26,835 research outputs found
Duality Symmetries and G^{+++} Theories
We show that the non-linear realisations of all the very extended algebras
G^{+++}, except the B and C series which we do not consider, contain fields
corresponding to all possible duality symmetries of the on-shell degrees of
freedom of these theories. This result also holds for G_2^{+++} and we argue
that the non-linear realisation of this algebra accounts precisely for the form
fields present in the corresponding supersymmetric theory. We also find a
simple necessary condition for the roots to belong to a G^{+++} algebra.Comment: 35 pages. v2: 2 appendices added, other minor corrections. v3: tables
corrected, other minor changes, one appendix added, refs. added. Version
published in Class. Quant. Gra
Superlinear Increase of Photocurrent due to Stimulated Scattering into a Polariton Condensate
We show that when a monopolar current is passed through an n-i-n structure,
superlinear photocurrent response occurs when there is a polariton condensate.
This is in sharp contrast to the previously observed behavior for a standard
semiconductor laser. Theoretical modeling shows that this is due to a
stimulated exciton-exciton scattering process in which one exciton relaxes into
the condensate, while another one dissociates into an electron-hole pair.Comment: 17 pages with 10 figure
Distinct Signatures For Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers
Two distinct types of magnetoresistance oscillations are observed in two
electronic Fabry-Perot interferometers of different sizes in the integer
quantum Hall regime. Measuring these oscillations as a function of magnetic
field and gate voltages, we observe three signatures that distinguish the two
types. The oscillations observed in a 2.0 square micron device are understood
to arise from the Coulomb blockade mechanism, and those observed in an 18
square micron device from the Aharonov-Bohm mechanism. This work clarifies,
provides ways to distinguish, and demonstrates control over, these distinct
physical origins of resistance oscillations seen in electronic Fabry-Perot
interferometers.Comment: related papers at http://marcuslab.harvard.ed
Asymptotic Freedom and Bound States in Hamiltonian Dynamics
We study a model of asymptotically free theories with bound states using the
similarity renormalization group for hamiltonians. We find that the
renormalized effective hamiltonians can be approximated in a large range of
widths by introducing similarity factors and the running coupling constant.
This approximation loses accuracy for the small widths on the order of the
bound state energy and it is improved by using the expansion in powers of the
running coupling constant. The coupling constant for small widths is order 1.
The small width effective hamiltonian is projected on a small subset of the
effective basis states. The resulting small matrix is diagonalized and the
exact bound state energy is obtained with accuracy of the order of 10% using
the first three terms in the expansion. We briefly describe options for
improving the accuracy.Comment: plain latex file, 15 pages, 6 latex figures 1 page each, 1 tabl
Evidence for the Hierarchical Formation of the Galactic Spheroid
The possibility that the Galactic spheroid was assembled from numerous
chemically-distinct, proto-Galactic fragments is investigated using a
Monte-Carlo technique designed to simulate the chemical evolution of the Galaxy
in hierarchical formation scenarios which involve no gas dissipation. By
comparing the observed and simulated metallicity distributions of Galactic
globular clusters and halo field stars, we estimate the level of fragmentation
in the collapsing proto-Galaxy. Although the formation process is highly
stochastic, the simulations often show good agreement with the observed
metallicity distributions, provided the luminosity function of proto-Galactic
fragments had a power-law form with exponent ~ -2. While this steep slope is
strongly at odds with the presently observed luminosity function of the Local
Group, it is in close agreement with the predictions of semi-analytic and
numerical models of hierarchical galaxy formation. We discuss a number of
possible explanations for this discrepancy. These simulations suggest that the
Galactic halo and its globular cluster system were assembled via the accretion
and disruption of approximately 1000 metal-poor, proto-Galactic fragments by
the dominant Galactic building block: a proto-bulge whose own metal-rich
globular clusters system has been preferentially eroded by dynamical processes.
We argue that the same process (ie, hierarchical growth involving little gas
dissipation) is responsible for the formation of both giant elliptical galaxies
and the bulge-halo components of spiral galaxies. (ABRIDGED).Comment: 20 pages, 9 postscript figures. Accepted for publication in the ApJ,
April 10 2000 issu
Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}
Materials that exhibit colossal magnetoresistance (CMR) are currently the
focus of an intense research effort, driven by the technological applications
that their sensitivity lends them to. Using the angular correlation of photons
from electron-positron annihilation, we present a first glimpse of the Fermi
surface of a material that exhibits CMR, supported by ``virtual crystal''
electronic structure calculations. The Fermi surface is shown to be
sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure
VLT Spectroscopy of Globular Clusters in Low Surface Brightness Dwarf Galaxies
We present VLT/FORS2 spectroscopic observations of globular clusters (GCs) in
five low surface brightness (LSB) dwarf galaxies: KK211 and KK221, which are
both dwarf spheroidal satellites (dSph) of NGC 5128, dSph KK84 located close to
the isolated S0 galaxy NGC 3115, and two isolated dwarf irregular (dIrr)
galaxies UGC 3755 and ESO 490-17. Our sample is selected from the Sharina et
al. (2005) database of Hubble Space Telescope WFPC2 photometry of GC candidates
in dwarf galaxies. For objects with accurate radial velocity measurements we
confirm 26 as genuine GCs out of the 27 selected candidates from our WFPC2
survey. Lick absorption line indices in the spectra of confirmed GCs and the
subsequent comparison with SSP model predictions show that all confirmed GCs in
dSphs are old, except GC KK211-3-149 (6 +/- 2 Gyr), which we consider to be the
nucleus of KK211. GCs in UGC 3755 and ESO 490-17 show a large spread in ages
ranging from old objects (t > 10 Gyr) to clusters with ages around 1 Gyr. Most
of our sample GCs have low metallicities [Z/H] <= -1. Two relatively metal-rich
clusters with [Z/H] ~ -0.3 are likely to be associated with NGC 3115. Our
sample GCs show in general a complex distribution of alpha-element enhancement
with a mean [alpha/Fe]=0.19 +/-0.04 derived with the chi2 minimization
technique and 0.18+/-0.12 dex computed with the iterative approach. These
values are slightly lower than the mean [alpha/Fe]=0.29+/-0.01 for typical
Milky Way GCs. We compare other abundance ratios with those of Local Group GCs
and find indications for systematic differences in N and Ca abundance. The
specific frequencies, S_N, of our sample galaxies are in line with the
predictions of a simple mass-loss model for dwarf galaxies and compare well
with S_N values found for dwarf galaxies in nearby galaxy clusters.Comment: accepted for publication in Ap
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