Evidence for a phase transition in glasses at very low temperature: A macroscopic quantum state of tunneling systems?

Dielectric measurements at very low temperature indicate that in the multicomponent glass BaO-Al2O3-SiO2 a phase transition occurs at 5.84 mK. Below that temperature small magnetic fields of the order of 10 mu T cause surprising changes of the dielectric constant although the glass is insensitive to fields up to 20 T above 50 mK. The experimental findings may be interpreted as the signature of the formation of a new phase in which many tunneling systems perform a coherent motion resulting in a macroscopic wave function.