Origins of low-temperature vibrational and thermal anomalies in glasses

Network glasses display universal vibrational and cryogenic thermal anomalies: a "boson peak", denoting an excess in the vibrational density of states over the Debye ω² law, and a nearly temperature-independent thermal conductivity plateau between 5 K and 25 K. These anomalies have so far lacked a quantitative first-principles explanation, owing to the prohibitive computational cost of lattice-dynamics methods for disordered systems, and the unreliability of classical molecular dynamics at cryogenic temperatures.

Here, we combine the Wigner formulation of thermal transport with GPU-accelerated machine-learning interatomic potentials and optimized sparse tensor-contraction algorithms to compute the anharmonic vibrational spectrum and thermal conductivity of amorphous silica models containing 1.5 million atoms. This is sufficiently large to capture the long-wavelength vibrations active at cryogenic temperatures. Leveraging these developments, we show that the conductivity plateau arises from a balance of two transport mechanisms: the temperature-inhibited, anharmonicity-damped propagation of vibrations below the boson peak, and the temperature-activated, disorder-mediated tunneling of vibrations above it.

Our approach opens a pathway to understanding and controlling heat transport in disordered solids at cryogenic temperatures. Results will be presented comparing our computational results with experiments for silica.