Optical signatures of electron correlations in the cuprates
The f-sum rule is introduced and its applications to electronic and vibrational modes are discussed. A related integral over the intra-band part of σ(ω) which is also valid for correlated electrons, becomes just the kinetic energy if the only hopping is between nearest neighbor sites. A summary is given of additional sum rule expressions for the optical conductivity and the dielectric function, including expressions for the first and second moment of the optical conductivity, and a relation between the Coulomb energy and the energy loss function. It is shown from various examples, that the optical spectra of high T c materials along the c-axis and in the ab-plane direction can be used to study the kinetic energy change due to the appearance of superconductivity. The results show, that the pairing mechanism is highly unconventional, and mostly associated with a lowering of kinetic energy parallel to the planes when pairs are formed.
Keywords: Optical conductivity, spectral weight, sum rules, reflectivity, dielectric function, inelastic scattering, energy loss function, inelastic electron scattering, Josephson plasmon, multi-layers, inter-layer tunneling, transverse optical plasmon, specific heat, pair correlation, kinetic energy, correlation energy, internal energy.
KeywordsElectron Correlation Superconducting State Optical Conductivity Fermi Liquid Spectral Weight
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