Pressure induced metallization and loss of surface magnetism in FeSi

Single crystalline FeSi samples with a conducting surface state (CSS) were studied under high pressure (P) and magnetic field (B) by means of electrical resistance (R) measurements to explore how the bulk semiconducting state and the surface state are tuned by the application of pressure. We found that the energy gap (Δ) associated with the semiconducting bulk phase begins to close abruptly at a critical pressure (Pcr) of ~10 GPa and the bulk material becomes metallic with no obvious sign of any emergent phases or non-Fermi liquid behavior in R(T) in the neighborhood of Pcr above 3 K. Moreover, the metallic phase appears to remain at near-ambient pressure upon release of the pressure. Interestingly, the hysteresis in the R(T) curve associated with the magnetically ordered CSS decreases with pressure and vanishes at Pcr, while the slope of the R(B) curve, dR/dB, which has a negative value for P < Pcr, decreases in magnitude with P and changes sign at Pcr. Thus, the CSS and the corresponding two-dimensional magnetic order collapse at Pcr where the energy gap Δ of the bulk material starts to close abruptly, revealing the connection between the CSS and the semiconducting bulk state in FeSi.