Quasi-particle density in Sr₂RuO₄ probed by means of the phonon thermal conductivity

The thermal conductivity of Sr₂RuO₄ along the least conducting direction perpendicular to the RuO₂ plane has been studied down to 0.3 K. In this configuration the phonons remain the dominant heat carriers down to the lowest temperature, and their conductivity in the normal state is determined by the scattering on conduction electrons. We show that the phonon mean free path in the superconducting state is sensitive to the density of the quasi-particles in the bulk. An unusual magnetic field dependence of the phonon thermal conductivity is ascribed to the anisotropic superconducting gap structure in Sr₂RuO₄.