Evolution of the density of states at the Fermi level across the metal-to-insulator crossover in alkali doped zeolite

TL;DR

This study uses NMR to investigate how the electronic density of states at the Fermi level evolves in sodium-loaded zeolite as it transitions from metal to insulator, revealing complex effects of doping, correlations, and disorder.

Contribution

It provides the first systematic NMR analysis of the DOS evolution across the metal-insulator transition in alkali-doped zeolite, highlighting the interplay of correlations and disorder.

Findings

High sodium loading shows metallic behavior with constant relaxation rates.

Decreasing sodium reduces the DOS at the Fermi level, indicating a crossover.

Residual DOS persists even without metallic optical signatures.

Abstract

We report a systematic nuclear magnetic resonance investigation of the $^{23}$Na spin-lattice relaxation rate, $1/T_1$, in sodium loaded low-silica X (LSX) zeolite, Na$_n$/Na$_{12}$-LSX, for various loading levels of sodium atoms $n$ across the metal-to-insulator crossover. For high loading levels of $n \geq 14.2$, $1/T_1T$ shows nearly temperature-independent behavior between 10 K and 25 K consistent with the Korringa relaxation mechanism and metallic ground state. As the loading levels decrease below $n \leq 11.6$, the extracted density of states (DOS) at the Fermi level sharply decreases, although a residual DOS at Fermi level is still observed even in samples that lack the metallic Drude-peak in the optical reflectance. The observed crossover is a result of a complex loading-level dependence of electric potential felt by the electrons confined to zeolite cages, where the electronic correlations and disorder both play an important role.