3,965 research outputs found
Absorption in atomic wires
The transfer matrix formalism is implemented in the form of the multiple
collision technique to account for dissipative transmission processes by using
complex potentials in several models of atomic chains. The absorption term is
rigorously treated to recover unitarity for the non-hermitian hamiltonians. In
contrast to other models of parametrized scatterers we assemble explicit
potentials profiles in the form of delta arrays, Poschl-Teller holes and
complex Scarf potentials. The techniques developed provide analytical
expressions for the scattering and absorption probabilities of arbitrarily long
wires. The approach presented is suitable for modelling molecular aggregate
potentials and also supports new models of continuous disordered systems. The
results obtained also suggest the possibility of using these complex potentials
within disordered wires to study the loss of coherence in the electronic
localization regime due to phase-breaking inelastic processes.Comment: 14 pages, 15 figures. To appear in Phys. Rev.
Semirelativistic Hamiltonians and the auxiliary field method
Approximate analytical closed energy formulas for semirelativistic
Hamiltonians of the form are obtained within
the framework of the auxiliary field method. This method, which is equivalent
to the envelope theory, has been recently proposed as a powerful tool to get
approximate analytical solutions of the Schr\"odinger equation. Various shapes
for the potential are investigated: power-law, funnel, square root, and
Yukawa. A comparison with the exact results is discussed in detail
Suppression of Superfluidity upon Overflow of Trapped Fermions. Quantal and Thomas-Fermi Studies
Two issues are treated in this work: (i) the generic fact that if a fermionic
superfluid in the BCS regime overflows from a narrow container into a much
wider one, pairing is much suppressed at the overflow point. Physical examples
where this feature may play an important role are discussed (ii) A Thomas-Fermi
(TF) approach to inhomogeneous superfluid Fermi-systems is presented and shown
that it works well in cases where the Local Density Approximation (LDA) breaks
down.Comment: 4 pages, 5 figure
Origin of adiabatic and non-adiabatic spin transfer torques in current-driven magnetic domain wall motion
A consistent theory to describe the correlated dynamics of quantum mechanical
itinerant spins and semiclassical local magnetization is given. We consider the
itinerant spins as quantum mechanical operators, whereas local moments are
considered within classical Lagrangian formalism. By appropriately treating
fluctuation space spanned by basis functions, including a zero-mode wave
function, we construct coupled equations of motion for the collective
coordinate of the center-of-mass motion and the localized zero-mode coordinate
perpendicular to the domain wall plane. By solving them, we demonstrate that
the correlated dynamics is understood through a hierarchy of two time scales:
Boltzmann relaxation time when a non-adiabatic part of the spin-transfer torque
appears, and Gilbert damping time when adiabatic part comes up.Comment: 4 pages, 2 figure
Multi-Atomic Mirror for Perfect Reflection of Single Photons in A Wide Band of Frequency
A resonant two level atom doped in one dimensional waveguide behaves as a
mirror, but this single-atom "mirror" can only reflect single photon perfectly
at a specific frequency. For a one dimensional coupled-resonator waveguide, we
propose to extend the perfect reflection region from a specific frequency to a
wide band by placing many atoms individually in the resonators in a finite
coordinate region of the waveguide. Such a doped resonator array promises us to
control the propagation of a practical photon wave packet with certain momentum
distribution instead of a single photon, which is ideally represented by a
plane wave with specific momentum. The studies based on the discrete-coordinate
scattering theory display that such hybrid structure indeed provides a
near-perfect reflection for single photon in a wide band. We also calculated
photon group velocity distribution, which shows that the perfect reflection
with wide band exactly corresponds to the stopping light region.Comment: 8 pages, 10 figure
Enhancement of pairing due to the presence of resonant cavities
A correlated fermion system is considered surrounding a finite cavity with
virtual levels. The pairing properties are calculated and the influence of the
cavity is demonstrated. To this end the Gell-Mann and Goldberger formula is
generalized to many-body systems. We find a possible enhancement of pairing
temperature if the Fermi momentum times the cavity radius fulfills a certain
resonance condition which suggests an experimental realization.Comment: 4 pages 2 figure
Collective Feshbach scattering of a superfluid droplet from a mesoscopic two-component Bose-Einstein condensate
We examine the collective scattering of a superfluid droplet impinging on a
mesoscopic Bose-Einstein condensate (BEC) as a target. The BEC consists of an
atomic gas with two internal electronic states, each of which is trapped by a
finite-depth external potential. An off-resonant optical laser field provides a
localized coupling between the BEC components in the trapping region. This
mesoscopic scenario matches the microscopic setup for Feshbach scattering of
two particles, when a bound state of one sub-manifold is embedded in the
scattering continuum of the other sub-manifold. Within the mean-field picture,
we obtain resonant scattering phase shifts from a linear response theory in
agreement with an exact numerical solution of the real time scattering process
and simple analytical approximations thereof. We find an energy-dependent
transmission coefficient that is controllable via the optical field between 0
and 100%.Comment: 4 Latex pages, including 4 figure
Extensions of the auxiliary field method to solve Schr\"{o}dinger equations
It has recently been shown that the auxiliary field method is an interesting
tool to compute approximate analytical solutions of the Schr\"{o}dinger
equation. This technique can generate the spectrum associated with an arbitrary
potential starting from the analytically known spectrum of a particular
potential . In the present work, general important properties of the
auxiliary field method are proved, such as scaling laws and independence of the
results on the choice of . The method is extended in order to find
accurate analytical energy formulae for radial potentials of the form , and several explicit examples are studied. Connections existing
between the perturbation theory and the auxiliary field method are also
discussed
The Woods-Saxon Potential in the Dirac Equation
The two-component approach to the one-dimensional Dirac equation is applied
to the Woods-Saxon potential. The scattering and bound state solutions are
derived and the conditions for a transmission resonance (when the transmission
coefficient is unity) and supercriticality (when the particle bound state is at
E=-m) are then derived. The square potential limit is discussed. The recent
result that a finite-range symmetric potential barrier will have a transmission
resonance of zero-momentum when the corresponding well supports a half-bound
state at E=-m is demonstrated.Comment: 8 pages, 4 figures. Submitted to JPhys
Solutions of the Klein-Gordon equation on manifolds with variable geometry including dimensional reduction
We develop the recent proposal to use dimensional reduction from the
four-dimensional space-time D=(1+3) to the variant with a smaller number of
space dimensions D=(1+d), d < 3, at sufficiently small distances to construct a
renormalizable quantum field theory. We study the Klein-Gordon equation on a
few toy examples ("educational toys") of a space-time with variable special
geometry, including a transition to a dimensional reduction. The examples
considered contain a combination of two regions with a simple geometry
(two-dimensional cylindrical surfaces with different radii) connected by a
transition region. The new technique of transforming the study of solutions of
the Klein-Gordon problem on a space with variable geometry into solution of a
one-dimensional stationary Schr\"odinger-type equation with potential generated
by this variation is useful. We draw the following conclusions: (1) The signal
related to the degree of freedom specific to the higher-dimensional part does
not penetrate into the smaller-dimensional part because of an inertial force
inevitably arising in the transition region (this is the centrifugal force in
our models). (2) The specific spectrum of scalar excitations resembles the
spectrum of the real particles; it reflects the geometry of the transition
region and represents its "fingerprints". (3) The parity violation due to the
asymmetric character of the construction of our models could be related to
violation of the CP symmetry.Comment: laTeX file, 9 pages, 8 figures. Significant corrections in the title,
abstract, text. Corrected formulas and figures. Added new references,
amendments in English. Acceptred for publication in Theoretical and
Mathematical Physics. To appear in vol. 167, may 201
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