Electronic transport and antiferromagnetism in the layered compound BaCoS2

BaCoS2 is a Mott insulator with a Neel temperature (7(N)) close to room tem perature. In the ordered antiferromagnetic phase, the magnetic moments are confined in the Co2S2 planes, and arranged in alternatingly directed ferrom agnetic chains. We report on resistivity and thermopower measurements and o n room-temperature cell constants of samples of BaCoSz with nominal sulfur content 1.9 less than or equal to z less than or equal to 2.1. We discuss t he effect of sulfur content on cell constants and band filling. The main ob servation is a crossover from activated electronic transport at high temper atures, to high resistivity almost independent of temperature, and accompan ied by a vanishingly small thermopower, at low temperatures (far below T-N) We interpret the transport mechanism in the low-temperature regime as tunn eling in the Co2S2 planes, to nearest neighbors along the ferromagnetic cha ins and to next-nearest neighbors in the perpendicular direction, or altern atively to nearest neighbors along the ferromagnetic chains with conducting inclusions acting as shorts. [S0163-1829(99)02410-8].