Incoherent tunneling in glasses at very low temperatures

It has been known for many years that the low temperature properties of amorphous solids are dominated by the presence of tunneling states which can be described phenomenologically by the tunneling model. However, new experimental results indicate that at very low temperatures the dynamics of these states differ significantly from that predicted by the model. Here we propose a new approach which overcomes these difficulties by taking into account the elastic interaction between the tunneling systems. It leads to a crossing from coherent to an incoherent tunneling motion for systems with a small tunnel splitting and allows one to resolve the discrepancies mentioned above.