7,858 research outputs found
Effects of a nonlinear bath at low temperatures
We use the numerical flow-equation renormalization method to study a
nonlinear bath at low temperatures. The model of our nonlinear bath consists of
a single two-level system coupled to a linear oscillator bath. The effects of
this nonlinear bath are analyzed by coupling it to a spin, whose relaxational
dynamics under the action of the bath is studied by calculating spin-spin
correlation functions. As a first result, we derive flow equations for a
general four-level system coupled to an oscillator bath, valid at low
temperatures. We then treat the two-level system coupled to our nonlinear bath
as a special case of the dissipative four-level system. We compare the effects
of the nonlinear bath with those obtained using an effective linear bath, and
study the differences between the two cases at low temperatures.Comment: 15 pages, 7 figure
Spin filter using a semiconductor quantum ring side-coupled to a quantum wire
We introduce a new spin filter based on spin-resolved Fano resonances due to
spin-split levels in a quantum ring (QR) side-coupled to a quantum wire (QW).
Spin-orbit coupling inside the QR, together with external magnetic fields,
induces spin splitting, and the Fano resonances due to the spin-split levels
result in perfect or considerable suppression of the transport of either spin
direction. Using the numerical renormalization group method, we find that the
Coulomb interaction in the QR enhances the spin filter operation by widening
the separation between dips in conductances for different spins and by allowing
perfect blocking for one spin direction and perfect transmission for the other.
The spin-filter effect persists as long as the temperature is less than the
broadening of QR levels due to the QW-QR coupling. We discuss realistic
conditions for the QR-based spin filter and its advantages to other similar
devices.Comment: 5 pages, 4 figure
Impulse control problem on finite horizon with execution delay
We consider impulse control problems in finite horizon for diffusions with
decision lag and execution delay. The new feature is that our general framework
deals with the important case when several consecutive orders may be decided
before the effective execution of the first one. This is motivated by financial
applications in the trading of illiquid assets such as hedge funds. We show
that the value functions for such control problems satisfy a suitable version
of dynamic programming principle in finite dimension, which takes into account
the past dependence of state process through the pending orders. The
corresponding Bellman partial differential equations (PDE) system is derived,
and exhibit some peculiarities on the coupled equations, domains and boundary
conditions. We prove a unique characterization of the value functions to this
nonstandard PDE system by means of viscosity solutions. We then provide an
algorithm to find the value functions and the optimal control. This easily
implementable algorithm involves backward and forward iterations on the domains
and the value functions, which appear in turn as original arguments in the
proofs for the boundary conditions and uniqueness results
Indistinguishability of quantum states and rotation counting
We propose a quantum system in which the winding number of rotations of a
particle around a ring can be monitored and emerges as a physical observable.
We explicitly analyze the situation when, as a result of the monitoring of the
winding number, the period of the orbital motion of the particle is extended to
full rotations, which leads to changes in the energy spectrum and in all
observable properties. In particular, we show that in this case, the usual
magnetic flux period of the Aharonov-Bohm effect is reduced to
.Comment: 5 pages, 3 embedded figure
Anderson-type model for a molecule adsorbed on a metal surface
We investigate a modified Anderson model to study the local density of states
(LDOS) of a molecular wire adsorbed on a metal. Using a self-consistent
mean-field type approach we find an exponential decay of the LDOS along the
molecule. A repulsive on-site interaction on the molecule suppresses the
tunneling and decreases the characteristic decay length.Comment: 7 pages (using europhys.sty), 5 EPS figures, To appear in Europhys.
Let
Foreign Nationality and Age - A Double Drawback for Reemployment in Germany?
We analyze reemployment prospects for Germans and non-Germans over the life course. Older foreigners may experience a double drawback due to health issues, discrimination or differences in occupational structure. This effect might be alleviated by accumulation of country-specific skills over time and selectivity effects. We apply a piecewise-constant hazard rate model on more than 270.000 unemployment episodes drawn from the social insurance register for male employees aged 25 to 65 years between 1975 to 2001. Foreign nationality lowers reemployment prospects by 7 percentage points. On average, the effect of aging on reemployment is stronger for non-Germans. The effect of nationality differs strongly between nationalities and ranges from minus 17 percentage points for Greeks up to plus 5 percentage points for people from Ex-Yugoslavia. Aging is particularly a problem for foreigners from Greece and Turkey: Until age 60, their prospects for reemployment are, on average, about 27 percent below that of natives.labor migration, aging workforce, reemployment, proportional hazard rate models, demographic change
Perturbative corrections to the Gutzwiller mean-field solution of the Mott-Hubbard model
We study the Mott-insulator transition of bosonic atoms in optical lattices.
Using perturbation theory, we analyze the deviations from the mean-field
Gutzwiller ansatz, which become appreciable for intermediate values of the
ratio between hopping amplitude and interaction energy. We discuss corrections
to number fluctuations, order parameter, and compressibility. In particular, we
improve the description of the short-range correlations in the one-particle
density matrix. These corrections are important for experimentally observed
expansion patterns, both for bulk lattices and in a confining trap potential.Comment: 10 pages, 10 figue
Aharonov-Bohm oscillations and resonant tunneling in strongly correlated quantum dots
We investigate Aharonov-Bohm oscillations of the current through a strongly
correlated quantum dot embedded in an arbitrary scattering geometry.
Resonant-tunneling processes lead to a flux-dependent renormalization of the
dot level. As a consequence we obtain a fine structure of the current
oscillations which is controlled by quantum fluctuations. Strong Coulomb
repulsion leads to a continuous bias voltage dependent phase shift and, in the
nonlinear response regime, destroys the symmetry of the differential
conductance under a sign change of the external flux.Comment: RevTex, 5 pages, 3 PostScript figures. Accepted for publication in
Phys. Rev. Let
- …