExpress, they report sharp E-versus-k relations near the Fermilevel of La1.2Sr1.8Mn2O7. These observations save thequasiparticle concept for the manganites and allow, for thefirst time, a detailed comparison with electronic structurecalculations and transport measurements. The spectra are sosharp and the effective mass so small that one might expect amuch larger bulk electrical conductivity than is actuallymeasured. However, the spectral weight of the quasiparticlefeatures in the ARPES data is very small, suggesting that onlya small fraction of the sample is metallic whereas theremainder is insulating. Complementary x-ray and neutrondiffraction experiments (9) on the same material haveprovided clues to the nature of these insulating regions byrevealing microscopic patches in which charge carriers arelocalized and form ordered arrays. The periodicity of the

Quasiparticles Survive—for NowBernhard Keimer

The author is at the Max-Planck-Institut fuer Festkörperforschung, Heisenbergstraße 1, Stuttgart, D-70569, Germany. E-mail:keimer@kmr.mpi-stuttgart.mpg.de

/ www.sciencexpress.org / 26 April 2001 / Page 2/ 10.1126/science.1061205

charge order within the arrays matches the wave vectorconnecting nearly flat segments of the Fermi surfacedetermined by Chuang et al. It is thus plausible that themetallic and insulating states are on either side of a zero-temperature Peierls-type (10) phase transition and thatLa1.2Sr1.8Mn2O7 is in the middle of a two-phase coexistenceregion. The unusual line shape of the ARPES spectra [whichare broad in energy but sharp in momentum space (8)], aswell as some aspects of the x-ray and neutron diffraction data(9), suggest that the coexistence may not be static and thatfluctuations between nanoscale domains may persist down to0 K. This remains a topic for future investigation.

The detailed microscopic picture obtained by Chuang et al.is new, but extended two-phase coexistence regimes havealso been reported for a wide variety of other manganites(11). The pronounced propensity toward phase separation isat least partly due to the fact that different d-orbitals, withtheir associated lattice distortions, are available to conductionelectrons in the manganites (12). This makes these materialshighly susceptible to lattice strain. In the copper oxides,where these orbital degrees of freedom are largely quenched,phase separation has also been reported (13) but appears to berelatively rare. Certainly, the extremely broad ARPES spectrain the normal state of underdoped and optimally doped high-temperature superconductors cannot be attributed to phaseseparation. Having barely survived in the manganese oxides,the quasi-electron is still in mortal danger in their copper-based sister materials.

References and Notes1. S. Y. Savrasov, D. Y. Savrasov, Phys. Rev. B 54, 16487

(1996).2. T. Valla et al., Phys. Rev. Lett. 83, 2085 (1999).3. A. G. Loeser et al., Science 273, 325 (1996).4. M. R. Norman et al., Nature 392, 157 (1998).5. P. W. Anderson, Science 288, 480 (2000).6. D. S. Dessau et al., Phys. Rev. Lett. 81, 192 (1998).7. R. Joynt, Science 284, 777 (1999).8. Y. D. Chuang et al., Science, published online 26 April

2001 (10.1126/science.1059255).9. C. P. Adams et al., Phys. Rev. Lett. 85, 3954 (2000).10.As proposed theoretically by Peierls and later shown

experimentally in many materials, the energy of low-dimensional metals can be reduced through a static chargeor spin modulation with periodicity equal to the Fermiwave vector.

11.A. Moreo et al., Science 283, 2034 (1999).12.Y. Tokura, N. Nagaosa, Science 288, 462 (2000).13.J.M. Tranquada et al., Phys. Rev. Lett. 78, 338 (1997).14.I acknowledge useful discussions with D. Dessau.

Published online 26 April 2001; 10.1126/science.1061205Include this information when citing this paper.

Detection of quasi-electrons. Illustration of the principle ofangle resolved photoemission spectroscopy (ARPES) (top).Electrons are emitted from the surface of the sample byphotons of energy h �DQG�GHWHFWHG�DV�D�IXQFWLRQ�RI�WKH

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electronic structure, the 2D wave vector k of a quasi-electronin the solid is proportional to sin . Traces of the intensity ofthe emitted electron beam as functions of (bottom, bluecurve) and binding energy E (red curve) provide detailedinformation about the E-versus-k relation and the lifetime ofthe quasi-electron.

Binding energy E

Emission angle θ

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