Transformation of soft localized modes in glasses under pressure

Experimentally observed narrowing of spectral holes in a glass under hydros tatic pressure confirms our theoretical finding that the external pressure, in addition to increasing the frequencies of soft localized modes, also re duces their number. This occurs because the majority of soft localized mode s in glasses is shown to have a negative cubic anharmonicity. For that reas on the applied pressure not only enhances the stiffness of these modes, but also transforms a fraction of them into tunneling two-level systems, where as the simultaneous reverse transformations of some other two-level systems into soft localized modes are less numerous.