Anomalous electronic transport in CuIr2S4 and CuIr2Se4

The resistivity (rho) and the thermopower (S) of spinel-type compounds CuIr 2S4 and CuIr2Se4 have been measured at temperatures from 2 to 900 K under m agnetic field from 0 to 15 T. The thermopower of both compounds is positive in the metallic phase except for the low-temperature region in CuIr2Se4. I t is also positive in the insulating state of CuIr2S4, implying p-type char ge carriers, in agreement with the recent photoemission results. The low-te mperature resistivity of CuIr2S4 is well described by Efros-Shklovskii vari able-range hopping conductivity mechanism: rho = rho(0) exp[(T*/T)(1/2)]. T he most unusual feature of the transport is that the resistivity of both co mpounds in the metallic state reveals an activation type of the temperature dependence. (C) 2000 Elsevier Science B.V. All rights reserved.