77,664 research outputs found
Making vortices in dipolar spinor condensates via rapid adiabatic passage
We propose to the create vortices in spin-1 condensates via magnetic
dipole-dipole interaction. Starting with a polarized condensate prepared under
large axial magnetic field, we show that by gradually inverting the field,
population transfer among different spin states can be realized in a controlled
manner. Under optimal condition, we generate a doubly quantized vortex state
containing nearly all atoms in the condensate. The resulting vortex state is a
direct manifestation of the dipole-dipole interaction and spin textures in
spinor condensates. We also point out that the whole process can be
qualitatively described by a simple rapid adiabatic passage model.Comment: 4 pages, 4 figure
Generating entangled photon pairs from a cavity-QED system
We propose a scheme for the controlled generation of Einstein-Podosky-Rosen
(EPR) entangled photon pairs from an atom coupled to a high Q optical cavity,
extending the prototype system as a source for deterministic single photons. A
thorough theoretical analysis confirms the promising operating conditions of
our scheme as afforded by currently available experimental setups. Our result
demonstrates the cavity QED system as an efficient and effective source for
entangled photon pairs, and shines new light on its important role in quantum
information science.Comment: It has recently come to our attention that the experiment by T. Wilk,
S. C. Webster, A. Kuhn and G. Rempe, published in Science 317, 488 (2007),
exactly realizes what we proposed in this article, which is published in Phy.
Rev. A 040302(R) (2005
Graphene formed on SiC under various environments: Comparison of Si-face and C-face
The morphology of graphene on SiC {0001} surfaces formed in various
environments including ultra-high vacuum, 1 atm of argon, and 10^-6 to 10^-4
Torr of disilane is studied by atomic force microscopy, low-energy electron
microscopy, and Raman spectroscopy. The graphene is formed by heating the
surface to 1100 - 1600 C, which causes preferential sublimation of the Si
atoms. The argon atmosphere or the background of disilane decreases the
sublimation rate so that a higher graphitization temperature is required, thus
improving the morphology of the films. For the (0001) surface, large areas of
monolayer-thick graphene are formed in this way, with the size of these areas
depending on the miscut of the sample. Results on the (000-1) surface are more
complex. This surface graphitizes at a lower temperature than for the (0001)
surface and consequently the growth is more three-dimensional. In an atmosphere
of argon the morphology becomes even worse, with the surface displaying
markedly inhomogeneous nucleation, an effect attributed to unintentional
oxidation of the surface during graphitization. Use of a disilane environment
for the (000-1) surface is found to produce improved morphology, with
relatively large areas of monolayer-thick graphene.Comment: 22 pages, 11 figures, Proceedings of STEG-2 Conference; eliminated
Figs. 4 and 7 from version 1, for brevity, and added Refs. 18, 29, 30, 31
together with associated discussio
An quantum approach of measurement based on the Zurek's triple model
In a close form without referring the time-dependent Hamiltonian to the total
system, a consistent approach for quantum measurement is proposed based on
Zurek's triple model of quantum decoherence [W.Zurek, Phys. Rev. D 24, 1516
(1981)]. An exactly-solvable model based on the intracavity system is dealt
with in details to demonstrate the central idea in our approach: by peeling off
one collective variable of the measuring apparatus from its many degrees of
freedom, as the pointer of the apparatus, the collective variable de-couples
with the internal environment formed by the effective internal variables, but
still interacts with the measured system to form a triple entanglement among
the measured system, the pointer and the internal environment. As another
mechanism to cause decoherence, the uncertainty of relative phase and its
many-particle amplification can be summed up to an ideal entanglement or an
Shmidt decomposition with respect to the preferred basis.Comment: 22pages,3figure
Post-Oligarchic Evolution of Protoplanetary Embryos and the Stability of Planetary Systems
We investigate the orbit-crossing time (T_c) of protoplanet systems both with
and without a gas-disk background. The protoplanets are initially with equal
masses and separation (EMS systems) scaled by their mutual Hill's radii. In a
gas-free environment, we find log (T_c/yr) = A+B \log (k_0/2.3). Through a
simple analytical approach, we demonstrate that the evolution of the velocity
dispersion in an EMS system follows a random walk. The stochastic nature of
random-walk diffusion leads to (i) an increasing average eccentricity ~
t^1/2, where t is the time; (ii) Rayleigh-distributed eccentricities
(P(e,t)=e/\sigma^2 \exp(-e^2/(2\sigma^2)) of the protoplanets; (iii) a
power-law dependence of T_c on planetary separation. As evidence for the
chaotic diffusion, the observed eccentricities of known extra solar planets can
be approximated by a Rayleigh distribution. We evaluate the isolation masses of
the embryos, which determine the probability of gas giant formation, as a
function of the dust and gas surface densities.Comment: 15 pages, 13 figures (2 color ones), accepted for publication in Ap
The Chemical and Ionization Conditions in Weak Mg II Absorbers
We present an analysis of the chemical and ionization conditions in a sample
of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar
spectra. Using a host of low ionization lines associated with each absorber in
this sample, and on the basis of ionization models, we infer that the
metallicity in a significant fraction of weak Mg II clouds is constrained to
values of solar or higher, if they are sub-Lyman limit systems. Based on the
observed constraints, we present a physical picture in which weak Mg II
absorbers are predominantly tracing two different astrophysical
processes/structures. A significant population of weak Mg II clouds, those in
which N(Fe II) is much less than N(Mg II), identified at both low (z ~ 1) and
high (z ~ 2) redshift, are potentially tracing gas in the extended halos of
galaxies, analogous to the Galactic high velocity clouds. These absorbers might
correspond to alpha-enhanced interstellar gas expelled from star-forming
galaxies, in correlated supernova events. On the other hand, N(FeII)
approximately equal to N(Mg II) clouds, which are prevalent only at lower
redshifts (z < 1.5), must be tracing Type Ia enriched gas in small, high
metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic
structures.Comment: 35 pages (including tables & figures). Accepted for publication in
ApJ. A high resolution version of the paper is available at
"http://www.astro.wisc.edu/~anand/weakMgII.pdf
Magnetization reversal in Kagome artificial spin ice studied by first-order reversal curves
Magnetization reversal of interconnected Kagome artificial spin ice was
studied by the first-order reversal curve (FORC) technique based on the
magneto-optical Kerr effect and magnetoresistance measurements. The
magnetization reversal exhibits a distinct six-fold symmetry with the external
field orientation. When the field is parallel to one of the nano-bar branches,
the domain nucleation/propagation and annihilation processes sensitively depend
on the field cycling history and the maximum field applied. When the field is
nearly perpendicular to one of the branches, the FORC measurement reveals the
magnetic interaction between the Dirac strings and orthogonal branches during
the magnetization reversal process. Our results demonstrate that the FORC
approach provides a comprehensive framework for understanding the magnetic
interaction in the magnetization reversal processes of spin-frustrated systems
The X-ray coronae of the two brightest galaxies in the Coma cluster
We use deep Chandra X-ray Observatory observations to examine the coronae of
the two brightest cluster galaxies in the Coma cluster of galaxies, NGC 4874
and NGC 4889. We find that NGC 4889 hosts a central depression in X-ray surface
brightness consistent with a cavity or pair of cavities of radius 0.6 kpc. If
the central cavity is associated with an AGN outburst and contains relativistic
material, its enthalpy should be around 5x10^55 erg. The implied heating power
of this cavity would be around an order of magnitude larger than the energy
lost by X-ray emission. It would be the smallest and youngest known cavity in a
brightest cluster galaxy and the lack of over pressuring implies heating is
still gentle. In contrast, NGC 4874 does not show any evidence for cavities,
although it hosts a well-known wide-angle-tail radio source which is visible
outside the region occupied by the X-ray corona. These two galaxies show that
AGN feedback can behave in varied ways in the same cluster environment.Comment: 11 pages, 11 figures, accepted by MNRA
- ā¦