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

## TL;DR

This study maps the complex phase diagram of CeOs$_4$Sb$_{12}$ under high magnetic fields, revealing an unusual wedge-shaped valence transition boundary and signs of quantum-critical behavior.

## Contribution

It uncovers a non-monotonic, wedge-shaped phase boundary in CeOs$_4$Sb$_{12}$ and links quantum-critical fluctuations to the valence transition's unusual shape.

## Key findings

- Unusual wedge-shaped phase boundary with non-monotonic gradient.
- Effective mass increases near the phase boundary, indicating quantum-criticality.
- Proximity to a topological semimetal phase at low fields and high temperatures.

## 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-$T$, $\cal{L}$ phase from the metallic high-$H$, high-$T$ $\cal{H}$ 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 $\cal{H}$ phase reveals an effective mass that increases as $H$ drops toward the $\cal{H-L}$ 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.