1,086 research outputs found

### Nonadiabatic quantum pumping in mesoscopic nanostructures

We consider a nonadiabatic quantum pumping phenomena in a ballistic narrow
constriction. The pumping is induced by a potential that has both spatial and
temporal periodicity characterized by $K$ and $\Omega$. In the zero frequency
($\Omega=0$) limit, the transmission through narrow constriction exhibits
valley structures due to the opening up of energy gaps in the pumping region --
a consequence of the $K$ periodicity. These valley structures remain robust in
the regime of finite $\Omega$, while their energies of occurrence are shifted
by about $\hbar\Omega/2$. The direction of these energy shifts depend on the
directions of both the phase-velocity of the pumping potential and the
transmitting electrons. This frequency dependent feature of the valley
structures gives rise to both the asymmetry in the transmission coefficients
and the pumping current. An experimental setup is suggested for a possible
observation of our nonadiabatic quantum pumping findings.Comment: 4 pages, 2 figure

### Differential conductance of a saddle-point constriction with a time-modulated gate-voltage

The effect of a time-modulated gate-voltage on the differential conductance
$G$ of a saddle-point constriction is studied. The constriction is modeled by a
symmetric saddle-point potential and the time-modulated gate-voltage is
represented by a potential of the form $V_{0} \theta(a/2-|x-x_{c}|) \cos
(\omega t)$. For $\hbar\omega$ less than half of the transverse subband energy
level spacing, gate-voltage-assisted (suppressed) feature occurs when the
chemical potential $\mu$ is less (greater) than but close to the threshold
energy of a subband. As $\mu$ increases, $G$ is found to exhibit,
alternatively, the assisted and the suppressed feature. For larger
$\hbar\omega$, these two features may overlap with one another. Dip structures
are found in the suppressed regime. Mini-steps are found in the assisted regime
only when the gate-voltage covers region far enough away from the center of the
constriction.Comment: 8 pages, 6 figure

### Finite Mass Effect on Two Photon Processes in Hydrogenic Systems: Effective Scalar Photon Interaction

We consider a hydrogenic system with a nucleus of finite mass. The coupling
of the radiation field to the center of mass motion gives rise to an effective
scalar type coupling. This induced scalar photon interaction emerges as a
correction in competition with the usual multipole interactions. This effect is
particularly important in positronium where the electric quadrupole interaction
is totally suppressed. We illustrate this effect with the two-photon decay of
metastable hydrogenic systems.Comment: 12 pages, Latex, no figure. Version to appear in Phys. Lett.

### Strings and D-Branes with Boundaries

The covariant field equations of ten-dimensional super D-branes are obtained
by considering fundamental strings whose ends lie in the superworldsurface of
the D-brane. By considering in a similar fashion Dp-branes ending on
D(p+2)-branes we derive equations describing D-branes with dual potentials, as
well as the vector potentials.Comment: 12 pages, Late

### Coherent quantum transport in the presence of a finite-range transversely polarized time-dependent field

This work investigates the quantum transport in a narrow constriction acted
upon by a finite-range transversely polarized time-dependent electric field. A
generalized scattering-matrix method is developed that has incorporated a
time-dependent mode-matching scheme. The transverse field induces coherent
inelastic scatterings that include both intersubband and intersideband
transitions. These scatterings give rise to the dc conductance $G$ a general
suppressed feature that escalates with the chemical potential. In addition,
particular suppressed features -- the dip structures -- are found in $G$. These
features are recognized as the quasi-bound-state (QBS) features that arise from
electrons making intersubband transitions to the vicinity of a subband bottom.
For the case of larger field intensities, the QBS features that involve more
photons are more evident. These QBS features are closely associated with the
singular density of states at the subband bottoms. An experimental setup is
proposed for the observation of these features.Comment: 8 pages, 4 figure

### Open Superbranes

Open branes ending on other branes, which may be referred to as the host
branes, are studied in the superembedding formalism. The open brane, host brane
and the target space in which they are both embedded are all taken to be
supermanifolds. It is shown that the superspace constraints satisfied by the
open brane are sufficient to determine the corresponding superspace constraints
for the host branes, whose dynamics are determined by these constraints. As a
byproduct, one also obtains information about the boundary of the open brane
propagating in the host brane.Comment: 12 pages, late

### Transport spectroscopy in a time-modulated open quantum dot

We have investigated the time-modulated coherent quantum transport phenomena
in a ballistic open quantum dot. The conductance $G$ and the electron dwell
time in the dots are calculated by a time-dependent mode-matching method. Under
high-frequency modulation, the traversing electrons are found to exhibit three
types of resonant scatterings. They are intersideband scatterings: into
quasibound states in the dots, into true bound states in the dots, and into
quasibound states just beneath the subband threshold in the leads. Dip
structures or fano structures in $G$ are their signatures. Our results show
structures due to 2$\hbar\omega$ intersideband processes. At the above
scattering resonances, we have estimated, according to our dwell time
calculation, the number of round-trip scatterings that the traversing electrons
undertake between the two dot openings.Comment: 8 pages, 5 figure

### Non-Linear Sigma Model on the Fuzzy Supersphere

In this note we develop fuzzy versions of the supersymmetric non-linear sigma
model on the supersphere S^(2,2). In hep-th/0212133 Bott projectors have been
used to obtain the fuzzy CP^1 model. Our approach utilizes the use of
supersymmetric extensions of these projectors. Here we obtain these (super)
-projectors and quantize them in a fashion similar to the one given in
hep-th/0212133. We discuss the interpretation of the resulting model as a
finite dimensional matrix model.Comment: 11 pages, LaTeX, corrected typo

### A toy model of open membrane field theory in constant 3-form flux

Based on an explicit computation of the scattering amplitude of four open
membranes in a constant 3-form background, we construct a toy model of the
field theory for open membranes in the large C field limit. It is a
generalization of the noncommutative field theories which describe open strings
in a constant 2-form flux. The noncommutativity due to the B-field background
is now replaced by a nonassociative triplet product. The triplet product
satisfies the consistency conditions of lattice 3d gravity, which is inherent
in the world-volume theory of open membranes. We show the UV/IR mixing of the
toy model by computing some Feynman diagrams. Inclusion of the internal degree
of freedom is also possible through the idea of the cubic matrix.Comment: 31 pages, latex, 2 eps figure

### Noncommutative Scalar Field Coupled to Gravity

A model for a noncommutative scalar field coupled to gravity is proposed via
an extension of the Moyal product. It is shown that there are solutions
compatible with homogeneity and isotropy to first non-trivial order in the
perturbation of the star-product, with the gravity sector described by a flat
Robertson-Walker metric. We show that in the slow-roll regime of a typical
chaotic inflationary scenario, noncommutativity has negligible impact.Comment: Revtex4, 6 pages. Final version to appear at Phys. Rev.

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