On the ferrorotational transition in solid hydrogen at very low ortho-fractions

On the basis of a model Hamiltonian for a dilute solid solution of ortho-para-hydrogen, a theory is suggested for the orientational phase transition which Gaines et al. recently discovered in NMR studies at temperatures of about 0.18°K and ortho-fractions of about 10−3. The paper discusses the possibility of a long-range orientational order (of the ferromagnetic, ferrorotational type) arising in the ortho-subsystem of solid hydrogen as the result of coupling of the rotations of ortho-molecules with the lattice vibrations. Equations for the order parameter and the critical temperature are derived in the simplest molecular-field approximation. The estimate of the critical temperature is in agreement with the experimental value to within the order of magnitude.

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