Electron density-of-states and the metal–insulator transition in LaHx

Abstract

The temperature dependence of the Gd3+ impurity ion spin relaxation rate, τi−1, in lanthanum hydrides, LaHx (2.0≤x≤3.0), probes the metal–insulator (MI) transition in this system. Because Gd3+ is an S-state ion, τi−1 depends on interaction with conduction electrons in the metallic state, resulting in Korringa-type behavior, τiT=constant, and (τiT)−1/2∝N(EF), the electronic density-of-states at the Fermi level. In the non-metallic state, weak phonon processes result in a temperature dependence, τi−1∝Tn, with 3<n<5. The rate τi−1 has been measured via the Gd3+-induced contribution, R1p, to the proton spin-lattice relaxation rate, R1, by comparing the measured rate in pure LaHx with that in LaHx containing controlled low levels of Gd. The results show that in the metallic state, N(EF)∝(2.91−x)1/3, consistent with earlier results based on the 139La Knight shift which also support N(EF)∝(2.91−x)1/3.