Proton Collective Quantum Tunneling Induces Anomalous Thermal Conductivity of Ice Under Pressure

Ice is one of the most ubiquitous substances in the universe. Understanding its thermal conduction properties is crucial for exploration of the evolution and dynamics of icy planets. Moreover, ice itself serves as an ideal model to study proton’s quantum effects in the hydrogen bond networks, which has received enormous attention in physical chemistry and condensed matter physics.<br/>It is believed that protons tunnel in the hydrogen bond network in ice and the tunneling is local compared with the wavelength of heavy phonon; as such, the quantum nature of the proton dynamics would not strongly affect the thermal conduction of ice, and the thermal conductivity as a function of pressure should increase following the classical Leibfried-Schlömann equation.<br/>In stark contrast, we observed anomalous, nonmonotonic behavior in the thermal conductivity of H₂O and D₂O ice above 20 GPa that breaks the classical rule. Such anomaly is revealed to be caused by global collective quantum tunneling involving several tens of molecules. It is the first time that quantum tunneling in ice is observed at such a large spatial scale, opposing to all previous studies which assumed that local or synchronized tunneling within a limited size (such as six-member rings) dominates in ice.<br/>Moreover, the tunneling dynamics in the ice VII-X phase transition region has been a long-standing challenge to understand using conventional spectroscopic methods. In this work, we provide a completely new approach that can directly probe the global dynamics of the phase transition with unambiguous experimental measurements.