Vacancy Assisted Motion of Charges in Quantum Crystals

Abstract

From comparison of results of measurements of diffusion coefficients D(T) of the positively and negatively charged complexes (charges), created under irradiation in perfect crystals, grown from pure helium or hydrogen at small pressures, with diffusion coefficients of the isotopic impurities or the self-diffusion coefficients known from NMR studies one can conclude that motion of the more mobile charges through these crystals (of positive charges in HCP ⁴ He, ³ He and D ₂ samples and of negative charges in BCC ⁴ He and ³ He and also in HCP crystals, grown from pure p–H ₂ ) is vacancy assisted. Thus strong departures of the temperature dependencies of diffusion coefficients of the positive charges D ₊(T) in HCP ⁴ He samples and of the negative charges D ₋(T) in p–H ₂ samples from the simple Arrhenius type of behavior D= D ₀ exp[−G/T] at low temperatures can be attributed to the change of the mechanism of diffusion of thermal vacancies: from classical thermally activated hopping of the localized vacancies near T _melt to the band motion of delocalized vacancions at T< T _melt/2. To explain the nature of the maxima on the D ₊(T) curves observed in perfect ⁴ He crystals, it may be assumed that the flow conditions of the vacancion gas around a positive charge (a probe particle with an effective radius of a few lattice constants) can change significantly with lowering the temperature: from a hydrodynamic flow of the viscous gas round the probe at the transition temperatures to a kinetic flow of the rarefied vacancion gas at low temperatures. In this case the bandwidth of the vacancions in studied ⁴ He samples is near Q _V ∼10^–4 K.