Nuclear spin coupling crossover in dense molecular hydrogen

Titelangaben

Meier, Thomas ; Laniel, Dominique ; Pena-Alvarez, Miriam ; Trybel, Florian ; Khandarkhaeva, Saiana ; Krupp, Alena ; Jacobs, Jeroen ; Dubrovinskaia, Natalia ; Dubrovinsky, Leonid:
Nuclear spin coupling crossover in dense molecular hydrogen.
In: Nature Communications. Bd. 11 (2020) Heft 1 . - No. 6334.
ISSN 2041-1723
DOI der Verlagsversion: https://doi.org/10.1038/s41467-020-19927-y

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Abstract

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 (Meier, Annu. Rep. NMR Spectrosc. 94:1–74, 2017; Meier, Prog. Nucl. Magn. Reson. Spectrosc. 106–107:26–36, 2018) 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 ortho-hydrogen at low pressures, above 70 GPa, 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.