702 research outputs found
Sound speed of a Bose-Einstein condensate in an optical lattice
The speed of sound of a Bose-Einstein condensate in an optical lattice is
studied both analytically and numerically in all three dimensions. Our
investigation shows that the sound speed depends strongly on the strength of
the lattice. In the one-dimensional case, the speed of sound falls
monotonically with increasing lattice strength. The dependence on lattice
strength becomes much richer in two and three dimensions. In the
two-dimensional case, when the interaction is weak, the sound speed first
increases then decreases as the lattice strength increases. For the three
dimensional lattice, the sound speed can even oscillate with the lattice
strength. These rich behaviors can be understood in terms of compressibility
and effective mass. Our analytical results at the limit of weak lattices also
offer an interesting perspective to the understanding: they show the lattice
component perpendicular to the sound propagation increases the sound speed
while the lattice components parallel to the propagation decreases the sound
speed. The various dependence of the sound speed on the lattice strength is the
result of this competition.Comment: 15pages 6 figure
Direct extraction of the Eliashberg function for electron-phonon coupling: A case study of Be(1010)
We propose a systematic procedure to directly extract the Eliashberg function
for electron-phonon coupling from high-resolution angle-resolved photoemission
data. The procedure is successfully applied to the Be(1010) surface, providing
new insights to electron-phonon coupling at this surface. The method is shown
to be robust against imperfections in experimental data and suitable for wider
applications.Comment: 4 pages, 4 figures. More details concerning the procedure are
include
Bragg spectroscopy of a superfluid Bose-Hubbard gas
Bragg spectroscopy is used to measure excitations of a trapped,
quantum-degenerate gas of 87Rb atoms in a 3-dimensional optical lattice. The
measurements are carried out over a range of optical lattice depths in the
superfluid phase of the Bose-Hubbard model. For fixed wavevector, the resonant
frequency of the excitation is found to decrease with increasing lattice depth.
A numerical calculation of the resonant frequencies based on Bogoliubov theory
shows a less steep rate of decrease than the measurements.Comment: 11 pages, 4 figure
Twisted and Nontwisted Bifurcations Induced by Diffusion
We discuss a diffusively perturbed predator-prey system. Freedman and
Wolkowicz showed that the corresponding ODE can have a periodic solution that
bifurcates from a homoclinic loop. When the diffusion coefficients are large,
this solution represents a stable, spatially homogeneous time-periodic solution
of the PDE. We show that when the diffusion coefficients become small, the
spatially homogeneous periodic solution becomes unstable and bifurcates into
spatially nonhomogeneous periodic solutions.
The nature of the bifurcation is determined by the twistedness of an
equilibrium/homoclinic bifurcation that occurs as the diffusion coefficients
decrease. In the nontwisted case two spatially nonhomogeneous simple periodic
solutions of equal period are generated, while in the twisted case a unique
spatially nonhomogeneous double periodic solution is generated through
period-doubling.
Key Words: Reaction-diffusion equations; predator-prey systems; homoclinic
bifurcations; periodic solutions.Comment: 42 pages in a tar.gz file. Use ``latex2e twisted.tex'' on the tex
files. Hard copy of figures available on request from
[email protected]
Surface Geometric and Electronic Structure of BaFe2As2(001)
BaFe2As2 exhibits properties characteristic of the parent compounds of the
newly discovered iron (Fe)-based high-TC superconductors. By combining the real
space imaging of scanning tunneling microscopy/spectroscopy (STM/S) with
momentum space quantitative Low Energy Electron Diffraction (LEED) we have
identified the surface plane of cleaved BaFe2As2 crystals as the As terminated
Fe-As layer - the plane where superconductivity occurs. LEED and STM/S data on
the BaFe2As2(001) surface indicate an ordered arsenic (As) - terminated
metallic surface without reconstruction or lattice distortion. It is surprising
that the STM images the different Fe-As orbitals associated with the
orthorhombic structure, not the As atoms in the surface plane.Comment: 12 pages, 4 figure
Landauer transport model for Hawking radiation from a Reissner-Nordstrom black hole
The recent work of Nation et al in which Hawking radiation energy and entropy
flow from a black hole can be regarded as a one-dimensional (1D) Landauer
transport process is extended to the case of a Reissner-Nordstrom (RN) black
hole. It is found that the flow of charge current can also be transported via a
1D quantum channel except the current of Hawking radiation. The maximum entropy
current, which is shown to be particle statistics independence, is also
obtained
Surface Phase Transitions Induced by Electron Mediated Adatom-Adatom Interaction
We propose that the indirect adatom-adatom interaction mediated by the
conduction electrons of a metallic surface is responsible for the
structural phase transitions
observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction
overwhelms the local stress field imposed by the substrate registry, the system
suffers a phonon instability, resulting in a structural phase transition in the
adlayer. Our theory is capable of explaining all the salient features of the
transitions observed in
Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of
systems whose surfaces are metallic before the transition.Comment: 4 pages, 5 figure
Time-Dependent Spintronic Transport and Current-Induced Spin Transfer Torque in Magnetic Tunnel Junctions
The responses of the electrical current and the current-induced spin transfer
torque (CISTT) to an ac bias in addition to a dc bias in a magnetic tunnel
junction are investigated by means of the time-dependent nonquilibrium Green
function technique. The time-averaged current (time-averaged CISTT) is
formulated in the form of a summation of dc current (dc CISTT) multiplied by
products of Bessel functions with the energy levels shifted by . The tunneling current can be viewed as to happen between the photonic
sidebands of the two ferromagnets. The electrons can pass through the barrier
easily under high frequencies but difficultly under low frequencies. The tunnel
magnetoresistance almost does not vary with an ac field. It is found that the
spin transfer torque, still being proportional to the electrical current under
an ac bias, can be changed by varying frequency. Low frequencies could yield a
rapid decrease of the spin transfer torque, while a large ac signal leads to
both decrease of the electrical current and the spin torque. If only an ac bias
is present, the spin transfer torque is sharply enhanced at the particular
amplitude and frequency of the ac bias. A nearly linear relation between such
an amplitude and frequency is observed.Comment: 13 pages,8 figure
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