Evolution of the Kondo insulating gap in Fe_1-xRu_xSi

Abstract

Electrical resistivity measurements are reported in Fe_1-xRu_xSi (x=0.0 to 0.30) as a function of temperature (4.2 K to 300 K) and as a function of temperature and pressure (0-5 GPa) for x=0.04 and 0.1. The temperature-dependent resistivity data have been analyzed based on a Model Density of States (DOS) taking into account both electron correlations and disorder. Using this model DOS and temperature-dependent mobility, the conductivity as a function of temperature is calculated and fitted to the experimental data for several external pressures and Ru concentrations. The parameters of the fit as a function of x, show extremal values at the Ru composition x=0.06. An effective band gap (Δ_eff) obtained from these parameters is seen to decrease with Ru substitution up to x=0.06 beyond which it increases again. Band-structure calculations performed for several stoichiometric compositions of x in Fe_1-xRu_xSi, indicate a broadening of the bands and an overall increase in the gap from 0.13 eV for x=0.0 to 0.3 eV for x=1.0. The existence of localized states within the gap leads to the description of the transport behavior at low temperature (5-25 K) in terms of variable range hopping (VRH) mechanism. The VRH parameter T₀ decreases by more than three orders of magnitude for x up to 0.06, beyond which it increases again. It is surmised that with Ru substitution, (FeRu)Si goes over from a Kondo insulator to a conventional band semiconductor via an intermediate metallic phase. The results of temperature-dependent resistivity under high pressure in the samples with compositions x=0.04 and 0.1 are also reported and analyzed based on the model.