Two-dimensional H-2 in Si: Raman scattering and modeling study

VV Melnikov and M Hiller and EV Lavrov, PHYSICAL REVIEW B, 97, 125307 (2018).

DOI: 10.1103/PhysRevB.97.125307

Molecular hydrogen trapped within 111-oriented platelets in silicon is studied by means of Raman scattering and first principles theory. The rotational transition S0(0) (J = 0 -> J = 2) of para-H-2 (nuclear spin I = 0) at 353 cm(-1) is used as a probe. We find that for temperatures below 100 K the S-0(0) Raman line starts to broaden asymmetrically, which is interpreted as the onset of a phase transition from a state with a short-range order ("gaseous" or "liquid" phase) to a two- dimensional molecular crystal lying in the 111 plane of silicon. The shape of the S-0(0) line at helium temperatures strongly depends on the relative content of ortho- (nuclear spin I = 1) and para-H-2 revealing the details of the intermolecular interaction. A comprehensive theoretical analysis based on ab initio calculations, molecular dynamics simulations, and rotational spectra modeling reveals that the phase transition to the crystalline state of the two-dimensional hydrogen does occur at temperatures substantially higher compared to those of bulk H-2.

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