Nuclear Spin Crossover in Dense Molecular Hydrogen

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.

References in corpus (6)

• Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen
• Quantum and Classical Orientational Ordering in Solid Hydrogen
• Observation of Nuclear Quantum Effects and Hydrogen Bond Symmetrisation in High Pressure Ice
• Structures of solid hydrogen at 300K
• Quadrupole Arrangements and the Ground State of Solid Hydrogen
• Improving Resolution of Solid State NMR in Dense Molecular Hydrogen

Cited by in corpus (4)

• High-Pressure Synthesis of Seven Lanthanum Hydrides with a Significant Variability of Hydrogen Content
• In-situ High Pressure Nuclear Magnetic Resonance Crystallography
• Absence of proton tunneling during the hydrogen bond symmetrization in $\delta$-AlOOH
• Magnetization and Polarization of Coupled Nuclear Spin Ensembles