R. Shiozaki

Impurity effect on charge-ordered and ferromagnetic–metallic state in manganese perovskites: Comparison between Cr and Al doping

The role of impurities in melting of the charge-ordered (CO) state in half-doped manganese perovskites is discussed based on the systematic study of Cr and Al doping on the CO state as well as on the ferromagnetic–metallic (FM) state. The CO state in Nd0.5Sr0.5MnO3 is destroyed and the FM state is restored by 2% high-spin (S=3/2) Cr3+, whereas the CO state survives doping of spinless Al3+. On the other hand, the doping effects on the FM state in La1−xSrxMnO3 are almost identical between Cr and Al, except that Cr degrades Curie temperature less. The slight resistance of the CO state against Cr doping can be explained by the magnetic interaction between the Cr and the Mn t2g spins.

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.

Optical reflectivity study on magnetoresistive manganese perovskites: impurity effect on the ferromagnetic–metallic and charge-ordered states

Abstract Impurity effect on manganese perovskites are investigated by the optical reflectivity measured on cleaved surfaces of the single crystals. The Al impurity affects optical conductivity σ ( ω ) of ferromagnetic–metallic La 1− x Sr x MnO 3 over a wide energy range up to 1 eV or higher, contrary to the simple-Drude picture. The energy gap of the charge-ordered (CO) state in Nd 0.5 Sr 0.5 MnO 3 is about 0.5 eV , much larger than that suggested by the previous studies using polished surfaces. This indicates that the CO state can be easily damaged by external disorder.

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.

Incoherent metallic state in a double-exchange ferromagnet: transport and optical properties of La1−xSrxMnO3

Abstract Transport and optical studies on La0.825Sr0.175MnO3 indicate that the charge dynamics undergoes a coherent–incoherent crossover at a certain critical region of the dc conductivity. The present result suggests that the ferromagnetic metallic phase below the Curie temperature TC is divided into two regions, a “high-temperature incoherent” metallic and a “low-temperature coherent” metallic region. “Colossal magnetoresistance” is a characteristic of the former region.