1,710 research outputs found
Ground-State Phase Diagram of the Two-Dimensional Extended Bose-Hubbard Model
We investigate the ground-state phase diagram of the soft-core Bose-Hubbard
model with the nearest-neighbor repulsion on a square lattice by using an
unbiased quantum Monte Carlo method. In contrast to the previous study[P.
Sengupta {\it et. al.}, Phys. Rev. Lett. {\bf 94}, 207202 (2005)], we present
the ground-state phase diagrams up to large hopping parameters. As a result, in
addition to the known supersolid above half-filling, we find supersolid even
below and at half-filling for large hopping parameters. Furthermore, for the
strong nearest-neighbor repulsion, we show that the supersolid phase occupies a
remarkably broad region in the phase diagram. The results are in qualitative
agreement with that obtained by the Gutzwiller mean-field approximation[M.
Iskin, Phys. Rev. A {\bf 83}, 051606(R) (2011) and T. Kimura, Phys. Rev. A {\bf
84}, 063630 (2011)]
Critical escape velocity of black holes from branes
In recent work we have shown that a black hole stacked on a brane escapes
once it acquires a recoil velocity. This result was obtained in the {\it
probe-brane} approximation, {\it i.e.}, when the tension of the brane is
negligibly small. Therefore, it is not clear whether the effect of the brane
tension may prevent the black hole from escaping for small recoil velocities.
The question is whether a critical escape velocity exists. Here, we analyze
this problem by studying the interaction between a Dirac-Nambu-Goto brane and a
black hole assuming adiabatic (quasi-static) evolution. By describing the brane
in a fixed black hole spacetime, which restricts our conclusions to lowest
order effects in the tension, we find that the critical escape velocity does
not exist for co-dimension one branes, while it does for higher co-dimension
branes.Comment: 10 pages, revte
Dynamical scaling analysis of the optical Hall conductivity in the quantum Hall regime
Dynamical scaling analysis is theoretically performed for the ac (optical)
Hall conductivity as a function of Fermi
energy and frequency for the two-dimensional electron
gas and for graphene. In both systems, results based on exact diagonalization
show that displays a well-defined dynamical
scaling, for which the dynamical critical exponent as well as the localization
exponent are fitted and plugged in. A crossover from the dc-like bahavior to
the ac regime is identified. The dynamical scaling analysis has enabled us to
quantify the plateau in the ac Hall conductivity previously obtained, and to
predict that the plateaux structure in ac is robust enough to be observed in
the THz regime.Comment: 5 pages, 3 figure
Dislocation nucleation in shocked fcc solids: effects of temperature and preexisting voids
Quantitative behaviors of shock-induced dislocation nucleation are
investigated by means of molecular dynamics simulations on fcc Lennard-Jones
solids: a model Argon. In perfect crystals, it is found that Hugoniot elastic
limit (HEL) is a linearly decreasing function of temperature: from near-zero to
melting temperatures. In a defective crystal with a void, dislocations are
found to nucleate on the void surface. Also HEL drastically decreases to 15
percent of the perfect crystal when a void radius is 3.4 nanometer. The
decrease of HEL becomes larger as the void radius increases, but HEL becomes
insensitive to temperature.Comment: 4 pages. (ver.2) All figures have been revised. Two citations are
newly added. Numerical unit is unified in the context of solid argon. (ver.
3) A minor revision including new reference
Where are Milky Way's Hadronic PeVatrons?
Observations of the Milky Way at TeV-PeV energies reveal a bright diffuse
flux of hadronic cosmic rays and also bright point sources of gamma rays. If
the gamma-ray sources are hadronic cosmic-ray accelerators, then they must also
be neutrino sources. However, no neutrino sources have been detected. Where are
they? We introduce a new population-based approach to probe Milky Way hadronic
PeVatrons, demanding consistency between diffuse and point-source PeV-range
data on cosmic rays, gamma rays, and neutrinos. For the PeVatrons, two extreme
scenarios are allowed: (1) the hadronic cosmic-ray accelerators and the
gamma-ray sources are the same objects, so that bright neutrino sources exist
and improved telescopes can detect them, versus (2) the hadronic cosmic-ray
accelerators and the gamma-ray sources are distinct, so that there are no
detectable neutrino sources. The latter case is possible if hadronic
accelerators have sufficiently thin column densities. We quantify present
constraints and future prospects, showing how to reveal the nature of the
hadronic PeVatrons.Comment: Main text 16 pages, 8 figures. Accepted by PR
Rheology and dynamical heterogeneity in frictionless beads at jamming density
We investigate the rheological properties of an assembly of inelastic (but
frictionless) particles close to the jamming density using numerical
simulation, in which uniform steady states with a constant shear rate
is realized. The system behaves as a power-law fluid and the
relevant exponents are estimated; e.g., the shear stress is proportional to
, where . It is also found that
the relaxation time and the correlation length of the velocity
increase obeying power laws: and
, where and
Where is the End of the Cosmic-Ray Electron Spectrum?
Detecting the end of the cosmic-ray (CR) electron spectrum would provide
important new insights. While we know that Milky Way sources can accelerate
electrons up to at least 1PeV, the observed CR electron spectrum at Earth
extends only up to 5TeV (possibly 20TeV), a large discrepancy. The question of
the end of the CR electron spectrum has received relatively little attention,
despite its importance. We take a comprehensive approach, showing that there
are multiple steps at which the observed CR electron spectrum could be cut off.
At the highest energies, the accelerators may not have sufficient luminosity,
or the sources may not allow sufficient escape, or propagation to Earth may not
be sufficiently effective, or present detectors may not have sufficient
sensitivity. For each step, we calculate a rough range of possibilities.
Although all of the inputs are uncertain, a clear vista of exciting
opportunities emerges. We outline strategies for progress based on CR electron
observations and auxiliary multi-messenger observations. In addition to
advancing our understanding of CRs in the Milky Way, progress will also sharpen
sensitivity to dark matter annihilation or decay.Comment: Main text 10 pages, 7 figure
Escape of black holes from the brane
TeV-scale gravity theories allow the possibility of producing small black
holes at energies that soon will be explored at the LHC or at the Auger
observatory. One of the expected signatures is the detection of Hawking
radiation, that might eventually terminate if the black hole, once perturbed,
leaves the brane. Here, we study how the `black hole plus brane' system evolves
once the black hole is given an initial velocity, that mimics, for instance,
the recoil due to the emission of a graviton. The results of our dynamical
analysis show that the brane bends around the black hole, suggesting that the
black hole eventually escapes into the extra dimensions once two portions of
the brane come in contact and reconnect. This gives a dynamical mechanism for
the creation of baby branes.Comment: 4 pages, 6 figure
The Distance to the Galactic Center Derived From Infrared Photometry of Bulge Red Clump Stars
On the basis of the near infrared observations of bulge red clump stars near
the Galactic center, we have determined the galactocentric distance to be R_0 =
7.52 +- 0.10 (stat) +- 0.35 (sys) kpc. We observed the red clump stars at |l| <
1.0 deg and 0.7 deg < |b| < 1.0 deg with the IRSF 1.4 m telescope and the
SIRIUS camera in the H and Ks bands. After extinction and population
corrections, we obtained (m - M)_0 = 14.38 +- 0.03 (stat) +- 0.10 (sys). The
statistical error is dominated by the uncertainty of the intrinsic local red
clump stars' luminosity. The systematic error is estimated to be +- 0.10
including uncertainties in extinction and population correction, zero-point of
photometry, and the fitting of the luminosity function of the red clump stars.
Our result, R_0 = 7.52 kpc, is in excellent agreement with the distance
determined geometrically with the star orbiting the massive black hole in the
Galactic center. The recent result based on the spatial distribution of
globular clusters is also consistent with our result. In addition, our study
exhibits that the distance determination to the Galactic center with the red
clump stars, even if the error of the population correction is taken into
account, can achieve an uncertainty of about 5%, which is almost the same level
as that in recent geometrical determinations.Comment: 14 pages, 4 figures, accepted by Ap
- …