512 research outputs found
Measurement of Turbulence in Superfluid 3He-B
The experimental investigation of superfluid turbulence in 3He-B is generally
not possible with the techniques which have been developed for 4He-II. We
describe a new method by which a transient burst of turbulent vortex expansion
can be generated in 3He-B. It is based on the injection of a few vortex loops
into rotating vortex-free flow. The time-dependent evolution of the quantized
vorticity is then monitored with NMR spectroscopy. Using these techniques the
transition between regular (i.e. vortex number conserving) and turbulent vortex
dynamics can be recorded at T ~ 0.6 Tc and a number of other characteristics of
turbulence can be followed down to a temperature of T ~ 0.4 Tc.Comment: 31 pages, 10 figure
Onset of turbulence in superfluid 3He-B and its dependence on vortex injection in applied flow
Vortex dynamics in 3He-B is divided by the temperature dependent damping into
a high-temperature regime, where the number of vortices is conserved, and a
low-temperature regime, where rapid vortex multiplication takes place in a
turbulent burst. We investigate experimentally the hydrodynamic transition
between these two regimes by injecting seed vortex loops into vortex-free
rotating flow. The onset temperature of turbulence is dominated by the roughly
exponential temperature dependence of vortex friction, but its exact value is
found to depend on the injection method.Comment: To be published in the proceedings of the 24th International
Conference on Low Temperature Physics - LT24, in Conference Proceedings of
the American Institute of Physic
Vortex Multiplication in Applied Flow: the Precursor to Superfluid Turbulence
The dynamics of quantized vortices in rotating He-B is investigated in
the low density (single-vortex) regime as a function of temperature. An abrupt
transition is observed at . Above this temperature the number of
vortex lines remains constant, as they evolve to their equilibrium positions.
Below this temperature the number of vortices increases linearly in time until
the vortex density has grown sufficiently for turbulence to switch on. On the
basis of numerical calculations we suggest a mechanism responsible for vortex
formation at low temperatures and identify the mutual friction parameter which
governs its abrupt temperature dependence.Comment: 5 pages, 4 figures; version submitted to Phys. Rev. Let
Classical and quantum regimes of the superfluid turbulence
We argue that turbulence in superfluids is governed by two dimensionless
parameters. One of them is the intrinsic parameter q which characterizes the
friction forces acting on a vortex moving with respect to the heat bath, with
1/q playing the same role as the Reynolds number Re=UR/\nu in classical
hydrodynamics. It marks the transition between the "laminar" and turbulent
regimes of vortex dynamics. The developed turbulence described by Kolmogorov
cascade occurs when Re >> 1 in classical hydrodynamics, and q << 1 in the
superfluid hydrodynamics. Another parameter of the superfluid turbulence is the
superfluid Reynolds number Re_s=UR/\kappa, which contains the circulation
quantum \kappa characterizing quantized vorticity in superfluids. This
parameter may regulate the crossover or transition between two classes of
superfluid turbulence: (i) the classical regime of Kolmogorov cascade where
vortices are locally polarized and the quantization of vorticity is not
important; and (ii) the quantum Vinen turbulence whose properties are
determined by the quantization of vorticity. The phase diagram of the dynamical
vortex states is suggested.Comment: 12 pages, 1 figure, version accepted in JETP Letter
Structure of surface vortex sheet between two rotating 3He superfluids
We study a two-phase sample of superfluid 3He where vorticity exists in one
phase (3He-A) but cannot penetrate across the interfacial boundary to a second
coherent phase (3He-B). We calculate the bending of the vorticity into a
surface vortex sheet on the interface and solve the internal structure of this
new type of vortex sheet. The compression of the vorticity from three to two
dimensions enforces a structure which is made up of half-quantum units,
independently of the structure of the source vorticity in the bulk. These
results are consistent with our NMR measurements.Comment: 4 pages, 4 figure
Superconducting Nb-film LC resonator
Sputtered Nb thin-film LC resonators for low frequencies at 0.5 MHz have been
fabricated and tested in the temperature range 0.05--1 K in magnetic fields up
to 30 mT. Their Q value increases towards decreasing temperature as sqrt(T) and
reaches 10^3 at 0.05 K. As a function of magnetic field Q is unstable and
displays variations up to 50%, which are reproducible from one field sweep to
the next. These instabilities are attributed to dielectric losses in the plasma
deposited SiO_2 insulation layer, since the thin-film coil alone reaches a Q >
10^5 at 0.05 K.Comment: 6 pages, 7 figures, submitted to Review of Scientific Instrument
System size scaling of topological defect creation in a second-order dynamical quantum phase transition
We investigate the system size scaling of the net defect number created by a
rapid quench in a second-order quantum phase transition from an O(N) symmetric
state to a phase of broken symmetry. Using a controlled mean-field expansion
for large N, we find that the net defect number variance in convex volumina
scales like the surface area of the sample for short-range correlations. This
behaviour follows generally from spatial and internal symmetries. Conversely,
if spatial isotropy is broken, e.g., by a lattice, and in addition long-range
periodic correlations develop in the broken-symmetry phase, we get the rather
counterintuitive result that the scaling strongly depends on the dimension
being even or odd: For even dimensions, the net defect number variance scales
like the surface area squared, with a prefactor oscillating with the system
size, while for odd dimensions, it essentially vanishes.Comment: 20 pages of IOP style, 6 figures; as published in New Journal of
Physic
Transition to superfluid turbulence governed by an intrinsic parameter
Hydrodynamic flow in both classical and quantum fluids can be either laminar
or turbulent. To describe the latter, vortices in turbulent flow are modelled
with stable vortex filaments. While this is an idealization in classical
fluids, vortices are real topologically stable quantized objects in
superfluids. Thus superfluid turbulence is thought to hold the key to new
understanding on turbulence in general. The fermion superfluid 3He offers
further possibilities owing to a large variation in its hydrodynamic
characteristics over the experimentally accessible temperatures. While studying
the hydrodynamics of the B phase of superfluid 3He, we discovered a sharp
transition at 0.60Tc between two regimes, with regular behaviour at
high-temperatures and turbulence at low-temperatures. Unlike in classical
fluids, this transition is insensitive to velocity and occurs at a temperature
where the dissipative vortex damping drops below a critical limit. This
discovery resolves the conflict between existing high- and low-temperature
measurements in 3He-B: At high temperatures in rotating flow a vortex loop
injected into superflow has been observed to expand monotonically to a single
rectilinear vortex line, while at very low temperatures a tangled network of
quantized vortex lines can be generated in a quiescent bath with a vibrating
wire. The solution of this conflict reveals a new intrinsic criterion for the
existence of superfluid turbulence.Comment: Revtex file; 5 pages, 2 figure
- …