Glasslike thermal properties and isotope effect in Rb1-x(NH4)xH2PO4 mixed crystals

Abstract

Low-temperature thermal conductivity and time-dependent specific heat have been measured on mixed crystals of Rb1-x(NH4)zH2PO4 (x=0.35,0.72), in the composition range in which an electric dipole glass state forms below 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. © 1991 The American Physical Society.