125 research outputs found
Theoretical analysis of the transmission phase shift of a quantum dot in the presence of Kondo correlations
We study the effects of Kondo correlations on the transmission phase shift of
a quantum dot coupled to two leads in comparison with the experimental
determinations made by Aharonov-Bohm (AB) quantum interferometry. We propose
here a theoretical interpretation of these results based on scattering theory
combined with Bethe ansatz calculations. We show that there is a factor of 2
difference between the phase of the S-matrix responsible for the shift in the
AB oscillations, and the one controlling the conductance. Quantitative
agreement is obtained with experimental results for two different values of the
coupling to the leads.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Time-dependent approach to many-particle tunneling in one-dimension
Employing the time-dependent approach, we investigate a quantum tunneling
decay of many-particle systems. We apply it to a one-dimensional three-body
problem with a heavy core nucleus and two valence protons. We calculate the
decay width for two-proton emission from the survival probability, which well
obeys the exponential decay-law after a sufficient time. The effect of the
correlation between the two emitted protons is also studied by observing the
time evolution of the two-particle density distribution. It is shown that the
pairing correlation significantly enhances the probability for the simultaneous
diproton decay.Comment: 9 pages, 10 eps figure
Dark matter and stable bound states of primordial black holes
We present three reasons for the formation of gravitational bound states of
primordial black holes,called holeums,in the early universe.Using Newtonian
gravity and nonrelativistic quantum mechanics we find a purely quantum
mechanical mass-dependant exclusion property for the nonoverlap of the
constituent black holes in a holeum.This ensures that the holeum occupies space
just like ordinary matter.A holeum emits only gravitational radiation whose
spectrum is an exact analogue of that of a hydrogen atom. A part of this
spectrum lies in the region accessible to the detectors being built.The holeums
would form haloes around the galaxies and would be an important component of
the dark matter in the universe today.They may also be the constituents of the
invisible domain walls in the universe.Comment: 13 pages,2tables,for wider circulation,PD
Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium
We present a theoretical study of transient absorption and reshaping of
extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately
strong infrared (IR) laser field. We formulate the atomic response using both
the frequency-dependent absorption cross section and a time-frequency approach
based on the time-dependent dipole induced by the light fields. The latter
approach can be used in cases when an ultrafast dressing pulse induces
transient effects, and/or when the atom exchanges energy with multiple
frequency components of the XUV field. We first characterize the dressed atom
response by calculating the frequency-dependent absorption cross section for
XUV energies between 20 and 24 eV for several dressing wavelengths between 400
and 2000 nm and intensities up to 10^12 W/cm^2. We find that for dressing
wavelengths near 1600 nm, there is an Autler-Townes splitting of the 1s ---> 2p
transition that can potentially lead to transparency for absorption of XUV
light tuned to this transition. We study the effect of this XUV transparency in
a macroscopic helium gas by incorporating the time-frequency approach into a
solution of the coupled Maxwell-Schr\"odinger equations. We find rich temporal
reshaping dynamics when a 61 fs XUV pulse resonant with the 1s ---> 2p
transition propagates through a helium gas dressed by an 11 fs, 1600 nm laser
pulse.Comment: 13 pages, 8 figures, 1 table, RevTeX4, revise
Theory of x-ray absorption by laser-aligned symmetric-top molecules
We devise a theory of x-ray absorption by symmetric-top molecules which are
aligned by an intense optical laser. Initially, the density matrix of the
system is composed of the electronic ground state of the molecules and a
thermal ensemble of rigid-rotor eigenstates. We formulate equations of motion
of the two-color (laser plus x rays) rotational-electronic problem. The
interaction with the laser is assumed to be nonresonant; it is described by an
electric dipole polarizability tensor. X-ray absorption is approximated as a
one-photon process. It is shown that the equations can be separated such that
the interaction with the laser can be treated independently of the x rays. The
laser-only density matrix is propagated numerically. After each time step, the
x-ray absorption is calculated. We apply our theory to study adiabatic
alignment of bromine molecules (Br2). The required dynamic polarizabilities are
determined using the ab initio linear response methods coupled-cluster singles
(CCS), second-order approximate coupled-cluster singles and doubles (CC2), and
coupled-cluster singles and doubles (CCSD). For the description of x-ray
absorption on the sigma_g 1s --> sigma_u 4p resonance, a parameter-free
two-level model is used for the electronic structure of the molecules. Our
theory opens up novel perspectives for the quantum control of x-ray radiation.Comment: 14 pages, 4 figures, 1 table, RevTeX4, revise
Evolution of Liouville density of a chaotic system
An area-preserving map of the unit sphere, consisting of alternating twists
and turns, is mostly chaotic. A Liouville density on that sphere is specified
by means of its expansion into spherical harmonics. That expansion initially
necessitates only a finite number of basis functions. As the dynamical mapping
proceeds, it is found that the number of non-negligible coefficients increases
exponentially with the number of steps. This is to be contrasted with the
behavior of a Schr\"odinger wave function which requires, for the analogous
quantum system, a basis of fixed size.Comment: LaTeX 4 pages (27 kB) followed by four short PostScript files (2 kB +
2 kB + 1 kB + 4 kB
Quantum Mechanics of Multi-Prong Potentials
We describe the bound state and scattering properties of a quantum mechanical
particle in a scalar -prong potential. Such a study is of special interest
since these situations are intermediate between one and two dimensions. The
energy levels for the special case of identical prongs exhibit an
alternating pattern of non-degeneracy and fold degeneracy. It is shown
that the techniques of supersymmetric quantum mechanics can be used to generate
new solutions. Solutions for prongs of arbitrary lengths are developed.
Discussions of tunneling in -well potentials and of scattering for piecewise
constant potentials are given. Since our treatment is for general values of
, the results can be studied in the large limit. A somewhat surprising
result is that a free particle incident on an -prong vertex undergoes
continuously increased backscattering as the number of prongs is increased.Comment: 17 pages. LATEX. On request, TOP_DRAW files or hard copies available
for 7 figure
Non-linear dynamics, entanglement and the quantum-classical crossover of two coupled SQUID rings
We explore the quantum-classical crossover of two coupled, identical,
superconducting quantum interference device (SQUID) rings. We note that the
motivation for this work is based on a study of a similar system comprising two
coupled Duffing oscillators. In that work we showed that the entanglement
characteristics of chaotic and periodic (entrained) solutions differed
significantly and that in the classical limit entanglement was preserved only
in the chaotic-like solutions. However, Duffing oscillators are a highly
idealised toy model. Motivated by a wish to explore more experimentally
realisable systems we now extend our work to an analysis of two coupled SQUID
rings. We observe some differences in behaviour between the system that is
based on SQUID rings rather than on Duffing oscillators. However, we show that
the two systems share a common feature. That is, even when the SQUID ring's
trajectories appear to follow (semi) classical orbits entanglement persists.Comment: 9 pages, 4 figures. Published as part of the proceedings of the 32nd
International Workshop on Condensed Matter Theories (CMT32) Loughborough
University, 2008 (invited paper
"Gauging" the Fluid
A consistent framework has been put forward to quantize the isentropic,
compressible and inviscid fluid model in the Hamiltonian framework, using the
Clebsch parameterization. The naive quantization is hampered by the
non-canonical (in particular field dependent) Poisson Bracket algebra. To
overcome this problem, the Batalin-Tyutin \cite{12} quantization formalism is
adopted in which the original system is converted to a local gauge theory and
is embedded in a {\it canonical} extended phase space. In a different reduced
phase space scheme \cite{vy} also the original model is converted to a gauge
theory and subsequently the two distinct gauge invariant formulations of the
fluid model are related explicitly. This strengthens the equivalence between
the relativistic membrane (where a gauge invariance is manifest) and the fluid
(where the gauge symmetry is hidden). Relativistic generalizations of the
extended model is also touched upon.Comment: Version to appear in J.Phys. A: Mathematical and Genera
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