Memory effects in superfluid vortex dynamics

Abstract

The dissipative dynamics of a vortex line in a superfluid is investigated within the frame of a non-Markovian quantal Brownian motion model. Our starting point is a recently proposed interaction Hamiltonian between the vortex and the superfluid quasiparticle excitations, whis is generalized to incorporate the effect of scattering from fermion inpurities (3He atoms). Thus, a non-Markovian equation of motion for the mean value of the vortex position operator is derived within a weak-coupling approximation. Such an eqaution is shown to yield, in the Markovian and elastic scattering limits, a 3He contribution to the longitudinal friction coefficient equivalent to that arising from the Rayfield-Reif formula. Simultaneous Markov and elastic scattering limits are found, however, to be incompatible, since an unexpected breakdown of the Markovian approximation is detected at low cyclotron frequencies. Then, a non-Markovian expression for the longitudinal friction coefficient is derived and computed as a function of temperature and 3He concentration. Such calculations show that cyclotron frequencies within the range 0.01-0.03 ps -1 yield a very good agreement to the longitudinal friction figures computed from te Iordanskii and Rayfield-Reif formulas for pure 4He, up to temperatures near 1 K. A similar performance is found for nonvanishing 3He concentrations, where the comparison is also shown to be very favourable with respect to the available experimental data. Memory effects are shown to be weak and increasing with temperature and concentration.