arXiv
(422.153 recursos)
This is one of the most extensive subject based repositories in the world in the field of physics, mathematics, astronomy, computer sciences and quantitative biology. This is the principal site with almost 20 mirror versions around the globe. The site is supported by an extensive collection of information and background documentation. An RSS feed is available for anyone interested in keeping up-to-date with newly added materials.
Mostrando recursos 1 - 20 de 123
1.
Chaotic features in classical scattering processes between ions and
atoms - Sattin, Fabio; Salasnich, Luca
A numerical study has been done of collisions between protons and hydrogen
atoms, treated as classical particles, at low impact velocities. The presence
of chaos has been looked for by investigating the processes with standard
techniques of the chaotic--scattering theory. The evidence of a sharp
transition from nearly regular scattering to fully developed chaos has been
found at the lower velocities.
2.
Phase ordering kinetics of the Bose gas - Damle, Kedar; Majumdar, Satya; Sachdev, Subir
We study the approach to equilibrium of a Bose gas to a superfluid state. We
point out that dynamic scaling, characteristic of far from equilibrium
phase-ordering systems, should hold. We stress the importance of a
non-dissipative Josephson precession term in driving the system to a new
universality class. A model of coarsening in dimension $d=2$, involving a
quench between two temperatures below the equilibrium superfluid transition
temperature ($T_c$), is exactly solved and demonstrates the relevance of the
Josephson term. Numerical results on quenches from above $T_c$ in $d=2,3$
provide evidence for the scaling picture postulated.
3.
Some exact analytic results for the linear and non-linear response of
atoms in a trap with a model interaction - Benjamin, Simon C.; Johnson, Neil F.; Quiroga, Luis
We present an exact expression for the evolution of the wavefunction of $N$
interacting atoms in an arbitrarily time-dependent, $d$-dimensional parabolic
trap potential $\omega(t)$. The interaction potential between atoms is taken to
be of the form $\xi/r^2$ with $\xi>0$. For a constant trap potential
$\omega(t)=\omega_0$, we find an exact, infinite set of relative mode
excitations. These excitations are relevant to the linear response of the
system; they are universal in that their frequencies are independent of the
initial state of the system (e.g. Bose-Einstein condensate), the strength $\xi$
of the atom-atom interaction, the dimensionality $d$ of the trap and the number
of atoms $N$. The time evolution of the...
4.
Collective excitations of atomic Bose-Einstein condensates - Edwards, Mark; Ruprecht, P. A.; Burnett, K.; Dodd, R. J.; Clark, Charles W.
We apply linear-response analysis of the Gross-Pitaevskii equation to obtain
the excitation frequencies of a Bose-Einstein condensate confined in a
time-averaged orbiting potential trap. Our calculated values are in excellent
agreement with those observed in a recent experiment.
5.
Excited states of a static dilute spherical Bose condensate in a trap - Fetter, Alexander L.
The Bogoliubov approximation is used to study the excited states of a dilute
gas of $N$ atomic bosons trapped in an isotropic harmonic potential
characterized by a frequency $\omega_0$ and an oscillator length $d_0 =
\sqrt{\hbar/m\omega_0}$. The self-consistent static Bose condensate has
macroscopic occupation number $N_0 \gg 1$, with nonuniform spherical condensate
density $n_0(r)$; by assumption, the depletion of the condensate is small ($N'
\equiv N - N_0\ll N_0$). The linearized density fluctuation operator $\hat
\rho'$ and velocity potential operator $\hat\Phi'$ satisfy coupled equations
that embody particle conservation and Bernoulli's theorem. For each angular
momentum $l$, introduction of quasiparticle operators yields coupled eigenvalue
equations for the excited states; they can...
6.
Stability of Bose condensed atomic Li-7 - Houbiers, M.; Stoof, H. T. C.
We study the stability of a Bose condensate of atomic $^7$Li in a (harmonic
oscillator) magnetic trap at non-zero temperatures. In analogy to the stability
criterion for a neutron star, we conjecture that the gas becomes unstable if
the free energy as a function of the central density of the cloud has a local
extremum which conserves the number of particles. Moreover, we show that the
number of condensate particles at the point of instability decreases with
increasing temperature, and that for the temperature interval considered, the
normal part of the gas is stable against density fluctuations at this point.
7.
Renormalization Group Theory of the Three-Dimensional Dilute Bose Gas - Bijlsma, M.; Stoof, H. T. C.
We study the three-dimensional atomic Bose gas using renormalization group
techniques. Using our knowledge of the microscopic details of the interatomic
interaction, we determine the correct initial values of our renormalization
group equations and thus obtain also information on nonuniversal properties. As
a result, we can predict for instance the critical temperature of the gas and
the superfluid and condensate density of the Bose-Einstein condensed phase in
the regime $na\Lambda_{th}^2\ll 1$.
8.
Initial Stages of Bose-Einstein Condensation - Stoof, H. T. C.
We present the quantum theory for the nucleation of Bose-Einstein
condensation in a dilute atomic Bose gas. This quantum theory comfirms the
results of the semiclassical treatment, but has the important advantage that
both the kinetic and coherent stages of the nucleation process can now be
described in a unified way by a single Fokker-Planck equation.
9.
Binary Boosts - Haeckl, R.; Hund, V.; Pilkuhn, H.
The relativistic motion of an isolated two--body system (bound or unbound) of
given lab energy $K^{0}$ in QED is separated into cms motion and relative
motion. The relative motion equation ${\cal K}_{L} \psi_{L} ({\bf r}_{L} ) =0$
contains the momentum eigenvalue ${\bf K}$ of the cms motion. It is greatly
simplified by a binary boost to the atomic rest frame, where $K^{0}$ and ${\bf
K}$ appear only in a Lorentz--invariant combination. This boost is not a
product of single--particle boosts, which are useful only for perturbative
interactions. CPT--invariance is demonstrated, and orthogonality relations are
derived.
10.
Dyon mass bounds from electric dipole moments - Osland, Per; Shnir, Ya. M.
Dyon loops give a contribution to the matrix element for light-by-light
scattering that violates parity and time-reversal symmetry. This effect induces
an electric dipole moment for the electron, of order $M^{-2}$, where $M$ is the
dyon mass. The current limit on the electric dipole moment of the electron
yields the lower mass bound $M>{\cal O}(1)~\mbox{TeV}$.
11.
Relativistic corrections to the positronium decay rate revisited - Khriplovich, I. B.; Milstein, A. I.
We rederive here in a simple and transparent way the master formula for the
dominant part of large relativistic corrections to the positronium decay rate.
12.
Effective field theories for QED bound states: extending Nonrelativistic
QED to study retardation effects - Labelle, Patrick
Nonrelativistic QED bound states are difficult to study because of the
presence of at least three widely different scales: the masses, three-momenta
($p_i$) and kinetic energies ($K_i$) of the constituents. Nonrelativistic QED
(NRQED), an effective field theory developed by Caswell and Lepage, simplifies
greatly bound state calculations by eliminating the masses as dynamical scales.
As we demonstrate, NRQED diagrams involving only photons of energy $E_\gamma
\simeq p_i$ contribute, in any calculation, to a unique order in $\alpha$. This
is not the case, however, for diagrams involving photons with energies
$E_\gamma \simeq K_i$ (``retardation effects"), for which no simple counting
counting rules can be given. We present a new effective...
13.
Quantum state measurement by realistic heterodyne detection - Paris, Matteo G. A.
The determination of the quantum properties of a single mode radiation field
by heterodyne or double homodyne detection is studied. The realistic case of
not fully efficient photodetectors is considered. It is shown that a large
amount of quite {\em precise} information is avalaible whereas the completeness
of such information is also discussed. Some examples are given and the special
case of states expressed as a finite superposition of number states is
considered in some detail.
14.
Quantum State Diffusion and Time Correlation Functions - Brun, Todd A.; Gisin, Nicolas
In computing the spectra of quantum mechanical systems one encounters the
Fourier transforms of time correlation functions, as given by the quantum
regression theorem for systems described by master equations. Quantum state
diffusion (QSD) gives a useful method of solving these problems by unraveling
the master equation into stochastic trajectories; but there is no generally
accepted definition of a time correlation function for a single QSD trajectory.
In this paper we show how QSD can be used to calculate these spectra directly;
by formally solving the equations which arise, we arrive at a natural
definition for a two-time correlation function in QSD, which depends explicitly
on both the stochastic...
15.
Negative entropy and information in quantum mechanics - Cerf, N. J.; Adami, C.
A framework for a quantum mechanical information theory is introduced that is
based entirely on density operators, and gives rise to a unified description of
classical correlation and quantum entanglement. Unlike in classical (Shannon)
information theory, quantum (von Neumann) conditional entropies can be negative
when considering quantum entangled systems, a fact related to quantum
non-separability. The possibility that negative (virtual) information can be
carried by entangled particles suggests a consistent interpretation of quantum
informational processes.
16.
A Proposed Experiment Showing that Classical Fields Can Violate Bell's
Inequalities - Suppes, Patrick; de Barros, J. Acacio; Sant'Anna, Adonai S.
We show one can use classical fields to modify a quantum optics experiment so
that Bell's inequalities will be violated. This happens with continuous random
variables that are local, but we need to use the correlation matrix to prove
there can be no joint probability distribution of the observables.
17.
Violation of Bell's Inequalities with a Local Theory of Photons - Suppes, Patrick; de Barros, J. Acacio; Sant'Anna, Adonai S.
We use a local theory of photons purely as particles to model the
single-photon experiment proposed by Tan, Walls, and Collett. Like Tan et al.
we are able to derive a violation of Bell's inequalities for photon counts
coincidence measurements. Our local probabilistic theory does not use any
specific quantum mechanical calculations.
19.
Motion and Ionization Equilibrium of Hydrogen Atoms in Superstrong
Magnetic Field - Lai, Dong; Salpeter, Edwin E.
We study the effects of finite proton mass on the energy levels of hydrogen
atoms moving transverse to a superstrong magnetic field $B$ with generalized
pseudomomentum $K_\perp$. Field strengths of order $B\sim 10^{12}$ Gauss are
typically found on the surfaces of neutron stars, but we also study the regime
$B\go B_{crit}= 4.23\times 10^{13}$ Gauss, where the Landau excitation energy
of the proton is large. We adopt two different approaches to the two-body
problem in strong magnetic field, and obtain an approximate, but complete
solution of the atomic energy as a function of $B$ and $K_\perp$. We show that,
for $B>>B_{crit}$, there is an orthogonal set of bound states,...
20.
Hydrogen Molecules In Superstrong Magnetic Field: II. Excitation Levels - Lai, Dong; Salpeter, Edwin E.
We study the energy levels of H$_2$ molecules in a superstrong magnetic field
($B\go 10^{12}$ G), typically found on the surfaces of neutron stars. The
interatomic interaction potentials are calculated by a Hartree-Fock method with
multi-configurations assuming electrons are in the ground Landau state. Both
the aligned configurations and arbitrary orientations of the molecular axis
with respect to the magnetic field axis are considered. Different types of
molecular excitations are then studied: electronic excitations, aligned (along
the magnetic axis) vibrational excitations, transverse vibrational excitations
(a constrained rotation of the molecular axis around the magnetic field line).
Similar results for the molecular ion H$_2^+$ are also obtained and compared
with previous...
1.
