Abstract
Measurements are reported of the rate
v
at which negative ions nucleate quantized vortices in isotopically pure superfluid
4
He for electric fields
E
, temperatures
T
and pressures
P
within the range 10
3
⩽ E ⩽ 10
6
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.
Subject
Pharmacology (medical),Complementary and alternative medicine,Pharmaceutical Science
Cited by
28 articles.
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