9,654 research outputs found
Three Dimensional Simulation of Jet Formation in Collapsing Condensates
We numerically study the behavior of collapsing and exploding condensates
using the parameters of the experiments by E.A. Donley et al. [Nature,
412, 295, (2001)]. Our studies are based on a full three-dimensional
numerical solution of the Gross-Pitaevskii equation (GPE) including three body
loss. We determine the three body loss rate from the number of remnant
condensate atoms and collapse times and obtain only one possible value which
fits with the experimental results. We then study the formation of jet atoms by
interrupting the collapse and find very good agreement with the experiment.
Furthermore we investigate the sensitivity of the jets to the initial
conditions. According to our analysis the dynamics of the burst atoms is not
described by the GPE with three body loss incorporated.Comment: 9 pages, 10 figure
Self-trapping of impurities in Bose-Einstein condensates: Strong attractive and repulsive coupling
We study the interaction-induced localization -- the so-called self-trapping
-- of a neutral impurity atom immersed in a homogeneous Bose-Einstein
condensate (BEC). Based on a Hartree description of the BEC we show that --
unlike repulsive impurities -- attractive impurities have a singular ground
state in 3d and shrink to a point-like state in 2d as the coupling approaches a
critical value. Moreover, we find that the density of the BEC increases
markedly in the vicinity of attractive impurities in 1d and 2d, which strongly
enhances inelastic collisions between atoms in the BEC. These collisions result
in a loss of BEC atoms and possibly of the localized impurity itself.Comment: 7 pages, 5 figure
Magnetic structure of antiferromagnetic NdRhIn5
The magnetic structure of antiferromagnetic NdRhIn5 has been determined using
neutron diffraction. It has a commensurate antiferromagnetic structure with a
magnetic wave vector (1/2,0,1/2) below T_N = 11K. The staggered Nd moment at
1.6K is 2.6mu_B aligned along the c-axis. We find the magnetic structure to be
closely related to that of its cubic parent compound NdIn3 below 4.6K. The
enhanced T_N and the absence of additional transitions below T_N for NdRhIn5
are interpreted in terms of an improved matching of the
crystalline-electric-field (CEF), magnetocrystalline, and exchange interaction
anisotropies. In comparison, the role of these competing anisotropies on the
magnetic properties of the structurally related compound CeRhIn5 is discussed.Comment: 4 pages, 4 figure
Crossover from commensurate to incommensurate antiferromagnetism in stoichiometric NaFeAs revealed by single-crystal 23Na,75As-NMR experiments
We report results of 23Na and 75As nuclear magnetic resonance (NMR)
experiments on a self-flux grown high-quality single crystal of stoichiometric
NaFeAs. The NMR spectra revealed a tetragonal to twinned-orthorhombic
structural phase transition at T_O = 57 K and an antiferromagnetic (AF)
transition at T_AF = 45 K. The divergent behavior of nuclear relaxation rate
near T_AF shows significant anisotropy, indicating that the critical slowing
down of stripe-type AF fluctuations are strongly anisotropic in spin space. The
NMR spectra at low enough temperatures consist of sharp peaks showing a
commensurate stripe AF order with a small moment \sim 0.3 muB. However, the
spectra just below T_AF exhibits highly asymmetric broadening pointing to an
incommensurate modulation. The commensurate-incommensurate crossover in NaFeAs
shows a certain similarity to the behavior of SrFe2As2 under high pressure.Comment: 5 pages, 5 figures, revised version to appear in J. Phys. Soc. Jp
Evolution of superconductivity in isovalent Te-substituted KxFe2-ySe2 crystals
We report the evolution of superconductivity and the phase diagram of the
KxFe2-ySe2-zTez (z=0-0.6) crystals grown by a simple one-step synthesis. No
structural transition is observed in any crystals, while lattice parameters
exhibit a systematic expansion with Te content. The Tc exhibits a gradual
decrease with increasing Te content from Tconset = 32.9 K at z = 0 to Tconset =
27.9 K at z = 0.5, followed by a sudden suppression of superconductivity at z =
0.6. Upon approaching a Te concentration of 0.6, the shielding volume fraction
decreases and eventually drops to zero. Simultaneously, hump positions in r-T
curve shift to lower temperatures. These results suggest that isovalent
substitution of Te for Se in KxFe2-ySe2 crystals suppresses the
superconductivity in this system.Comment: 10 pages, 1 table, 8 figure
Constraining a possible time variation of the gravitational constant G with terrestrial nuclear laboratory data
Testing the constancy of the gravitational constant G has been a longstanding
fundamental question in natural science. As first suggested by Jofr\'{e},
Reisenegger and Fern\'{a}ndez [1], Dirac's hypothesis of a decreasing
gravitational constant with time due to the expansion of the Universe would
induce changes in the composition of neutron stars, causing dissipation and
internal heating. Eventually, neutron stars reach their quasi-stationary states
where cooling due to neutrino and photon emissions balances the internal
heating. The correlation of surface temperatures and radii of some old neutron
stars may thus carry useful information about the changing rate of G. Using the
density dependence of the nuclear symmetry energy constrained by recent
terrestrial laboratory data on isospin diffusion in heavy-ion reactions at
intermediate energies and the size of neutron skin in within the
gravitochemical heating formalism, we obtain an upper limit of the relative
changing rate of consistent with the
best available estimates in the literature.Comment: 27 pages, 11 figures, and 2 tables. Accepted version to appear in PRC
(2007
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