Specific-Heat Evidence of the First-Order Superconducting Transition in Sr₂RuO₄

Abstract

We investigate the specific heat of ultra-pure single crystals of Sr 2 RuO 4 , 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.

A note on the upper critical field of Sr2RuO4 under strain

“…This would be inconsistent with NMR [20,21,22] and neutron scattering [23] experiments which show no observable change in the spin susceptibility through the superconducting transition for in-plane fields. An explanation for this unusual behaviour, compatible with all experimental results, has not been established so far, and experimentalists have been suggesting that it is due to a new pair breaking mechanism [16,18]. In our recent work we suggest that the mechanism could be related to IOE, which can potentially account for both the anisotropy and the change in the character of the transition, as we sumamrize in the following.…”

“…As can be seen in Fig. 2.1 top, the EMM (blue, full line) describes the angular dependence of the upper critical field very well for angles larger than 2 o , but for angles within 2 o of the plane there are clear deviations from the EMM [16,17,18]. The extra suppression of the SC state in this region indicates that there should be a second mechanism which is most effective for in-plane fields.…”

“…Given the fact that the critical temperature was enhanced by a factor of ∼ 2.3, an enhancement of the upper critical field is expected, but the rather different factors for different orientations is unusual; • Less anisotropy: the ratio of the upper critical field (in-plane/z-axis) for unstrained Sr 2 RuO 4 is around 25 [16,17,18], while for the strained sample it is approximately 4 [15]. It is difficult to understand this dramatic reduction of the anisotropy purely within orbital effects by a reduction in the mass anisotropy.…”

“…• Larger critical fields: compared to the unstrained material, the critical field along the z-axis is enhanced by a factor of ∼ 20, while the in-plane critical field is enhanced by a factor of ∼ 3 [15,16,17,18]. Given the fact that the critical temperature was enhanced by a factor of ∼ 2.3, an enhancement of the upper critical field is expected, but the rather different factors for different orientations is unusual; • Less anisotropy: the ratio of the upper critical field (in-plane/z-axis) for unstrained Sr 2 RuO 4 is around 25 [16,17,18], while for the strained sample it is approximately 4 [15].…”

Higher-Tc Superconducting Phase in Sr2RuO4 Induced by In-Plane Uniaxial Pressure

“…Under an in-plane magnetic field, the possibility of SC double transitions has been reported. [12][13][14][15][16][17] However, recent thermal measurements 18,19 revealing the first-order SC transition without a clear second anomaly have suggested that the previously reported anomalies may have originated from the broadening of the first-order transition or from sample mosaicity. Thus, the existence of double transitions in the in-plane magnetic field is still unclear.…”