Softons, Softarons and Bisoftarons in Amorphous Solids
Many amorphous solids as well as other non-crystalline substances exhibit1 an anomalously high specific heat which seems to vary linearly with temperature T at very low T. This implies a finite density of states down to energies comparable to kT, where T is the lowest temperature in these experiments. Anderson et al.2 and Phillips3 have attributed this high density of states to the existence, locally, of another configuration having about the same energy as the ground state. In other words the potential energy of the system as a function of some appropriate local atomic coordinates is assumed to exhibit a double minimum form instead of the single minimum of the crystalline solids. If the potential barrier between the two minima is not too high, tunneling from one minimum to the other is possible, and it can account for the linear specific heat. This picture has been further supported by experiments on thermal conductivity,1 ultrasonic attenuation,4 sound velocity5 and nuclear spin lattice relaxation.6,7 Note that the existence of the double minimum structure is associated with the presence of a soft (≡ easily deformable) chemical arrangement of comparable energy with an alternative local chemical bonding.
KeywordsValence Band Chalcogenide Glass Amorphous Solid Ultrasonic Attenuation Amorphous Semiconductor
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