Unconventional strain dependence of superconductivity in Sr2RuO4

In the transition-metal oxide Sr2RuO4 unconventional superconductivity emerges presumably near a ferromagnetic ground state. As a result it is generally believed that critical magnetic fluctuations form the pairing mechanism leading to p-wave symmetry of the order parameter. To investigate the coupling of superconductivity to the crystal lattice we measured the thermal expansion of Sr2RuO4 single crystals in a temperature range between 20 mK and 10 K. The uniaxial pressure and strain dependences of the superconducting transition temperature along the tetragonal a and c axes were determined by using the Ehrenfest relation and the elastic constants. Although the layered crystal structure suggests a strongly anisotropic behavior, the strain dependences of Tc are nearly of the same size for both axes. In contrast to many conventional superconductors, the temperature dependence of the linear thermal expansion coefficients along the a and c axes are not proportional to each other. Such a difference indicates that the superconducting ground state is controlled by more than one characteristic energy scale.