The breakdown of superfluidity in liquid 4HE VI. Macroscopic quantum tunnelling by vortices in isotopically pure He II

Abstract

Abstract Measurements are reported of the rate v at which negative ions nucleate quantized vortices in isotopically pure superfluid 4He for electric fields E, temperatures T and pressures P within the range 103 ⩽ E ⩽ 106 V m-1, 75 ⩽ T ⩽ 500 mK, 12 ⩽ P ⩽ 23 bar (= 2.3 MPa). The form of v(E, T) differs in unexpected ways from that observed in earlier work at higher P, exhibiting: a pronounced dip in v(T) at ca. 0.3 K whose depth and precise position depends on E and P ; an exponential increase in v(T) at higher T, with an activation energy considerably smaller than the roton energy gap; and distinct structure in v(E). The experimental data are discussed and analysed in terms of the macroscopic quantum tunnelling model proposed by Muirhead et al. (Phil. Trans. R. Soc. Lond. A 311, 433 (1984)). The relatively small barrier heights of ca. 2—3 K deduced from the data on this basis are construed as confirmation that the initial vortex is a loop rather than an encircling ring. The temperature dependence of v at low pressures is interpreted in terms of a phonon-driven vortex nucleation mechanism, and values for its cross section are extracted from the data. The minima in v(T) are ascribed to phonon damping of the tunnelling process, and the kinks observed in some of the low-temperature v(E) curves are attributed to tunnelling of the system into the first excited state of the nascent vortex loop.