Unusual phase boundary of the magnetic-field-tuned valence transition in CeOs4Sb12

The phase diagram of the filled skutterudite CeOs4Sb12 has been mapped in fields μ0H 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 L phase from the metallic high-H, high-T 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 H2â T2 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 CeOs4Sb12 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 CeOs4Sb12 to a topological semimetal phase induced by uniaxial stress.