The critical velocity for the vortex nucleation of positive ions is determined experimentally in isotopically purified $$^4$$ 4 He at temperatures as low as 50 mK. Systematic IV characteristic measurements for a two-dimensional positive ion pool at a depth of 37.6 nm from the surface are carried out with extremely fine control of driving electric fields. The critical velocity of $${\sim }$$ ∼ 32 ms$$^{-1}$$ - 1 at 500 mK decreases with decrease in temperature and approaches a temperature-independent value of $${\sim }$$ ∼ 18 ms$$^{-1}$$ - 1 below 200 mK. The decrease in critical velocity corresponds to the increase in nucleation rate. The temperature dependence of critical velocity is qualitatively attributed to the “superohmic” macroscopic quantum tunneling of the Caldeira–Leggett theory. The reduction in tunneling rate with increase in temperature is evidence for quantum friction.

The critical velocity for vortex nucleation of snowball (positive ion) is determined experimentally in isotopically purified 4He at temperatures down to 50 mK. Systematic I-V characteristic measurements for the two-dimensional snowball pool at the depth of 37.6 nm from the surface are carried out with an extremely fine control of driving electric fields. The critical velocity of ∼ 32 ms-1 at 500 mK decreases with lowering temperature and approaches a temperature independent value of ∼ 18 ms-1 below 200 mK. The decrease of critical velocity corresponds to the increase of the nucleation rate. The temperature dependence of the critical velocity is qualitatively attributed to the "superohmic" macroscopic quantum tunnelling developed by Caldeira and Leggett. The reduction of the tunnelling rate by increasing temperature is an evidence for the quantum friction.