Electronic transport across the insulator-metal transition in Co-doped pyrite FeS2 single crystals

Pyrite FeS2 is a low-cost, sustainable, nontoxic, ∼1-eV-band gap semiconductor with unrealized potential in several application arenas, including photovoltaics. From the fundamental perspective, issues such as surface conduction and the deep-donor nature of S vacancies have hindered the study of low-temperature electronic phenomena in pyrite, including the insulator-metal transition (IMT). Here, we leverage a recently developed CoS2-based contact scheme in tandem with wide-range doping via shallow Co donors to directly access low-temperature bulk FeS2 transport properties and thus probe the IMT. Thoroughly characterized FeS2:Co single crystals are studied over broad ranges of temperature (0.4-400 K) and Hall electron density (8.6×1016-2.0×1020cm-3) through resistivity, Hall effect, and magnetoresistance measurements. The IMT is found to occur near 2×1017cm-3, with Efros-Shklovskii variable-range hopping below this, weak-localization-corrected metallic conductivity above this, and the onset of magnetic effects at the highest doping levels. Most significantly, unexpected additional phenomena are found near the IMT, including a nonlinear Hall effect with nonmonotonic temperature and doping dependence, and a nonsaturating, linear positive magnetoresistance at low temperatures. Quantitative analysis points to unusually strong disorder effects in the vicinity of the IMT, further elucidating the electronic behavior of this unique semiconductor.