J. Nohara

Correlation between Raman sum and optical conductivity sum in La2−xSrxCuO4

In a strongly correlated electron system, the single-particle spectral function changes into a coherent peak and incoherent humps which extend over 1 eV. The incoherent parts lose the symmetry and k dependence, so that the Raman spectra with different symmetries become identical and they are expressed by the optical conductivity. We found that the B1g and B2g spectra in La(2-x)Sr(x)CuO4 become identical above 2000 cm(-1) in the underdoped phase, if Fleury-Loudon type B1g two-magnon scattering is removed. The first Raman susceptibility moment correlates with the generalized optical conductivity moment. The good correlation arises from the incoherent states of a hump from 1000 to 4000 cm(-1). The hump is the only structure of the incoherent electronic states in the mid-infrared absorption spectra below 1.4 eV at low carrier densities. The energy is twice the separated dispersion segments of the spin wave in the k(perpendicular) stripe direction. The incoherent state is formed by the magnetic excitations created by the hole hopping in the antiferromagnetic spin stripes in the real space picture.

Electric resistivity and phase diagram viewed from charges in La2- xSrxCuO4

The electric transport changes from the power-law temperature dependent resistivity ρ= a + b T c to the logarithmic temperature dependent conductivity σ= d + e ln T as temperature decreases across 90 K in the underdoped phase. The crossover temperature is independent of the carrier density differently from the pseudo-spin-gap. The crossover is not caused by the simple relaxation time, but by the change of the density of states near the Fermi energy in the quasiparticle (resonant) peak at (π/2,π/2). The exponent c decreases from 2 at x =0.03 to 1 at x =0.16 and then increases to 1.4 at x =0.25.

Raman and Optical Reflection Studies of Electronic States in La2−xSrxCuO4

The electronic states in La2−xSrxCuO4 are systematically investigated by Raman scattering, infrared‐ultraviolet reflection spectroscopy, and electric resistivity. A narrow quasi‐particle band is created at EF and sharply grows up by collecting the density of states from the high energy region at 1–1.5 eV as temperature decreases near the insulator‐metal transition. The relaxation time of conducting carriers is limited by the quasi‐particle band width near the insulator‐metal transition, but it drastically increases in the overdoped phase. The quasi‐particle band gives the anomalous electronic properties.The electronic states in La2−xSrxCuO4 are systematically investigated by Raman scattering, infrared‐ultraviolet reflection spectroscopy, and electric resistivity. A narrow quasi‐particle band is created at EF and sharply grows up by collecting the density of states from the high energy region at 1–1.5 eV as temperature decreases near the insulator‐metal transition. The relaxation time of conducting carriers is limited by the quasi‐particle band width near the insulator‐metal transition, but it drastically increases in the overdoped phase. The quasi‐particle band gives the anomalous electronic properties.

Coherent-to-incoherent crossover in optical conductivity of bad-metallic La2-xSrxCuO4

Abstract The in-plane charge dynamics of La1.92Sr0.08CuO4 are examined. The in-plane resistivity ρab(T) is metallic up to 1000 K without saturation at the Mott criterion, whereas the in-plane optical conductivity σab(ω) shows a Drude peak only below a certain temperature T ∗ ∼300 K. Above T ∗ the Drude peak shifts to finite energy. The relation between the shift of the Drude peak and the Mott Criterion indicates the “dynamical” localization of the carriers.