396 research outputs found
The X-ray edge singularity in Quantum Dots
In this work we investigate the X-ray edge singularity problem realized in
noninteracting quantum dots. We analytically calculate the exponent of the
singularity in the absorption spectrum near the threshold and extend known
analytical results to the whole parameter regime of local level detunings.
Additionally, we highlight the connections to work distributions and to the
Loschmidt echo.Comment: 7 pages, 2 figures; version as publishe
Birefringence and Dichroism of the QED Vacuum
We use an analytic form for the Heisenberg-Euler Lagrangian to calculate the
birefringent and dichroic properties of the vacuum for arbitrarily strong
wrenchless fields.
PACS : 12.20.Ds, 42.25.Lc 97.60.Jd, 98.70.RzComment: 8 pages, 2 figures, to appear in Journal of Physics
Real-time dynamics of string breaking in quantum spin chains
String breaking is a central dynamical process in theories featuring
confinement, where a string connecting two charges decays at the expense of the
creation of new particle-antiparticle pairs. Here, we show that this process
can also be observed in quantum Ising chains where domain walls get confined
either by a symmetry-breaking field or by long-range interactions. We find that
string breaking occurs, in general, as a two-stage process: First, the initial
charges remain essentially static and stable. The connecting string, however,
can become a dynamical object. We develop an effective description of this
motion, which we find is strongly constrained. In the second stage, which can
be severely delayed due to these dynamical constraints, the string finally
breaks. We observe that the associated time scale can depend crucially on the
initial separation between domain walls and can grow by orders of magnitude by
changing the distance by just a few lattice sites. We discuss how our results
generalize to one-dimensional confining gauge theories and how they can be made
accessible in quantum simulator experiments such as Rydberg atoms or trapped
ions.Comment: 16 pages, 8 figures; version published in Physical Review
Electron-Positron Jets from a Critically Magnetized Black Hole
The curved spacetime surrounding a rotating black hole dramatically alters
the structure of nearby electromagnetic fields. The Wald field which is an
asymptotically uniform magnetic field aligned with the angular momentum of the
hole provides a convenient starting point to analyze the effects of radiative
corrections on electrodynamics in curved spacetime. Since the curvature of the
spacetime is small on the scale of the electron's Compton wavelength, the tools
of quantum field theory in flat spacetime are reliable and show that a rotating
black hole immersed in a magnetic field approaching the quantum critical value
of ~G cm is unstable. Specifically, a maximally rotating
three-solar-mass black hole immersed in a magnetic field of ~G would be a copious producer of electron-positron pairs with a
luminosity of erg s.Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Powering Anomalous X-ray Pulsars by Neutron Star Cooling
Using recently calculated analytic models for the thermal structure of
ultramagnetized neutron stars, we estimate the thermal fluxes from young
( yr) ultramagnetized ( G) cooling neutron stars.
We find that the pulsed X-ray emission from objects such as 1E 1841-045 and 1E
2259+586 as well as many soft-gamma repeaters can be explained by photon
cooling if the neutron star possesses a thin insulating envelope of matter of
low atomic weight at densities g/cm. The total mass
of this insulating layer is .Comment: 8 pages, 1 figure, to appear in Ap.J. Letters (one reference entry
corrected, no other changes
Quantifying and Controlling Prethermal Nonergodicity in Interacting Floquet Matter
The use of periodic driving for synthesizing many-body quantum states depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of heating. This dependence motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. Using a double-quench protocol, we measure an inverse participation ratio quantifying the degree of prethermal localization as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering
Anomalous X-ray Pulsars and Soft gamma-ray Repeaters: Spectral Fits and the Magnetar Model
The energy source powering the X-ray emission from anomalous X-ray pulsars
(AXPs) and soft gamma-ray repeaters (SGRs) is still uncertain. In one scenario,
the presence of an ultramagnetized neutron star, or ``magnetar'', with B on the
order of 10^{14} - 10^{15} G is invoked. To investigate this hypothesis, we
have analyzed archival ASCA data for several known AXPs and SGRs, and fitted
them with a model in which all or part of the X-ray flux originates as thermal
emission from a magnetar. Our magnetar spectral model includes the effects of
the anisotropy of the heat flow through an ultramagnetized neutron star
envelope, reprocessing by a light element atmosphere, and general relativistic
corrections to the observed spectrum. We obtain good fits to the data with
radii for the emitting areas which are generally consistent with those expected
for neutron stars, in contrast to blackbody (BB) fits, which imply much smaller
radii. Furthermore, the inclusion of atmospheric effects results in inferred
temperatures which are lower than those implied by BB fits, but however still
too high to be accounted by thermal cooling alone. An extra source of heating
(possibly due to magnetic field decay) is needed. Despite the harder tail in
the spectrum produced by reprocessing of the outgoing flux through the
atmosphere, spectral fits still require a considerable fraction of the flux to
be in a power-law component.Comment: 14 pages, 2 tables, 1 figure, ApJ in press; note added to Table
Characterizing Entanglement Entropy Produced by Non-Linear Scalar Interactions During Inflation
The density fluctuations that we observe in the universe today are thought to
originate from quantum fluctuations produced during a phase of the early
universe called inflation. By evolving a wavefunction describing two coupled
Fourier modes of a scalar field forward through an inflationary epoch, we
demonstrate that non-linear effects can result in a generation of entanglement
entropy between modes with different momenta in a scalar field during the
inflationary period when just one of the modes is observed. Through this
mechanism, the field would experience decoherence and appear more like a
classical distribution today; however the mechanism is not sufficiently
efficient to explain classicality. We find that the amount of entanglement
entropy generated scales roughly as a power law S \propto \lambda^{1.75}, where
\lambda is the coupling coefficient of the non-linear potential term. We also
investigate how the entanglement entropy scales with the duration of inflation
and compare various entanglement measures from the literature with the von
Neumann entropy. This demonstration explicitly follows particle creation and
interactions between modes; consequently, the mechanism contributing to the
generation of the von Neumann entropy can be easily seen.Comment: 11 pages, 9 figures, version accepted by Phys. Rev.
Strange Star Heating Events as a Model for Giant Flares of Soft Gamma-ray Repeaters
Two giant flares were observed on 5 March 1979 and 27 August 1998 from the
soft gamma-ray repeaters SGR 0526-66 and SGR 1900+14, respectively. The
striking similarity between these remarkable bursts strongly implies a common
nature. We show that the light curves of the giant bursts may be easily
explained in the model where the burst radiation is produced by the bare quark
surface of a strange star heated, for example, by impact of a massive
comet-like object.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Letter
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