Decrease of Itinerant Holes near the Metal to Insulator Crossover in Superconducting La_1.85Sr_0.15CuO₄

Abstract

Polarized Cu L-edge X-ray absorption spectra (XAS) measurements of the unoccupied electronic state of 3d_x²-y² symmetry in superconducting La_1.85Sr_0.15CuO₄ single crystals show a temperature dependent anomalous decrease of the density of itinerant holes below -- 100K. The anomalous temperature coincides with the temperature of the metal to insulator (MI) transition and local lattice distortions in the CuO₂ plane. The results give direct evidence of decrease of itinerant states in the CuO₂ plane due to local lattice distortions across the MI crossover in the normal state.