Nuclear spin correlations and collective excitations in supercritical H2
Abstract
Nuclear spins are known to experience spontaneous long-range correlations only below 2.5 mili-Kelvin in superfluid 3He. Here we present the first evidence of nuclear spin coupling in molecular hydrogen (H2) at 74-92 Kelvin using neutron scattering, showing a fundamental change in nature from the incoherent scattering universally expected from hydrogen, which reflects single particle properties of uncorrelated nuclear spins, to coherent, with a peak materializing on the elastic line, indicating H2-H2 nuclear spin correlations. In this novel phase, the dynamic response of the system also changes nature, and collective excitations with an effective mass of nine H2 are observed with inelastic scattering at momentum transfers up to 37 -1, corresponding to length scales smaller than the H-H bond, where previous experiments have always found single atom excitations. This novel behavior has only been observed from H2 within the subnanometer sized graphitic pores of a carbon material, marking the first demonstration that a confined materials environment can be used to control nuclear spin correlations.
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