Mapping delocalization of impurity bands across archetypal Mott-Anderson transition

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.