Towards the identification of a quantum critical line in the (p, B) phase diagram of CeCoIn5

TL;DR

This study investigates the thermal expansion of CeCoIn5 and suggests the existence of a quantum critical line in its (p, B) phase diagram, challenging the idea of a quantum critical point at the upper critical field.

Contribution

It provides experimental evidence and a phase diagram proposal indicating a quantum critical line rather than a point in CeCoIn5.

Findings

Deviations from Fermi-liquid behavior within the superconducting region

Incompatibility of thermal expansion data with a QCP at Bc2

Proposal of a quantum critical line extending to negative pressures

Abstract

The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient $\beta$. Considerable deviations of $\beta(T)$ from Fermi-liquid behavior occur already within the superconducting region of the (B, T) phase diagram and become maximal at the upper critical field $B^0_{c2}$. However, $\beta(T)$ and the Gr\"uneisen parameter $\Gamma$ are incompatible with a quantum critical point (QCP) at $B^0_{c2}$, but allow for a QCP shielded by superconductivity and extending to negative pressures for $B < B^0_{c2}$. Together with literature data we construct a tentative (p, B, T) phase diagram of CeCoIn5 suggesting a quantum critical line in the (p, B) plane.