Pressure-induced insulator-metal transition of localized states in FeSi1-xGex - art. no. 115103

Resistivity measurements have been carried out in FeSi1-xGex (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 y ielding the measured transport gap Delta. The trends in the pressure variat ion of Delta seems to depend on the measured resistivity ratio R(4.2 K)/R(3 00 K) at ambient pressure. The observed behavior of Delta with increase in pressure is argued to arise from two competing factors that contribute to D elta, a decrease due to shift in the mobility edge E-mu toward E-F and an i ncrease due to changes in the electronic structure of the bulk. A remarkabl e feature of the experimental results, however, is the drastic change in th e temperature dependence of conductivity sigma (T) in the 4-50 K range. In this temperature range, while sigma (T) fits the variable range hopping tra nsport 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 derivati ve w=d In sigma /d In T as a function of T-1/2 for various external pressur es reveals that w is a decreasing function of temperature for low pressure and gradually becomes an increasing function of temperature at higher press ures, in both FeSi and FeSi0.95Ge0.05. These results indicate that the loca lized states in the gap delocalize, giving rise to an insulator to metal tr ansition as a function of pressure. From the nature of the temperature depe ndence of w across the transition, it can be surmised that the insulator to metal transition in FeSi is possibly continuous as in doped semiconductors .