Unusual phase boundary of the magnetic-field-tuned valence transition in CeOs 4 Sb 12

The phase diagram of the filled skutterudite ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ has been mapped in fields ${\mu }_{0}H$ of up to 60 T and temperatures $T$ down to 0.5 K using resistivity, magnetostriction, and megahertz conductivity. The valence transition separating the semimetallic low-$H$, low-$T$ $\mathcal{L}$ phase from the metallic high-$H$, high-$T$ $\mathcal{H}$ phase exhibits a very unusual, wedge-shaped phase boundary, with a nonmonotonic gradient alternating between positive and negative. The expected “elliptical” behavior of the phase boundary of a valence transition with $H^2\propto T^2$ originates in the $H$ and $T$ dependence of the free energy of the $f$ multiplet. Here, quantum oscillation measurements suggest that additional energy scales associated with a quantum critical point are responsible for the deviation of the phase boundary of ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ from this textbook behavior at high $H$ and low $T$. The distortion of the low-$H$, high-$T$ portion of the phase boundary may be associated with the proximity of ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ to a topological semimetal phase induced by uniaxial stress.