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 $n>1$ 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 $\Phi_0=h/q$ of the Aharonov-Bohm effect is reduced to $\Phi_0/n$.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