Symmetry field breaking effects in Sr$_2$RuO$_4$

Abstract

In this work, after reviewing the theory of the elastic properties of \sr, an extension suitable to explain the sound speed experiments of Lupien et al. \protect\cite{lup2} and Clifford et al. \protect\cite{clif1} is carried out. It is found that the discontinuity in the elastic constant C$_{66}$ gives unambiguous experimental evidence that the \sr superconducting order parameter $\Psi$ has two components and shows a broken time-reversal symmetry state. A detailed study of the elastic behavior is performed by means of a phenomenological theory employing the Ginzburg-Landau formalism.