66,960 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
On the Electronic Spectroscopy of Closed Shell Cations Derived From Resonance Stabilized Radicals: Insights From Theory and Franck-Condon Analysis
Context. Recent attention has been directed on closed-shell aromatic cations as potential carriers of the diffuse interstellar bands. The spectra of mass-selected, matrix-isolated benzylium, and tropylium cations were recently reported. The visible spectrum of benzylium exhibits a large Franck-Condon (FC) envelope, inconsistent with diffuse interstellar band carriers.
Aims. We perform a computational analysis of the experimentally studied benzylium spectrum before extending the methods to a range of larger, closed-shell aromatic cations to determine the potential for this class of systems as diffuse interstellar band carriers.
Methods. Density functional theory (DFT), time-dependant ((TD)DFT), and multi-configurational self-consistent field second-order perturbation theory (MRPT2) methods in concert with multidimensional FC analysis is used to model the benzylium spectrum. These methods are extended to larger closed-shell aromatic hydrocarbon cations derived from resonance-stabilized radicals, which are predicted to show strong S0 → Sn transitions in the visible region. The ionization energies of a range of these systems are also calculated by DFT.
Results. The simulated benzylium spectrum was found to yield excellent agreement with the experimental spectrum showing an extended progression in a low frequency (510 cm-1) ring distortion mode. The FC progression was found to be significantly quenched in the larger species: 1-indanylium, 1-naphthylmethylium, and fluorenium. Excitation and ionization energies of the closed-shell cations were found to be consistent with diffuse interstellar band carriers, with the former lying in the visible range and the latter straddling the Lyman limit in the 13−14 eV range.
Conclusions. Large closed-shell polycyclic aromatic hydrocarbon cations remain viable candidate carriers of the diffuse interstellar bands
Spectral Properties of Magnetic Excitations in Cuprate Two-Leg Ladder Systems
This article summarizes and extends the recent developments in the
microscopic modeling of the magnetic excitations in cuprate two-leg ladder
systems. The microscopic Hamiltonian comprises dominant Heisenberg exchange
terms plus an additional four-spin interaction which is about five times
smaller. We give an overview over the relevant energies like the one-triplon
dispersion, the energies of two-triplon bound states and the positions of
multi-triplon continua and over relevant spectral properties like spectral
weights and spectral densities in the parameter regime appropriate for cuprate
systems. It is concluded that an almost complete understanding of the magnetic
excitations in undoped cuprate ladders has been obtained as measured by
inelastic neutron scattering, inelastic light (Raman) scattering and infrared
absorption.Comment: 26 pages, 10 figures, review for Mod. Phys. Lett.
Computer simulations of colloidal transport on a patterned magnetic substrate
We study the transport of paramagnetic colloidal particles on a patterned
magnetic substrate with kinetic Monte Carlo and Brownian dynamics computer
simulations. The planar substrate is decorated with point dipoles in either
parallel or zigzag stripe arrangements and exposed to an additional external
magnetic field that oscillates in time. For the case of parallel stripes we
find that the magnitude and direction of the particle current is controlled by
the tilt angle of the external magnetic field. The effect is reliably obtained
in a wide range of ratios between temperature and magnetic permeability.
Particle transport is achieved only when the period of oscillation of the
external field is greater than a critical value. For the case of zigzag stripes
a current is obtained using an oscillating external field normal to the
substrate. In this case, transport is only possible in the vertex of the
zigzag, giving rise to a narrow stream of particles. The magnitude and
direction of the particle current are found to be controlled by a combination
of the zigzag angle and the distance of the colloids from the substrate.
Metropolis Monte Carlo and Brownian dynamics simulations predict results that
are in good agreement with each other. Using kinetic Monte Carlo we find that
at high density the particle transport is hindered by jamming.Comment: 8 pages, 9 figure
Spin Effects in the Local Density of States of GaAs
We present spin-resolved measurements of the local density of states in Si
doped GaAs. Both spin components exhibit strong mesoscopic fluctuations. In the
magnetic quantum limit, the main features of the spin-up and spin-down
components of the local density of states are found to be identical apart from
Zeeman splitting. Based on this observation, we introduce a mesoscopic method
to measure the -factor in a material where macroscopic methods are severely
restricted by disorder. Differences between the spin-up and spin-down
components are discussed in terms of spin relaxation due to spin-orbit
coupling.Comment: 4 pages and 5 figure
Theoretical Model for the Semimetal Yb_4As_3
We present a model which can explain semiquantitatively a number of the
unusual properties of \mbox{YbAs}. The structural phase transition at
T_{\text{c}}\simeq300\,\mbox{K} is described by a band Jahn-Teller effect of
correlated electrons and is interpreted as a charge ordering of the Yb ions.
The low carrier concentration in the low-temperature phase follows from the
strong electron correlations of the 4f-holes on the Yb sites and can be viewed
as self-doping of charge-ordered chains. The observed heavy-fermion behaviour
is on a scale of T^\ast\simeq50\,\mbox{K} and is due to spinon-like
excitations in the Yb-chains. The appearance of a second low-energy
scale around 0.2\,K is due to the Fermi energy of the low-density carriers.Comment: 7 pages, REVTeX, 1 Postscript-figure separatel
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