892 research outputs found
Condensation and vortex formation in Bose-gas upon cooling
The mechanism for the transition of a Bose gas to the superfluid state via
thermal fluctuations is considered. It is shown that in the process of external
cooling some critical fluctuations (instantons) are formed above the critical
temperature. The probability of the instanton formation is calculated in the
three and two-dimensional cases. It is found that this probability increases as
the system approaches the transition temperature. It is shown that the
evolution of an individual instanton is impossible without the formation of
vortices in its superfluid part
Mechanism of electron localization in a quantum wire
We show that quasi-bound electron states are formed in a quantum wire as a
result of electron backscattering in the transition regions between the wire
and the electron reservoirs, to which the wire is coupled. The backscattering
mechanism is caused by electron density oscillations arising even in smooth
transitions due to the reflection of electrons not transmitting through the
wire. The quasi-bound states reveal themselves in resonances of the electron
transmission probability through the wire. The calculations were carried out
within the Hartree-Fock approximation using quasiclassic wavefunctions.Comment: 7 pages, IOP style, 4 figures, typos corrected, published versio
Time-reversal symmetric resolution of unity without background integrals in open quantum systems
We present a new complete set of states for a class of open quantum systems,
to be used in expansion of the Green's function and the time-evolution
operator. A remarkable feature of the complete set is that it observes
time-reversal symmetry in the sense that it contains decaying states (resonant
states) and growing states (anti-resonant states) parallelly. We can thereby
pinpoint the occurrence of the breaking of time-reversal symmetry at the choice
of whether we solve Schroedinger equation as an initial-condition problem or a
terminal-condition problem. Another feature of the complete set is that in the
subspace of the central scattering area of the system, it consists of
contributions of all states with point spectra but does not contain any
background integrals. In computing the time evolution, we can clearly see
contribution of which point spectrum produces which time dependence. In the
whole infinite state space, the complete set does contain an integral but it is
over unperturbed eigenstates of the environmental area of the system and hence
can be calculated analytically. We demonstrate the usefulness of the complete
set by computing explicitly the survival probability and the escaping
probability as well as the dynamics of wave packets. The origin of each term of
matrix elements is clear in our formulation, particularly the exponential
decays due to the resonance poles.Comment: 62 pages, 13 figure
Delayed Recombination and Standard Rulers
Measurements of Baryonic Acoustic Oscillations in galaxy surveys have been
recognized as a powerful tool for constraining dark energy. However, this
method relies on the knowledge of the size of the acoustic horizon at
recombination derived from Cosmic Microwave Background Anisotropy measurements.
This estimate is typically derived assuming a standard recombination scheme;
additional radiation sources can delay recombination altering the cosmic
ionization history and the cosmological inferences drawn from CMB and BAO data.
In this paper we quantify the effect of delayed recombination on the
determination of dark energy parameters from future BAO surveys such as BOSS
and WFMOS. We find the impact to be small but still not negligible. In
particular, if recombination is non-standard (to a level still allowed by CMB
data), but this is ignored, future surveys may incorrectly suggest the presence
of a redshift dependent dark energy component. On the other hand, in the case
of delayed recombination, adding to the analysis one extra parameter describing
deviations from standard recombination, does not significantly degrade the
error-bars on dark energy parameters and yields unbiased estimates.Comment: 8 pages, 5 figure
Unbalanced Renormalization of Tunneling in MOSFET-type Structures in Strong High-Frequency Electric Fields
Two-dimensional electron gas coupled to adjacent impurity sites in
high-frequency out-of-plane ac control electric field is investigated.
Modification of tunneling rates as a function of the field amplitude is
calculated. Nonlinear dependence on the ac field strength is reported for the
conductivity of two-dimensional electron gas. It develops a periodic peak
structure.Comment: 9 pages, 4 figure
X-ray diffraction from shock-loaded polycrystals
X-ray diffraction was demonstrated from shock-compressed polycrystalline
metal on nanosecond time scales. Laser ablation was used to induce shock waves
in polycrystalline foils of Be, 25 to 125 microns thick. A second laser pulse
was used to generate a plasma x-ray source by irradiation of a Ti foil. The
x-ray source was collimated to produce a beam of controllable diameter, and the
beam was directed at the Be sample. X-rays were diffracted from the sample, and
detected using films and x-ray streak cameras. The diffraction angle was
observed to change with shock pressure. The diffraction angles were consistent
with the uniaxial (elastic) and isotropic (plastic) compressions expected for
the loading conditions used. Polycrystalline diffraction will be used to
measure the response of the crystal lattice to high shock pressures and through
phase changes
Constraints On The Topology Of The Universe From The WMAP First-Year Sky Maps
We compute the covariance expected between the spherical harmonic
coefficients of the cosmic microwave temperature anisotropy if the
universe had a compact topology. For fundamental cell size smaller than the
distance to the decoupling surface, off-diagonal components carry more
information than the diagonal components (the power spectrum). We use a maximum
likelihood analysis to compare the Wilkinson Microwave Anisotropy Probe
first-year data to models with a cubic topology. The data are compatible with
finite flat topologies with fundamental domain times the distance to
the decoupling surface at 95% confidence. The WMAP data show reduced power at
the quadrupole and octopole, but do not show the correlations expected for a
compact topology and are indistinguishable from infinite models.Comment: 16 pages, 5 figure
Sources of Radiation in the Early Universe: The Equation of Radiative Transfer and Optical Distances
We have derived the radiative-transfer equation for a point source with a
specified intensity and spectrum, originating in the early Universe between the
epochs of annihilation and recombination, at redshifts z_\s =10^8\div 10^4.
The direct radiation of the source is separated from the diffuse radiation it
produces. Optical distances from the source for Thomson scattering and
bremsstrahlung absorption at the maximum of the thermal background radiation
are calculated as a function of the redshift z.The distances grow sharply with
decreasing z, approaching asymptotic values, the absorption distance increasing
more slowly and reaching their limiting values at lower z. For the adopted z
values, the optical parameters of the Universe can be described in a flat model
with dusty material and radiation, and radiative transfer can be treated in a
grey approximation.Comment: 14 pages, 2 figure
On decay of large amplitude bubble of disoriented chiral condensate
The time evolution of initially formed large amplitude bubble of disoriented
chiral condensate (DCC) is studied. It is found that the evolution of this
object may have a relatively long pre-decay stage. Simple explanation of such
delay of the DCC bubble decay is given. This delay is related to the existence
of the approximate solutions of multi-soliton type of the corresponding radial
sine-Gordon equation in (3+1) dimensions at large bubble radius.Comment: 6 pages, LaTeX, 5 PostScript figure
Scalar and Spinor Particles with Low Binding Energy in the Strong Stationary Magnetic Field Studied by Means of Two-and Three-Dimensional Models
On the basis of analytic solutions of Schrodinger and Pauli equations for a
uniform magnetic field and a single attractive -potential the
equations for the bound one-active electron states are discussed. It is vary
important that ground electron states in the magnetic field essentially
different from the analog state of spin-0 particles that binding energy has
been intensively studied at more then forty years ago. We show that binding
energy equations for spin-1/2 particles can be obtained without using of a
well-known language of boundary conditions in the model of -potential
that has been developed in pioneering works. Obtained equations are used for
the analytically calculation of the energy level displacements, which
demonstrate nonlinear dependencies on field intensities. It is shown that in a
case of the weak intensity a magnetic field indeed plays a stabilizing role in
considering systems. However the strong magnetic field shows the opposite
action. We are expected that these properties can be of importance for real
quantum mechanical fermionic systems in two- and three-dimensional cases.Comment: 18 page
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