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