427 research outputs found
Universal asymptotic behavior in flow equations of dissipative systems
Based on two dissipative models, universal asymptotic behavior of flow
equations for Hamiltonians is found and discussed. Universal asymptotic
behavior only depends on fundamental bath properties but not on initial system
parameters, and the integro-differential equations possess an universal
attractor. The asymptotic flow of the Hamiltonian can be characterized by a
non-local differential equation which only depends on one parameter -
independent of the dissipative system or truncation scheme. Since the fixed
point Hamiltonian is trivial, the physical information is completely
transferred to the transformation of the observables. This yields a more stable
flow which is crucial for the numerical evaluation of correlation functions.
Furthermore, the low energy behavior of correlation functions is determined
analytically. The presented procedure can also be applied if relevant
perturbations are present as is demonstrated by evaluating dynamical
correlation functions for sub-Ohmic environments. It can further be generalized
to other dissipative systems.Comment: 15 pages, 9 figures; to appear in Phys. Rev.
Parallaxes and Infrared Photometry of three Y0 dwarfs
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We have followed up the three Y0 dwarfs WISEPA J041022.71+150248.5, WISEPA J173835.53+273258.9 and WISEPC J205628.90+145953.3 using the UKIRT/WFCAM telescope/instruments. We find parallaxes that are more consistent and accurate than previously published values. We estimate absolute magnitudes in photometric pass-bands from to and find them to be consistent between the three Y0 dwarfs indicating the inherent cosmic absolute magnitude spread of these objects is small. We examine the MKO magnitudes over the four year time line and find small but significant monotonic variations. Finally we estimate physical parameters from a comparison of spectra and parallax to equilibrium and non-equilibrium models finding values consistent with solar metallicity, an effective temperature of 450-475\,K and log~g of 4.0-4.5.Peer reviewe
2D superconductivity with strong spin-orbit interaction
We consider superconductivity confined at a two-dimensional interface with a
strong surface spin-orbit (Rashba) interaction. Some peculiar properties of
this system are investigated. In particular, we show that an in-plane Zeeman
field can induce a supercurrent flow.Comment: latex, 1 figure in ep
Bose-Einstein condensate collapse: a comparison between theory and experiment
We solve the Gross-Pitaevskii equation numerically for the collapse induced
by a switch from positive to negative scattering lengths. We compare our
results with experiments performed at JILA with Bose-Einstein condensates of
Rb-85, in which the scattering length was controlled using a Feshbach
resonance. Building on previous theoretical work we identify quantitative
differences between the predictions of mean-field theory and the results of the
experiments. Besides the previously reported difference between the predicted
and observed critical atom number for collapse, we also find that the predicted
collapse times systematically exceed those observed experimentally. Quantum
field effects, such as fragmentation, that might account for these
discrepancies are discussed.Comment: 4 pages, 2 figure
Real-Time-RG Analysis of the Dynamics of the Spin-Boson Model
Using a real-time renormalization group method we determine the complete
dynamics of the spin-boson model with ohmic dissipation for coupling strengths
. We calculate the relaxation and dephasing time, the
static susceptibility and correlation functions. Our results are consistent
with quantum Monte Carlo simulations and the Shiba relation. We present for the
first time reliable results for finite cutoff and finite bias in a regime where
perturbation theory in or in tunneling breaks down. Furthermore, an
unambigious comparism to results from the Kondo model is achieved.Comment: 4 pages, 5 figures, 1 tabl
Mean-field description of collapsing and exploding Bose-Einstein condensates
We perform numerical simulation based on the time-dependent mean-field
Gross-Pitaevskii equation to understand some aspects of a recent experiment by
Donley et al. on the dynamics of collapsing and exploding Bose-Einstein
condensates of Rb atoms. They manipulated the atomic interaction by an
external magnetic field via a Feshbach resonance, thus changing the repulsive
condensate into an attractive one and vice versa. In the actual experiment they
changed suddenly the scattering length of atomic interaction from positive to a
large negative value on a pre-formed condensate in an axially symmetric trap.
Consequently, the condensate collapses and ejects atoms via explosion. We find
that the present mean-field analysis can explain some aspects of the dynamics
of the collapsing and exploding Bose-Einstein condensates.Comment: 9 Latex pages, 10 ps and eps files, version accepted in Physical
Review A, minor changes mad
Cooper pair dispersion relation for weak to strong coupling
Cooper pairing in two dimensions is analyzed with a set of renormalized
equations to determine its binding energy for any fermion number density and
all coupling assuming a generic pairwise residual interfermion interaction. \
Also considered are Cooper pairs (CPs) with nonzero center-of-mass momentum
(CMM)--usually neglected in BCS theory--and their binding energy is expanded
analytically in powers of the CMM up to quadratic terms. A Fermi-sea-dependent
{\it linear} term in the CMM dominates the pair excitation energy in weak
coupling (also called the BCS regime) while the more familiar quadratic term
prevails in strong coupling (the Bose regime). The crossover, though strictly
unrelated to BCS theory {\it per se,} is studied numerically as it is expected
to play a central role in a model of superconductivity as a Bose-Einstein
condensation of CPs where the transition temperature vanishes for all
dimensionality for quadratic dispersion, but is {\it nonzero} for all
for linear dispersion.Comment: 11 pages plus 3 figures, revised version accepted in Physical Review
Methane and ammonia in the near-infrared spectra of late-T dwarfs
Analysis of T dwarfs using model atmospheres has been hampered by the absence of reliable line lists for methane and ammonia. Newly computed high-temperature line lists for both of these important molecules are now available, so it is timely to investigate the appearance of the various absorption features in T dwarfs in order to better understand their atmospheres and validate the new line lists. We present high-quality R ∼ 5000 Gemini/NIFS 1.0–2.4 μm spectra of the T8 standard 2MASS 0415−0935 and the T9 standard UGPS 0722−0540. We use these spectra to identify numerous methane and ammonia features not previously seen and we discuss the implications for our understanding of T dwarf atmospheres. Among our results, we find that ammonia is the dominant opacity source between ∼1.233–1.266 μm in UGPS 0722−0540, and we tentatively identify several absorption features in this wavelength range in the T9's spectrum which may be due entirely to ammonia opacity. Our results also suggest that water rather than methane is the dominant opacity source in the red half of the J band of the T8 dwarf. Water appears to be the main absorber in this wavelength region in the T9 dwarf until ∼1.31 μm, when methane starts to dominate.Peer reviewe
Dynamical properties of the unitary Fermi gas: collective modes and shock waves
We discuss the unitary Fermi gas made of dilute and ultracold atoms with an
infinite s-wave inter-atomic scattering length. First we introduce an efficient
Thomas-Fermi-von Weizsacker density functional which describes accurately
various static properties of the unitary Fermi gas trapped by an external
potential. Then, the sound velocity and the collective frequencies of
oscillations in a harmonic trap are derived from extended superfluid
hydrodynamic equations which are the Euler-Lagrange equations of a
Thomas-Fermi-von Weizsacker action functional. Finally, we show that this
amazing Fermi gas supports supersonic and subsonic shock waves.Comment: 9 pages, 3 figures, invited talk at the International Workshop
"Critical Stability 2011" (Erice, October 2011), to be published in the
journal Few Body System
Linear to quadratic crossover of Cooper pair dispersion relation
Cooper pairing is studied in three dimensions to determine its binding energy
for all coupling using a general separable interfermion interaction. Also
considered are Cooper pairs (CPs) with nonzero center-of-mass momentum (CMM). A
coupling-independent {\it linear} term in the CMM dominates the pair excitation
energy in weak coupling and/or high fermion density, while the more familiar
quadratic term prevails only in the extreme low-density (i.e., vacuum) limit
for any nonzero coupling. The linear-to-quadratic crossover of the CP
dispersion relation is analyzed numerically, and is expected to play a central
role in a model of superconductivity (and superfluidity) simultaneously
accommodating a BCS condensate as well as a Bose-Einstein condensate of CP
bosons.Comment: 13 pages plus 2 figure
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