Investigation of the Metallic State in Cubic FeGe beyond its Quantum Phase Transition

FeGe and MnSi are prominent examples where the Dzyaloshinskii-Moriya interaction causes a modulation of the ferromagnetic structure as a consequence of the lack of inversion symmetry in the $B20$ structure (space group $P2_13 $). In FeGe, helimagnetism sets in through a first order phase transition at $T_{\rm C}=280\,$K with a saturated moment of $m=1 \mu_B$ per Fe atom. The helical modulation has a period of about $700\,$\AA\ and propagates along the spiral propagation vector ${\bf k} \parallel [1 0 0]$. It alters its direction to ${\bf k}\parallel [1 1 1]$ at $T_2\approx 211-245\,$K without a change in the period. In MnSi, however, the helical order occurs below $T_{\rm C}=29\,$K. The modulation has a wavelength of $175\,$\AA\ and the ordered moments of about $m=0.4\,\mu_B$ per Mn atom are perpendicular to ${\bf k}\parallel [1 1 1]$. It is well established that the second order phase transition is driven first order for a sufficiently weak magnetic interaction close to the critical pressure, $p_{\rm c}=1.46\,$GPa. In light of these structural and magnetic similarities between FeGe and MnSi, a volume compression in FeGe could tune its $T_C$ to zero temperature with the chance to reveal peculiar electronic ground state properties at the verge of the magnetic order. Indeed, the electrical resistivity measurements, $\rho(T) $, show a suppression of the helical order at $p_c\approx 19 \,$GPa. The strong deviations from a Fermi-liquid behavior in a wide pressure range above $p_c$ suggest that the suppression of $T_C$ disagrees with the standard notion of a quantum critical phase transition. Our band-structure calculations suggest that disorder due to zero-point motion is strong enough to close the narrow gap expected for compressed FeGe, stabilizing a new magnetic ground state above $p_c$. An anomaly observed at $T_X$ in the $\rho(T)$ curves recorded above $p_c$ might be related to this magnetic phase. The isothermal structural data at low temperature revealed a discontinuous change in the pressure dependence of the shortest Fe-Ge interatomic distance close to the $T_C(p)$ phase line. The $(T,V)$ phase diagram will be discussed and the connection with MnSi and the semiconducting properties of FeSi will be addressed.