138 research outputs found
Doublon relaxation in the Bose-Hubbard model
Decay of a high-energy double occupancy state, doublon, in a narrow-band
lattice requires creation of a coherent many-particle excitation. This leads to
an exponentially long relaxation time of such a state. We show that, if the
average occupation number is sufficiently small, the corresponding exponent may
be evaluated exactly. To this end we develop the quasiclassical approach to
calculation of the high-order tree-level decay amplitudes.Comment: 4 pages, 1 figur
Weak antilocalization in HgTe quantum wells and topological surface states: Massive versus massless Dirac fermions
HgTe quantum wells and surfaces of three-dimensional topological insulators
support Dirac fermions with a single-valley band dispersion. In the presence of
disorder they experience weak antilocalization, which has been observed in
recent transport experiments. In this work we conduct a comparative theoretical
study of the weak antilocalization in HgTe quantum wells and topological
surface states. The difference between these two single-valley systems comes
from a finite band gap (effective Dirac mass) in HgTe quantum wells in contrast
to gapless (massless) surface states in topological insulators. The finite
effective Dirac mass implies a broken internal symmetry, leading to suppression
of the weak antilocalization in HgTe quantum wells at times larger than certain
t_M, inversely proportional to the Dirac mass. This corresponds to the opening
of a relaxation gap 1/t_M in the Cooperon diffusion mode which we obtain from
the Bethe-Salpeter equation including relevant spin degrees of freedom. We
demonstrate that the relaxation gap exhibits an interesting nonmonotonic
dependence on both carrier density and band gap, vanishing at a certain
combination of these parameters. The weak-antilocalization conductivity
reflects this nonmonotonic behavior which is unique to HgTe QWs and absent for
topological surface states. On the other hand, the topological surface states
exhibit specific weak-antilocalization magnetoconductivity in a parallel
magnetic field due to their exponential decay in the bulk.Comment: 14 pages, 10 figures, version as publishe
Superflow in Solid 4He
Kim and Chan have recently observed Non-Classical Rotational Inertia (NCRI)
for solid He in Vycor glass, gold film, and bulk. Their low value of
the superfluid fraction, , is consistent with what
is known of the atomic delocalization in this quantum solid. By including a
lattice mass density distinct from the normal fluid density
, we argue that , and we
develop a model for the normal fluid density with contributions from
longitudinal phonons and ``defectons'' (which dominate). The Bose-Einstein
Condensation (BEC) and macroscopic phase inferred from NCRI implies quantum
vortex lines and quantum vortex rings, which may explain the unusually low
critical velocity and certain hysteretic phenomena.Comment: 4 page pdf, 1 figur
Order from Disorder in Graphene Quantum Hall Ferromagnet
Valley-polarized quantum Hall states in graphene are described by a
Heisenberg O(3) ferromagnet model, with the ordering type controlled by the
strength and sign of valley anisotropy. A mechanism resulting from electron
coupling to strain-induced gauge field, giving leading contribution to the
anisotropy, is described in terms of an effective random magnetic field aligned
with the ferromagnet z axis. We argue that such random field stabilizes the XY
ferromagnet state, which is a coherent equal-weight mixture of the and
valley states. Other implications such as the Berezinskii-Kosterlitz-Thouless
ordering transition and topological defects with half-integer charge are
discussed.Comment: 4 pages, 2 figure
Bound States in a Quantized Hall Ferromagnet
We report on a study of the quasielectron-quasihole and skyrmion-antiskyrmion
bound states in the quantum Hall regime. The short range attraction
potential is assumed to be determined by a point magnetic impurity. The
calculations are performed within the strong field approximation when the
binding energy and the characteristic electron-electron interaction energy are
smaller than the Landau level spacing. The Excitonic Representation technique
is used in that case.Comment: 8 page
Electron spin-orbit splitting in InGaAs/InP quantum well studied by means of the weak antilocalization and spin-zero effects in tilted magnetic fields
The coupling between Zeeman spin splitting and Rashba spin-orbit terms has
been studied experimentally in a gated InGaAs/InP quantum well structure by
means of simultaneous measurements of the weak antilocalization (WAL) effect
and beating in the SdH oscillations. The strength of the Zeeman splitting was
regulated by tilting the magnetic field with the spin-zeros in the SdH
oscillations, which are not always present, being enhanced by the tilt. In
tilted fields the spin-orbit and Zeeman splittings are not additive, and a
simple expression is given for the energy levels. The Rashba parameter and the
electron g-factor were extracted from the position of the spin zeros in tilted
fields. A good agreement is obtained for the spin-orbit coupling strength from
the spin-zeros and weak antilocalization measurements.Comment: Accepted for publication in Semiconductors Science and Technolog
Magnetic Force Exerted by the Aharonov-Bohm Line
The problem of the scattering of a charge by the Aharonov-Bohm (AB) flux line
is reconsidered in terms of finite width beams. It is shown that despite the
left-right symmetry in the AB scattering cross-section, the charge is scattered
asymmetrically. The asymmetry (i.e. magnetic force) originates from almost
forward scattering within the angular size of the incident wave. In the
paraxial approximation, the real space solution to the scattering problem of a
beam is found as well as the scattering S-matrix. The Boltzmann kinetics and
the Landau quantization in a random AB array are considered.Comment: 5 pages, RevTeX. Discussions of paraxial approximation to the
Aharonov-Bohm solution (Cornu spiral) and S-matrix, are extended. References
are adde
Decay of metastable current states in one-dimensional resonant tunneling devices
Current switching in a double-barrier resonant tunneling structure is studied
in the regime where the current-voltage characteristic exhibits intrinsic
bistability, so that in a certain range of bias two different steady states of
current are possible. Near the upper boundary V_{th} of the bistable region the
upper current state is metastable, and because of the shot noise it eventually
decays to the stable lower current state. We find the time of this switching
process in strip-shaped devices, with the width small compared to the length.
As the bias V is tuned away from the boundary value V_{th} of the bistable
region, the mean switching time \tau increases exponentially. We show that in
long strips \ln\tau \propto (V_{th} -V)^{5/4}, whereas in short strips \ln\tau
\propto (V_{th} -V)^{3/2}. The one-dimensional geometry of the problem enables
us to obtain analytically exact expressions for both the exponential and the
prefactor of \tau. Furthermore, we show that, depending on the parameters of
the system, the switching can be initiated either inside the strip, or at its
ends.Comment: 12 pages, 5 figures, update to published versio
Sound and Heat Absorption by a 2D Electron Gas in an Odd-Integer Quantized-Hall Regime
The absorption of bulk acoustic phonons in a two-dimensional (2D) GaAs/AlGaAs
heterostructure is studied (in the clean limit) where the 2D electron-gas
(2DEG), being in an odd-integer quantum-Hall state, is in fact a spin
dielectric. Of the two channels of phonon absorption associated with excitation
of spin waves, one, which is due to the spin-orbit (SO) coupling of electrons,
involves a change of the spin state of the system and the other does not. We
show that the phonon-absorption rate corresponding to the former channel (in
the paper designated as the second absorption channel) is finite at zero
temperature (), whereas that corresponding to the latter (designated as the
first channel) vanishes for . The long-wavelength limit, being the
special case of the first absorption channel, corresponds to sound (bulk and
surface) attenuation by the 2DEG. At the same time, the ballistic phonon
propagation and heat absorption are determined by both channels. The 2DEG
overheat and the attendant spin-state change are found under the conditions of
permanent nonequilibrium phonon pumping.Comment: 26 pages, 2 figure
Dimensional Control of Antilocalisation and Spin Relaxation in Quantum Wires
The spin relaxation rate in disordered quantum wires with
Rashba and Dresselhaus spin-orbit coupling is derived analytically as a
function of wire width . It is found to be diminished when is smaller
than the bulk spin-orbit length . Only a small spin relaxation rate
due to cubic Dresselhaus coupling is found to remain in this limit. As
a result, when reducing the wire width the quantum conductivity correction
changes from weak anti- to weak localization and from negative to positive
magnetoconductivity.Comment: 4.0 pages, 3 figures, final version, Refs. updated, introduction and
formula for spin relaxation rate adde
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