Quantum Theory of Glasses

Abstract

The quantum excitations in glasses have long presented a set of puzzles for condensed matter scientists. A common view is that they are largely disordered analogs of elementary excitations in crystals, supplemented by two level systems which are chemically local entities coming from disorder. This thesis suggests a radical revision of this picture. We argue that the excitations in low temperature glasses are deeply connected to the energy landscape of the glass when it vitrifies: the excitations are not low excited states built on a single ground state but locally defined resonances, high in the energy spectrum of a solid. Two level systems involve resonant collective tunneling motions of around two hundred molecular units. The Boson Peak and the plateau in thermal conductuvity, observed at higher temperatures, arise from the same motions but the motions are no longer fully coherent.