We have over 12,000 students, from over 100 countries, within one of the safest campuses in the UK


97% of Lancaster students go into work or further study within six months of graduating

Home > Research > Publications & Outputs > The breakdown of superfluidity in liquid 4He. I...
View graph of relations

« Back

The breakdown of superfluidity in liquid 4He. II : an investigation of excitation emission from negative ions travelling at extreme supercritical velocities.

Research output: Contribution to journalJournal article


<mark>Journal publication date</mark>18/04/1980
<mark>Journal</mark>Philosophical Transactions of the Royal Society of London A
Number of pages15
<mark>Original language</mark>English


We have measured the drift velocity, v, of negative ions in He II at a temperature of 0.34 K and a pressure of 25 bar for electric fields, E, in the range 5 x 10^3 < E < 6 X 10^6 V/m. For if v < 70 m/s, the results are in good agreement with the predictions of the roton pair-emission theory of Bowley and Sheard. Even better agreement is obtained by extending the theory to take explicit account of departures from parabolicity of the real dispersion curve at large momenta, variations in the pole strength of high momentum excitations, and the slight increase in the average momentum of excitations emitted at large values of v. For v > 70 m/s, however, experiment and theory are in clear disagreement: by if = 80 m/s, the measured drag on the ion has become about 100% larger than the theoretical prediction. We tentatively attribute this discrepancy to the onset at if ~ 70 m/sl of a new and, so far, unidentified dissipation mechanism.