High-sensitivity heat-capacity measurements on Sr₂RuO₄under uniaxial pressure

A key question regarding the unconventional superconductivityof Sr2RuO4remains whether the order parameter is single- ortwo-component. Under a hypothesis of two-component super-conductivity, uniaxial pressure is expected to lift their degeneracy,resulting in a split transition. The most direct and fundamentalprobe of a split transition is heat capacity. Here, we report mea-surement of heat capacity of samples subject to large and highlyhomogeneous uniaxial pressure. We place an upper limit on theheat-capacity signature of any second transition of a few percentof that of the primary superconducting transition. The normalizedjump in heat capacity,∆C/C, grows smoothly as a function ofuniaxial pressure, favoring order parameters which are allowedto maximize in the same part of the Brillouin zone as the well-studied van Hove singularity. Thanks to the high precision ofour measurements, these findings place stringent constraints ontheories of the superconductivity of Sr2RuO4