859 research outputs found
Evidence from K2 for rapid rotation in the descendant of an intermediate-mass star
Using patterns in the oscillation frequencies of a white dwarf observed by
K2, we have measured the fastest rotation rate, 1.13(02) hr, of any isolated
pulsating white dwarf known to date. Balmer-line fits to follow-up spectroscopy
from the SOAR telescope show that the star (SDSSJ0837+1856, EPIC 211914185) is
a 13,590(340) K, 0.87(03) solar-mass white dwarf. This is the highest mass
measured for any pulsating white dwarf with known rotation, suggesting a
possible link between high mass and fast rotation. If it is the product of
single-star evolution, its progenitor was a roughly 4.0 solar-mass
main-sequence B star; we know very little about the angular momentum evolution
of such intermediate-mass stars. We explore the possibility that this rapidly
rotating white dwarf is the byproduct of a binary merger, which we conclude is
unlikely given the pulsation periods observed.Comment: 5 pages, 4 figure, 1 table; accepted for publication in The
Astrophysical Journal Letter
Anomalous Low Temperature Behavior of Superconducting Nd(1.85)Ce(0.15)CuO(4-y)
We have measured the temperature dependence of the in-plane London
penetration depth lambda(T) and the maximum Josephson current Ic(T) using
bicrystal grain boundary Josephson junctions of the electron-doped cuprate
superconductor Nd(1.85)Ce(0.15)CuO(4-y). Both quantities reveal an anomalous
temperature dependence below about 4 K. In contrast to the usual monotonous
decrease (increase) of lambda(T) (Ic(T)) with decreasing temperature, lambda(T)
and Ic(T) are found to increase and decrease, respectively, with decreasing
temperature below 4 K resulting in a non-monotonous overall temperature
dependence. This anomalous behavior was found to be absent in analogous
measurements performed on Pr(1.85)Ce(0.15)CuO(4-y). From this we conclude that
the anomalous behavior of Nd(1.85)Ce(0.15)CuO(4-y) is caused by the presence of
the Nd3+ paramagnetic moments. Correcting the measured lambda(T) dependence of
Nd(1.85)Ce(0.15)CuO(4-y) for the temperature dependent susceptibility due to
the Nd moments, an exponential dependence is obtained indicating isotropic
s-wave pairing. This result is fully consistent with the lambda(T) dependence
measured for Pr(1.85)Ce(0.15)CuO(4-y).Comment: 4 pages including 4 figures, to appear in Phys. Rev. Let
Coil Formation in Multishell Carbon Nanotubes: Competition between Curvature Elasticity and Interlayer Adhesion
To study the shape formation process of carbon nanotubes, a string equation
describing the possible existing shapes of the axis-curve of multishell carbon
tubes (MCTs) is obtained in the continuum limit by minimizing the shape energy,
that is the difference between the MCT energy and the energy of the
carbonaceous mesophase (CM). It is shown that there exists a threshold relation
of the outmost and inmost radii, that gives a parameter regime in which a
straight MCT will be bent or twisted. Among the deformed shapes, the regular
coiled MCTs are shown being one of the solutions of the string equation. In
particular,the optimal ratio of pitch and radius for such a coil is
found to be equal to , which is in good agreement with recent
observation of coil formation in MCTs by Zhang et al.Comment: RevTeX, no figure, 12 pages, to appear in Phys. Rev. Let
Quantum-state control in optical lattices
We study the means to prepare and coherently manipulate atomic wave packets
in optical lattices, with particular emphasis on alkali atoms in the
far-detuned limit. We derive a general, basis independent expression for the
lattice operator, and show that its off-diagonal elements can be tailored to
couple the vibrational manifolds of separate magnetic sublevels. Using these
couplings one can evolve the state of a trapped atom in a quantum coherent
fashion, and prepare pure quantum states by resolved-sideband Raman cooling. We
explore the use of atoms bound in optical lattices to study quantum tunneling
and the generation of macroscopic superposition states in a double-well
potential. Far-off-resonance optical potentials lend themselves particularly
well to reservoir engineering via well controlled fluctuations in the
potential, making the atom/lattice system attractive for the study of
decoherence and the connection between classical and quantum physics.Comment: 35 pages including 8 figures. To appear in Phys. Rev. A. March 199
Effective and Asymptotic Critical Exponents of Weakly Diluted Quenched Ising Model: 3d Approach Versus -Expansion
We present a field-theoretical treatment of the critical behavior of
three-dimensional weakly diluted quenched Ising model. To this end we analyse
in a replica limit n=0 5-loop renormalization group functions of the
-theory with O(n)-symmetric and cubic interactions (H.Kleinert and
V.Schulte-Frohlinde, Phys.Lett. B342, 284 (1995)). The minimal subtraction
scheme allows to develop either the -expansion series or to
proceed in the 3d approach, performing expansions in terms of renormalized
couplings. Doing so, we compare both perturbation approaches and discuss their
convergence and possible Borel summability. To study the crossover effect we
calculate the effective critical exponents providing a local measure for the
degree of singularity of different physical quantities in the critical region.
We report resummed numerical values for the effective and asymptotic critical
exponents. Obtained within the 3d approach results agree pretty well with
recent Monte Carlo simulations. -expansion does not allow
reliable estimates for d=3.Comment: 35 pages, Latex, 9 eps-figures included. The reference list is
refreshed and typos are corrected in the 2nd versio
Tunneling control and localization for Bose-Einstein condensates in a frequency modulated optical lattice
The similarity between matter waves in periodic potential and solid-state
physics processes has triggered the interest in quantum simulation using
Bose-Fermi ultracold gases in optical lattices. The present work evidences the
similarity between electrons moving under the application of oscillating
electromagnetic fields and matter waves experiencing an optical lattice
modulated by a frequency difference, equivalent to a spatially shaken periodic
potential. We demonstrate that the tunneling properties of a Bose-Einstein
condensate in shaken periodic potentials can be precisely controlled. We take
additional crucial steps towards future applications of this method by proving
that the strong shaking of the optical lattice preserves the coherence of the
matter wavefunction and that the shaking parameters can be changed
adiabatically, even in the presence of interactions. We induce reversibly the
quantum phase transition to the Mott insulator in a driven periodic potential.Comment: Laser Physics (in press
A quantum point contact for neutral atoms
We show that the conductance of neutral atoms through a tightly confining
waveguide constriction is quantized in units of lambda_dB^2/pi, where lambda_dB
is the de Broglie wavelength of the incident atoms. Such a constriction forms
the atom analogue of an electron quantum point contact and is an example of
quantum transport of neutral atoms in an aperiodic system. We present a
practical constriction geometry that can be realized using a microfabricated
magnetic waveguide, and discuss how a pair of such constrictions can be used to
study the quantum statistics of weakly interacting gases in small traps.Comment: 5 pages with 3 figures. To appear in Phys. Rev. Let
Nonlinearity and disorder: Classification and stability of nonlinear impurity modes
We study the effects produced by competition of two physical mechanisms of
energy localization in inhomogeneous nonlinear systems. As an example, we
analyze spatially localized modes supported by a nonlinear impurity in the
generalized nonlinear Schr\"odinger equation and describe three types of
nonlinear impurity modes --- one- and two-hump symmetric localized modes and
asymmetric localized modes --- for both focusing and defocusing nonlinearity
and two different (attractive or repulsive) types of impurity. We obtain an
analytical stability criterion for the nonlinear localized modes and consider
the case of a power-law nonlinearity in detail. We discuss several scenarios of
the instability-induced dynamics of the nonlinear impurity modes, including the
mode decay or switching to a new stable state, and collapse at the impurity
site.Comment: 18 pages, 22 figure
Surface reconstruction induced geometries of Si clusters
We discuss a generalization of the surface reconstruction arguments for the
structure of intermediate size Si clusters, which leads to model geometries for
the sizes 33, 39 (two isomers), 45 (two isomers), 49 (two isomers), 57 and 61
(two isomers). The common feature in all these models is a structure that
closely resembles the most stable reconstruction of Si surfaces, surrounding a
core of bulk-like tetrahedrally bonded atoms. We investigate the energetics and
the electronic structure of these models through first-principles density
functional theory calculations. These models may be useful in understanding
experimental results on the reactivity of Si clusters and their shape as
inferred from mobility measurements.Comment: 9 figures (available from the author upon request) Submitted to Phys.
Rev.
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