204 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

### Thomas-Fermi generalized approach for studying systems under pressure

In a previous work one of the authors proposed a simple model for studying
systems under pressure based on the Thomas-Fermi (TF) model of single atom. In
this work we intend to extend the previous work to more general Thomas-Fermi
models where electronic exchange and correlation are introduced. To do so, we
first study numerically the equation obtained by H.W.Lewis (TFDL) which
introduces the effects of exchange and correlation into the original TF
equation; next the procedure followed in the previous work is extended to the
new approach and a specific example is illustrated. Although one could expect
that no big differences were produced by the generalized TF model, we show the
qualitative as well as quantitative equivalence with detailed numerical
results. These results support the robustness of our conclusions with regards
to the model proposed in the previous work and give the character of
universality (i.e. to pass from one atom to another, the quantities calculated
must be simply scaled by a numerical factor) to the properties of compressed
systems shown in this work.Comment: 16 pages and 5 figure

### 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

### 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

### 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

### Path Integral Quantization of Cosmological Perturbations

We derive the first order canonical formulation of cosmological perturbation
theory in a Universe filled by a few scalar fields. This theory is quantized
via well-defined Hamiltonian path integral. The propagator which describes the
evolution of the initial (for instance, vacuum) state, is calculated.Comment: 16 pages, ETH-TH/94-0

### Interpolating the Nucleon's Spin-Dependent Sum Rules at High and Low Energies

I discuss a few interpolating sum rules for spin structure functions of the
nucleon. Using the concept of duality, I argue that the $G_1$ sum rule,
including the elastic contribution, is useful for learning higher twist matrix
elements of the nucleon.Comment: 9 pages in Plain TeX, MIT CTP #220

### BRS and Anti-BRS Symmetry in Topological Yang--Mills Theory

We incorporate both BRS symmetry and anti-BRS symmetry into the quantisation
of topological Yang--Mills theory. This refines previous treatments which
consider only the BRS symmetry. Our formalism brings out very clearly the
geometrical meaning of topological Yang--Mills theory in terms of connections
and curvatures in an enlarged superspace; and its simple relationship to the
geometry of ordinary Yang--Mills theory. We also discover a certain SU(3)
triality between physical spacetime, and the two ghost directions of
superspace. Finally, we demonstrate how to recover the usual gauge-fixed
topological Yang--Mills action from our formalism.Comment: 17 pages, harvmac, DAMTP R92/3

### 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 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.

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