121 research outputs found
Current behavior of a quantum Hamiltonian ratchet in resonance
We investigate the ratchet current that appears in a kicked Hamiltonian
system when the period of the kicks corresponds to the regime of quantum
resonance. In the classical analogue, a spatial-temporal symmetry should be
broken to obtain a net directed current. It was recently discovered that in
quantum resonance the temporal symmetry can be kept, and we prove that breaking
the spatial symmetry is a necessary condition to find this effect.
Moreover, we show numerically and analytically how the direction of the
motion is dramatically influenced by the strength of the kicking potential and
the value of the period. By increasing the strength of the interaction this
direction changes periodically, providing us with a non-expected source of
current reversals in this quantum model. These reversals depend on the kicking
period also, though this behavior is theoretically more difficult to analyze.
Finally, we generalize the discussion to the case of a non-uniform initial
condition.Comment: 6 pages, 4 figure
Equilibrium Chemical Engines
An equilibrium reversible cycle with a certain engine to transduce the energy
of any chemical reaction into mechanical energy is proposed. The efficiency for
chemical energy transduction is also defined so as to be compared with Carnot
efficiency. Relevance to the study of protein motors is discussed. KEYWORDS:
Chemical thermodynamics, Engine, Efficiency, Molecular machine.Comment: 5 pages, late
Zitterbewegung of relativistic electrons in a magnetic field and its simulation by trapped ions
One-electron 3+1 and 2+1 Dirac equations are used to calculate the motion of
a relativistic electron in a vacuum in the presence of an external magnetic
field. First, calculations are carried on an operator level and exact
analytical results are obtained for the electron trajectories which contain
both intraband frequency components, identified as the cyclotron motion, as
well as interband frequency components, identified as the trembling motion
(Zitterbewegung, ZB). Next, time-dependent Heisenberg operators are used for
the same problem to compute average values of electron position and velocity
employing Gaussian wave packets. It is shown that the presence of a magnetic
field and the resulting quantization of the energy spectrum has pronounced
effects on the electron Zitterbewegung: it introduces intraband frequency
components into the motion, influences all the frequencies and makes the motion
stationary (not decaying in time) in case of the 2+1 Dirac equation. Finally,
simulations of the 2+1 Dirac equation and the resulting electron ZB in the
presence of a magnetic field are proposed and described employing trapped ions
and laser excitations. Using simulation parameters achieved in recent
experiments of Gerritsma and coworkers we show that the effects of the
simulated magnetic field on ZB are considerable and can certainly be observed.Comment: 19 pages, 9 figures, published versio
Fidelity for displaced squeezed states and the oscillator semigroup
The fidelity for two displaced squeezed thermal states is computed using the
fact that the corresponding density operators belong to the oscillator
semigroup.Comment: 3 pages, REVTEX, no figures, submitted to Journal of Physics A, May
5, 199
Quantum simulation of manybody effects in steady-state nonequilibrium: electron-phonon coupled quantum dots
We develop a mapping of quantum steady-state nonequilibrium to an effective
equilibrium and solve the problem using a quantum simulation technique. A
systematic implementation of the nonequilibrium boundary condition in
steady-state is made in the electronic transport on quantum dot structures.
This formulation of quantum manybody problem in nonequilibrium enables the use
of existing numerical quantum manybody techniques. The algorithm coherently
demonstrates various transport behaviors from phonon-dephasing to I-V staircase
and phonon-assisted tunneling.Comment: 5 pages, 4 figure
Quantum effect in the diffusion along a potential barrier: Comments on the synthesis of superheavy elements
We discuss a quantum effect in the diffusion process by developing a theory,
which takes the finite curvature of the potential field into account. The
transport coefficients of our theory satisfy the well-known
fluctuation-dissipation theorem in the limit of Markovian approximation in the
cases of diffusion in a flat potential and in a potential well. For the
diffusion along a potential barrier, the diffusion coefficient can be related
to the friction coefficient by an analytic continuation of the
fluctuation-dissipation theorem for the case of diffusion along a potential
well in the asymptotic time, but contains strong non-Markovian effects at short
times. By applying our theory to the case of realistic values of the
temperature, the barrier curvature, and the friction coefficient, we show that
the quantum effects will play significant roles in describing the synthesis of
superheavy elements, i.e., the evolution from the fusion barrier to the
conditional saddle, in terms of a diffusion process. We especially point out
the importance of the memory effect, which increases at lower temperatures. It
makes the net quantum effects enhance the probability of crossing the
conditional saddle.Comment: 12 pages, 3 figures, accepted for publication in Phys. Rev.
Anharmonic oscillation effect on the Davydov-Scott monomer in thermal bath
The dynamics of Davydov-Scott monomer in a thermal bath with higher order
amide-site's displacement leads to anharmonic oscillation effect is
investigated using full-quantum approach and the Lindblad formulation of master
equation. The specific heat is calculated based on the thermodynamic partition
function using the path integral method. The temperature dependence of the
specific heat is studied. In the model the specific heat anomaly as pointed out
in recent works by Ingold et.al is also observed. However it is found that the
anomaly occurs at high temperature region, and the anharmonic oscillation
restores the positivity of specific heat.Comment: 11 pages, 5 figure
Nonequilibrium Dephasing in an Electronic Mach-Zehnder Interferometer
We study nonequilibrium dephasing in an electronic Mach-Zehnder
interferometer. We demonstrate that the shot noise at the beam splitter of the
interferometer generates an ensemble of nonequilibrium electron density
configurations and that electron interactions induce configuration-specific
phase shifts of an interfering electron. The resulting dephasing exhibits two
characteristic features, a lobe pattern in the visibility and phase jumps of
, in good agreement with experimental data.Comment: 4 pages, 3 figures; some typos are corrected; published versio
Brownian Motors driven by Particle Exchange
We extend the Langevin dynamics so that particles can be exchanged with a
particle reservoir. We show that grand canonical ensembles are realized at
equilibrium and derive the relations of thermodynamics for processes between
equilibrium states. As an application of the proposed evolution rule, we devise
a simple model of Brownian motors driven by particle exchange. KEYWORDS:
Langevin Dynamics, Thermodynamics, Open SystemsComment: 5 pages, late
Interfacial fluctuations near the critical filling transition
We propose a method to describe the short-distance behavior of an interface
fluctuating in the presence of the wedge-shaped substrate near the critical
filling transition. Two different length scales determined by the average
height of the interface at the wedge center can be identified. On one length
scale the one-dimensional approximation of Parry et al. \cite{Parry} which
allows to find the interfacial critical exponents is extracted from the full
description. On the other scale the short-distance fluctuations are analyzed by
the mean-field theory.Comment: 13 pages, 3 figure
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