Dynamics of glasses at very low temperatures

The low temperature dynamic behavior of glasses is determined by tunneling systems which can be considered as small clusters of atoms moving in double-well potentials. Below 1 K this motion is generally described as a coherent tunneling through the barrier between the two wells. However, the tunneling motion of systems with a very small energy splitting is incoherent because of the interaction between the tunneling systems. This means that at very low temperature the dynamic behavior of glasses is determined by systems exhibiting incoherent tunneling. Furthermore, due to this interaction new phenomena occur in the Millikelvin range which have not been observed in crystalline solids so far.