Unusual phase boundary of the magnetic-field-tuned valence transition in CeOs${}_4$Sb${}_{12}$

Abstract

The phase diagram of the filled skutterudite CeOs${}_4$Sb${}_{12}$ has been mapped in fields $H$ of up to 60 T and temperatures $T$ down to 0.5 K using resistivity, magnetostriction, and MHz 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 non-monotonic gradient alternating between positive and negative. This is quite different from the text-book "elliptical" phase boundary usually followed by valence transitions. Analysis of Shubnikov-de Haas oscillations within the $\mathcal{H}$ phase reveals an effective mass that increases as $H$ drops toward the $\mathcal{H}-\mathcal{L}$ phase boundary, suggesting proximity to a quantum-critical point. The associated magnetic fluctuations may be responsible for the anomalous $H,T$ dependence of the valence transition at high $H$, whereas the low$-H$, high$-T$ portion of the phase boundary may rather be associated with the proximity of CeOs${}_4$Sb${}_{12}$ to a topological semimetal phase induced by uniaxial stress.

7 pages, 4 figures and Supplementary Information of 4 pages, 2 figures