Unconventional Strain Dependence of Superconductivity in Spin-Triplet Superconductor Sr₂RuO₄

Ultrasonic measurements have been performed on high quality single crystals of Sr₂RuO₄ across the superconducting transition temperature T_c (≈ 1.37 K). A discontinuous jump at T_c was observed in a longitudinal elastic modulus C₁₁ as is seen in conventional superconductors, and the strain dependence of T_c was estimated from the jump by using the Ehrenfest relation. The uniaxial pressure dependence of T_c in Sr₂RuO₄ was found to be negative along the a axis and positive along the c axis, respectively. Thermodynamic analysis has cleanly shown that the superconductivity of Sr ₂RuO₄ is much affected by the deformation of RuO ₂ planes. The strain effects on superconductivity are explained by large anisotropy of the lattice and electronic structure in Sr ₂RuO₄.