High-sensitivity heat-capacity measurements on Sr <sub>2</sub> RuO <sub>4</sub> under uniaxial pressure
You-Sheng Li, Naoki Kikugawa, D. A. Sokolov, Fabian Jerzembeck, Alexandra S. Gibbs, Y. Maeno, Clifford W. Hicks, Jörg Schmalian, M. Nicklas, A. P. Mackenzie
Abstract
A key question regarding the unconventional superconductivity of [Formula: see text] remains whether the order parameter is single- or two-component. Under a hypothesis of two-component superconductivity, uniaxial pressure is expected to lift their degeneracy, resulting in a split transition. The most direct and fundamental probe of a split transition is heat capacity. Here, we report measurement of heat capacity of samples subject to large and highly homogeneous uniaxial pressure. We place an upper limit on the heat-capacity signature of any second transition of a few percent of that of the primary superconducting transition. The normalized jump in heat capacity, [Formula: see text], grows smoothly as a function of uniaxial pressure, favoring order parameters which are allowed to maximize in the same part of the Brillouin zone as the well-studied van Hove singularity. Thanks to the high precision of our measurements, these findings place stringent constraints on theories of the superconductivity of [Formula: see text].