Low-energy Excitations In Disordered Solids: A Story Of The 'Universal' Phenomena Of Structural Tunneling

The subject of low-energy excitations has evolved since two-level-tunneling systems were first proposed ~50 years ago. Initially they were used to explain the common anomalous properties of oxide glasses and polymers; now the subject includes a wide range of other materials containing disorder: amorphous semiconductors and metals, doped- mixed- and quasi-crystals, surface adsorbates, ... and topic...