Researchers from the University of Warwick together with collaborators from the USA, Japan and the Netherlands have mapped the temperature – magnetic field phase diagram of the filled skutterudite material CeOs4Sb12 in fields up to60 T and temperatures down to 0.5 K. They found that the valence transition separating the semimetallic low-field, low-temperature ‘L’ phase from the metallic high-field, high-temperature ‘H’ phase exhibits a very unusual, wedge-shaped phase boundary, contrary to expectations from thermodynamic constraints which predict an elliptical phase boundary. The low-temperature, high-field behaviour of the phase boundary is attributed to magnetic fluctuations associated with a quantum critical point identified by a marked enhancement of the electronic effective mass. In addition, a strong sample dependence of the high-temperature, low-field side of the valence transition is ascribed toCeOs4Sb12 being on the verge of becoming a topological semimetal triggered by uniaxial strain.

Valence transitions, driven by magnetic field, pressure and/or temperature are a highly and widely debated topic in the condensed-matter community. The phase diagrams of a number of important systems, such as Pu and URu2Si2, are discussed in the context of valence transitions and their underlying energy scales. Field-induced valence transitions usually follow an elliptical phase boundary for which H^2 is proportional to T^2. CeOs4Sb12, a close relative of the unconventional superconductor PrOs4Sb12 and the ferromagnetic NdOs4Sb12,deviates significantly from this universal behaviour and shows a strongly altered boundary of its field-tuned valence transition. This alteration of a so far well-understood phase boundary will advance the understanding of valence transitions tuned by external parameters and stimulate further discussion on valence transitions affected by quantum criticality and topology.

• Caption to figure: Temperature-magnetic field phase diagram of CeOs4Sb12 derived from resistivity, magnetostriction, and MHz conductivity. Top inset: example of the elliptical phase boundary expected for valence transitions. Bottom inset: field dependence of the effective mass.
• Publication: “Unusual phase boundary of the magnetic-field-tuned valence transition in CeOs4Sb12”, K. Götze, M. J. Pearce, P. A. Goddard, M. Jaime, M. B. Maple, K. Sasmal, T. Yanagisawa, A. McCollam, T. Khouri, P.-C. Ho, and J. Singleton, Phys. Rev. B 101, 075102 (2020)
• DOI: https://doi.org/10.1103/PhysRevB.101.075102