Electronic properties of GeTe and Ag- or Sb-substituted GeTe studied by low temperature 125Te NMR

TL;DR

This study uses $^{125}$Te NMR across a broad temperature range to analyze the electronic properties of GeTe and its Ag- or Sb-substituted variants, revealing dual-band contributions and effects of substitution on carrier concentration.

Contribution

It provides a microscopic understanding of the electronic structure of GeTe-based materials, highlighting the impact of Ag and Sb substitutions on their electronic properties through NMR and first-principles calculations.

Findings

Two electronic bands contribute to properties, one metallic and one semiconducting.

Ag substitution increases hole concentration, Sb decreases it.

No strong electron correlations observed in the metallic band.

Abstract

We have carried out $^{125}$Te nuclear magnetic resonance (NMR) in a wide temperature range of 1.5 -- 300 K to investigate electronic properties of Ge$_{50}$Te$_{50}$, Ag$_{2}$Ge$_{48}$Te$_{50}$ and Sb$_{2}$Ge$_{48}$Te$_{50}$ from a microscopic point of view. From the temperature dependence of NMR shift ($K$) and nuclear spin lattice relaxation rate (1/$T_1$), we found that two bands contribute to the physical properties of the materials. One band overlaps the Fermi level providing the metallic state where no strong electron correlations are revealed by Korringa analysis. The other band is separated from the Fermi level by an energy gap of $E_{\rm g}/k_{\rm B}$ $\sim$ 67 K, which gives rise to the semiconductor-like properties. First principle calculation revealed that the metallic band originates from the Ge vacancy while the semiconductor-like band may be related to the fine structure of the density of states near the Fermi level. Low temperature $^{125}$Te NMR data for the materials studied here clearly show that the Ag substitution increases hole concentration while Sb substitution decreases it.