Symmetry Field Breaking Effects in Sr${}_2$RuO${}_4$

In this work, after reviewing the theory of the elastic properties of Strontium ruthenate, 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 $\mathrm{\Psi }$ 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.