187 research outputs found
Nonequilibrium and nonperturbative dynamics of ultrastrong coupling in open lines
The time and space resolved dynamics of a qubit with an Ohmic coupling to propagating 1D photons is studied, from weak coupling to the ultrastrong coupling regime. A nonperturbative study based on matrix product states shows the following results, (i) The ground state of the combined systems contains excitations of both the qubit and the surrounding bosonic field. (ii) An initially excited qubit equilibrates through spontaneous emission to a state, which under certain conditions is locally close to that ground state, both in the qubit and the field. (iii) The resonances of the combined qubit-photon system match those of the spontaneous emission process and also the predictions of the adiabatic renormalization [A. J. Leggett et al., Rev. Mod. Phys. 59, 1 (1987)]. Finally, nonperturbative ab initio calculations show that this physics can be studied using a flux qubit galvanically coupled to a superconducting transmission line
A comprehensive study of shower to shower fluctuations
By means of Monte Carlo simulations of extensive air showers (EAS), we have
performed a comprehensive study of the shower to shower fluctuations affecting
the longitudinal and lateral development of EAS. We split the fluctuations into
physical fluctuations and those induced by the thinning procedure customarily
applied to simulate showers at EeV energies and above. We study the influence
of thinning on the calculation of the shower to shower fluctuations in the
simulations. For thinning levels larger than 10^(-5) - 10^(-6), the
determination of the shower to shower fluctuations is hampered by the
artificial fluctuations induced by the thinning procedure. However, we show
that shower to shower fluctuations can still be approximately estimated, and we
provide expressions to calculate them. The influence of fluctuations of the
depth of first interaction on the determination of shower to shower
fluctuations is also addressed.Comment: 17 pages, 15 figure
A finite element method with mesh adaptivity for computing vortex states in fast-rotating Bose-Einstein condensates
Numerical computations of stationary states of fast-rotating Bose-Einstein
condensates require high spatial resolution due to the presence of a large
number of quantized vortices. In this paper we propose a low-order finite
element method with mesh adaptivity by metric control, as an alternative
approach to the commonly used high order (finite difference or spectral)
approximation methods. The mesh adaptivity is used with two different numerical
algorithms to compute stationary vortex states: an imaginary time propagation
method and a Sobolev gradient descent method. We first address the basic issue
of the choice of the variable used to compute new metrics for the mesh
adaptivity and show that simultaneously refinement using the real and imaginary
part of the solution is successful. Mesh refinement using only the modulus of
the solution as adaptivity variable fails for complicated test cases. Then we
suggest an optimized algorithm for adapting the mesh during the evolution of
the solution towards the equilibrium state. Considerable computational time
saving is obtained compared to uniform mesh computations. The new method is
applied to compute difficult cases relevant for physical experiments (large
nonlinear interaction constant and high rotation rates).Comment: to appear in J. Computational Physic
Morphometric Comparison of Blue Catfish Ictalurus furcatus (Lesueur, 1840) from Northern and Southern Atlantic Drainages of Me´xico
A morphometric comparison was performed on specimens of blue catfish (Ictalurus furcatus) from northern (Lower Rı´o Bravo) and southern (Chiapas) Me´xico in order to identify diagnostic characters that allow their discrimination. The discriminant function analysis determined three characters to be highly diagnostic to separate the two groups of specimens: the southern group [SG] has a shorter anal base (mean 5 3.4 times in standard length [SL], range 5 3.1 to 3.7) vs northern group [NG] (mean 5 2.9 times in SL, range 5 2.7 to 3.1), a lesser head width (mean 5 6.0 times in SL, range 5.1 to 6.8) vs NG (mean 5 5.6 times, range 5 5.3 to 6.0), and a lower number of anal rays (mean 5 26, range 5 24 to 28) vs NG (mean 5 31, range 5 29 to 34). Additionally, 14 other characters were also different (P , 0.01) between both groups. All these characters support the taxonomic validation of Ictalurus meridionalis (Gu¨ nther 1864) for the individuals of SG that are currently included in I. furcatus Lesueur. Studies on comparative osteology and molecular genetics of both forms are needed for the clarification of their taxonomic status
Improved Gauge Actions on Anisotropic Lattices I
On anisotropic lattices with the anisotropy the
following basic parameters are calculated by perturbative method: (1) the
renormalization of the gauge coupling in spatial and temporal directions,
and , (2) the parameter, (3) the ratio of the
renormalized and bare anisotropy and (4) the derivatives of
the coupling constants with respect to , and . We employ the improved gauge
actions which consist of plaquette and six-link rectangular loops, . This class of actions
covers Symanzik, Iwasaki and DBW2 actions. The ratio shows an impressive
behavior as a function of , i.e., for the standard Wilson and
Symanzik actions, while for Iwasaki and DBW2 actions. This is
confirmed non-perturbatively by numerical simulations in weak coupling regions.
The derivatives and also changes sign as increases. For
Iwasaki and DBW2 actions they become opposite sign to those for standard and
Symanzik actions. However, their sum is independent of the type of actions due
to Karsch's sum rule
Renormalization of the isovector amplitude in pionic atoms
The extraction of the isovector s-wave pi N amplitude from pionic atoms is
studied with special emphasis on uncertainties and their dependence on the
assumptions made regarding the neutron density distributions in nuclei and on
the size of the data base used . Only `global' analyses of pionic-atom data
reveal a discrepancy between the extracted isovector s-wave pi N amplitude
b_1=-0.108\pm0.007 m_\pi^{-1} and its free pi N counterpart
b_1^{free}=-0.0885^{+0.0010}_{-0.0021} m_\pi ^{-1}, where the uncertainty in
the neutron densities is included in the error analysis. The role of `deeply
bound' pionic atom states is discussed and the reason for failure of these
states to provide new information is explained.Comment: 17 pages, 4 figures, slightly extended, accepted by NP
The Puzzling Stability of Monatomic Gold Wires
We have examined theoretically the spontaneous thinning process of
tip-suspended nanowires, and subsequently studied the structure and stability
of the monatomic gold wires recently observed by Transmission Electron
Microscopy (TEM). The methods used include thermodynamics, classical many-body
force simulations, Local Density (LDA) and Generalized Gradient (GGA)
electronic structure calculations as well as ab-initio simulations including
the two tips. The wire thinning is well explained in terms of a thermodynamic
tip suction driving migration of surface atoms from the wire to the tips. For
the same reason the monatomic wire becomes progressively stretched.
Surprisingly, however, all calculations so far indicate that the stretched
monatomic gold wire should be unstable against breaking, contrary to the
apparent experimental stability. The possible reasons for the observed
stability are discussed.Comment: 4 figure
Ion size effects on the electrokinetics of salt-free concentrated suspensions in ac fields
We analyze the influence of finite ion size effects in the response of a
salt-free concentrated suspension of spherical particles to an oscillating
electric field. Salt-free suspensions are just composed of charged colloidal
particles and the added counterions released by the particles to the solution,
that counterbalance their surface charge. In the frequency domain, we study the
dynamic electrophoretic mobility of the particles and the dielectric response
of the suspension. We find that the Maxwell-Wagner-O'Konski process associated
with the counterions condensation layer, is enhanced for moderate to high
particle charges, yielding an increment of the mobility for such frequencies.
We also find that the increment of the mobility grows with ion size and
particle charge. All these facts show the importance of including ion size
effects in any extension attempting to improve standard electrokinetic models.Comment: J. Colloid Interface Sci., in press, 13 pages, 9 figure
A travelling wave model of ripple formation on ion bombarded surfaces
NOTICE: this is the author’s version of a work that was accepted for publication in Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Vol 303, May 2013, DOI:10.1016/j.nimb.2012.11.031We present a mathematical model describing surface modification resulting from atomic motion after ion bombardment. The model considers only the defect production and recovery process induced by the local atom rearrangement and is essentially independent of surface topography changes formed by both sputtering and surface diffusion. A stable analytic, travelling wave solution is presented for a specific incident angle, which agrees with experimental observation excellently. © 2013 Elsevier B.V. All rights reserved
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