118 research outputs found
Superfluid vortex front at T -> 0: Decoupling from the reference frame
Steady-state turbulent motion is created in superfluid 3He-B at low
temperatures in the form of a turbulent vortex front, which moves axially along
a rotating cylindrical container of 3He-B and replaces vortex-free flow with
vortex lines at constant density. We present the first measurements on the
thermal signal from dissipation as a function of time, recorded at 0.2 Tc
during the front motion, which is monitored using NMR techniques. Both the
measurements and the numerical calculations of the vortex dynamics show that at
low temperatures the density of the propagating vortices falls well below the
equilibrium value, i.e. the superfluid rotates at a smaller angular velocity
than the container. This is the first evidence for the decoupling of the
superfluid from the container reference frame in the zero-temperature limit.Comment: 4 pages, 4 figure
Superflow-Stabilized Nonlinear NMR in Rotating 3He-B
Nonlinear spin precession has been observed in 3He-B in large counterflow of
the normal and superfluid fractions. The new precessing state is stabilized at
high rf excitation level and displays frequency-locked precession over a large
range of frequency shifts, with the magnetization at its equilibrium value.
Comparison to analytical and numerical calculation indicates that in this state
the orbital angular momentum L of the Cooper pairs is oriented transverse to
the external magnetic field in a ``non-Leggett'' configuration with broken
spin-orbit coupling. The resonance shift depends on the tipping angle theta of
the magnetization as omega - omega_L = (Omega_B^2 / 2 omega_L)(cos(theta) -
1/5). The phase diagram of the precessing modes with arbitrary orientation of L
is constructed.Comment: Revtex file, 5 pages, 4 figures, version submitted to Phys. Rev. Let
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
Vortex core contribution to textural energy in 3He-B below 0.4Tc
Vortex lines affect the spatial order-parameter distribution in superfluid
3He-B owing to superflow circulating around vortex cores and due to the
interaction of the order parameter in the core and in the bulk as a result of
superfluid coherence over the whole volume. The step-like change of the latter
contribution at 0.6Tc (at a pressure of 29bar) signifies the transition from
axisymmetric cores at higher temperatures to broken-symmetry cores at lower
temperatures. We extended earlier measurements of the core contribution to
temperatures below 0.2Tc, in particular searching for a possible new core
transition to lower symmetries. As a measuring tool we track the energy levels
of magnon condensate states in a trap formed by the order-parameter texture.Comment: 13 pages, 10 figures, submitted to proceedings of the QFS2010
conferenc
Transition to Superfluid Turbulence
Turbulence in superfluids depends crucially on the dissipative damping in
vortex motion. This is observed in the B phase of superfluid 3He where the
dynamics of quantized vortices changes radically in character as a function of
temperature. An abrupt transition to turbulence is the most peculiar
consequence. As distinct from viscous hydrodynamics, this transition to
turbulence is not governed by the velocity-dependent Reynolds number, but by a
velocity-independent dimensionless parameter 1/q which depends only on the
temperature-dependent mutual friction -- the dissipation which sets in when
vortices move with respect to the normal excitations of the liquid. At large
friction and small values of 1/q < 1 the dynamics is vortex number conserving,
while at low friction and large 1/q > 1 vortices are easily destabilized and
proliferate in number. A new measuring technique was employed to identify this
hydrodynamic transition: the injection of a tight bundle of many small vortex
loops in applied vortex-free flow at relatively high velocities. These vortices
are ejected from a vortex sheet covering the AB interface when a two-phase
sample of 3He-A and 3He-B is set in rotation and the interface becomes unstable
at a critical rotation velocity, triggered by the superfluid Kelvin-Helmholtz
instability.Comment: Short review; to be published in Journal of Low Temperature Physics
(2006
The dynamics of vortex generation in superfluid 3He-B
A profound change occurs in the stability of quantized vortices in externally
applied flow of superfluid 3He-B at temperatures ~ 0.6 Tc, owing to the rapidly
decreasing damping in vortex motion with decreasing temperature. At low damping
an evolving vortex may become unstable and generate a new independent vortex
loop. This single-vortex instability is the generic precursor to turbulence. We
investigate the instability with non-invasive NMR measurements on a rotating
cylindrical sample in the intermediate temperature regime (0.3 - 0.6) Tc. From
comparisons with numerical calculations we interpret that the instability
occurs at the container wall, when the vortex end moves along the wall in
applied flow.Comment: revised & extended version. Journal of Low Temperature Physics,
accepted (2008
Asymptotic motion of a single vortex in a rotating cylinder
We study numerically the behavior of a single quantized vortex in a rotating
cylinder. We study in particular the spiraling motion of a vortex in a cylinder
that is parallel to the rotation axis. We determine the asymptotic form of the
vortex and its axial and azimuthal propagation velocities under a wide range of
parameters. We also study the stability of the vortex line and the effect of
tilting the cylinder from the rotation axis.Comment: 9 pages, 10 figures. Considerable changes, now close to the published
versio
Topological nodal line in superfluid He and the Anderson theorem
We have found an experimental evidence for the existence of the Dirac nodal
line in the quasiparticle spectrum of the polar phase of superfluid He. The
polar phase is stabilized by confinement of He between nm-sized cylinders.
The temperature dependence of the gap, measured via frequency shift in the NMR
spectrum, follows expected dependence. The results support the
Fomin extension of the Anderson theorem to the polar phase with columnar
defects: perfect columnar non-magnetic defects do no perturb the magnitude of
the gap in the polar phase. The existence of the node line opens possibilities
to study Bogoliubov Fermi surfaces and flat-band fermions in the polar phase.Comment: 6 pages, 4 figures, references adde
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