Factors influencing quantum evaporation of helium from polar semiconductors from first principles

TL;DR

This paper uses first-principles calculations to understand helium evaporation from polar semiconductors, aiming to optimize detection schemes for low-mass dark matter through phonon interactions.

Contribution

It provides a detailed ab initio analysis of helium adsorption and evaporation on NaI surfaces, revealing how surface and chemical modifications influence evaporation rates.

Findings

Surface termination affects helium adsorption energies.

Chemical modifications can tune evaporation rates.

Optimal target features enhance detection sensitivity.

Abstract

While there is much indirect evidence for the existence of dark matter (DM), to date it has evaded detection. Current efforts focus on DM masses over $\sim$GeV -- to push the sensitivity of DM searches to lower masses, new DM targets and detection schemes are needed. In this work, we focus on the latter - a novel detection scheme recently proposed to detect ~10-100 meV phonons in polar target materials. Previous work showed that well-motivated models of DM can interact with polar semiconductors to produce an athermal population of phonons. This new sensing scheme proposes that these phonons then facilitate quantum evaporation of $^3$He from a van der Waals film deposited on the target material. However, a fundamental understanding of the underlying process is still unclear, with several uncertainties related to the precise rate of evaporation and how it can be controlled. In this work, we use \textit{ab initio} density functional theory (DFT) calculations to compare the adsorption energies of helium atoms on a polar target material, sodium iodide (NaI), to understand the underlying evaporation physics. We explore the role of surface termination, monolayer coverage and elemental species on the rate of He evaporation from the target material. Using this, we discuss the optimal target features for He-evaporation experiments and their range of tunability through chemical and physical modifications such as applied field and surface termination.