2,436 research outputs found
Instantaneous modulations in time-varying complex optical potentials
We study the impact of a spatially homogeneous yet non-stationary dielectric permittivity on the dynamical and spectral properties of light. Our choice of potential is motivated by the interest in -symmetric systems as an extension of quantum mechanics. Because we consider a homogeneous and non-stationary medium, symmetry reduces to time-reversal symmetry in the presence of balanced gain and loss. We construct the instantaneous amplitude and angular frequency of waves within the framework of Maxwell's equations and demonstrate the modulation of light amplification and attenuation associated with the well-defined temporal domains of gain and loss, respectively. Moreover, we predict the splitting of extrema of the angular frequency modulation and demonstrate the associated shrinkage of the modulation period. Our theory can be extended for investigating similar time-dependent effects with matter and acoustic waves in -symmetric structures
Nonuniform currents and spins of relativistic electron vortices in a magnetic field
We present a relativistic description of electron vortex beams in a
homogeneous magnetic field. Including spin from the beginning reveals that
spin-polarized electron vortex beams have a complicated azimuthal current
structure, containing small rings of counterrotating current between rings of
stronger corotating current. Contrary to many other problems in relativistic
quantum mechanics, there exists a set of vortex beams with exactly zero
spin-orbit mixing in the highly relativistic and nonparaxial regime. The well
defined phase structure of these beams is analogous to simpler scalar vortex
beams, owing to the protection by the Zeeman effect. For states that do show
spin-orbit mixing, the spin polarization across the beam is nonuniform
rendering the spin and orbital degrees of freedom inherently inseparable.Comment: 5 pages + supplemental materia
Instantaneous modulations in time-varying complex optical potentials
We study the impact of a spatially homogeneous yet non-stationary dielectric
permittivity on the dynamical and spectral properties of light. Our choice of
potential is motivated by the interest in PT-symmetric systems as an extension
of quantum mechanics. Because we consider a homogeneous and non-stationary
medium, PT symmetry reduces to time-reversal symmetry in the presence of
balanced gain and loss. We construct the instantaneous amplitude and angular
frequency of waves within the framework of Maxwell's equations and demonstrate
the modulation of light amplification and attenuation associated with the
well-defined temporal domains of gain and loss, respectively. Moreover, we
predict the splitting of extrema of the angular frequency modulation and
demonstrate the associated shrinkage of the modulation period. Our theory can
be extended for investigating similar time-dependent effects with matter and
acoustic waves in PT-symmetric structures.Comment: 13 pages, 4 figure
Electromagnetic wave propagation in spatially homogeneous yet smoothly time-varying dielectric media
We explore the propagation and transformation of electromagnetic waves
through spatially homogeneous yet smoothly time-dependent media within the
framework of classical electrodynamics. By modelling the smooth transition,
occurring during a finite period {\tau}, as a phenomenologically realistic and
sigmoidal change of the dielectric permittivity, an analytically exact solution
to Maxwell's equations is derived for the electric displacement in terms of
hypergeometric functions. Using this solution, we show the possibility of
amplification and attenuation of waves and associate this with the decrease and
increase of the time-dependent permittivity. We demonstrate, moreover, that
such an energy exchange between waves and non-stationary media leads to the
transformation (or conversion) of frequencies. Our results may pave the way
towards controllable light-matter interaction in time-varying structures.Comment: 5 figure
Color-magnetic flux tubes in quark matter cores of neutron stars
We argue that if color-superconducting quark matter exists in the core of a
neutron star, it may contain a high density of flux tubes, carrying flux that
is mostly color-magnetic, with a small admixture of ordinary magnetic flux. We
focus on the two-flavor color-superconducting ("2SC") phase, and assume that
the flux tubes are energetically stable, although this has not yet been
demonstrated. The density of flux tubes depends on the nature of the transition
to the color-superconducting phase, and could be within an order of magnitude
of the density of magnetic flux tubes that would be found if the core were
superconducting nuclear matter. We calculate the cross-section for
Aharonov-Bohm scattering of gapless fermions off the flux tubes, and the
associated collision time and frictional force on a moving flux tube. We
discuss the other forces on the flux tube, and find that if we take in to
account only the forces that arise within the 2SC core region then the
timescale for expulsion of the color flux tubes from the 2SC core is of order
10^10 years.Comment: 28 pages, LaTeX, 1 figure, 2 appendices; added discussion of
energetic stability of flux tube
Two-Center Black Holes Duality-Invariants for stu Model and its lower-rank Descendants
We classify 2-center extremal black hole charge configurations through
duality-invariant homogeneous polynomials, which are the generalization of the
unique invariant quartic polynomial for single-center black holes based on
homogeneous symmetric cubic special Kaehler geometries. A crucial role is
played by an horizontal SL(p,R) symmetry group, which classifies invariants for
p-center black holes. For p = 2, a (spin 2) quintet of quartic invariants
emerge. We provide the minimal set of independent invariants for the rank-3 N =
2, d = 4 stu model, and for its lower-rank descendants, namely the rank-2 st^2
and rank-1 t^3 models; these models respectively exhibit seven, six and five
independent invariants. We also derive the polynomial relations among these and
other duality invariants. In particular, the symplectic product of two charge
vectors is not independent from the quartic quintet in the t^3 model, but
rather it satisfies a degree-16 relation, corresponding to a quartic equation
for the square of the symplectic product itself.Comment: 1+31 pages; v2: amendments in Sec. 9, App. C added, other minor
refinements, Refs. added; v3: Ref. added, typos fixed. To appear on
J.Math.Phy
Optimal refrigerator
We study a refrigerator model which consists of two -level systems
interacting via a pulsed external field. Each system couples to its own thermal
bath at temperatures and , respectively ().
The refrigerator functions in two steps: thermally isolated interaction between
the systems driven by the external field and isothermal relaxation back to
equilibrium. There is a complementarity between the power of heat transfer from
the cold bath and the efficiency: the latter nullifies when the former is
maximized and {\it vice versa}. A reasonable compromise is achieved by
optimizing the product of the heat-power and efficiency over the Hamiltonian of
the two system. The efficiency is then found to be bounded from below by
(an analogue of the Curzon-Ahlborn
efficiency), besides being bound from above by the Carnot efficiency
. The lower bound is reached in the
equilibrium limit . The Carnot bound is reached (for a finite
power and a finite amount of heat transferred per cycle) for . If
the above maximization is constrained by assuming homogeneous energy spectra
for both systems, the efficiency is bounded from above by and
converges to it for .Comment: 12 pages, 3 figure
Off-shell pairing correlations from meson-exchange theory of nuclear forces
We develop a model of off-mass-shell pairing correlations in nuclear systems,
which is based on the meson-exchange picture of nuclear interactions. The
temporal retardations in the model are generated by the Fock-exchange diagrams.
The kernel of the complex gap equation for baryons is related to the in-medium
spectral function of mesons, which is evaluated nonperturbatively in the random
phase approximation. The model is applied to the low-density neutron matter in
neutron star crusts by separating the interaction into a long-range
one-pion-exchange component and a short-range component parametrized in terms
of Landau Fermi liquid parameters. The resulting Eliashberg-type coupled
nonlinear integral equations are solved by an iterative procedure.We find that
the self-energies extend to off-shell energies of the order of several tens of
MeV. At low energies the damping of the neutron pair correlations due to the
coupling to the pionic modes is small, but becomes increasingly important as
the energy is increased. We discuss an improved quasiclassical approximation
under which the numerical solutions are obtained.Comment: 15 pages, 7 figures, uses RevTeX 4; v2: substantially expanded
version to appear in PR
Work extremum principle: Structure and function of quantum heat engines
We consider a class of quantum heat engines consisting of two subsystems
interacting via a unitary transformation and coupled to two separate baths at
different temperatures . The purpose of the engine is to extract
work due to the temperature difference. Its dynamics is not restricted to the
near equilibrium regime. The engine structure is determined by maximizing the
extracted work under various constraints. When this maximization is carried out
at finite power, the engine dynamics is described by well-defined temperatures
and satisfies the local version of the second law. In addition, its efficiency
is bounded from below by the Curzon-Ahlborn value and from
above by the Carnot value . The latter is reached|at finite
power|for a macroscopic engine, while the former is achieved in the equilibrium
limit . When the work is maximized at a zero power, even a small
(few-level) engine extracts work right at the Carnot efficiency.Comment: 16 pages, 5 figure
Multi-Centered Black Hole Flows
We describe the systematical construction of the first order formalism for
multi-centered black holes with flat three dimensional base-space, within the
so-called model of N=2, D=4 ungauged Maxwell-Einstein supergravity. The
three possible flow classes (BPS, composite non-BPS and almost-BPS) are
analyzed in detail, and various solutions, such as single-centered (static or
under-rotating) and all known multi-centered black holes, are recovered in this
unified framework. We also consider the possibility of obtaining new solutions.
The almost-BPS class is proved to split into two general sub-classes,
corresponding to a positive or negative value of the duality-invariant
polynomial for the total charge; the well known almost BPS system is shown to
be a particular solution of the second sub-class.Comment: 17 pages,no figure
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