5,959 research outputs found
Timing and Spectral Properties of X-ray Emission from the Converging Flows onto Black hole: Monte-Carlo Simulations
We demonstrate that a X-ray spectrum of a converging inflow (CI) onto a black
hole is the sum of a thermal (disk) component and the convolution of some
fraction of this component with the Comptonization spread (Green's) function.
The latter component is seen as an extended power law at energies much higher
than the characteristic energy of the soft photons. We show that the high
energy photon production (source function) in the CI atmosphere is distributed
with the characteristic maximum at about the photon bending radius, 1.5r_S,
independently of the seed (soft) photon distribution. We show that high
frequency oscillations of the soft photon source in this region lead to the
oscillations of the high energy part of the spectrum but not of the thermal
component. The high frequency oscillations of the inner region are not
significant in the thermal component of the spectrum. We further demonstrate
that Doppler and recoil effects (which are responsible for the formation of the
CI spectrum) are related to the hard (positive) and soft (negative) time lags
between the soft and hard photon energy channels respectively.Comment: 9 pages and 4 figures, to be published in the Astrophysical Journal
Letter
The Cluster Processor: New Results
We present a progress report on the Cluster Processor, a special-purpose
computer system for the Wolff simulation of the three-dimensional Ising model,
including an analysis of simulation results obtained thus far. These results
allow, within narrow error margins, a determination of the parameters
describing the phase transition of the simple-cubic Ising model and its
universality class. For an improved determination of the correction-to-scaling
exponent, we include Monte Carlo data for systems with nearest-neighbor and
third-neighbor interactions in the analysis.Comment: 14 pages, latex2
Deviation from one-dimensionality in stationary properties and collisional dynamics of matter-wave solitons
By means of analytical and numerical methods, we study how the residual
three-dimensionality affects dynamics of solitons in an attractive
Bose-Einstein condensate loaded into a cigar-shaped trap. Based on an effective
1D Gross-Pitaevskii equation that includes an additional quintic self-focusing
term, generated by the tight transverse confinement, we find a family of exact
one-soliton solutions and demonstrate stability of the entire family, despite
the possibility of collapse in the 1D equation with the quintic self-focusing
nonlinearity. Simulating collisions between two solitons in the same setting,
we find a critical velocity, , below which merger of identical in-phase
solitons is observed. Dependence of on the strength of the transverse
confinement and number of atoms in the solitons is predicted by means of the
perturbation theory and investigated in direct simulations. Symmetry breaking
in collisions of identical solitons with a nonzero phase difference is also
shown in simulations and qualitatively explained by means of an analytical
approximation.Comment: 10 pages, 7 figure
Measuring spectrum of spin wave using vortex dynamics
We propose to measure the spectrum of magnetic excitation in magnetic
materials using motion of vortex lattice driven by both ac and dc current in
superconductors. When the motion of vortex lattice is resonant with oscillation
of magnetic moments, the voltage decreases at a given current. From transport
measurement, one can obtain frequency of the magnetic excitation with the wave
number determined by vortex lattice constant. By changing the lattice constant
through applied magnetic fields, one can obtains the spectrum of the magnetic
excitation up to a wave vector of order .Comment: 4 pages, 2 figure
Dynamic polarizabilities and magic wavelengths for dysprosium
We theoretically study dynamic scalar polarizabilities of the ground and
select long-lived excited states of dysprosium, a highly magnetic atom recently
laser cooled and trapped. We demonstrate that there are a set of magic
wavelengths of the unpoarized lattice laser field for each pair of states which
includes the ground state and one of these excited states. At these
wavelengths, the energy shift due to laser field is the same for both states,
which can be useful for resolved sideband cooling on narrow transitions and
precision spectroscopy. We present an analytical formula which, near
resonances, allows for the determination of approximate values of the magic
wavelengths without calculating the dynamic polarizabilities of the excited
states.Comment: 6 papers, 3 figure
Frustration and glassiness in spin models with cavity-mediated interactions
We show that the effective spin-spin interaction between three-level atoms
confined in a multimode optical cavity is long-ranged and sign-changing, like
the RKKY interaction; therefore, ensembles of such atoms subject to frozen-in
positional randomness can realize spin systems having disordered and frustrated
interactions. We argue that, whenever the atoms couple to sufficiently many
cavity modes, the cavity-mediated interactions give rise to a spin glass. In
addition, we show that the quantum dynamics of cavity-confined spin systems is
that of a Bose-Hubbard model with strongly disordered hopping but no on-site
disorder; this model exhibits a random-singlet glass phase, absent in
conventional optical-lattice realizations. We briefly discuss experimental
signatures of the realizable phases.Comment: 5 pages, 2 figure
Frequency-Domain Coherent Control of Femtosecond Two-Photon Absorption: Intermediate-Field vs. Weak-Field Regime
Coherent control of femtosecond two-photon absorption in the
intermediate-field regime is analyzed in detail in the powerful frequency
domain using an extended 4th-order perturbative description. The corresponding
absorption is coherently induced by the weak-field non-resonant two-photon
transitions as well as by four-photon transitions involving three absorbed
photons and one emitted photons. The interferences between these two groups of
transitions lead to a difference between the intermediate-field and weak-field
absorption dynamics. The corresponding interference nature (constructive or
destructive) strongly depends on the detuning direction of the pulse spectrum
from half the two-photon transition frequency. The model system of the study is
atomic sodium, for which both experimental and theoretical results are
obtained. The detailed understanding obtained here serves as a basis for
coherent control with rationally-shaped femtosecond pulses in a regime of
sizable absorption yields.Comment: 25 pages, 5 figure
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