243 research outputs found
Pseudo Goldstone Bosons Phenomenology in Minimal Walking Technicolor
We construct the non-linear realized Lagrangian for the Goldstone Bosons
associated to the breaking pattern of SU(4) to SO(4). This pattern is expected
to occur in any Technicolor extension of the standard model featuring two Dirac
fermions transforming according to real representations of the underlying gauge
group. We concentrate on the Minimal Walking Technicolor quantum number
assignments with respect to the standard model symmetries. We demonstrate that
for, any choice of the quantum numbers, consistent with gauge and Witten
anomalies the spectrum of the pseudo Goldstone Bosons contains electrically
doubly charged states which can be discovered at the Large Hadron Collider.Comment: 25 pages, 5 figure
First Principles Simulations of Boron Diffusion in Graphite
Boron strongly modifies electronic and diffusion properties of graphite. We report the first ab initio study of boron interaction with the point defects in graphite, which includes structures, thermodynamics, and diffusion. A number of possible diffusion mechanisms of boron in graphite are suggested. We conclude that boron diffuses in graphite by a kick-out mechanism. This mechanism explains the common activation energy, but large magnitude difference, for the rate of boron diffusion parallel and perpendicular to the basal plane. © 2007 The American Physical Society
Models of core reconstruction for the 90-degree partial dislocation in semiconductors
We compare the models that have been proposed in the literature for the
atomic structure of the 90-degree partial dislocation in the homopolar
semiconductors, silicon, diamond, and germanium. In particular, we examine the
traditional single-period and our recently proposed double-period core
structures. Ab-initio and tight-binding results on the core energies are
discussed, and the geometries are compared in light of the available
experimental information about dislocations in these systems. The double-period
geometry is found to be the ground-state structure in all three materials. We
address boundary-conditions issues that have been recently raised about these
results. The structures of point excitations (kinks, solitons, and kink-soliton
complexes) in the two geometries are also reviewed.Comment: 9 pages, with 3 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/rn_eds/index.htm
Atomic structure of dislocation kinks in silicon
We investigate the physics of the core reconstruction and associated
structural excitations (reconstruction defects and kinks) of dislocations in
silicon, using a linear-scaling density-matrix technique. The two predominant
dislocations (the 90-degree and 30-degree partials) are examined, focusing for
the 90-degree case on the single-period core reconstruction. In both cases, we
observe strongly reconstructed bonds at the dislocation cores, as suggested in
previous studies. As a consequence, relatively low formation energies and high
migration barriers are generally associated with reconstructed
(dangling-bond-free) kinks. Complexes formed of a kink plus a reconstruction
defect are found to be strongly bound in the 30-degree partial, while the
opposite is true in the case of 90-degree partial, where such complexes are
found to be only marginally stable at zero temperature with very low
dissociation barriers. For the 30-degree partial, our calculated formation
energies and migration barriers of kinks are seen to compare favorably with
experiment. Our results for the kink energies on the 90-degree partial are
consistent with a recently proposed alternative double-period structure for the
core of this dislocation.Comment: 12 pages, two-column style with 8 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#rn_di
Metastable Frenkel pair defect in graphite: source of Wigner energy?
The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitial-vacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation
Rates and Characteristics of Intermediate Mass Ratio Inspirals Detectable by Advanced LIGO
Gravitational waves (GWs) from the inspiral of a neutron star (NS) or
stellar-mass black hole (BH) into an intermediate-mass black hole (IMBH) with
mass between ~50 and ~350 solar masses may be detectable by the planned
advanced generation of ground-based GW interferometers. Such intermediate mass
ratio inspirals (IMRIs) are most likely to be found in globular clusters. We
analyze four possible IMRI formation mechanisms: (1) hardening of an NS-IMBH or
BH-IMBH binary via three-body interactions, (2) hardening via Kozai resonance
in a hierarchical triple system, (3) direct capture, and (4) inspiral of a
compact object from a tidally captured main-sequence star; we also discuss
tidal effects when the inspiraling object is an NS. For each mechanism we
predict the typical eccentricities of the resulting IMRIs. We find that IMRIs
will have largely circularized by the time they enter the sensitivity band of
ground-based detectors. Hardening of a binary via three-body interactions,
which is likely to be the dominant mechanism for IMRI formation, yields
eccentricities under 10^-4 when the GW frequency reaches 10 Hz. Even among
IMRIs formed via direct captures, which can have the highest eccentricities,
around 90% will circularize to eccentricities under 0.1 before the GW frequency
reaches 10 Hz. We estimate the rate of IMRI coalescences in globular clusters
and the sensitivity of a network of three Advanced LIGO detectors to the
resulting GWs. We show that this detector network may see up to tens of IMRIs
per year, although rates of one to a few per year may be more plausible. We
also estimate the loss in signal-to-noise ratio that will result from using
circular IMRI templates for data analysis and find that, for the eccentricities
we expect, this loss is negligible.Comment: Accepted for publication in ApJ; revised version reflects changes
made to the article during the acceptance proces
Four-Body Effects in Globular Cluster Black Hole Coalescence
In the high density cores of globular clusters, multibody interactions are
expected to be common, with the result that black holes in binaries are
hardened by interactions. It was shown by Sigurdsson & Hernquist (1993) and
others that 10 solar mass black holes interacting exclusively by three-body
encounters do not merge in the clusters themselves, because recoil kicks the
binaries out of the clusters before the binaries are tight enough to merge.
Here we consider a new mechanism, involving four-body encounters. Numerical
simulations by a number of authors suggest that roughly 20-50% of binary-binary
encounters will eject one star but leave behind a stable hierarchical triple.
If the orbital plane of the inner binary is strongly tilted with respect to the
orbital plane of the outer object, a secular Kozai resonance, first
investigated in the context of asteroids in the Solar System, can increase the
eccentricity of the inner body significantly. We show that in a substantial
fraction of cases the eccentricity is driven to a high enough value that the
inner binary will merge by gravitational radiation, without a strong
accompanying kick. Thus the merged object remains in the cluster; depending on
the binary fraction of black holes and the inclination distribution of
newly-formed hierarchical triples, this mechanism may allow massive black holes
to accumulate through successive mergers in the cores of globular clusters. It
may also increase the likelihood that stellar-mass black holes in globular
clusters will be detectable by their gravitational radiation.Comment: Submitted to ApJ Letters (includes emulateapj.sty
Gravitational Radiation from Intermediate-Mass Black Holes
Recent X-ray observations of galaxies with ROSAT, ASCA, and Chandra have
revealed numerous bright off-center point sources which, if isotropic emitters,
are likely to be intermediate-mass black holes, with hundreds to thousands of
solar masses. The origin of these objects is under debate, but observations
suggest that a significant number of them currently reside in young
high-density stellar clusters. There is also growing evidence that some
Galactic globular clusters harbor black holes of similar mass, from
observations of stellar kinematics. In such high-density stellar environments,
the interactions of intermediate-mass black holes are promising sources of
gravitational waves for ground-based and space-based detectors. Here we explore
the signal strengths of binaries containing intermediate-mass black holes or
stellar-mass black holes in dense stellar clusters. We estimate that a few to
tens per year of these objects will be detectable during the last phase of
their inspiral with the advanced LIGO detector, and up to tens per year will be
seen during merger, depending on the spins of the black holes. We also find
that if these objects reside in globular clusters then tens of sources will be
detectable with LISA from the Galactic globular system in a five year
integration, and similar numbers will be detectable from more distant galaxies.
The signal strength depends on the eccentricity distribution, but we show that
there is promise for strong detection of pericenter precession and
Lense-Thirring precession of the orbital plane. We conclude by discussing what
could be learned about binaries, dense stellar systems, and strong gravity if
such signals are detected.Comment: Minor changes, accepted by ApJ (December 10, 2002
Black holes and core expansion in massive star clusters
We present the results from realistic N-body modelling of massive star
clusters in the Magellanic Clouds. We have computed eight simulations with N ~
10^5 particles; six of these were evolved for at least a Hubble time. The aim
of this modelling is to examine the possibility of large-scale core expansion
in massive star clusters and search for a viable dynamical origin for the
radius-age trend observed for such objects in the Magellanic Clouds. We
identify two physical processes which can lead to significant and prolonged
cluster core expansion: mass-loss due to rapid stellar evolution in a
primordially mass segregated cluster, and heating due to a retained population
of stellar-mass black holes. These two processes operate over different
time-scales - the former occurs only at early times and cannot drive core
expansion for longer than a few hundred Myr, while the latter typically does
not begin until several hundred Myr have passed but can result in core
expansion lasting for many Gyr. We investigate the behaviour of these expansion
mechanisms in clusters with varying degrees of primordial mass segregation and
in clusters with varying black hole retention fractions. In combination, the
two processes can lead to a wide variety of evolutionary paths on the
radius-age plane, which fully cover the observed cluster distribution and hence
define a dynamical origin for the radius-age trend in the Magellanic Clouds. We
discuss the implications of core expansion for various aspects of globular
cluster research, as well as the possibility of observationally inferring the
presence of a population of stellar-mass black holes in a cluster.Comment: Accepted for publication in MNRA
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