Nuclear Spin Crossover in Dense Molecular Hydrogen

TL;DR

This study uses high-pressure nuclear magnetic resonance to observe a nuclear spin crossover in molecular hydrogen at pressures up to 123 GPa, revealing the loss of spin isomer distinction under extreme conditions.

Contribution

First direct measurement of nuclear spin states in hydrogen at high pressures, demonstrating a crossover from spin-1 to spin-1/2 system at 70 GPa.

Findings

Confirmation of ortho-hydrogen at low pressures

Observation of spin crossover above 70 GPa

Evidence of loss of spin isomer distinction

Abstract

The laws of quantum mechanics are often tested against the behaviour of the lightest element in the periodic table, hydrogen. One of the most striking properties of molecular hydrogen is the coupling between molecular rotational properties and nuclear spin orientations, giving rise to the spin isomers ortho- and para-hydrogen. At high pressure, as intermolecular interactions increase significantly, the free rotation of H2 molecules is increasingly hindered, and consequently a modification of the coupling between molecular rotational properties and the nuclear spin system can be anticipated. To date, high-pressure experimental methods have not been able to observe nuclear spin states at pressures approaching 100 GPa and consequently the effect of high pressure on the nuclear spin statistics could not be directly measured. Here, we present in-situ high-pressure nuclear magnetic resonance data on molecular hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our measurements confirm the presence of I=1 ortho-hydrogen at low pressures, above 70 GPa, where inter- and intramolecular distances become comparable, we observe a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These observations represent a unique case of a nuclear spin crossover phenomenon in quantum solids.