Ultrasonic Absorption in Polymethylmethacrylate at Low Temperatures
In a series of experiments performed at low temperatures it has been demonstrated that amorphous solids exhibit thermal, acoustic and dielectric properties which are not observed in perfect crystals.1 These anomalies are caused by a broad distribution of low-energy excitations which have their microscopic origin in structural rearrangements of small groups of atoms via quantum mechanical tunnelling.2 These tunnelling states interact strongly with phonons resulting in a strong ultrasonic absorption and a temperature dependent sound velocity even at temperatures of a few mK.3
KeywordsEnergy Splitting Deformation Potential Tunnelling Model Tunnelling System Tunnelling State
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