Orbital-driven Variation of Electronic Structures in Tetragonal La_1/2Sr_1/2MnO₃ as Investigated by Optical Spectroscopy

Abstract

Optical conductivity spectra (σ(ω)) were investigated for coherently strained epitaxial films of tetragonal La_1/2Sr_1/2MnO₃ with various c/a ratios (1.04, 1.00, and 0.98). With decreasing c/a, σ(ω) in the ground state shows a successive dramatic change from an insulating gap-like to metallic but highly incoherent spectrum, then to a pseudo-gap one, which reflects the strain-induced e_g orbital ordering/disordering and the resultant different spin structures. The variation of the electronic structure induced by such a strain-orbital coupling is discussed in comparison with some theoretical models.