Temperature-dependent vortex nucleation in isotopically pure superfluid ⁴He

The authors have investigated the temperature dependence of the rate nu at which negative ions nucleate vortex rings in isotopically pure superfluid ⁴He. Measurements of nu (ET) are presented for a pressure of 23 bar with electric fields E in 5.8<E<1.2*10⁶ V m^-1 and temperatures T in 0.3<T<0.8K. Provided that due allowance is made for the drag arising from ion-excitation scattering, ( nu - nu ₀) infinity N_r, where N_r is the thermal roton density and nu ₀ is the temperature-dependent limiting value of nu below 0.5K. They show that this discovery is consistent with the existence of an additional vortex nucleation mechanism, characterised by a critical velocity reduced by roton absorption, but ultimately determined by a requirement for the conservation of energy and momentum.