139 research outputs found
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
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
Dry demagnetization cryostat for sub-millikelvin helium experiments: Refrigeration and thermometry
We demonstrate successful “dry” refrigeration of quantum fluids down to T = 0.16 mK by using copper nuclear demagnetization stage that is pre-cooled by a pulse-tube-based dilution refrigerator. This type of refrigeration delivers a flexible and simple sub-mK solution to a variety of needs including experiments with superfluid 3He. Our central design principle was to eliminate relative vibrations between the high-field magnet and the nuclear refrigeration stage, which resulted in the minimum heat leak of Q = 4.4 nW obtained in field of 35 mT. For thermometry, we employed a quartz tuning fork immersed into liquid 3He. We show that the fork oscillator can be considered as self-calibrating in superfluid 3He at the crossover point from hydrodynamic into ballistic quasiparticle regime.Peer reviewe
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
Growth of ³He crystals at different magnetic fields
The experiments on ³He crystal growth are carried out in magnetic field up 9 T. The data were analyzed
and compared with the results found at zero magnetic field. It was found that the interface and the crystal lattice
couple weakly in the presence of an external magnetic field, and we could set an upper limit of the step
energy of the , and facets at different magnetic fields
Experiments on the twisted vortex state in superfluid 3He-B
We have performed measurements and numerical simulations on a bundle of
vortex lines which is expanding along a rotating column of initially
vortex-free 3He-B. Expanding vortices form a propagating front: Within the
front the superfluid is involved in rotation and behind the front the twisted
vortex state forms, which eventually relaxes to the equilibrium vortex state.
We have measured the magnitude of the twist and its relaxation rate as function
of temperature above 0.3Tc. We also demonstrate that the integrity of the
propagating vortex front results from axial superfluid flow, induced by the
twist.Comment: prepared for proceedings of the QFS2007 symposium in Kaza
Vortex lines or sheets - what is formed in dynamic drives?
In isotropic macroscopic quantum systems vortex lines can be formed while in
anisotropic systems also vortex sheets are possible. Based on measurements of
superfluid 3He-A, we present the principles which select between these two
competing forms of quantized vorticity: sheets displace lines if the frequency
of the external field exceeds a critical limit. The resulting topologically
stable state consists of multiple vortex sheets and has much faster dynamics
than the state with vortex lines.Comment: RevTex, 5 pages, sumbitted to Phys. Rev. Let
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
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