Unusual Pressure-Induced Metallic State In The Correlated Narrow Band-Gap Semiconductor Fesi

Compressing FeSi induces a progressive semiconductor to metal transition, onset at P >= 15 GPa at temperatures below T-max determined by the degree of disorder in the sample. At high pressure preceding charge-gap closure, a broad maximum manifests in the rho(T) data at T-max and is a feature which persists into the metallic state. The extremum in rho(T) occurs at T-max similar to 40 K at similar to 11 GPa and shifts monotonically to similar to 240 K as pressure is increased to similar to 32 GPa, in the most detailed example of three series of measurements involving pressurized FeSi with different degrees of disorder. The transition to a metallic phase is an electronic change only, in that the B 20-type crystal structure is retained up to 30 GPa, with no evidence of a discontinuity in the volume-pressure equation of state data. Samples from the same ingot subjected to different quasihydrostatic conditions reveal different values of the critical pressure of the electronic transition, its width, and pressure dependences of T-max. This attests to sensitivity of the electronic transition to the degree of disorder in the investigated sample. The metallic state has neither Fermi-liquid nor non-Fermi-liquid behavior. Such an unusual pressure-induced correlated metallic state in FeSi is attributed to extended states within the 3d-3p hybridization gap originating from disorder and compression tuning of the mobility edge relative to the Fermi level. The metallic state has also been investigated in external magnetic fields up to 8 T at low temperatures (2 K <= T <= 20 K) at 15 and 19 GPa. This reveals a positive magnetoresistance, as observed in doped Fe1-xCoxSi samples at ambient pressure, suggesting that in the majority and minority spin bands there is a field-induced modification of the respective magnitudes of charge-carrier populations which have different mobilities.