# Multiple Diffusion-Freezing Mechanisms in Molecular Hydrogen Films

## TL;DR

This study investigates the diffusion mechanisms in thin molecular hydrogen films, revealing multiple freezing processes including classical, quantum, and surface diffusion, with implications for quantum phase transitions.

## Contribution

It identifies and characterizes three distinct diffusion mechanisms in hydrogen films and links surface diffusion activity to quantum phase transition proximity.

## Key findings

- Multiple diffusion mechanisms identified: classical, quantum tunneling, surface diffusion.
- Surface diffusion remains active down to 1 K, indicating near quantum phase transition.
- Hydrogen films exhibit anomalies in elasticity related to diffusion freezing.

## Abstract

Molecular hydrogen is a fascinating candidate for quantum fluid showing bosonic and fermionic superfluidity. We have studied diffusion dynamics of thin films of H$_2$, HD and D$_2$ adsorbed on a glass substrate by measurements of elasticity. The elasticity shows multiple anomalies well below bulk triple point. They are attributed to three different diffusion mechanisms of admolecules and their "freezing" into localized state: classical thermal diffusion of vacancies, quantum tunneling of vacancies, and diffusion of molecules in the uppermost surface. The surface diffusion is active down to 1 K, below which the molecules become localized. This suggests that the surface layer of hydrogen films is on the verge of quantum phase transition to superfluid state.