Macroscopic quantum state of tunneling systems

At low temperatures glasses exhibit a variety of surprising properties. They are attributed to the existence of low-energy excitations which are caused by the tunneling of small atomic clusters. With decreasing temperature the interaction between the tunneling systems becomes increasingly important and a transition from a coherent to an incoherent tunneling motion occurs. Very recently a new and unexpected phenomenon was found. Below a critical temperature T-c = 5.8 mK the dielectric constant of the multicomponent glass BaO-Al2O3-SiO2 becomes highly sensitive to very weak magnetic fields. In addition, the dielectric constant shows a kink at that temperature. Both observations indicate the occurrence of a phase transition from a state of uncorrelated and incoherent tunneling to a highly correlated motion of a macroscopic number of tunneling systems. Above T-c, the dielectric constant exhibits a maximum at moderate magnetic fields. This dependence vanishes rapidly with increasing temperature. (C) 1999 Elsevier Science B.V. All rights reserved.