Phillips, A. and McClintock, Peter V. E. (1974) Breaking the roton barrier:an experimental study of motion faster than the Landau critical velocity for roton creation in He II. Physical Review Letters, 33 (25). pp. 1468-1471. ISSN 1079-7114
We report the first observation of objects moving through He II with equilibrium drift velocities v beyond the Landau critical velocity vc,for roton creation. With T about 0.4 K, P = 25 bar, F =2 kV/cm, (v - vc) for negative ions is larger than a theoretical prediction by a factor of about 10^5. The vortex-ring nucleation rate is found to decrease with F above 300 V/cm, thus resolving apparent inconsistencies between earlier experiments.
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