Insulator-Metal Transition in Co-Doped Pyrite FeS₂ Single Crystals

Iron pyrite (FeS₂) is a low-cost, earth-abundant, non-toxic semiconductor with attractive electronic and optical properties, both for fundamentals, and applications (e.g. photovoltaics). Sulfur vacancies (V_S)[1] and Co[2] are the only well-established n-dopants in FeS₂. V_S are deep donors, however, preventing facile study of phenomena such as the insulator-metal transition (IMT), while surface conduction complicates transport at low Co doping. In this work the problem of surface conduction is circumvented via a contacting scheme that provides access to the bulk, enabling wide-T-range transport studies of Co-doped FeS₂ single crystals. An IMT is found at ~4Χ10¹⁷cm^-3 Hall density, with Efros-Shklovskii variable-range hopping and/or activated transport below this, and electron-electron interaction-corrected metallic conductivity above it. A number of unexpected features also occur, however, including non-monotonic T dependence of the Hall coefficient, resistivity anomalies at intermediate T, and non-saturating non-parabolic positive magnetoresistance. These will be discussed in detail.
1.B. Voigt et al.,ACS Appl.Mater.Interfaces,11,15552(2019).
2.S. Guo et al.,Phys.Rev.B,81,144424,(2010).