Specific-Heat Evidence of the First-Order Superconducting Transition in Sr_2RuO_4

摘要

We investigate the specific heat of ultra pure single crystals of Sr_2RuO_4, a leading candidate spin-triplet superconductor. For the first time, we 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 totally disappears in a sample with slightly lower quality. These thermodynamic observations provide crucial bases for understanding the unconventional pair-breaking effect responsible for the first-order transition.