Optical Study of Metal-Insulator Transition in the Ferromagnetic Ground State of La1-xSrxMnO3

Abstract

Optical spectra for the ferromagnetic ground states have been investigated for single crystals of perovskite-type manganese oxides La 1- x Sr x MnO 3 with finely varying hole-doping concentration ( x ) around the metal-insulator (MI) phase boundary ( x c ∼ 0.16). For such a fully spin-polarized state, orbital degree of freedom may play an important role in the MI transition, which is anticipated to be figured out in terms of optical spectroscopy. In the ferromagnetic ground state, the low-energy optical conductivity spectra are observed to vary from Drude-like (typically for x =0.4) to incoherent broad feature (for x < 0.2) with decreasing x . The spectral-weight analysis shows that the kinetic energy of conduction electron scarcely changes with change of x when viewed on a large energy scale (about 1 eV) whereas the coherent response on a low energy scale (∼0.1 eV) is critically suppressed toward the insulating phase. Orbital correlation and its coupling with lattice dynamics are likely responsible for such an anomalous variation of charge dynamics. Far-infrared pseudo gap structure observed in the ferromagnetic metallic phase of x =0.16 on the verge of MI transition suggests the charge localization affected by randomness and non-local interaction in the course of orbital and charge ordering.