Specific-heat evidence of the first-order superconducting transition in Sr2RuO4

TL;DR

This study provides thermodynamic evidence of a first-order superconducting transition in ultra-pure Sr2RuO4, revealing the transition's sensitivity to sample quality and its implications for unconventional pairing mechanisms.

Contribution

First demonstration of specific-heat evidence for a first-order transition in Sr2RuO4, highlighting the role of sample quality and thermodynamic signatures.

Findings

First-order transition evidenced by divergent peaks and hysteresis in specific heat.

Transition occurs for all in-plane magnetic field directions.

Hysteresis disappears in lower-quality samples.

Abstract

We investigate the specific heat of ultra-pure single crystals of Sr2RuO4, a leading candidate of a spin-triplet superconductor. We for the first time obtained specific-heat evidence of the first-order superconducting transition below 0.8 K, namely divergent-like peaks and clear hysteresis in the specific heat at the upper critical field. The first-order transition occurs for all in-plane field directions. The specific-heat features for the first-order transition are found to be highly sensitive to sample quality; in particular, the hysteresis becomes totally absent in a sample with slightly lower quality. These thermodynamic observations provide crucial bases to understand the unconventional pair-breaking effect responsible for the first-order transition.