We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

/pdf/electrodynamics-of-correlated-electron-materials-2qg1x9aaaz.pdf

Electrodynamics of correlated electron materials

Recent studies of the electromagnetic response of high-Tc superconductors using terahertz, infrared, and optical spectroscopies are reviewed. In combination these experimental techniques provide a comprehensive picture of the low-energy excitations and charge dynamics in this class of materials. These results are discussed with an emphasis on conceptual issues, including evolution of the electronic spectral weight in doped Mott-Hubbard insulators, the d-wave superconducting energy gap and the normal-state pseudogap, anisotropic superfluid response, electronic phase segregation, emergence of coherent electronic state as a function of both temperature and doping, the vortex state, and the energetics of the superconducting transition. Because the theoretical understanding of these issues is still evolving the review is focused on the analysis of the universal trends that are emerging out of a large body of work carried on by many research teams. Where possible data generated by infrared/optical techniques are compared with the data from other spectroscopic and transport methods.

/pdf/electrodynamics-of-high-t-c-superconductors-1p3dck85rg.pdf

Electrodynamics of high- T c superconductors

We report on a study of the electromagnetic response of three different families of high- superconductors that in combination allowed us to cover the whole doping range from under- to overdoped. The discussion is focused on the ab-plane charge dynamics in the pseudogap state which is realized in underdoped materials below a characteristic temperature , a temperature that can significantly exceed the superconducting transition temperature . We explore the evolution of the pseudogap response by changing the doping level, by varying the temperature from above to below , or by introducing impurities in the underdoped compounds. We employ a memory function analysis of the ab-plane optical data that allows us to observe the effect of the pseudogap most clearly. We compare the infrared data with other experimental results, including the c-axis optical response, dc transport, and angle-resolved photoemission.

/pdf/the-pseudogap-state-in-high-superconductors-an-infrared-3m5xqnkd02.pdf

The pseudogap state in high- superconductors: an infrared study

A fairly sophisticated description of electron-phonon superconductivity has existed since the early 1960’s, following the work of Eliashberg [1], Nambu [2], Morel and Anderson [3], and Schrieffer et al. [4]. All of this work extended the original ideas of Bardeen, Cooper, and Schrieffer [5] on superconductivity, to include dynamical phonon exchange as the root cause of the effective attractive interaction between electrons in a metal. For certain superconducting materials, Eliashberg theory (as this description is generally called) provides a very accurate description of the superconducting state. Nonetheless, as B.T. Matthias was fond of iterating [6], this description was never considered (by him and others) particularly helpful for discovering new, high temperature superconductors [7]. Part of the problem remains that a truly accurate description of the normal state has not been forthcoming. Part of that problem is the “curse” of Fermi Liquid Theory. To the extent that the electron-phonon coupling causes relatively innocuous corrections to most normal state properties, its underlying characteristics remain undetectable (indeed, as will be reviewed here, the characteristics of the electron-phonon interaction are made more apparent in the superconducting state). An exception may be the A15 compounds, whose anomalous normal state properties might help us achieve further understanding of the electron-phonon interaction in these materials [10].

Electron - Phonon Superconductivity

Theoretical and experimental work concerning high-temperature superconductors in general and cuprates in particular is reviewed. A detailed analysis of the current knowledge of the subject suggests that when in the normal state, superconducting cuprates behave very much the same as 'conventional' metals. Experimental evidence is presented for the existence of strong relaxation processes in the normal state of HTSC systems at low energy. Ab initio calculations of the optical spectra and the electron–phonon interaction (EPI) show that the electron-phonon mechanism explains many features of low-energy relaxation processes in HTSC systems, including the high critical temperature. However, many properties of the superconducting state, for example, the anisotropic d pairing in cuprates, cannot be explained with the EPI mechanism alone. A number of models for cuprate superconductors with EPI and Coulomb repulsion are discussed in detail.

High-temperature superconductivity: the current state

Calculations of the effect of intermediate bosons on the optical properties of the normal state are used to analyze the spectra of these bosons. For high temperature superconductors these spectra are in agreement with the standard electron-phonon model (the end of the spectrum is near 800 cm−1) with a moderate coupling constant λ=1–1.5. For the heavy fermion compound UPt3 the spectrum of intermediate bosons is consistent with neutron data for spin fluctuations.

S. V. Shulga

Papers

Analysis of intermediate boson spectra from FIR data for HTSC and heavy fermion systems