Leggett's Mode in Mg$_{1-x}$Al$_x$B$_2$

A detailed investigation of multiband superconductivity and Leggett's mode in the Mg$_{1-x}$Al$_x$B$_2$ $(0\leq x\leq0.45)$ system was carried out using tunneling and Andreev spectroscopy. Temperature dependences of superconducting gaps $\Delta_\sigma$ and $\Delta_\pi$ and their variation upon the degree of disorder and the Al concentration were studied. The dependence of the Leggett's mode energy $\varepsilon_0$ upon the values of the gaps $\Delta_\sigma$ and $\Delta_\pi$ has been derived.