Unconventional electronic transport near the insulator-metal transition in Co-doped pyrite FeS₂ single crystals

Pyrite FeS₂ is a low-cost, sustainable, non-toxic 1-eV-gap semiconductor with unrealized potential in applications such as photovoltaics. From the fundamental perspective, issues such as surface conduction and the deep-donor nature of sulfur vacancies have strongly hindered the study of low T transport and phenomena such as the insulator-metal transition (IMT). In this work, a novel CoS₂-based contact scheme is used to access bulk transport, enabling wide-T-range transport studies of thoroughly characterized Co-doped FeS₂ single crystals. The IMT is found to occur near 3 × 10¹⁷ cm^-3 Hall density, with Efros-Shklovskii variable-range hopping below this, and electron-electron-interaction-corrected conductivity above this. In the vicinity of the IMT a host of intriguing phenomena emerge, including non-linear Hall effect with a non-monotonic T dependence, and non-saturating non-parabolic positive low-T magnetoresistance. Quantitative analysis of these effects points to strong effects of disorder in the vicinity of the IMT.