Inelastic light scattering is an intensively used tool in the study of electronic properties of solids. Triggered by the discovery of high-temperature superconductivity in the cuprates and by new developments in instrumentation, light scattering in both the visible (Raman effect) and x-ray part of the electromagnetic spectrum has become a method complementary to optical (infrared) spectroscopy while providing additional and relevant information. The main purpose of the review is to position Raman scattering with regard to single-particle methods like angle-resolved photoemission spectroscopy, and other transport and thermodynamic measurements in correlated materials. Particular focus will be placed on photon polarizations and the role of symmetry to elucidate the dynamics of electrons in different regions of the Brillouin zone. This advantage over conventional transport (usually measuring averaged properties) provides new insights into anisotropic and complex many-body behavior of electrons in various systems. Recent developments in the theory of electronic Raman scattering in correlated systems and experimental results in paradigmatic materials such as the A15 superconductors, magnetic and paramagnetic insulators, compounds with competing orders, as well as the cuprates with high superconducting transition temperatures are reviewed. An overview of the manifestations of complexity in the Raman response due to the impact of correlations and developing competing orders is presented. In a variety of materials, observations which may be understood and a summary of important open questions that pave the way to a detailed understanding of correlated electron systems, are discussed.

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Inelastic light scattering from correlated electrons

A recent paper (Nature 447, 565—568 2007) described the electronic structure of a hole–doped high–temperature copper oxide superconductor in its normal metallic state. The Fermi surface observed in underdoped materials — made up of small pockets — was at odds with that of their overdoped counterparts, and quite unlike that of conventional metals. Now David LeBoeuf et al. use measurements of the Hall effect to show that these structures in the Fermi surface are electron pockets, not the 'hole' pockets suspected. This pocket formation could be a result of an enigmatic symmetry–breaking phase, similar to that seen in some electron-doped copper oxides. The implicated phase seems to be a generic property of the copper oxides, so may be expected to play a central role in shaping the properties of high–temperature superconductors. The observation of a negative Hall resistance in the magnetic-field-induced normal state of underdoped 'YBCO'materials, which reveals that these pockets are electron-like rather than hole-like. It is proposed that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry1. The Fermi surface of the electronic states in the underdoped ‘YBCO’ materials YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets2,3,4, in contrast with the large cylinder that characterizes the overdoped regime5, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order6,7. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order8,9,10,11 suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (Tc) superconductors.

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors.

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004479.pdf

Electron pockets in the Fermi surface of hole-doped high-$T$c superconductors

At low temperatures, low dimensional metals are commonly found to exhibit superlattices incommensurate with the primitive lattice (periodic lattice distortion coupled to charge density waves, PLD — CDW). Transitions to such states are driven by strong electron phonon coupling and favoured by the particular forms of the Fermi surfaces of these metals. Experimental investigations of the structural distortions, and of the changes in electronic structure around the Fermi level produced by the CDW — PLD state, will be discussed for the layered dichalcogenides, the linear chain platinum complex KCP, and the organic charge transfer salts.

https://iopscience.iop.org/article/10.1088/0022-3719/12/8/009/pdf

Periodic lattice distortions and charge density waves in one- and two-dimensional metals

Evidence for a connection between charge density waves and the pressure enhancement of superconductivity in 2H-NbSe2☆

The Hall coefficient, magnetoresistance, and thermoelectric power of several specimens of NbSe2 have been measured as a function of temperature for various crystal orientations. A range of behaviou...

D. J. Huntley

Papers

Transport Properties of NbSe2