Pressure-induced insulator-metal transition of localized states inFeSi1−xGex

Abstract

Resistivity measurements have been carried out in ${\mathrm{FeSi}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ $(x=0.0,$ 0.05, and 0.20) in the 4--300 K range under the application of pressure up to 6.4 GPa. The resistivity data between 100 and 200 K fit an activated behavior yielding the measured transport gap \ensuremath{\Delta}. The trends in the pressure variation of \ensuremath{\Delta} seems to depend on the measured resistivity ratio $R(4.2\mathrm{K})/R(300\mathrm{K})$ at ambient pressure. The observed behavior of \ensuremath{\Delta} with increase in pressure is argued to arise from two competing factors that contribute to \ensuremath{\Delta}, a decrease due to shift in the mobility edge ${E}_{\ensuremath{\mu}}$ toward ${E}_{F}$ and an increase due to changes in the electronic structure of the bulk. A remarkable feature of the experimental results, however, is the drastic change in the temperature dependence of conductivity $\ensuremath{\sigma}(T)$ in the 4--50 K range. In this temperature range, while $\ensuremath{\sigma}(T)$ fits the variable range hopping transport mechanism in pristine FeSi, significant deviation from such a fit is seen with Ge substitution and under the application of pressure. The data in these cases fit better to power laws. A plot of the logarithmic derivative $w=d\mathrm{ln}\ensuremath{\sigma}/d\mathrm{ln}T$ as a function of ${T}^{1/2}$ for various external pressures reveals that w is a decreasing function of temperature for low pressure and gradually becomes an increasing function of temperature at higher pressures, in both FeSi and ${\mathrm{FeSi}}_{0.95}{\mathrm{Ge}}_{0.05}.$ These results indicate that the localized states in the gap delocalize, giving rise to an insulator to metal transition as a function of pressure. From the nature of the temperature dependence of w across the transition, it can be surmised that the insulator to metal transition in FeSi is possibly continuous as in doped semiconductors.