Resistivity studies in the Kondo insulating system, FeSi_1-xGe_x

Abstract

Electrical resistivity measurements have been carried out in the FeSi_1-xGe_x system in the 1.4-300 K temperature range for 0.0≤x 0.25, the composition range in which the samples are monophasic. In FeSi, the resistance shows a two-step increase with decrease in temperature; the high-temperature region (200-100K) showing an activated semiconducting behaviour corresponding to a gap value, Δ, of 707 K. With Ge substitution at the Si site, the resistivity continues to show a semiconducting behaviour and the gap values obtained from the experimental data systematically decrease to 115 K for x=0.25. The observed decrease in Δ is discussed in conjuction with band-structure calculations. In FeSi and in the Ge substituted sample with x=0.05, the resistivity in the 5-50 K range could be fitted either to an activated behaviour with lower Δ or to the variable-range hopping mechanism. In the samples with x>0.05, the data in the 5-50 K region fits better to the variable-range hopping mechanism. Below 5 K, the resistance tends to saturate to a constant value in FeSi and the low-temperature saturation of resistance persists upon Ge substitution, possibly implying a metallic ground state as T→0.