13,311 research outputs found
Symmetry Classes in Graphene Quantum Dots: Universal Spectral Statistics, Weak Localization, and Conductance Fluctuations
We study the symmetry classes of graphene quantum dots, both open and closed,
through the conductance and energy level statistics. For abrupt termination of
the lattice, these properties are well described by the standard orthogonal and
unitary ensembles. However, for smooth mass confinement, special time-reversal
symmetries associated with the sublattice and valley degrees of freedom are
critical: they lead to block diagonal Hamiltonians and scattering matrices with
blocks belonging to the unitary symmetry class even at zero magnetic field.
While the effect of this structure is clearly seen in the conductance of open
dots, it is suppressed in the spectral statistics of closed dots, because the
intervalley scattering time is shorter than the time required to resolve a
level spacing in the closed systems but longer than the escape time of the open
systems.Comment: 4 pages, 4 figures, RevTex, submitted to Phys. Rev. Let
The Mystery Deepens: Spitzer Observations of Cool White Dwarfs
We present 4.5m and 8m photometric observations of 18 cool white
dwarfs obtained with the Spitzer Space Telescope. Our observations demonstrate
that four white dwarfs with T_eff< 6000 K show slightly depressed mid-infrared
fluxes relative to white dwarf models. In addition, another white dwarf with a
peculiar optical and near-infrared spectral energy distribution (LHS 1126) is
found to display significant flux deficits in Spitzer observations. These
mid-infrared flux deficits are not predicted by the current white dwarf models
including collision induced absorption due to molecular hydrogen. We postulate
that either the collision induced absorption calculations are incomplete or
there are other unrecognized physical processes occuring in cool white dwarf
atmospheres. The spectral energy distribution of LHS 1126 surprisingly fits a
Rayleigh-Jeans spectrum in the infrared, mimicking a hot white dwarf with
effective temperature well in excess of 10 K. This implies that the source
of this flux deficit is probably not molecular absorption but some other
process.Comment: 17 pages, 4 figures, ApJ in press, 10 May 200
Glassy transition and metastability in four-spin Ising model
Using Monte Carlo simulations we show that the three-dimensional Ising model
with four-spin (plaquette) interactions has some characteristic glassy
features. The model dynamically generates diverging energy barriers, which give
rise to slow dynamics at low temperature. Moreover, in a certain temperature
range the model possesses a metastable (supercooled liquid) phase, which is
presumably supported by certain entropy barriers. Although extremely strong,
metastability in our model is only a finite-size effect and sufficiently large
droplets of stable phase divert evolution of the system toward the stable
phase. Thus, the glassy transitions in this model is a dynamic transition,
preceded by a pronounced peak in the specific heat.Comment: extensively revised, with further simulations of metastability
properties, response to referees tactfully remove
The Case for an Accelerating Universe from Supernovae
The unexpected faintness of high-redshift Type Ia supernovae (SNe Ia), as
measured by two teams, has been interpreted as evidence that the expansion of
the Universe is accelerating. We review the current challenges to this
interpretation and seek to answer whether the cosmological implications are
compelling. We discuss future observations of SNe Ia which could offer
extraordinary evidence to test acceleration.Comment: To appear as an Invited Review for PASP 20 pages, 13 figure
Parasite-associated mortality in a long-lived mammal: Variation with host age, sex, and reproduction.
Parasites can cause severe host morbidity and threaten survival. As parasites are generally aggregated within certain host demographics, they are likely to affect a small proportion of the entire population, with specific hosts being at particular risk. However, little is known as to whether increased host mortality from parasitic causes is experienced by specific host demographics. Outside of theoretical studies, there is a paucity of literature concerning dynamics of parasite-associated host mortality. Empirical evidence mainly focuses on short-lived hosts or model systems, with data lacking from long-lived wild or semi-wild vertebrate populations. We investigated parasite-associated mortality utilizing a multigenerational database of mortality, health, and reproductive data for over 4,000 semi-captive timber elephants (Elephas maximus), with known causes of death for mortality events. We determined variation in mortality according to a number of host traits that are commonly associated with variation in parasitism within mammals: age, sex, and reproductive investment in females. We found that potentially parasite-associated mortality varied significantly across elephant ages, with individuals at extremes of lifespan (young and old) at highest risk. Mortality probability was significantly higher for males across all ages. Female reproducers experienced a lower probability of potentially parasite-associated mortality than females who did not reproduce at any investigated time frame. Our results demonstrate increased potentially parasite-associated mortality within particular demographic groups. These groups (males, juveniles, elderly adults) have been identified in other studies as susceptible to parasitism, stressing the need for further work investigating links between infection and mortality. Furthermore, we show variation between reproductive and non-reproductive females, with mothers being less at risk of potentially parasite mortality than nonreproducers
Efficiency in Multi-objective Games
In a multi-objective game, each agent individually evaluates each overall
action-profile on multiple objectives. I generalize the price of anarchy to
multi-objective games and provide a polynomial-time algorithm to assess it.
This work asserts that policies on tobacco promote a higher economic
efficiency
Localized D-dimensional global k-defects
We explicitly demonstrate the existence of static global defect solutions of
arbitrary dimensionality whose energy does not diverge at spatial infinity, by
considering maximally symmetric solutions described by an action with
non-standard kinetic terms in a D+1 dimensional Minkowski space-time. We
analytically determine the defect profile both at small and large distances
from the defect centre. We verify the stability of such solutions and discuss
possible implications of our findings, in particular for dark matter and charge
fractionalization in graphene.Comment: 6 pages, published versio
N-representability and stationarity in time-dependent density functional theory
To construct an N-representable time-dependent density-functional theory, a
generalization to the time domain of the Levy-Lieb (LL) constrained search
algorithm is required. That the action is only stationary in the Dirac-Frenkel
variational principle eliminates the possibility of basing the search on the
action itself. Instead, we use the norm of the partial functional derivative of
the action in the Hilbert space of the wave functions in place of the energy of
the LL search. The electron densities entering the formalism are
-representable, and the resulting universal action functional has a unique
stationary point in the density at that corresponding to the solution of the
Schr\"{o}dinger equation. The original Runge-Gross (RG) formulation is subsumed
within the new formalism. Concerns in the literature about the meaning of the
functional derivatives and the internal consistency of the RG formulation are
allayed by clarifying the nature of the functional derivatives entering the
formalism.Comment: 9 pages, 0 figures, Phys. Rev. A accepted. Introduction was expanded,
subsections reorganized, appendix and new references adde
The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics
We present the discovery of the first L-type subdwarf, 2MASS
J05325346+8246465. This object exhibits enhanced collision-induced H
absorption, resulting in blue NIR colors (). In
addition, strong hydride bands in the red optical and NIR, weak TiO absorption,
and an optical/J-band spectral morphology similar to the L7 DENIS 02051159AB
imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both
a high proper motion, = 2\farcs60\pm0\farcs15 yr, and a
substantial radial velocity, km s, and its
probable proximity to the Sun (d = 10--30 pc) is consistent with halo
membership. Comparison to subsolar-metallicity evolutionary models strongly
suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 M
0.085 M_{\sun} for ages 10--15 Gyr and metallicities Z_{\sun}. The discovery of this object clearly indicates that star
formation occurred below the Hydrogen burning mass limit at early times,
consistent with prior results indicating a flat or slightly rising mass
function for the lowest-mass stellar subdwarfs. Furthermore, 2MASS 0532+8246
serves as a prototype for a new spectral class of subdwarfs, additional
examples of which could be found in NIR proper motion surveys.Comment: 9 pages, 3 figures, accepted to Ap
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