Quantum effects in the dynamics of proton glasses

The dynamics of proton and deuteron glasses has been studied via the NMR spin-lattice relaxation (T-1) of Rb-87 and O-D ... O deuterons down to T = 1.6 K in Rb-0.50(NH4)(0.50)H2PO4,Rb-0.58(ND4)(0.42)D2PO4, and Rb-0.68(ND4)(0.32)D2AsO4. In the glassy phase the relaxation rate was found to be anomalously short and temperature independent as T-->0, whereas this effect is absent in pure ferroelectric RbH2PO4. The temperature independence of T-1 at low temperatures demonstrates the presence of phonon-assisted tunneling of the proton and the deuteron between the two potential minima in the H bond. The proton and deuteron glass phases are thus quantum rather than classical glasses.