407 research outputs found

### Transverse force on a vortex and vortex mass: effects of free bulk and vortex-core bound quasiparticles

The paper reassesses the old but still controversial problem of the
transverse force on a vortex and the vortex mass. The transverse force from
free bulk quasiparticles on the vortex, both in the Bose and the Fermi liquid,
originates from the Aharonov--Bohm effect. However, in the Fermi liquid one
should take into account peculiarities of the Aharonov--Bohm effect for BCS
quasiparticles described by two-component spinor wave functions. There is no
connection between the transverse force (either from free bulk quasiparticles
or from vortex-core bound quasiparticles) and the spectral flow in the vortex
core in superfluid Fermi liquid, in contrast to widely known claims. In fact,
there is no steady spectral flow in the core of the moving vortex, and the
analogy with the Andreev bound states in the SNS junction, where the spectral
flow is really possible, is not valid in this respect.
The role of the backflow on the vortex mass is clarified. The backflow is an
inevitable consequence of a mismatch between the currents inside and outside
the vortex core and restores the conservation of the particle number (charge)
violated by this mismatch. In the Fermi liquid the backflow compensates the
current through the core bound states, which is a source of the vortex mass
(the Kopnin mass). This results in renormalization of the Kopnin vortex mass by
a numerical factor.Comment: 20 pages, 4 figures, the version accepted for publication in Phys.
Rev.

### Spin and mass superfluidity in ferromagnetic spin-1 Bose-Einstein condensate

The paper investigates the coexistence and interplay of spin and mass
superfluidity in a ferromagnetic spin-1 Bose-Einstein condensate. Superfluidity
is possible only in the presence of uniaxial anisotropy (linear and quadratic
Zeeman effect). This follows from the topology of the order-parameter space
(vacuum manifold). According to the Landau criterion, the critical phase
gradients, both for mass and spin supercurrents, vanish at the phase transition
from the easy-plane to the easy-axis anisotropy. However, mass superfluidity is
still possible at the phase transition. This is because the Landau criterion
signals instability only with respect to nonsingular vortices with special
ratio between circulations of mass and spin currents. Phase slips produced by
these vortices are not sufficient for complete decay of supercurrents. Full
decay of supercurrents requires phase slips with vortices of another
topological class and larger energy. These phase slips are suppressed by
energetic barriers up to the upper critical velocity (gradient) exceeding the
Landau critical velocity. The upper critical velocity does not vanish nor has
any anomaly in the critical point at the phase transition from the easy-plane
to the easy-axis anisotropy.Comment: 13 pages, 6 figures, revised version published in Phys. Rev.

### Charge transport and shot noise in ballistic graphene sheet

The current and the shot noise in a graphene sheet were analyzed in the
ballistic regime for arbitrary voltage drops between leads and the sheet in the
limit of infinite aspect ratio of the sheet width to its length, when
quantization of transversal wave vectors is not essential. The cases of
coherent and incoherent ballistic transport were compared. At high voltages the
difference with coherent transport is not essential. But at low voltages
conductance and Fano-factor dependences for incoherent transport become
non-monotonous so that the conductance has a minimum and the Fano factor has a
maximum at non-zero voltage bias.Comment: 6 pages, 2 figure

### Transverse force on a vortex in lattice models of superfluids

The paper derives the transverse forces (the Magnus and the Lorentz forces)
in the lattice models of superfluids in the continuous approximation. The
continuous approximation restores translational invariance absent in the
original lattice model, but the theory is not Galilean invariant. As a result,
calculation of the two transverse forces on the vortex, Magnus force and
Lorentz force, requires the analysis of two balances, for the true momentum of
particles in the lattice (Magnus force) and for the quasimomentum (Lorentz
force) known from the Bloch theory of particles in the periodic potential.
While the developed theory yields the same Lorentz force, which was well known
before, a new general expression for the Magnus force was obtained. The theory
demonstrates how a small Magnus force emerges in the Josephson-junction array
if the particle-hole symmetry is broken. The continuous approximation for the
Bose--Hubbard model close to the superfluid-insulator transition was developed,
which was used for calculation of the Magnus force. The theory shows that there
is an area in the phase diagram for the Bose--Hubbard model, where the Magnus
force has an inverse sign with respect to that which is expected from the sign
of velocity circulation.Comment: 8 pages, 1 figur

### Tunneling into 1D and quasi-1D conductors and Luttinger-liquid behavior

The paper addresses the problem whether and how it is possible to detect the
Luttinger-liquid behavior from the $IV$ curves for tunneling to 1D or quasi-1D
conductors. The power-law non-ohmic $IV$ curve, which is usually considered as
a manifestation of the Luttinger-liquid behavior, can be also deduced from the
theory of the Coulomb blockaded junction between 3D conductors affected by the
environment effect. In both approaches the power-law exponents are determined
by the ratio of the impedance of an effective electric circuit to the quantum
resistance. Though two approaches predict different power-law exponents
(because of a different choice of effective circuits), the difference becomes
negligible for a large number of conductance channels.Comment: 13 pages, submitted to Proceedings of Symposium on Ultra Low Energy
Physics, Helsinki, January 200

### Effect of Klein tunneling on conductance and shot noise in ballistic graphene

The conductance and the Fano factor in a graphene sheet in the ballistic
regime are calculated. The electrostatic potential in the sheet is modeled by a
trapezoid barrier, which allows to use the exact solution of the Dirac equation
in a uniform electric field in the slope areas (the two lateral sides of the
trapezoid). A special attention is devoted to asymmetry with respect to the
sign of the gate voltage, which is connected with the difference between the
Klein tunneling and the over-barrier reflection. The comparison of the
developed theory with the experiment supports the conclusion that the Klein
tunneling was revealed experimentally.Comment: 12 pages, 6 figures, typos correcte

### Symmetry of Kelvin-wave dynamics and the Kelvin-wave cascade in the T=0 superfluid turbulence

The article considers implications of tilt symmetry (symmetry with respect to
tilting of the coordinate axis with respect to which vortex motion is studied)
in the non-linear dynamics of Kelvin waves. The conclusion is that although the
spectrum of Kelvin wave is not tilt-invariant, this does not compromise tilt
invariance of the Kelvin-wave cascade vividly argued now in the theory of
superfluid turbulence. The article investigates the effect of strong kelvon
interaction on the power-law exponent for the Kelvin-wave cascade and suggests
a simple picture of the crossover from the classical Kolmogorov cascade to the
quantum Kelvin-wave cascade, which does not encounter with mismatch of the
energy distributions at the crossover and does not require a broad intermediate
interval for realization of the crossover.Comment: 7 pages, 1 figur

### Reply to Comment on "Symmetry of Kelvin-wave dynamics and the Kelvin-wave cascade in the T=0 superfluid turbulence" [arXiv:1208.4593]

I discuss the Comment by L'vov and Nazarenko (arXiv:1208.4593) aiming at
refutation of my perviously published criticism of their mechanism of the
Kelvin-wave cascade. It is important that in their Comment L'vov and Nazarenko
admitted that the Hamiltonian, from which they derived their mechanism, is not
tilt-invariant. This provides full ammunition to their critics, who believe
that their mechanism is in conflict with tilt symmetry of the Kelvin-wave
dynamics in an isotropic space.Comment: 2 page

### Superfluid spin transport in ferro- and antiferromagnets

This paper focuses on spin superfluid transport, observation of which was
recently reported in antiferromagnet Cr$_2$O$_3$ [Yuan et al., Sci. Adv. 4,
eaat1098 (2018)]. This paper analyzes the role of dissipation in transformation
of spin current injected with incoherent magnons to a superfluid spin current
near the interface where spin is injected. The Gilbert damping parameter in the
Landau-Lifshitz-Gilbert theory does not describe dissipation properly, and the
dissipation parameters are calculated from the Boltzmann equation for magnons
scattered by defects. The two-fluid theory is developed similar to the
two-fluid theory for superfluids. This theory shows that the influence of
temperature variation in bulk on the superfluid spin transport (bulk Seebeck
effect) is weak at low temperatures. The scenario that the results of Yuan et
al. are connected with the Seebeck effect at the interface between the spin
detector and the sample is also discussed.
The Landau criterion for an antiferromagnet put in a magnetic field is
derived from the spectrum of collective spin modes. The Landau instability
starts in the gapped mode earlier than in the Goldstone gapless mode, in
contrast to easy-plane ferromagnets where the Goldstone mode becomes unstable.
The structure of the magnetic vortex in the geometry of the experiment is
determined. The vortex core has the skyrmion structure with finite
magnetization component normal to the magnetic field. This magnetization
creates stray magnetic fields around the exit point of the vortex line from the
sample, which can be used for experimental detection of vortices.Comment: 15 pages, 3 figures, the version after corrections of some equation

### Superfluid spin transport in magnetically ordered solids

The paper reviews the theory of the long-distance spin superfluid transport
in solid ferro- and antiferromagnets based on the analysis of the topology, the
Landau criterion, and phase slips. Experiments reporting evidence of the
existence of spin superfluidity are also overviewed.Comment: Review article, 29 pages, 10 figure

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