Insulator-Metal Transition in La_2-xSr_xNiO₄

抄録

Successful fabrication of epitaxial thin films of La_2-xSr_xNiO₄ (0.5≤ x ≤ 1.4) has enabled us to investigate the change in charge dynamics upon the insulator-metal transition (IMT). The charge gap induced by (π,π) charge ordering as observed in optical spectra for x=0.5 is decreased with the increase of x, while the pseudo-gap feature subsists at least up to x=1.0 beyond IMT critical point (x_c ≈ 0.9). In the metallic phase near IMT, the Hall coefficient shows remarkable enhancement at low temperatures. The results indicate the predominant role of the charge ordering/correlation in the IMT.