1,028 research outputs found
Numerical Study of Velocity Statistics in Steady Counterflow Quantum Turbulence
We investigate the velocity statistics by calculating the Biot--Savart
velocity induced by vortex filaments in steady counterflow turbulence
investigated in a previous study [Phys. Rev. B {\bf 81}, 104511 (2010)]. The
probability density function (PDF) obeys a Gaussian distribution in the
low-velocity region and a power-law distribution in the high-velocity
region. This transition between the two distributions occur at the velocity
characterized by the mean inter-vortex distance. Counterflow turbulence causes
anisotropy of the vortex tangle, which leads to a difference in the PDF for the
velocities perpendicular to and parallel to the counterflow.Comment: 4 pages, 7 figure
Aharonov-Bohm Effect at liquid-nitrogen temperature: Frohlich superconducting quantum device
The Aharonov-Bohm (AB) effect has been accepted and has promoted
interdisciplinary scientific activities in modern physics. To observe the AB
effect in condensed matter physics, the whole system needs to maintain phase
coherence, in a tiny ring of the diameter 1 micrometer and at low temperatures
below 1 K. We report that AB oscillations have been measured at high
temperature 79 K by use of charge-density wave (CDW) loops in TaS3 ring
crystals. CDW condensate maintained macroscopic quantum coherence, which
extended over the ring circumference 85 micrometer. The periodicity of the
oscillations is h/2e in accuracy within a 10 percent range. The observation of
the CDW AB effect implies Frohlich superconductivity in terms of macroscopic
coherence and will provide a novel quantum interference device running at room
temperature.Comment: 11 pages, 4 figure
Instability of vortex array and transitions to turbulent states in rotating helium II
We consider superfluid helium inside a container which rotates at constant
angular velocity and investigate numerically the stability of the array of
quantized vortices in the presence of an imposed axial counterflow. This
problem was studied experimentally by Swanson {\it et al.}, who reported
evidence of instabilities at increasing axial flow but were not able to explain
their nature. We find that Kelvin waves on individual vortices become unstable
and grow in amplitude, until the amplitude of the waves becomes large enough
that vortex reconnections take place and the vortex array is destabilized. The
eventual nonlinear saturation of the instability consists of a turbulent tangle
of quantized vortices which is strongly polarized. The computed results compare
well with the experiments. Finally we suggest a theoretical explanation for the
second instability which was observed at higher values of the axial flow
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
Momentum Dependence of Charge Excitations in the Electron-Doped Superconductor Nd1.85Ce0.15CuO4: a RIXS Study
We report a resonant inelastic x-ray scattering (RIXS) study of charge
excitations in the electron-doped high-Tc superconductor Nd1.85Ce0.15CuO4. The
intraband and interband excitations across the Fermi energy are separated for
the first time by tuning the experimental conditions properly to measure charge
excitations at low energy. A dispersion relation with q-dependent width emerges
clearly in the intraband excitation, while the intensity of the interband
excitation is concentrated around 2 eV near the zone center. The experimental
results are consistent with theoretical calculation of the RIXS spectra based
on the Hubbard model
Zel'dovich-Starobinsky Effect in Atomic Bose-Einstein Condensates: Analogy to Kerr Black Hole
We consider circular motion of a heavy object in an atomic Bose-Einstein
condensate (BEC) at . Even if the linear velocity of the object is
smaller than the Landau critical velocity, the object may radiate
quasiparticles and thus experience the quantum friction. The radiation process
is similar to Zel'dovich-Starobinskii (ZS) effect -- the radiation by a
rotating black hole. This analogy emerges when one introduces the effective
acoustic metric for quasiparticles. In the rotating frame this metric has an
ergosurface, which is similar to the ergosurface in the metric of a rotating
black hole. In a finite size BEC, the quasiparticle creation takes place when
the ergosurface is within the condensate and occurs via quantum tunneling from
the object into the ergoregion. The dependence of the radiation rate on the
position of the ergosurface is investigated.Comment: 6 pages, 3 figures,submitted to JLT
Charge density wave soliton liquid
We investigate the charge density wave transport in a quasi-one-dimensional
conductor, orthorhombic tantalum trisulfide (-TaS), by applying a
radio-frequency ac voltage. We find a new ac-dc interference spectrum in the
differential conductance, which appear on both sides of the zero-bias peak. The
frequency and amplitude dependences of the new spectrum do not correspond to
those of any usual ac-dc interference spectrum (Shapiro steps). The results
suggest that CDW phase dynamics has a hidden degree of freedom. We propose a
model in which phase solitons behave as liquid. The origin of the new
spectrum is that the solitons are depinned from impurity potentials assisted by
an ac field when small dc field is applied. Our results provide a new insight
as regards our understanding of an elementary process in CDW dynamics.Comment: 16 pages, 5 figure
Hydrogenation of Nd-Fe-B magnet powder under a high pressure of hydrogen
The hydrogenation of Nd2Fe14B under a high pressure of hydrogen has been
investigated for the first time. At the heat-treatment temperature of 600
degree C, the almost complete decomposition of Nd2Fe14B into NdH2+x and
alpha-Fe is observed, although a rather long heat-treatment time is necessary
to achieve the sufficient hydrogenation. The desorption of hydrogen from NdH2+x
does not occur in the furnace-cooling process.Comment: to appear in Results in Physic
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