23,472 research outputs found
Bose-Einstein Condensates with Large Number of Vortices
We show that as the number of vortices in a three dimensional Bose-Einstein
Condensate increases, the system reaches a "quantum Hall" regime where the
density profile is a Gaussian in the xy-plane and an inverted parabolic profile
along z. The angular momentum of the system increases as the vortex lattice
shrinks. However, Coriolis force prevents the unit cell of the vortex lattice
from shrinking beyond a minimum size. Although the recent MIT experiment is not
exactly in the quantum Hall regime, it is close enough for the present results
to be used as a guide. The quantum Hall regime can be easily reached by
moderate changes of the current experimental parameters.Comment: 4 pages, no figure
The role of electron-electron interactions in two-dimensional Dirac fermions
The role of electron-electron interactions on two-dimensional Dirac fermions
remains enigmatic. Using a combination of nonperturbative numerical and
analytical techniques that incorporate both the contact and long-range parts of
the Coulomb interaction, we identify the two previously discussed regimes: a
Gross-Neveu transition to a strongly correlated Mott insulator, and a
semi-metallic state with a logarithmically diverging Fermi velocity accurately
described by the random phase approximation. Most interestingly, experimental
realizations of Dirac fermions span the crossover between these two regimes
providing the physical mechanism that masks this velocity divergence. We
explain several long-standing mysteries including why the observed Fermi
velocity in graphene is consistently about 20 percent larger than the best
values calculated using ab initio and why graphene on different substrates show
different behavior.Comment: 11 pages, 4 figure
Thermal stability of titanium nitride for shallow junction solar cell contacts
To demonstrate the thermal stability of titanium nitride as a high-temperature diffusion barrier, the TiN-Ti-Ag metallization scheme has been tested on shallow-junction (~2000 Å) Si solar cells. Electrical measurements on reference samples with the Ti-Ag metallization scheme show serious degradation after a 600 °C, 10-min annealing. With the TiN-Ti-Ag scheme, no degradation of cell performance is observed after the same heat treatment if the TiN layer is >~1700 Å. The glass encapsulation of cells by electrostatic bonding requires such a heat treatment
Interaction driven metal-insulator transition in strained graphene
The question of whether electron-electron interactions can drive a metal to
insulator transition in graphene under realistic experimental conditions is
addressed. Using three representative methods to calculate the effective
long-range Coulomb interaction between -electrons in graphene and solving
for the ground state using quantum Monte Carlo methods, we argue that without
strain, graphene remains metallic and changing the substrate from SiO to
suspended samples hardly makes any difference. In contrast, applying a rather
large -- but experimentally realistic -- uniform and isotropic strain of about
seems to be a promising route to making graphene an antiferromagnetic
Mott insulator.Comment: Updated version: 6 pages, 3 figure
SMA observations of the proto brown dwarf candidate SSTB213 J041757
Context. The previously identified source SSTB213 J041757 is a proto brown
dwarf candidate in Taurus, which has two possible components A and B. It was
found that component B is probably a class 0/I proto brown dwarf associated
with an extended envelope.
Aims. Studying molecular outflows from young brown dwarfs provides important
insight into brown dwarf formation mechanisms, particularly brown dwarfs at the
earliest stages such as class 0, I. We therefore conducted a search for
molecular outflows from SSTB213 J041757.
Methods. We observed SSTB213 J041757 with the Submillimeter Array to search
for CO molecular outflow emission from the source.
Results. Our CO maps do not show any outflow emission from the proto brown
dwarf candidate.
Conclusions. The non-detection implies that the molecular outflows from the
source are weak; deeper observations are therefore needed to probe the outflows
from the source.Comment: 7 pages, 4 figures, accepted for publication in A&
Metric-affine gauge theory of gravity II. Exact solutions
In continuing our series on metric-affine gravity (see Gronwald IJMP D6
(1997) 263 for Part I), we review the exact solutions in this theory.Comment: Revtex file, 25 pages, final version to appear in IJMP
The Circumstellar Structure and Excitation Effects around the Massive Protostar Cepheus A HW 2
We report SMA 335 GHz continuum observations with angular resolution of
~0.''3, together with VLA ammonia observations with ~1'' resolution toward Cep
A HW 2. We find that the flattened disk structure of the dust emission observed
by Patel et al. is preserved at the 0.''3 scale, showing an elongated structure
of ~$0.''6 size (450 AU) peaking on HW 2. In addition, two ammonia cores are
observed, one associated with a hot-core previously reported, and an elongated
core with a double peak separated by ~1.''3 and with signs of heating at the
inner edges of the gas facing HW 2. The double-peaked ammonia structure, as
well as the double-peaked CH3CN structure reported previously (and proposed to
be two independent hot-cores), surround both the dust emission as well as the
double-peaked SO2 disk structure found by Jimenez-Serra et al. All these
results argue against the interpretation of the elongated dust-gas structure as
due to a chance-superposition of different cores; instead, they imply that it
is physically related to the central massive object within a disk-protostar-jet
system.Comment: 12 pages, 3 figures; accepted for publication in the Astrophysical
Journa
Ab-initio calculation of all-optical time-resolved calorimetry of nanosized systems: Evidence of nanosecond-decoupling of electron and phonon temperatures
The thermal dynamics induced by ultrashort laser pulses in nanoscale systems,
i.e. all-optical time-resolved nanocalorimetry is theoretically investigated
from 300 to 1.5 K. We report ab-initio calculations describing the temperature
dependence of the electron-phonon interactions for Cu nanodisks supported on
Si. The electrons and phonons temperatures are found to decouple on the ns time
scale at 10 K, which is two orders of magnitude in excess with respect to that
found for standard low-temperature transport experiments. By accounting for the
physics behind our results we suggest an alternative route for overhauling the
present knowledge of the electron-phonon decoupling mechanism in nanoscale
systems by replacing the mK temperature requirements of conventional
experiments with experiments in the time-domain.Comment: 5 pages, 3 figures. Accepted on Physical Review B
Possible Evidence for Truncated Thin Disks in the Low-Luminosity Active Galactic Nuclei M81 and NGC 4579
M81 and NGC 4579 are two of the few low-luminosity active galactic nuclei
which have an estimated mass for the central black hole, detected hard X-ray
emission, and detected optical/UV emission. In contrast to the canonical ``big
blue bump,'' both have optical/UV spectra which decrease with increasing
frequency in a plot. Barring significant reddening by dust and/or
large errors in the black hole mass estimates, the optical/UV spectra of these
systems require that the inner edge of a geometrically thin, optically thick,
accretion disk lies at roughly 100 Schwarzschild radii. The observed X-ray
radiation can be explained by an optically thin, two temperature,
advection-dominated accretion flow at smaller radii.Comment: emulateapj.sty, to appear in ApJ Letter
Multiplpe Choice Minority Game With Different Publicly Known Histories
In the standard Minority Game, players use historical minority choices as the
sole public information to pick one out of the two alternatives. However,
publishing historical minority choices is not the only way to present global
system information to players when more than two alternatives are available.
Thus, it is instructive to study the dynamics and cooperative behaviors of this
extended game as a function of the global information provided. We numerically
find that although the system dynamics depends on the kind of public
information given to the players, the degree of cooperation follows the same
trend as that of the standard Minority Game. We also explain most of our
findings by the crowd-anticrowd theory.Comment: Extensively revised, to appear in New J Phys, 7 pages with 4 figure
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