Surface instability and isotopic impurities in quantum solids

TL;DR

This paper investigates how surface melting and local melting near isotopic impurities occur at temperatures below the bulk critical temperature in quantum solids, with implications for solid helium.

Contribution

It introduces a self-consistent harmonic approximation to study surface and impurity-induced melting in quantum solids, highlighting isotope effects.

Findings

Surface melting can occur at temperatures below the bulk critical temperature.

Lighter isotopes induce stronger local lattice fluctuations leading to local melting.

Implications discussed for solid helium experiments.

Abstract

In this paper we employ a self-consistent harmonic approximation to investigate surface melting and local melting close to quantum impurities in quantum solids. We show that surface melting can occur at temperatures much lower than the critical temperature $T_c$ of the solid phase instability in the bulk. Similar effects are driven by the presence of an isotope substitution. In this latter case, we show that stronger local lattice fluctuations, induced by a lighter isotope atom, can induce local melting of the host bulk phase. Experimental consequences and the possible relevance in solid helium are discussed.