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

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We report a systematic nuclear magnetic resonance investigation of the ²³Na spin-lattice relaxation rate, 1/T₁, in sodium loaded low-silica X (LSX) zeolite, Naₙ/Na₁₂-LSX, for various loading levels of sodium atoms n across the metal-to-insulator crossover. For high loading levels of n ≥ 14.2, 1/T₁T 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 ≤ 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.