Date of Original Version
We calculated the time of pairing by quantum diffusion of ortho-H2 impurities in solid para-H2. The important feature of the pairing process is a strong directional bias associated with the dependence of the hopping rates on energy mismatches caused by the interaction of the pairing particles. This bias at moderate temperatures is against a mutual approach of particles and creates a ‘‘kinetic barrier.’’ At lower temperatures, the corresponding diffusion mechanism freezes out, which leads to a rapid increase in pairing rates. This explains a well-developed, experimentally observed maximum in the pairing time as a function of temperature: a maximum that exists in spite of a monotonic temperature dependence of individual hopping rates. Our results are in good agreement with experimental data.
M.Tammaro, M.P. Nightingale, A.E. Meyerovich. Quantum Pairing of Impurities in Quantum Crystals, Phys.Rev. B 47, 2573 (1993). Available at: http://dx.doi.org/10.1103/PhysRevB.47.2573