Abstract

CeOs4Sb12, a member of the filled skutterudite family and close relative of the unconventional superconductor PrOs4Sb12, has a very unusual H-T phase diagram in which the boundary of the valence transition between the semimetallic low-H, low-T L phase and the metallic high-H, high-T H phase exhibits an uncommon, wedge-like shape. Contrary to the “elliptical” text-book behaviour for which H2 is proportional to T2, the boundary between H and L phase is strongly altered with a non-monotonic gradient that alternates between positive and negative. The phase diagram was mapped out by resistivity, magnetostriction, and MHz conductivity in magnetic fields up to 60 T. We ascribe the unusual behaviour of the phase boundary at low T to the presence of additional energy scales associated with a quantum critical point. Field-dependent, increasing effective masses derived from quantum oscillations close to the intersection of L, H and the ordered spin-density wave phase are interpreted as indicators of this quantum criticality. The high T, low H portion of the phase diagram may instead be associated with the proximity of CeOs4Sb12 to a topological semimetal phase induced by uniaxial stress.

In this talk I will present the experimental results that indicate the boundaries in the phase diagram and discuss the valence transition in CeOs4Sb12 in context with other Ce and Yb based systems.