GLASSLIKE THERMAL-PROPERTIES AND ISOTOPE EFFECT IN RB1-X(NH4)XH2PO4 MIXED-CRYSTALS

Low-temperature thermal conductivity and time-dependent specific heat have been measured on mixed crystals of Rb(1-x)(NH4)xH2PO4 (x = 0.35,0.72), in the composition range in which an electric dipole glass state forms below approximately 100 K. Glasslike excitations have been observed. In the fully deuterated isomorph with x = 0.62, however, the thermal conductivity is that of a crystal, and glasslike excitations are only seen in the specific heat at very long times. This striking isotope effect shows that the low-energy excitations in these crystals relate to tunneling of protons or deuterons, respectively.