69,081 research outputs found
Transient dynamics of a molecular quantum dot with a vibrational degree of freedom
We investigate the transient effects occurring in a molecular quantum dot
described by an Anderson-Holstein Hamiltonian which is instantly coupled to two
fermionic leads biased by a finite voltage. In the limit of weak
electron-phonon interaction, we use perturbation theory to determine the
time-dependence of the dot population and the average current. The limit of
strong coupling is accessed by means of a self-consistent time-dependent
mean-field approximation. These comple- mentary approaches allow us to
investigate the dynamics of the inelastic effects occurring when the applied
bias voltage exceeds the phonon frequency and the emergence of bistability.Comment: 7 pages, 4 figure
FORTRAN optical lens design program
Computer program uses the principles of geometrical optics to design optical systems containing up to 100 planes, conic or polynomial aspheric surfaces, 7 object points, 6 colors, and 200 rays. This program can be used for the automatic design of optical systems or for the evaluation of existing optical systems
Magnetic susceptibility in quasi one-dimensional Ba2V3O9: chain segmentation versus the staggered field effect
A pronounced Curie-like upturn of the magnetic susceptibility chi(T) of the
quasi one-dimensional spin chain compound Ba2V3O9 has been found recently.
Frequently this is taken as a signature for a staggered field mechanism due to
the presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We
calculate this contribution within a realistic structure of vanadium 3d- and
oxygen 2p-orbitals and conclude that this mechanism is far too small to explain
experimental results. We propose that the Curie term is rather due to a
segmentation of spin chains caused by broken magnetic bonds which leads to
uncompensated S=1/2 spins of segments with odd numbers of spins. Using a
finite-temperature Lanczos method we calculate their effective moment and show
that ~1% of broken magnetic bonds is sufficient to reproduce the anomalous
low-T behavior of chi(T) in Ba2V3O9.Comment: 5 pages, 5 figures, REVTeX 4, minor corrections to the text,
references adde
Dynamic response functions and helical gaps in interacting Rashba nanowires with and without magnetic fields
A partially gapped spectrum due to the application of a magnetic field is one
of the main probes of Rashba spin-orbit coupling in nanowires. Such a "helical
gap" manifests itself in the linear conductance, as well as in dynamic response
functions such as the spectral function, the structure factor, or the
tunnelling density of states. In this paper, we investigate theoretically the
signature of the helical gap in these observables with a particular focus on
the interplay between Rashba spin-orbit coupling and electron-electron
interactions. We show that in a quasi-one-dimensional wire, interactions can
open a helical gap even without magnetic field. We calculate the dynamic
response functions using bosonization, a renormalization group analysis, and
the exact form factors of the emerging sine-Gordon model. For special
interaction strengths, we verify our results by refermionization. We show how
the two types of helical gaps, caused by magnetic fields or interactions, can
be distinguished in experiments.Comment: 15 pages, 7 figures, v2 refs adde
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation
Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the
coherent splitting of Cooper pairs from a superconductor to two spatially
separated quantum dots has been predicted to generate nonlocal pairs of
entangled electrons. In order to test this hypothesis, we propose a scheme to
transfer the spin state of a split Cooper pair onto the polarization state of a
pair of optical photons. We show that the produced photon pairs can be used to
violate a Bell inequality, unambiguously demonstrating the entanglement of the
split Cooper pairs.Comment: 11 pages, 9 figures, v3 with added reference
Multiple phase slips phenomena in mesoscopic superconducting rings
We investigate the behavior of a mesoscopic one-dimensional ring in an
external magnetic field by simulating the time dependent Ginzburg-Landau
equations with periodic boundary conditions. We analyze the stability and the
different possible evolutions for the phase slip phenomena starting from a
metastable state. We find a stability condition relating the winding number of
the initial solution and the number of flux quanta penetrating the ring. The
analysis of multiple phase slips solutions is based on analytical results and
simulations. The role of the ratio of two characteristic times u is studied for
the case of a multiple phase slips transition. We found out that if u>>1,
consecutive multiple phase slips will be more favorable than simultaneous ones.
If u>1 is often a necessary
condition to reach the ground state. The influence of the Langevin noise on the
kinetics of the phase transition is discussed.Comment: 8 pages, 6 figure
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