Structure, superfluidity, and quantum melting of hydrogen clusters

TL;DR

This study uses Path Integral Monte Carlo simulations to explore superfluidity, quantum melting, and structural stability in small hydrogen and deuterium clusters at very low temperatures, revealing size-dependent quantum phenomena.

Contribution

It provides the first detailed theoretical analysis of superfluidity and quantum melting in hydrogen clusters up to 30 molecules, highlighting size effects and structural transitions.

Findings

Clusters up to 21 molecules are nearly fully superfluid below 1 K.

Quantum melting occurs in clusters with 21-30 molecules due to quantum exchanges.

Superfluid response observed up to 27 molecules in para-hydrogen clusters.

Abstract

We present results of a theoretical study of para-hydrogen and ortho-deuterium clusters at low temperature (0.5 K < T < 3.5 K), based on Path Integral Monte Carlo simulations. Clusters of size up to N=21 para-hydrogen molecules are nearly entirely superfluid at T < 1 K. For 21 < N < 30, the superfluid response displays strong variations with N, reflecting structural changes that occur on adding or removing even a single molecule. Some clusters in this size range display quantum melting, going from solid- to liquid-like as T tends to 0. Melting is caused by quantum exchanges of molecules. The largest para-hydrogen cluster for which a significant superfluid response is observed comprises 27 molecules. Evidence of a finite superfluid response is presented for ortho-deuterium clusters of size up to 14 molecules. Magic numbers are observed, at which both types of clusters feature pronounced stability.