Quantum Critical Behavior of Nanoconfined Water Molecules

We have studied terahertz and radio-frequency dielectric response of single water molecules periodically arranged in nano-cages formed by the crystal lattice of beryl. Below ≈20 K, quantum effects start to dominate the properties of the H2O dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature behavior of the dielectric permittivity e'. Analyzing the temperature evolution of the inverse permittivity (e')-1 down to T = 0.3 K, we discover signatures of quantum critical behavior of coupled water molecular dipoles: its quadratic variation with temperature at T=3-12 K and a shallow minimum in the (e')-1(T) dependence below 3 K.