Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules.

Abstract

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below <i>T</i> = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(<i>T</i>). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε')^-1 down to <i>T</i> ≈ 0.3 K and comparing it with the data obtained for conventional quantum paraelectrics, like SrTiO₃, KTaO₃, we discovered clear signatures of a quantum-critical behavior of the interacting water molecular dipoles: Between <i>T</i> = 6 and 14 K, the reciprocal permittivity follows a quadratic temperature dependence and displays a shallow minimum below 3 K. This is the first observation of "dielectric fingerprints" of quantum-critical phenomena in a paraelectric system of coupled <i>point</i> electric dipoles.