Identifying multi-component superconductivity with resonant ultrasound spectroscopyIdentifying multi-component superconductivity with resonant ultrasound spectroscopy*

Superconductors can break time-reversal symmetry (TRS) when their order parameter has two degenerate components, or “representations”—perhaps the most famous example of this is the p_x+ip_y state. Most experiments that investigate TRS-breaking superconductivity look for signatures of magnetism—very few experiments are capable of determining the order parameter degeneracy directly. A discontinuity in a shear elastic modulus at T_c is one of the few direct experimental test for two-component superconductivity, and recent advances in resonant ultrasound spectroscopy (RUS) have allowed us to measure the entire elastic tensor with high precision and at low temperature. Using RUS we have discovered that Sr₂RuO₄ has a two-component order parameter, even though recent NMR measurements rule out the p_x+ip_y state. This suggests an unconventional {d_xz, d_yz} order parameter, or perhaps even the accidental degeneracy of d_x2-y2 and g_xy(x2-y2). We have also investigated the proposed TRS-breaking superconductor UTe₂, where heat capacity measurements find two superconducting transitions that suggest two nearly-degenerate order parameters.