Constraints on the superconducting state of Sr₂RuO₄ from elastocaloric measurements

Strontium ruthenate Sr₂RuO₄ is an unconventional superconductor whose pairing symmetry has not been fully clarified, despite more than two decades of intensive research. Recent NMR Knight shift experiments have rekindled the Sr₂RuO₄ pairing debate by giving strong evidence against all odd-parity pairing states, including chiral p-wave pairing that was for a long time the leading pairing candidate. Here, we exclude additional pairing states by analyzing recent elastocaloric measurements [Y.S. Li et al., Nature 607, 276 (2022)]. To be able to explain the elastocaloric experiment, we find that unconventional even-parity pairings must include either large d_x2−y2 -wave or large {d_xz | d_yz}-wave admixtures, where the latter possibility arises because of the body-centered point group symmetry. These {d_xz | d_yz}-wave admixtures take the form of distinctively body-centered-periodic harmonics that have horizontal line nodes. Hence g_{xy(x2−y2 )}-wave and d_xy-wave pairings are excluded as possible dominant even pairing states.