Mapping delocalization of impurity bands across archetypal Mott-Anderson transition

TL;DR

This study investigates how impurity states evolve into a conducting band across a Mott-Anderson transition in Fe-V-Al Heusler compounds, revealing the localization-delocalization process and its impact on resistivity at high disorder levels.

Contribution

It provides a detailed mapping of the impurity band delocalization process during the Mott-Anderson transition using combined theoretical and experimental approaches.

Findings

Impurity states are initially Anderson localized at low V concentrations.

Mobility edges form as V content increases, indicating a transition.

Resistivity becomes temperature-independent at high disorder, reaching the Mott-Ioffe-Regel limit.

Abstract

Tailoring charge transport in solids on demand is the overarching goal of condensed-matter research as it is crucial for electronic applications. Yet, often the proper tuning knob is missing and extrinsic factors such as impurities and disorder impede coherent conduction. Here we control the very buildup of an electronic band from impurity states within the pseudogap of ternary Fe$_{2-x}$V$_{1+x}$Al Heusler compounds via reducing the Fe content. Our density functional theory calculations combined with specific heat and electrical resistivity experiments reveal that, initially, these states are Andersonlocalized at low V concentrations $0 < x < 0.1$. As x increases, we monitor the formation of mobility edges upon the archetypal Mott-Anderson transition and map the increasing bandwidth of conducting states by thermoelectric measurements. Ultimately, delocalization of charge carriers in fully disordered V$_3$Al results in a resistivity exactly at the Mott-Ioffe-Regel limit that is perfectly temperature-independent up to 700 K - more constant than constantan.