6,447 research outputs found
Relaxation of superflow in a network: an application to the dislocation model of supersolidity of helium crystals
We have considered the dislocation network model for the supersolid state in
He-4 crystals. In difference with uniform 2D and 3D systems, the temperature of
superfluid transition T_c in the network is much smaller than the degeneracy
temperature T_d. It is shown that a crossover into a quasi superfluid state
occurs in the temperature interval between T_c and T_d. Below the crossover
temperature the time of decay of the flow increases exponentially under
decrease of the temperature. The crossover has a continuous character and the
crossover temperature does not depend on the density of dislocations.Comment: Corrected typo
Non-dissipative drag of superflow in a two-component Bose gas
A microscopic theory of a non-dissipative drag in a two-component superfluid
Bose gas is developed. The expression for the drag current in the system with
the components of different atomic masses, densities and scattering lengths is
derived. It is shown that the drag current is proportional to the square root
of the gas parameter. The temperature dependence of the drag current is studied
and it is shown that at temperature of order or smaller than the interaction
energy the temperature reduction of the drag current is rather small. A
possible way of measuring the drag factor is proposed. A toroidal system with
the drag component confined in two half-ring wells separated by two Josephson
barriers is considered. Under certain condition such a system can be treated as
a Bose-Einstein counterpart of the Josephson charge qubit in an external
magnetic field. It is shown that the measurement of the difference of number of
atoms in two wells under a controlled evolution of the state of the qubit
allows to determine the drag factor.Comment: 13 pages, 3 figures. This preprint is extended and substantially
revised variant of related preprint cond-mat/040456
Bose-Einstein condensation in a decorated lattice: an application to supersolid
The Bose-Einstein condensation of vacancies in a three-dimensional decorated
lattice is considered. The model describes possible scenario of superfluidity
of solid helium, caused by the presence of zero-point vacancies in a
dislocation network. It is shown that the temperature of Bose-Einstein
condensation decreases under increase of the length of the segments of the
network, and the law of decrease depends essentially on the properties of the
vertexes of the network. If the vertexes correspond to barriers with a small
transparency, the critical temperature is inversely as the square of the length
of the segment. On the contrary, if the vertexes correspond to traps for the
vacancies (it is energetically preferable for the vacancies to localize at the
vertexes), an exponential lowering of the temperature of transition takes
place. The highest temperature of Bose-Einstein condensation is reached in the
intermediate case of vertexes with large transparency, but in the absence of
tendency of localization in them. In the latter case the critical temperature
is inversely as the length of the segment.Comment: 7 page
Drag of superfluid current in bilayer Bose systems
An effect of nondissipative drag of a superfluid flow in a system of two Bose
gases confined in two parallel quasi two-dimensional traps is studied. Using an
approach based on introduction of density and phase operators we compute the
drag current at zero and finite temperatures for arbitrary ratio of densities
of the particles in the adjacent layers. We demonstrate that in a system of two
ring-shape traps the "drag force" influences on the drag trap in the same way
as an external magnetic flux influences on a superconducting ring. It allows to
use the drag effect to control persistent current states in superfluids and
opens a possibility for implementing a Bose analog of the superconducting
Josephson flux qubit.Comment: 12 pages, 2 figures, new section is added, refs are adde
Dislocation-induced superfluidity in a model supersolid
Motivated by recent experiments on the supersolid behavior of He, we
study the effect of an edge dislocation in promoting superfluidity in a Bose
crystal. Using Landau theory, we couple the elastic strain field of the
dislocation to the superfluid density, and use a linear analysis to show that
superfluidity nucleates on the dislocation before occurring in the bulk of the
solid. Moving beyond the linear analysis, we develop a systematic perturbation
theory in the weakly nonlinear regime, and use this method to integrate out
transverse degrees of freedom and derive a one-dimensional Landau equation for
the superfluid order parameter. We then extend our analysis to a network of
dislocation lines, and derive an XY model for the dislocation network by
integrating over fluctuations in the order parameter. Our results show that the
ordering temperature for the network has a sensitive dependence on the
dislocation density, consistent with numerous experiments that find a clear
connection between the sample quality and the supersolid response.Comment: 10 pages, 6 figure
Kinetic simulations of X-B and O-X-B mode conversion
We have performed fully-kinetic simulations of X-B and O-X-B mode conversion
in one and two dimensional setups using the PIC code EPOCH. We have recovered
the linear dispersion relation for electron Bernstein waves by employing
relatively low amplitude incoming waves. The setups presented here can be used
to study non-linear regimes of X-B and O-X-B mode conversion.Comment: 4 pages, 3 figure
Phases of the excitonic condensate in two-layer graphene
Two graphene monolayers that are oppositely charged and placed close to each
other are considered. Taking into account valley and spin degeneracy of
electrons we analyze the symmetry of the excitonic insulator states in such a
system and build a phase diagram that takes into account the effect of the
symmetry breaking due to the external in-plane magnetic field and the carrier
density imbalance between the layers.Comment: 12 pages, 6 figures, 1 tabl
Luttinger Liquid in the Core of Screw Dislocation in Helium-4
On the basis of first-principle Monte Carlo simulations we find that the
screw dislocation along the hexagonal axis of an hcp He4 crystal features a
superfluid core. This is the first example of a regular quasi-one-dimensional
supersolid, and one of the cleanest cases of a regular Luttinger-liquid system.
In contrast, the same type of screw dislocation in solid Hydrogen is
insulating.Comment: replaced with revised versio
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