6,651 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
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
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
Electromagnetic multipole theory for optical nanomaterials
Optical properties of natural or designed materials are determined by the
electromagnetic multipole moments that light can excite in the constituent
particles. In this work we present an approach to calculate the multipole
excitations in arbitrary arrays of nanoscatterers in a dielectric host medium.
We introduce a simple and illustrative multipole decomposition of the electric
currents excited in the scatterers and link this decomposition to the classical
multipole expansion of the scattered field. In particular, we find that
completely different multipoles can produce identical scattered fields. The
presented multipole theory can be used as a basis for the design and
characterization of optical nanomaterials
Metastable bound state of a pair of two-dimensional spatially separated electrons in anti-parallel magnetic fields
We propose a new mechanism for binding of two equally charged carriers in a
double-layer system subjected by a magnetic field of a special form. A field
configuration for which the magnetic fields in adjacent layers are equal in
magnitude and opposite in direction is considered. In such a field an
additional integral of motion - the momentum of the pair P arises. For the case
when in one layer the carrier is in the zero (n=0) Landau level while in the
other layer - in the first (n=1) Landau level the dependence of the energy of
the pair on its momentum E(P} is found. This dependence turns out to be
nonmonotonic one : a local maximum and a local minimum appears, indicating the
emergence of a metastable bound state of two carrier with the same sign of
electrical charge.Comment: 7 page
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
Distinguishing quantum from classical oscillations in a driven phase qubit
Rabi oscillations are coherent transitions in a quantum two-level system
under the influence of a resonant perturbation, with a much lower frequency
dependent on the perturbation amplitude. These serve as one of the signatures
of quantum coherent evolution in mesoscopic systems. It was shown recently [N.
Gronbech-Jensen and M. Cirillo, Phys. Rev. Lett. 95, 067001 (2005)] that in
phase qubits (current-biased Josephson junctions) this effect can be mimicked
by classical oscillations arising due to the anharmonicity of the effective
potential. Nevertheless, we find qualitative differences between the classical
and quantum effect. First, while the quantum Rabi oscillations can be produced
by the subharmonics of the resonant frequency (multiphoton processes), the
classical effect also exists when the system is excited at the overtones.
Second, the shape of the resonance is, in the classical case,
characteristically asymmetric; while quantum resonances are described by
symmetric Lorentzians. Third, the anharmonicity of the potential results in the
negative shift of the resonant frequency in the classical case, in contrast to
the positive Bloch-Siegert shift in the quantum case. We show that in the
relevant range of parameters these features allow to confidently distinguish
the bona fide Rabi oscillations from their classical Doppelganger.Comment: 8 pages, 4 figures; v2: minor corrections, Fig.1 added, introduction
expande
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