titles
stringlengths 1
382
| abstracts
stringlengths 6
6.09k
|
|---|---|
Hecke-Clifford algebras and spin Hecke algebras I: the classical affine
type | Associated to the classical Weyl groups, we introduce the notion of
degenerate spin affine Hecke algebras and affine Hecke-Clifford algebras. For
these algebras, we establish the PBW properties, formulate the intertwiners,
and describe the centers. We further develop connections of these algebras with
the usual degenerate (i.e. graded) affine Hecke algebras of Lusztig by
introducing a notion of degenerate covering affine Hecke algebras.
|
Towards Minimal Resources of Measurement-based Quantum Computation | We improve the upper bound on the minimal resources required for
measurement-based quantum computation. Minimizing the resources required for
this model is a key issue for experimental realization of a quantum computer
based on projective measurements. This new upper bound allows also to reply in
the negative to the open question about the existence of a trade-off between
observable and ancillary qubits in measurement-based quantum computation.
|
A Spitzer census of the IC 348 nebula | We present a Spitzer based census of the IC 348 nebula and embedded star
cluster. Our Spitzer census supplemented by ground based spectra has added 42
class II T-Tauri sources to the cluster membership and identified ~20 class 0/I
protostars. The population of IC 348 likely exceeds 400 sources after
accounting statistically for unidentified diskless members. Our Spitzer census
of IC 348 reveals a population of protostars that is anti-correlated spatially
with the T-Tauri members, which comprise the centrally condensed cluster around
a B star. The protostars are instead found mostly at the cluster periphery
about 1 pc from the B star and spread out along a filamentary ridge. We find
that the star formation rate in this protostellar ridge is consistent with that
rate which built the exposed cluster while the presence of fifteen cold,
starless, millimeter cores intermingled with this protostellar population
indicates that the IC 348 nebula has yet to finish forming stars. We show that
the IC 348 cluster is of order 3-5 crossing times old, and, as evidenced by its
smooth radial profile and confirmed mass segregation, is likely relaxed. While
it seems apparent that the current cluster configuration is the result of
dynamical evolution and its primordial structure has been erased, our findings
support a model where embedded clusters are built up from numerous smaller
sub-clusters. Finally, the results of our Spitzer census indicate that the
supposition that star formation must progress rapidly in a dark cloud should
not preclude these observations that show it can be relatively long lived.
|
Non-Equilibrium Josephson and Andreev Current through Interacting
Quantum Dots | We present a theory of transport through interacting quantum dots coupled to
normal and superconducting leads in the limit of weak tunnel coupling. A
Josephson current between two superconducting leads, carried by first-order
tunnel processes, can be established by non-equilibrium proximity effect. Both
Andreev and Josephson current is suppressed for bias voltages below a threshold
set by the Coulomb charging energy. A $\pi$-transition of the supercurrent can
be driven by tuning gate or bias voltages.
|
Discovery of X-ray emission from the young radio pulsar PSR J1357-6429 | We present the first X-ray detection of the very young pulsar PSR J1357-6429
(characteristic age of 7.3 kyr) using data from the XMM-Newton and Chandra
satellites. We find that the spectrum is well described by a power-law plus
blackbody model, with photon index Gamma=1.4 and blackbody temperature kT=160
eV. For the estimated distance of 2.5 kpc, this corresponds to a 2-10 keV
luminosity of about 1.2E+32 erg/s, thus the fraction of the spin-down energy
channeled by PSR J1357-6429 into X-ray emission is one of the lowest observed.
The Chandra data confirm the positional coincidence with the radio pulsar and
allow to set an upper limit of 3E+31 erg/s on the 2-10 keV luminosity of a
compact pulsar wind nebula. We do not detect any pulsed emission from the
source and determine an upper limit of 30% for the modulation amplitude of the
X-ray emission at the radio frequency of the pulsar.
|
Resonant activation in bistable semiconductor lasers | We theoretically investigate the possibility of observing resonant activation
in the hopping dynamics of two-mode semiconductor lasers. We present a series
of simulations of a rate-equations model under random and periodic modulation
of the bias current. In both cases, for an optimal choice of the modulation
time-scale, the hopping times between the stable lasing modes attain a minimum.
The simulation data are understood by means of an effective one-dimensional
Langevin equation with multiplicative fluctuations. Our conclusions apply to
both Edge Emitting and Vertical Cavity Lasers, thus opening the way to several
experimental tests in such optical systems.
|
Quark matter and the astrophysics of neutron stars | Some of the means through which the possible presence of nearly deconfined
quarks in neutron stars can be detected by astrophysical observations of
neutron stars from their birth to old age are highlighted.
|
Some non-braided fusion categories of rank 3 | We classify all fusion categories for a given set of fusion rules with three
simple object types. If a conjecture of Ostrik is true, our classification
completes the classification of fusion categories with three simple object
types. To facilitate the discussion we describe a convenient, concrete and
useful variation of graphical calculus for fusion categories, discuss
pivotality and sphericity in this framework, and give a short and elementary
re-proof of the fact that the quadruple dual functor is naturally isomorphic to
the identity.
|
Chandra Observations of Supernova 1987A | We have been monitoring Supernova (SN) 1987A with {\it Chandra X-Ray
Observatory} since 1999. We present a review of previous results from our {\it
Chandra} observations, and some preliminary results from new {\it Chandra} data
obtained in 2006 and 2007. High resolution imaging and spectroscopic studies of
SN 1987A with {\it Chandra} reveal that X-ray emission of SN 1987A originates
from the hot gas heated by interaction of the blast wave with the ring-like
dense circumstellar medium (CSM) that was produced by the massive progenitor's
equatorial stellar winds before the SN explosion. The blast wave is now
sweeping through dense CSM all around the inner ring, and thus SN 1987A is
rapidly brightening in soft X-rays. At the age of 20 yr (as of 2007 January),
X-ray luminosity of SN 1987A is $L_{\rm X}$ $\sim$ 2.4 $\times$ 10$^{36}$ ergs
s$^{-1}$ in the 0.5$-$10 keV band. X-ray emission is described by two-component
plane shock model with electron temperatures of $kT$ $\sim$ 0.3 and 2 keV. As
the shock front interacts with dense CSM all around the inner ring, the X-ray
remnant is now expanding at a much slower rate of $v$ $\sim$ 1400 km s$^{-1}$
than it was until 2004 ($v$ $\sim$ 6000 km s$^{-1}$).
|
Classification of superpotentials | We extend our previous classification of superpotentials of ``scalar
curvature type" for the cohomogeneity one Ricci-flat equations. We now consider
the case not covered in our previous paper, i.e., when some weight vector of
the superpotential lies outside (a scaled translate of) the convex hull of the
weight vectors associated with the scalar curvature function of the principal
orbit. In this situation we show that either the isotropy representation has at
most 3 irreducible summands or the first order subsystem associated to the
superpotential is of the same form as the Calabi-Yau condition for submersion
type metrics on complex line bundles over a Fano K\"ahler-Einstein product.
|
Linkedness and ordered cycles in digraphs | The minimum semi-degree of a digraph D is the minimum of its minimum
outdegree and its minimum indegree. We show that every sufficiently large
digraph D with minimum semi-degree at least n/2 +k-1 is k-linked. The bound on
the minimum semi-degree is best possible and confirms a conjecture of
Manoussakis from 1990. We also determine the smallest minimum semi-degree which
ensures that a sufficiently large digraph D is k-ordered, i.e. that for every
ordered sequence of k distinct vertices of D there is a directed cycle which
encounters these vertices in this order.
|
Curvature and isocurvature perturbations in two-field inflation | We study cosmological perturbations in two-field inflation, allowing for
non-standard kinetic terms. We calculate analytically the spectra of curvature
and isocurvature modes at Hubble crossing, up to first order in the slow-roll
parameters. We also compute numerically the evolution of the curvature and
isocurvature modes from well within the Hubble radius until the end of
inflation. We show explicitly for a few examples, including the recently
proposed model of `roulette' inflation, how isocurvature perturbations affect
significantly the curvature perturbation between Hubble crossing and the end of
inflation.
|
Geometric Complexity Theory V: On deciding nonvanishing of a generalized
Littlewood-Richardson coefficient | This article has been withdrawn because it has been merged with the earlier
article GCT3 (arXiv: CS/0501076 [cs.CC]) in the series. The merged article is
now available as:
Geometric Complexity Theory III: on deciding nonvanishing of a
Littlewood-Richardson Coefficient, Journal of Algebraic Combinatorics, vol. 36,
issue 1, 2012, pp. 103-110. (Authors: Ketan Mulmuley, Hari Narayanan and Milind
Sohoni)
The new article in this GCT5 slot in the series is:
Geometric Complexity Theory V: Equivalence between blackbox derandomization
of polynomial identity testing and derandomization of Noether's Normalization
Lemma, in the Proceedings of FOCS 2012 (abstract), arXiv:1209.5993 [cs.CC]
(full version) (Author: Ketan Mulmuley)
|
A schematic model of scattering in PT-symmetric Quantum Mechanics | One-dimensional scattering problem admitting a complex, PT-symmetric
short-range potential V(x) is considered. Using a Runge-Kutta-discretized
version of Schroedinger equation we derive the formulae for the reflection and
transmission coefficients and emphasize that the only innovation emerges in
fact via a complexification of one of the potential-characterizing parameters.
|
The exact asymptotic of the collision time tail distribution for
independent Brownian particles with different drifts | In this note we consider the time of the collision $\tau$ for $n$ independent
Brownian motions $X^1_t,...,X_t^n$ with drifts $a_1,...,a_n$, each starting
from $x=(x_1,...,x_n)$, where $x_1<...<x_n$. We show the exact asymptotics of
$P_x(\tau>t) = C h(x)t^{-\alpha}e^{-\gamma t}(1 + o(1))$ as $t\to\infty$ and
identify $C,h(x),\alpha,\gamma$ in terms of the drifts.
|
Ab initio Study of Graphene on SiC | Employing density-functional calculations we study single and double graphene
layers on Si- and C-terminated 1x1-6H-SiC surfaces. We show that, in contrast
to earlier assumptions, the first carbon layer is covalently bonded to the
substrate, and cannot be responsible for the graphene-type electronic spectrum
observed experimentally. The characteristic spectrum of free-standing graphene
appears with the second carbon layer, which exhibits a weak van der Waals
bonding to the underlying structure. For Si-terminated substrate, the interface
is metallic, whereas on C-face it is semiconducting or semimetallic for single
or double graphene coverage, respectively.
|
Capacity of a Multiple-Antenna Fading Channel with a Quantized Precoding
Matrix | Given a multiple-input multiple-output (MIMO) channel, feedback from the
receiver can be used to specify a transmit precoding matrix, which selectively
activates the strongest channel modes. Here we analyze the performance of
Random Vector Quantization (RVQ), in which the precoding matrix is selected
from a random codebook containing independent, isotropically distributed
entries. We assume that channel elements are i.i.d. and known to the receiver,
which relays the optimal (rate-maximizing) precoder codebook index to the
transmitter using B bits. We first derive the large system capacity of
beamforming (rank-one precoding matrix) as a function of B, where large system
refers to the limit as B and the number of transmit and receive antennas all go
to infinity with fixed ratios. With beamforming RVQ is asymptotically optimal,
i.e., no other quantization scheme can achieve a larger asymptotic rate. The
performance of RVQ is also compared with that of a simpler reduced-rank scalar
quantization scheme in which the beamformer is constrained to lie in a random
subspace. We subsequently consider a precoding matrix with arbitrary rank, and
approximate the asymptotic RVQ performance with optimal and linear receivers
(matched filter and Minimum Mean Squared Error (MMSE)). Numerical examples show
that these approximations accurately predict the performance of finite-size
systems of interest. Given a target spectral efficiency, numerical examples
show that the amount of feedback required by the linear MMSE receiver is only
slightly more than that required by the optimal receiver, whereas the matched
filter can require significantly more feedback.
|
On Almost Periodicity Criteria for Morphic Sequences in Some Particular
Cases | In some particular cases we give criteria for morphic sequences to be almost
periodic (=uniformly recurrent). Namely, we deal with fixed points of
non-erasing morphisms and with automatic sequences. In both cases a
polynomial-time algorithm solving the problem is found. A result more or less
supporting the conjecture of decidability of the general problem is given.
|
The Radio Emission, X-ray Emission, and Hydrodynamics of G328.4+0.2: A
Comprehensive Analysis of a Luminous Pulsar Wind Nebula, its Neutron Star,
and the Progenitor Supernova Explosion | We present new observational results obtained for the Galactic non-thermal
radio source G328.4+0.2 to determine both if this source is a pulsar wind
nebula or supernova remnant, and in either case, the physical properties of
this source. Using X-ray data obtained by XMM, we confirm that the X-ray
emission from this source is heavily absorbed and has a spectrum best fit by a
power law model of photon index=2 with no evidence for a thermal component, the
X-ray emission from G328.4+0.2 comes from a region significantly smaller than
the radio emission, and that the X-ray and radio emission are significantly
offset from each other. We also present the results of a new high resolution (7
arcseconds) 1.4 GHz image of G328.4+0.2 obtained using the Australia Telescope
Compact Array, and a deep search for radio pulsations using the Parkes Radio
Telescope. We find that the radio emission has a flat spectrum, though some
areas along the eastern edge of G328.4+0.2 have a steeper radio spectral index
of ~-0.3. Additionally, we obtain a luminosity limit of the central pulsar of
L_{1400} < 30 mJy kpc^2, assuming a distance of 17 kpc. In light of these
observational results, we test if G328.4+0.2 is a pulsar wind nebula (PWN) or a
large PWN inside a supernova remnant (SNR) using a simple hydrodynamic model
for the evolution of a PWN inside a SNR. As a result of this analysis, we
conclude that G328.4+0.2 is a young (< 10000 years old) pulsar wind nebula
formed by a low magnetic field (<10^12 G) neutron star born spinning rapidly
(<10 ms) expanding into an undetected SNR formed by an energetic (>10^51 ergs),
low ejecta mass (M < 5 Solar Masses) supernova explosion which occurred in a
low density (n~0.03 cm^{-3}) environment.
|
Three Particle Correlations from STAR | Two-particle correlations have shown modification to the away-side shape in
central Au+Au collisions relative to $pp$, d+Au and peripheral Au+Au
collisions. Different scenarios can explain this modification including: large
angle gluon radiation, jets deflected by transverse flow, path length dependent
energy loss, Cerenkov gluon radiation of fast moving particles, and conical
flow generated by hydrodynamic Mach-cone shock-waves. Three-particle
correlations have the power to distinguish the scenarios with conical emission,
conical flow and Cerenkov radiation, from other scenarios. In addition, the
dependence of the observed shapes on the $p_T$ of the associated particles can
be used to distinguish conical emission from a sonic boom (Mach-cone) and from
QCD-Cerenkov radiation. We present results from STAR on 3-particle azimuthal
correlations for a high $p_T$ trigger particle with two softer particles.
Results are shown for $pp$, d+Au and high statistics Au+Au collisions at
$\sqrt{s_{NN}}$=200 GeV. An important aspect of the analysis is the subtraction
of combinatorial backgrounds. Systematic uncertainties due to this subtraction
and the flow harmonics v2 and v4 are investigated in detail. The implications
of the results for the presence or absence of conical flow from Mach-cones are
discussed.
|
The Return of a Static Universe and the End of Cosmology | We demonstrate that as we extrapolate the current $\Lambda$CDM universe
forward in time, all evidence of the Hubble expansion will disappear, so that
observers in our "island universe" will be fundamentally incapable of
determining the true nature of the universe, including the existence of the
highly dominant vacuum energy, the existence of the CMB, and the primordial
origin of light elements. With these pillars of the modern Big Bang gone, this
epoch will mark the end of cosmology and the return of a static universe. In
this sense, the coordinate system appropriate for future observers will perhaps
fittingly resemble the static coordinate system in which the de Sitter universe
was first presented.
|
Dark Matter annihilation in Draco: new considerations on the expected
gamma flux | A new estimation of the gamma-ray flux that we expect to detect from SUSY
dark matter annihilation from the Draco dSph is presented using the DM density
profiles compatible with the latest observations. This calculation takes also
into account the important effect of the Point Spread Function (PSF) of the
telescope. We show that this effect is crucial in the way we will observe and
interpret a possible signal detection. Finally, we discuss the prospects to
detect a possible gamma signal from Draco for MAGIC and GLAST.
|
Magnetohydrodynamic Rebound Shocks of Supernovae | We construct magnetohydrodynamic (MHD) similarity rebound shocks joining
`quasi-static' asymptotic solutions around the central degenerate core to
explore an MHD model for the evolution of random magnetic field in supernova
explosions. This provides a theoretical basis for further studying synchrotron
diagnostics, MHD shock acceleration of cosmic rays, and the nature of intense
magnetic field in compact objects. The magnetic field strength in space
approaches a limiting ratio, that is comparable to the ratio of the ejecta mass
driven out versus the progenitor mass, during this self-similar rebound MHD
shock evolution. The intense magnetic field of the remnant compact star as
compared to that of the progenitor star is mainly attributed to both the
gravitational core collapse and the radial distribution of magnetic field.
|
Are There Mach Cones in Heavy Ion Collisions? Three-Particle
Correlations from STAR | We present results from STAR on 3-particle azimuthal correlations for a
$3<p_T<4$ GeV/c trigger particle with two softer $1<p_T<2$ GeV/c particles.
Results are shown for pp, d+Au and high statistics Au+Au collisions at
$\sqrt{s_{NN}}=200 GeV$. We observe a 3-particle correlation in central Au+Au
collisions which may indicate the presence of conical emission. In addition,
the dependence of the observed signal angular position on the $p_T$ of the
associated particles can be used to distinguish conical flow from simple
QCD-\v{C}erenkov radiation. An important aspect of the analysis is the
subtraction of combinatorial backgrounds. Systematic uncertainties due to this
subtraction and the flow harmonics $v_2$ and $v_4$ are investigated in detail.
|
Exploring First Stars Era with GLAST | Cosmic infrared background (CIB) includes emissions from objects inaccessible
to current telescopic studies, such as the putative Population III, the first
stars. Recently, strong direct evidence for significant CIB levels produced by
the first stars came from CIB fluctuations discovered in deep Spitzer images.
Such CIB levels should have left a unique absorption feature in the spectra of
high-z GRBs and blazars as suggested in [4]. This is observable with GLAST
sources at z>2 and measuring this absorption will give important information on
energetics and constituents of the first stars era.
|
Correlated modulation between the redshifted Fe K alpha line and the
continuum emission in NGC 3783 | It has been suggested that X-ray observations of rapidly variable Seyfert
galaxies may hold the key to probe the gas orbital motions in the innermost
regions of accretion discs around black holes and, thus, trace flow patterns
under the effect of the hole strong gravitational field. We explore this
possibility analizing XMM-Newton observations of the seyfert 1 galaxy NGC 3783.
A detiled time-resolved spectral analysis is performed down to the shortest
possible time-scales (few ks) using "excess maps" and cross-correlating light
curves in different energy bands. In addition to a constant core of the Fe K
alpha line, we detected a variable and redshifted Fe K alpha emission feature
between 5.3-6.1 keV. The line exhibits a modulation on a time-scale of 27 ks
that is similar to and in phase with a modulation of the 0.3-10 keV source
continuum. The time-scale of the correlated variability of the redshifted Fe
line and continuum agrees with the local dynamical time-scale of the accretion
disc at 10 r_g around a black hole of 10^7 M_sun. Given the shape of the
redshfted line emission and the overall X-ray variability pattern, the line is
likely to arise from the relativistic region near the black hole.
|
Isotopic Effects in Nuclear Reactions at Relativistic Energies | A systematic study of isotopic effects in the break-up of projectile
spectators at relativistic energies has been performed at the GSI laboratory
with the ALADiN spectrometer coupled to the LAND neutron detector. Besides a
primary beam of 124Sn, also secondary beams of 124La and 107Sn produced at the
FRS fragment separator have been used in order to extend the range of isotopic
compositions. The gross properties of projectile fragmentation are very similar
for all the studied systems but specific isotopic effects have been observed in
both neutron and charged particle production. The breakup temperatures obtained
from the double ratios of isotopic yields have been extracted and compared with
the limiting-temperature expectation.
|
Einstein vs Maxwell: Is gravitation a curvature of space, a field in
flat space, or both? | Starting with a field theoretic approach in Minkowski space, the
gravitational energy momentum tensor is derived from the Einstein equations in
a straightforward manner. This allows to present them as {\it acceleration
tensor} = const. $\times$ {\it total energy momentum tensor}. For flat space
cosmology the gravitational energy is negative and cancels the material energy.
In the relativistic theory of gravitation a bimetric coupling between the
Riemann and Minkowski metrics breaks general coordinate invariance. The case of
a positive cosmological constant is considered. A singularity free version of
the Schwarzschild black hole is solved analytically. In the interior the
components of the metric tensor quickly die out, but do not change sign,
leaving the role of time as usual. For cosmology the $\Lambda$CDM model is
covered, while there appears a form of inflation at early times. Here both the
total energy and the zero point energy vanish.
|
Geometric Complexity Theory VI: the flip via saturated and positive
integer programming in representation theory and algebraic geometry | This article belongs to a series on geometric complexity theory (GCT), an
approach to the P vs. NP and related problems through algebraic geometry and
representation theory. The basic principle behind this approach is called the
flip. In essence, it reduces the negative hypothesis in complexity theory (the
lower bound problems), such as the P vs. NP problem in characteristic zero, to
the positive hypothesis in complexity theory (the upper bound problems):
specifically, to showing that the problems of deciding nonvanishing of the
fundamental structural constants in representation theory and algebraic
geometry, such as the well known plethysm constants--or rather certain relaxed
forms of these decision probelms--belong to the complexity class P. In this
article, we suggest a plan for implementing the flip, i.e., for showing that
these relaxed decision problems belong to P. This is based on the reduction of
the preceding complexity-theoretic positive hypotheses to mathematical
positivity hypotheses: specifically, to showing that there exist positive
formulae--i.e. formulae with nonnegative coefficients--for the structural
constants under consideration and certain functions associated with them. These
turn out be intimately related to the similar positivity properties of the
Kazhdan-Lusztig polynomials and the multiplicative structural constants of the
canonical (global crystal) bases in the theory of Drinfeld-Jimbo quantum
groups. The known proofs of these positivity properties depend on the Riemann
hypothesis over finite fields and the related results. Thus the reduction here,
in conjunction with the flip, in essence, says that the validity of the P vs.
NP conjecture in characteristic zero is intimately linked to the Riemann
hypothesis over finite fields and related problems.
|
Two new basaltic asteroids in the Outer Main Belt? | The identification of basaltic asteroids in the asteroid Main Belt and the
description of their surface mineralogy is necessary to understand the
diversity in the collection of basaltic meteorites. Basaltic asteroids can be
identified from their visible reflectance spectra and are classified as V-type
in the usual taxonomies. In this work, we report visible spectroscopic
observations of two candidate V-type asteroids, (7472) Kumakiri and (10537)
1991 RY16, located in the outer Main Belt (a > 2.85 UA). These candidate have
been previously identified by Roig and Gil-Hutton (2006, Icarus 183, 411) using
the Sloan Digital Sky Survey colors. The spectroscopic observations have been
obtained at the Calar Alto Observatory, Spain, during observational runs in
November and December 2006. The spectra of these two asteroids show the steep
slope shortwards of 0.70 microns and the deep absorption feature longwards of
0.75 microns that are characteristic of V-type asteroids. However, the presence
of a shallow but conspicuous absorption band around 0.65 microns opens some
questions about the actual mineralogy of these two asteroids. Such band has
never been observed before in basaltic asteroids with the intensity we detected
it. We discuss the possibility for this shallow absorption feature to be caused
by the presence of chromium on the asteroid surface. Our results indicate that,
together with (1459) Magnya, asteroids (7472) Kumakiri and (10537) 1991 RY16
may be the only traces of basaltic material found up to now in the outer Main
Belt.
|
Interpolating and sampling sequences in finite Riemann surfaces | We provide a description of the interpolating and sampling sequences on a
space of holomorphic functions with a uniform growth restriction defined on
finite Riemann surfaces.
|
New algebraic aspects of perturbative and non-perturbative Quantum Field
Theory | In this expository article we review recent advances in our understanding of
the combinatorial and algebraic structure of perturbation theory in terms of
Feynman graphs, and Dyson-Schwinger equations. Starting from Lie and Hopf
algebras of Feynman graphs, perturbative renormalization is rephrased
algebraically. The Hochschild cohomology of these Hopf algebras leads the way
to Slavnov-Taylor identities and Dyson-Schwinger equations. We discuss recent
progress in solving simple Dyson-Schwinger equations in the high energy sector
using the algebraic machinery. Finally there is a short account on a relation
to algebraic geometry and number theory: understanding Feynman integrals as
periods of mixed (Tate) motives.
|
Many-body interband tunneling as a witness for complex dynamics in the
Bose-Hubbard model | A perturbative model is studied for the tunneling of many-particle states
from the ground band to the first excited energy band, mimicking Landau-Zener
decay for ultracold, spinless atoms in quasi-one dimensional optical lattices
subjected to a tunable tilting force. The distributions of the computed
tunneling rates provide an independent and experimentally accessible signature
of the regular-chaotic transition in the strongly correlated many-body dynamics
of the ground band.
|
Comments on ``Are Swift Gamma-Ray Bursts consistent with the Ghirlanda
relation?", by Campana et al.(astro--ph/0703676) | In their recent paper, Campana et al. (2007) found that 5 bursts, among those
detected by Swift, are outliers with respect to the E_peak-E_gamma
("Ghirlanda") correlation. We instead argue that they are not.
|
The Determination of the Helicity of $W'$ Boson Couplings at the LHC | Apart from its mass and width, the most important property of a new charged
gauge boson, $W'$, is the helicity of its couplings to the SM fermions. Such
particles are expected to exist in many extensions of the Standard Model. In
this paper we explore the capability of the LHC to determine the $W'$ coupling
helicity at low integrated luminosities in the $\ell +E_T^{miss}$ discovery
channel. We find that measurements of the transverse mass distribution,
reconstructed from this final state in the $W-W'$ interference region, provides
the best determination of this quantity. To make such measurements requires
integrated luminosities of $\sim 10(60) fb^{-1}$ assuming $M_{W'}=1.5(2.5)$ TeV
and provided that the $W'$ couplings have Standard Model magnitude. This
helicity determination can be further strengthened by the use of various
discovery channel leptonic asymmetries, also measured in the same interference
regime, but with higher integrated luminosities.
|
Curvature flows in semi-Riemannian manifolds | We prove that the limit hypersurfaces of converging curvature flows are
stable, if the initial velocity has a weak sign, and give a survey of the
existence and regularity results.
|
Hydrodynamic and Spectral Simulations of HMXB Winds | We describe preliminary results of a global model of the radiatively-driven
photoionized wind and accretion flow of the high-mass X-ray binary Vela X-1.
The full model combines FLASH hydrodynamic calculations, XSTAR photoionization
calculations, HULLAC atomic data, and Monte Carlo radiation transport. We
present maps of the density, temperature, velocity, and ionization parameter
from a FLASH two-dimensional time-dependent simulation of Vela X-1, as well as
maps of the emissivity distributions of the X-ray emission lines.
|
Radio Astrometric Detection and Characterization of Extra-Solar Planets:
A White Paper Submitted to the NSF ExoPlanet Task Force | The extraordinary astrometric accuracy of radio interferometry creates an
important and unique opportunity for the discovery and characterization of
exo-planets. Currently, the Very Long Baseline Array can routinely achieve
better than 100 microarcsecond accuracy, and can approach 10 microarcsecond
with careful calibration. We describe here RIPL, the Radio Interferometric
PLanet search, a new program with the VLBA and the Green Bank 100 m telescope
that will survey 29 low-mass, active stars over 3 years with sub-Jovian planet
mass sensitivity at 1 AU. An upgrade of the VLBA bandwidth will increase
astrometric accuracy by an order of magnitude. Ultimately, the colossal
collecting area of the Square Kilometer Array could push astrometric accuracy
to 1 microarcsecond, making detection and characterizaiton of Earth mass
planets possible.
RIPL and other future radio astrometric planet searches occupy a unique
volume in planet discovery and characterization parameter space. The parameter
space of astrometric searches gives greater sensitivity to planets at large
radii than radial velocity searches. For the VLBA and the expanded VLBA, the
targets of radio astrometric surveys are by necessity nearby, low-mass, active
stars, which cannot be studied efficiently through the radial velocity method,
coronagraphy, or optical interferometry. For the SKA, detection sensitivity
will extend to solar-type stars. Planets discovered through radio astrometric
methods will be suitable for characterization through extreme adaptive optics.
The complementarity of radio astrometric techniques with other methods
demonstrates that radio astrometry can play an important role in the roadmap
for exoplanet discovery and characterization.
|
Interface dynamics of microscopic cavities in water | An analytical description of the interface motion of a collapsing
nanometer-sized spherical cavity in water is presented by a modification of the
Rayleigh-Plesset equation in conjunction with explicit solvent molecular
dynamics simulations. Quantitative agreement is found between the two
approaches for the time-dependent cavity radius $R(t)$ at different solvent
conditions while in the continuum picture the solvent viscosity has to be
corrected for curvature effects. The typical magnitude of the interface or
collapse velocity is found to be given by the ratio of surface tension and
fluid viscosity, $v\simeq\gamma/\eta$, while the curvature correction
accelerates collapse dynamics on length scales below the equilibrium crossover
scales ($\sim$1nm). The study offers a starting point for an efficient implicit
modeling of water dynamics in aqueous nanoassembly and protein systems in
nonequilibrium.
|
Viscosity, Black Holes, and Quantum Field Theory | We review recent progress in applying the AdS/CFT correspondence to
finite-temperature field theory. In particular, we show how the hydrodynamic
behavior of field theory is reflected in the low-momentum limit of correlation
functions computed through a real-time AdS/CFT prescription, which we
formulate. We also show how the hydrodynamic modes in field theory correspond
to the low-lying quasinormal modes of the AdS black p-brane metric. We provide
a proof of the universality of the viscosity/entropy ratio within a class of
theories with gravity duals and formulate a viscosity bound conjecture.
Possible implications for real systems are mentioned.
|
Superconducting states of the quasi-2D Holstein model: Effects of vertex
and non-local corrections | I investigate superconducting states in a quasi-2D Holstein model using the
dynamical cluster approximation (DCA). The effects of spatial fluctuations
(non-local corrections) are examined and approximations neglecting and
incorporating lowest-order vertex corrections are computed. The approximation
is expected to be valid for electron-phonon couplings of less than the
bandwidth. The phase diagram and superconducting order parameter are
calculated. Effects which can only be attributed to theories beyond
Migdal--Eliashberg theory are present. In particular, the order parameter shows
momentum dependence on the Fermi-surface with a modulated form and s-wave order
is suppressed at half-filling. The results are discussed in relation to
Hohenberg's theorem and the BCS approximation.
|
Masers and star formation | Recent observational and theoretical advances concerning astronomical masers
in star forming regions are reviewed. Major masing species are considered
individually and in combination. Key results are summarized with emphasis on
present science and future prospects.
|
Renormalized quasiparticles in antiferromagnetic states of the Hubbard
model | We analyze the properties of the quasiparticle excitations of metallic
antiferromagnetic states in a strongly correlated electron system. The study is
based on dynamical mean field theory (DMFT) for the infinite dimensional
Hubbard model with antiferromagnetic symmetry breaking. Self-consistent
solutions of the DMFT equations are calculated using the numerical
renormalization group (NRG). The low energy behavior in these results is then
analyzed in terms of renormalized quasiparticles. The parameters for these
quasiparticles are calculated directly from the NRG derived self-energy, and
also from the low energy fixed point of the effective impurity. They are found
to be in good agreement. We show that the main low energy features of the $\bf
k$-resolved spectral density can be understood in terms of the quasiparticle
picture. We also find that Luttinger's theorem is satisfied for the total
electron number in the doped antiferromagnetic state.
|
Comparison of exact-exchange calculations for solids in
current-spin-density- and spin-density-functional theory | The relative merits of current-spin-density- and spin-density-functional
theory are investigated for solids treated within the exact-exchange-only
approximation. Spin-orbit splittings and orbital magnetic moments are
determined at zero external magnetic field. We find that for magnetic (Fe, Co
and Ni) and non-magnetic (Si and Ge) solids, the exact-exchange
current-spin-density functional approach does not significantly improve the
accuracy of the corresponding spin-density functional results.
|
One-loop MHV Rules and Pure Yang-Mills | It has been known for some time that the standard MHV diagram formulation of
perturbative Yang-Mills theory is incomplete, as it misses rational terms in
one-loop scattering amplitudes of pure Yang-Mills. We propose that certain
Lorentz violating counterterms, when expressed in the field variables which
give rise to standard MHV vertices, produce precisely these missing terms.
These counterterms appear when Yang-Mills is treated with a regulator,
introduced by Thorn and collaborators, which arises in worldsheet formulations
of Yang-Mills theory in the lightcone gauge. As an illustration of our
proposal, we show that a simple one-loop, two-point counterterm is the
generating function for the infinite sequence of one-loop, all-plus helicity
amplitudes in pure Yang-Mills, in complete agreement with known expressions.
|
Fermi-liquid effects in the transresistivity in quantum Hall double
layers near $\nu= 1/2 $ | Here, we present theoretical studies of the temperature and magnetic field
dependences of the Coulomb drag transresistivity between two parallel layers of
two dimensional electron gases in quantum Hall regime near half filling of the
lowest Landau level. It is shown that Fermi-liquid interactions between the
relevant quasiparticles could give a significant effect on the
transresistivity, providing its independence of the interlayer spacing for
spacings taking on values reported in the experiments. Obtained results agree
with the experimental evidence.
|
Geometry of four-dimensional Killing spinors | The supersymmetric solutions of N=2, D=4 minimal ungauged and gauged
supergravity are classified according to the fraction of preserved
supersymmetry using spinorial geometry techniques. Subject to a reasonable
assumption in the 1/2-supersymmetric time-like case of the gauged theory, we
derive the complete form of all supersymmetric solutions. This includes a
number of new 1/4- and 1/2-supersymmetric possibilities, like gravitational
waves on bubbles of nothing in AdS_4.
|
A Rigorous Time-Domain Analysis of Full--Wave Electromagnetic Cloaking
(Invisibility) | There is currently a great deal of interest in the theoretical and practical
possibility of cloaking objects from the observation by electromagnetic waves.
The basic idea of these invisibility devices \cite{glu1, glu2, le},\cite{pss1}
is to use anisotropic {\it transformation media} whose permittivity and
permeability $\var^{\lambda\nu}, \mu^{\lambda\nu}$, are obtained from the ones,
$\var_0^{\lambda\nu}, \mu^{\lambda\nu}_0$, of isotropic media, by singular
transformations of coordinates. In this paper we study electromagnetic cloaking
in the time-domain using the formalism of time-dependent scattering theory.
This formalism allows us to settle in an unambiguous way the mathematical
problems posed by the singularities of the inverse of the permittivity and the
permeability of the {\it transformation media} on the boundary of the cloaked
objects. We write Maxwell's equations in Schr\"odinger form with the
electromagnetic propagator playing the role of the Hamiltonian. We prove that
the electromagnetic propagator outside of the cloaked objects is essentially
self-adjoint. Moreover, the unique self-adjoint extension is unitarily
equivalent to the electromagnetic propagator in the medium
$\var_0^{\lambda\nu}, \mu^{\lambda\nu}_0$. Using this fact, and since the
coordinate transformation is the identity outside of a ball, we prove that the
scattering operator is the identity. Our results give a rigorous proof that the
construction of \cite{glu1, glu2, le}, \cite{pss1} perfectly cloaks passive and
active devices from observation by electromagnetic waves. Furthermore, we prove
cloaking for general anisotropic materials. In particular, our results prove
that it is possible to cloak objects inside general crystals.
|
Non-perturbative conserving approximations and Luttinger's sum rule | Weak-coupling conserving approximations can be constructed by truncations of
the Luttinger-Ward functional and are well known as thermodynamically
consistent approaches which respect macroscopic conservation laws as well as
certain sum rules at zero temperature. These properties can also be shown for
variational approximations that are generated within the framework of the
self-energy-functional theory without a truncation of the diagram series.
Luttinger's sum rule represents an exception. We analyze the conditions under
which the sum rule holds within a non-perturbative conserving approximation.
Numerical examples are given for a simple but non-trivial dynamical two-site
approximation. The validity of the sum rule for finite Hubbard clusters and the
consequences for cluster extensions of the dynamical mean-field theory are
discussed.
|
2D-MIT as self-doping of a Wigner-Mott insulator | We consider an interaction-driven scenario for the two-dimensional
metal-insulator transition in zero magnetic field (2D-MIT), based on melting
the Wigner crystal through vacancy-interstitial pair formation. We show that
the transition from the Wigner-Mott insulator to a heavy Fermi liquid emerges
as an instability to self-doping, resembling conceptually the solid to normal
liquid transition in He3. The resulting physical picture naturally explains
many puzzling features of the 2D-MIT.
|
Entanglement of Subspaces and Error Correcting Codes | We introduce the notion of entanglement of subspaces as a measure that
quantify the entanglement of bipartite states in a randomly selected subspace.
We discuss its properties and in particular we show that for maximally
entangled subspaces it is additive. Furthermore, we show that maximally
entangled subspaces can play an important role in the study of quantum error
correction codes. We discuss both degenerate and non-degenerate codes and show
that the subspace spanned by the logical codewords of a non-degenerate code is
a 2k-totally (maximally) entangled subspace. As for non-degenerate codes, we
provide a mathematical definition in terms of subspaces and, as an example, we
analyze Shor's nine qubits code in terms of 22 mutually orthogonal subspaces.
|
Does the present data on B_s - bar B_s mixing rule out a large
enhancement in the branching ratio of B_s --> mu+ mu- ? | In this letter, we consider the constraints imposed by the recent measurement
of B_s - bar B_s mixing on the new physics contribution to the rare decay B_s
--> mu+ mu-. New physics in the form vector and axial-vector couplings is
already severely constrained by the data on B --> (K,K*) mu+ mu-. Here, we show
that B_s - bar B_s mixing data, together with the data on K0 - bar K0 mixing
and K_L --> mu+ mu- decay rate, strongly constrain the scalar-pseudoscalar
contribution to B_s --> mu+ mu-. We conclude that new physics can at best lead
to a factor of 2 increase in the branching ratio of B_s --> mu+ mu- compared to
its Standard Model expectation.
|
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds VIII.
Serpens Observed with MIPS | We present maps of 1.5 square degrees of the Serpens dark cloud at 24, 70,
and 160\micron observed with the Spitzer Space Telescope MIPS Camera. More than
2400 compact sources have been extracted at 24um, nearly 100 at 70um, and 4 at
160um. We estimate completeness limits for our 24um survey from Monte Carlo
tests with artificial sources inserted into the Spitzer maps. We compare source
counts, colors, and magnitudes in the Serpens cloud to two reference data sets,
a 0.50 deg^2 set on a low-extinction region near the dark cloud, and a 5.3
deg^2 subset of the SWIRE ELAIS N1 data that was processed through our
pipeline. These results show that there is an easily identifiable population of
young stellar object candidates in the Serpens Cloud that is not present in
either of the reference data sets. We also show a comparison of visual
extinction and cool dust emission illustrating a close correlation between the
two, and find that the most embedded YSO candidates are located in the areas of
highest visual extinction.
|
Unravelling the sbottom spin at the CERN LHC | Establishing that a signal of new physics is undoubtly supersymmetric
requires not only the discovery of the supersymmetric partners but also probing
their spins and couplings. We show that the sbottom spin can be probed at the
CERN Large Hadron Collider using only angular correlations in sbottom pair
production with subsequent decay of sbottoms into bottom quark plus the
lightest neutralino, which allow us to distinguish a universal extra
dimensional interpretation with a fermionic heavy bottom quark from
supersymmetry with a bosonic bottom squark. We demonstrate that this channel
provides a clear indication of the sbottom spin provided the sbottom production
rate and branching ratio into bottom quark plus the lightest neutralino are
sufficiently large to have a clear signal above Standard Model backgrounds.
|
Modeling the three-point correlation function | We present new predictions for the galaxy three-point correlation function
(3PCF) using high-resolution dissipationless cosmological simulations of a flat
LCDM Universe which resolve galaxy-size halos and subhalos. We create realistic
mock galaxy catalogs by assigning luminosities and colors to dark matter halos
and subhalos, and we measure the reduced 3PCF as a function of luminosity and
color in both real and redshift space. As galaxy luminosity and color are
varied, we find small differences in the amplitude and shape dependence of the
reduced 3PCF, at a level qualitatively consistent with recent measurements from
the SDSS and 2dFGRS. We confirm that discrepancies between previous 3PCF
measurements can be explained in part by differences in binning choices. We
explore the degree to which a simple local bias model can fit the simulated
3PCF. The agreement between the model predictions and galaxy 3PCF measurements
lends further credence to the straightforward association of galaxies with CDM
halos and subhalos.
|
Multi-spectral Observations of Lunar Occultations: I. Resolving The Dust
Shell Around AFGL 5440 | We present observations and modeling of a lunar occultation of the
dust-enshrouded carbon star AFGL 5440. The observations were made over a
continuous range of wavelengths from 1 - 4um with a high-speed
spectrophotometer designed expressly for this purpose. We find that the
occultation fringes cannot be fit by any single-size model. We use the DUSTY
radiative transfer code to model a circumstellar shell and fit both the
observed occultation light curves and the spectral energy distribution
described in the literature. We find a strong constraint on the inner radius of
the dust shell, Tmax = 950 K +/- 50K, and optical depth at 5um of 0.5 +/- 0.1.
The observations are best fit by models with a density gradient of r^-2 or the
gradient derived by Ivezic & Elitzur for a radiatively driven hydrodynamic
outflow. Our models cannot fit the observed IRAS 60um flux without assuming a
substantial abundance of graphite or by assuming a substantially higher
mass-loss rate in the past.
|
Orbifold cohomology of abelian symplectic reductions and the case of
weighted projective spaces | These notes accompany a lecture about the topology of symplectic (and other)
quotients. The aim is two-fold: first to advertise the ease of computation in
the symplectic category; and second to give an account of some new computations
for weighted projective spaces. We start with a brief exposition of how
orbifolds arise in the symplectic category, and discuss the techniques used to
understand their topology. We then show how these results can be used to
compute the Chen-Ruan orbifold cohomology ring of abelian symplectic
reductions. We conclude by comparing the several rings associated to a weighted
projective space. We make these computations directly, avoiding any mention of
a stacky fan or of a labeled moment polytope.
|
Correlation functions in the Non Perturbative Renormalization Group and
field expansion | The usual procedure of including a finite number of vertices in Non
Perturbative Renormalization Group equations in order to obtain $n$-point
correlation functions at finite momenta is analyzed. This is done by exploiting
a general method recently introduced which includes simultaneously all vertices
although approximating their momentum dependence. The study is performed using
the self-energy of the tridimensional scalar model at criticality. At least in
this example, low order truncations miss quantities as the critical exponent
$\eta$ by as much as 60%. However, if one goes to high order truncations the
procedure seems to converge rapidly.
|
Formation and Collisional Evolution of Kuiper Belt Objects | This chapter summarizes analytic theory and numerical calculations for the
formation and collisional evolution of KBOs at 20--150 AU. We describe the main
predictions of a baseline self-stirring model and show how dynamical
perturbations from a stellar flyby or stirring by a giant planet modify the
evolution. Although robust comparisons between observations and theory require
better KBO statistics and more comprehensive calculations, the data are broadly
consistent with KBO formation in a massive disk followed by substantial
collisional grinding and dynamical ejection. However, there are important
problems reconciling the results of coagulation and dynamical calculations.
Contrasting our current understanding of the evolution of KBOs and asteroids
suggests that additional observational constraints, such as the identification
of more dynamical families of KBOs (like the 2003 EL61 family), would provide
additional information on the relative roles of collisional grinding and
dynamical ejection in the Kuiper Belt. The uncertainties also motivate
calculations that combine collisional and dynamical evolution, a `unified'
calculation that should give us a better picture of KBO formation and
evolution.
|
On Existence of Boundary Values of Polyharmonic Functions | In trigonometric series terms all polyharmonic functions inside the unit disk
are described. For such functions it is proved the existence of their boundary
values on the unit circle in the space of hyperfunctions. The necessary and
sufficient conditions are presented for the boundary value to belong to certain
subspaces of the space of hyperfunctions.
|
Constraints on the Self-Interaction Cross-Section of Dark Matter from
Numerical Simulations of the Merging Galaxy Cluster 1E 0657-5 | (Abridged) We compare recent results from X-ray, strong lensing, weak
lensing, and optical observations with numerical simulations of the merging
galaxy cluster 1E0657-56. X-ray observations reveal a bullet-like subcluster
with a prominent bow shock, while lensing results show that the positions of
the total mass peaks are consistent with the centroids of the collisionless
galaxies (and inconsistent with the X-ray brightness peaks). Previous studies,
based on older observational datasets, have placed upper limits on the
self-interaction cross-section of dark matter per unit mass, sigma/m, using
simplified analytic techniques. In this work, we take advantage of new,
higher-quality observational datasets by running N-body simulations of
1E0657-56 that include the effects of self-interacting dark matter, and
comparing the results with observations. Furthermore, the recent data allow for
a new independent method of constraining sigma/m, based on the non-observation
of an offset between the bullet subcluster mass peak and galaxy centroid. This
new method places an upper limit (68% confidence) of sigma/m < 1.25 cm^2/g. If
we make the assumption that the subcluster and the main cluster had equal
mass-to-light ratios prior to the merger, we derive our most stringent
constraint of sigma/m < 0.7 cm^2/g, which comes from the consistency of the
subcluster's observed mass-to-light ratio with the main cluster's, and with the
universal cluster value, ruling out the possibility of a large fraction of dark
matter particles being scattered away due to collisions. Our limit is a slight
improvement over the previous result from analytic estimates, and rules out
most of the 0.5 - 5cm^2/g range invoked to explain inconsistencies between the
standard collisionless cold dark matter model and observations.
|
Stringy Instantons at Orbifold Singularities | We study the effects produced by D-brane instantons on the holomorphic
quantities of a D-brane gauge theory at an orbifold singularity. These effects
are not limited to reproducing the well known contributions of the gauge theory
instantons but also generate extra terms in the superpotential or the
prepotential. On these brane instantons there are some neutral fermionic
zero-modes in addition to the ones expected from broken supertranslations. They
are crucial in correctly reproducing effects which are dual to gauge theory
instantons, but they may make some other interesting contributions vanish. We
analyze how orientifold projections can remove these zero-modes and thus allow
for new superpotential terms. These terms contribute to the dynamics of the
effective gauge theory, for instance in the stabilization of runaway
directions.
|
Turbulent Diffusion of Lines and Circulations | We study material lines and passive vectors in a model of turbulent flow at
infinite-Reynolds number, the Kraichnan-Kazantsev ensemble of velocities that
are white-noise in time and rough (Hoelder continuous) in space. It is argued
that the phenomenon of ``spontaneous stochasticity'' generalizes to material
lines and that conservation of circulations generalizes to a ``martingale
property'' of the stochastic process of lines.
|
Gluon Radiation of an Expanding Color Skyrmion in the Quark-Gluon Plasma | The density of states and energy spectrum of the gluon radiation are
calculated for the color current of an expanding hydrodynamic skyrmion in the
quark gluon plasma with a semiclassical method. Results are compared with those
in literatures.
|
REM near-IR and optical multiband observations of PKS2155-304 in 2005 | Spectral variability is the main tool for constraining emission models of BL
Lac objects. By means of systematic observations of the BL Lac prototype PKS
2155-304 in the infrared-optical band, we explore variability on the scales of
months, days and hours. We made our observations with the robotic 60 cm
telescope REM located at La Silla, Chile. VRIJHK filters were used. PKS
2155-304 was observed from May to December 2005. The wavelength interval
explored, the total number of photometric points and the short integration time
render our photometry substantially superior to previous ones for this source.
On the basis of the intensity and colour we distinguish three different states
of the source, each of duration of months, which include all those described in
the literature. In particular, we report the highest state ever detected in the
H band. The source varied by a factor of 4 in this band, much more than in the
V band (a factor ~2). The source softened with increasing intensity, contrary
to the general pattern observed in the UV-X-ray bands. On five nights of
November we had nearly continuous monitoring for 2-3 hours. A variability
episode with a time scale of ~24 h is well documented, a much more rapid flare
with t=1-2 h, is also apparent, but is supported by relatively few points.
|
Supernova Polarization and the Type IIn Classification | While the members of the Type IIn category of supernovae are united by the
presence of strong multicomponent Balmer emission lines in their spectra, they
are quite heterogeneous with respect to other properties such as Balmer line
profiles, light curves, strength of radio emission, and intrinsic brightness.
We are now beginning to see variety among SNe IIn in their polarimetric
characteristics as well, some but not all of which may be due to inclination
angle effects. The increasing number of known "hybrid" SNe with IIn-like
emission lines suggests that circumstellar material may be more common around
all types of SNe than previously thought. Investigation of the correlations
between spectropolarimetric signatures and other IIn attributes will help us
address the question of classification of "interacting SNe" and the possibility
of distinguishing different groups within the diverse IIn subclass.
|
Near-Infrared Spectra of the Black Hole X-Ray Binary, A0620-00 | We present broadband NIR spectra of A0620-00 obtained with SpeX on the IRTF.
The spectrum is characterized by a blue continuum on which are superimposed
broad emission lines of HI and HeII and a host of narrower absorption lines of
neutral metals and molecules. Spectral type standard star spectra scaled to the
dereddened spectrum of A0620-00 in K exceed the A0620-00 spectrum in J and H
for all stars of spectral type K7V or earlier, demonstrating that the donor
star, unless later than K7V, cannot be the sole NIR flux source in A0620-00. In
addition, the atomic absorption lines in the K3V spectrum are too weak with
respect to those of A0620-00 even at 100% donor star contribution, restricting
the spectral type of the donor star in A0620-00 to later than K3V. Comparison
of the A0620-00 spectrum to scaled K star spectra indicates that the CO
absorption features are significantly weaker in A0620-00 than in field dwarf
stars. Fits of scaled model spectra of a Roche lobe-filling donor star to the
spectrum of A0620-00 show that the best match to the CO absorption lines is
obtained when the C abundance is reduced to [C/H] = -1.5. The donor star
contribution in the H waveband is determined to be 82+-2%. Combined with
previous published results from Froning & Robinson (2001) and Marsh et al.
(1994), this gives a precise mass for the black hole in A0620-00 of M_BH =
9.7+-0.6 M_solar.
|
Automated Generation of Layout and Control for Quantum Circuits | We present a computer-aided design flow for quantum circuits, complete with
automatic layout and control logic extraction. To motivate automated layout for
quantum circuits, we investigate grid-based layouts and show a performance
variance of four times as we vary grid structure and initial qubit placement.
We then propose two polynomial-time design heuristics: a greedy algorithm
suitable for small, congestion-free quantum circuits and a dataflow-based
analysis approach to placement and routing with implicit initial placement of
qubits. Finally, we show that our dataflow-based heuristic generates better
layouts than the state-of-the-art automated grid-based layout and scheduling
mechanism in terms of latency and potential pipelinability, but at the cost of
some area.
|
Modeling the Spectral Energy Distribution and Variability of 3C 66A
during the WEBT campaign of 2003 -- 2004 | The BL Lac object 3C 66A was observed in an extensive multiwavelength
monitoring campaign from July 2003 till April 2004. The spectral energy
distribution (SED) was measured over the entire electromagnetic spectrum, with
flux measurements from radio to X-ray frequencies and upper limits in the very
high energy (VHE) gamma-ray regime. Here, we use a time-dependent leptonic jet
model to reproduce the SED and optical spectral variability observed during our
multiwavelength campaign. Our model simulations could successfully reproduce
the observed SED and optical light curves and predict an intrinsic cutoff value
for the VHE gamma-ray emission at ~ 4 GeV. The effect of the optical depth due
to the intergalactic infrared background radiation (IIBR) on the peak of the
high-energy component of 3C 66A was found to be negligible. Also, the presence
of a broad line region (BLR) in the case of 3C 66A may play an important role
in the emission of gamma-ray photons when the emission region is very close to
the central engine, but further out, the production mechanism of hard X-ray and
gamma-ray photons becomes rapidly dominated by synchrotron self-Compton
emission. We further discuss the possibility of an observable X-ray spectral
variability pattern. The simulated results do not predict observable hysteresis
patterns in the optical or soft X-ray regimes for major flares on multi-day
time scales.
|
The HARPS search for southern extra-solar planets. X. A m sin i = 11
Mearth planet around the nearby spotted M dwarf GJ 674 | Context: How planet properties depend on stellar mass is a key diagnostic of
planetary formation mechanisms. Aims: This motivates planet searches around
stars which are significantly more massive or less massive than the Sun, and in
particular our radial velocity search for planets around very-low mass stars.
Methods: As part of that program, we obtained measurements of GJ 674, an M2.5
dwarf at d=4.5 pc, which have a dispersion much in excess of their internal
errors. An intensive observing campaign demonstrates that the excess dispersion
is due to two superimposed coherent signals, with periods of 4.69 and 35 days.
Results: These data are well described by a 2-planet Keplerian model where each
planet has a ~11 Mearth minimum mass. A careful analysis of the (low level)
magnetic activity of GJ 674 however demonstrates that the 35-day period
coincides with the stellar rotation period. This signal therefore originates in
a spot inhomogeneity modulated by stellar rotation. The 4.69-day signal on the
other hand is caused by a bona-fide planet, GJ 674b. Conclusion: Its detection
adds to the growing number of Neptune-mass planets around M-dwarfs, and
reinforces the emerging conclusion that this mass domain is much more populated
than the jovian mass range. We discuss the metallicity distributions of M dwarf
with and without planets and find a low 11% probability that they are drawn
from the same parent distribution. Moreover, we find tentative evidence that
the host star metallicity correlates with the total mass of their planetary
system.
|
An individual based model with global competition interaction:
fluctuations effects in pattern formation | We present some numerical results obtained from a simple individual based
model that describes clustering of organisms caused by competition. Our aim is
to show how, even when a deterministic description developed for continuum
models predicts no pattern formation, an individual based model displays well
defined patterns, as a consequence of fluctuations effects caused by the
discrete nature of the interacting agents.
|
A Comparison between Anomalous 6-cm H$_2$CO Absorption and CO(1-0)
Emission in the L1204/S140 | We report observations of the dust cloud L1204 with the Onsala 25-m telescope
in the 6 cm (1$_{11}-1_{10}$) transition of \htco. The observed region includes
the
S140 H${\alpha}$ arc. This spectral line is seen here in absorption against
the cosmic microwave background, indicating the presence of widespread warm
molecular gas at intermediate densities. Overall, the distributions of H$_2$CO
and CO (taken from the literature) are fairly similar, though significant
differences exist at small scales. Most notably, while the CO peak is nearly
coincident with the S140 H${\alpha}$ arc, the maximum H$_2$CO absorption is
clearly separated from it by a full 10$'$ beam ($\sim$ 3 pc). We argue that
these differences result from differing abundances and excitation requirements.
The CO(1-0) line is more optically thick and more biased towards warm gas than
the H$_2$CO 6 cm line. On the other hand, formaldehyde is more easily
photodissociated and is, therefore, a poorer tracer of the molecular gas
located immediately behind Photon Dominated Regions.
|
Dimers on surface graphs and spin structures. II | In a previous paper, we showed how certain orientations of the edges of a
graph G embedded in a closed oriented surface S can be understood as discrete
spin structures on S. We then used this correspondence to give a geometric
proof of the Pfaffian formula for the partition function of the dimer model on
G. In the present article, we generalize these results to the case of compact
oriented surfaces with boundary. We also show how the operations of cutting and
gluing act on discrete spin structures and how they change the partition
function. These operations allow to reformulate the dimer model as a quantum
field theory on surface graphs.
|
New version announcement for TaylUR, an arbitrary-order diagonal
automatic differentiation package for Fortran 95 | We present a new version of TaylUR, a Fortran 95 module to automatically
compute the numerical values of a complex-valued function's derivatives with
respect to several variables up to an arbitrary order in each variable, but
excluding mixed derivatives. The new version fixes a potentially serious bug in
the code for exponential-related functions that could corrupt the imaginary
parts of derivatives, as well as being compatible with a wider range of
compilers.
|
Mapping radii of metric spaces | It is known that every closed curve of length \leq 4 in R^n (n>0) can be
surrounded by a sphere of radius 1, and that this is the best bound. Letting S
denote the circle of circumference 4, with the arc-length metric, we here
express this fact by saying that the "mapping radius" of S in R^n is 1.
Tools are developed for estimating the mapping radius of a metric space X in
a metric space Y. In particular, it is shown that for X a bounded metric space,
the supremum of the mapping radii of X in all convex subsets of normed metric
spaces is equal to the infimum of the sup norms of all convex linear
combinations of the functions d(x,-): X --> R (x\in X).
Several explicit mapping radii are calculated, and open questions noted.
|
A New Model For The Loop-I (The North Polar Spur) Region | The North Polar Spur (NPS) is the brightest filament of Loop I, a large
circular feature in the radio continuum sky. In this paper, a model consisting
of two synchrotron emitting shells is presented that reproduces large-scale
structures revealed by recent polarization surveys. The polarized emission of
the NPS is reproduced by one of these shells. The other shell, which passes
close to the Sun, gives rise to polarized emission towards the Galactic poles.
It is proposed that X-ray emission seen towards the NPS is produced by
interaction of the two shells. Two OB-associations coincide with the centers of
the shells. A formation scenario of the Loop I region is suggested.
|
Leray numbers of projections and a topological Helly type theorem | Let X be a simplicial complex on the vertex set V. The rational Leray number
L(X) of X is the minimal d such that the rational reduced homology of any
induced subcomplex of X vanishes in dimensions d and above. Let \pi be a
simplicial map from X to a simplex Y, such that the cardinality of the preimage
of any point in |Y| is at most r. It is shown that L(\pi(X)) \leq r L(X)+r-1.
One consequence is a topological extension of a Helly type result of Amenta.
|
q-Deformed spin foam models of quantum gravity | We numerically study Barrett-Crane models of Riemannian quantum gravity. We
have extended the existing numerical techniques to handle q-deformed models and
arbitrary space-time triangulations. We present and interpret expectation
values of a few selected observables for each model, including a spin-spin
correlation function which gives insight into the behaviour of the models. We
find the surprising result that, as the deformation parameter q goes to 1
through roots of unity, the limit is discontinuous.
|
HI velocity dispersion in NGC 1058 | We present excellent resolution and high sensitivity Very Large Array (VLA)
observations of the 21cm HI line emission from the face-on galaxy NGC 1058,
providing the first reliable study of the HI profile shapes throughout the
entire disk of an external galaxy. Our observations show an intriguing picture
of the interstellar medium; throughout this galaxy velocity-- dispersions range
between 4 to 15 km/sec but are not correlated with star formation, stars or the
gaseous spiral arms. The velocity dispersions decrease with radius, but this
global trend has a large scatter as there are several isolated, resolved
regions of high dispersion. The decline of star light with radius is much
steeper than that of the velocity dispersions or that of the energy in the gas
motions.
|
Common Envelope Evolution Redux | Common envelopes form in dynamical time scale mass exchange, when the
envelope of a donor star engulfs a much denser companion, and the core of the
donor plus the dense companion star spiral inward through this dissipative
envelope. As conceived by Paczynski and Ostriker, this process must be
responsible for the creation of short-period binaries with degenerate
components, and, indeed, it has proven capable of accounting for short-period
binaries containing one white dwarf component. However, attempts to reconstruct
the evolutionary histories of close double white dwarfs have proven more
problematic, and point to the need for enhanced systemic mass loss, either
during the close of the first, slow episode of mass transfer that produced the
first white dwarf, or during the detached phase preceding the final, common
envelope episode. The survival of long-period interacting binaries with massive
white dwarfs, such as the recurrent novae T CrB and RS Oph, also presents
interpretative difficulties for simple energetic treatments of common envelope
evolution. Their existence implies that major terms are missing from usual
formulations of the energy budget for common envelope evolution. The most
plausible missing energy term is the energy released by recombination in the
common envelope, and, indeed, a simple reformulation the energy budget
explicitly including recombination resolves this issue.
|
The Source of Turbulence in Astrophysical Disks: An Ill-posed Problem. | An critical overview of the current state of research in turbulence in
astrophysical disks.
|
On Punctured Pragmatic Space-Time Codes in Block Fading Channel | This paper considers the use of punctured convolutional codes to obtain
pragmatic space-time trellis codes over block-fading channel. We show that good
performance can be achieved even when puncturation is adopted and that we can
still employ the same Viterbi decoder of the convolutional mother code by using
approximated metrics without increasing the complexity of the decoding
operations.
|
On the Markov trace for Temperley--Lieb algebras of type $E_n$ | We show that there is a unique Markov trace on the tower of Temperley--Lieb
type quotients of Hecke algebras of Coxeter type $E_n$ (for all $n \geq 6$). We
explain in detail how this trace may be computed easily using tom Dieck's
calculus of diagrams. As applications, we show how to use the trace to show
that the diagram representation is faithful, and to compute leading
coefficients of certain Kazhdan--Lusztig polynomials.
|
Second Order Perturbative Calculation of Quasinormal Modes of
Schwarzschild Black Holes | We analytically calculate to second order the correction to the asymptotic
form of quasinormal frequencies of four dimensional Schwarzschild black holes
based on the monodromy analysis proposed by Motl and Neitzke. Our results are
in good agreement with those obtained from numerical calculation.
|
Epitaxial graphene | Graphene multilayers are grown epitaxially on single crystal silicon carbide.
This system is composed of several graphene layers of which the first layer is
electron doped due to the built-in electric field and the other layers are
essentially undoped. Unlike graphite the charge carriers show Dirac particle
properties (i.e. an anomalous Berry's phase, weak anti-localization and square
root field dependence of the Landau level energies). Epitaxial graphene shows
quasi-ballistic transport and long coherence lengths; properties which may
persists above cryogenic temperatures. Paradoxically, in contrast to exfoliated
graphene, the quantum Hall effect is not observed in high mobility epitaxial
graphene. It appears that the effect is suppressed due to absence of localized
states in the bulk of the material.Epitaxial graphene can be patterned using
standard lithography methods and characterized using a wide array of
techniques. These favorable features indicate that interconnected room
temperature ballistic devices may be feasible for low dissipation high-speed
nanoelectronics.
|
Mathematics of thermoacoustic tomography | The paper presents a survey of mathematical problems, techniques, and
challenges arising in the Thermoacoustic and Photoacoustic Tomography.
|
Search for Very High Energy Emission from Gamma-Ray Bursts using Milagro | Gamma-Ray Bursts (GRBs) have been detected at GeV energies by EGRET and
models predict emission at > 100 GeV. Milagro is a wide field (2 sr) high duty
cycle (> 90 %) ground based water Cherenkov detector that records extensive air
showers in the energy range 100 GeV to 100 TeV. We have searched for very high
energy emission from a sample of 106 gamma-ray bursts (GRB) detected since the
beginning of 2000 by BATSE, BeppoSax, HETE-2, INTEGRAL, Swift or the IPN. No
evidence for emission from any of the bursts has been found and we present
upper limits from these bursts.
|
Specific heat and bimodality in canonical and grand canonical versions
of the thermodynamic model | We address two issues in the thermodynamic model for nuclear disassembly.
Surprisingly large differences in results for specific heat were seen in
predictions from the canonical and grand canonical ensembles when the nuclear
system passes from liquid-gas co-existence to the pure gas phase. We are able
to pinpoint and understand the reasons for such and other discrepancies when
they appear. There is a subtle but important difference in the physics
addressed in the two models. In particular if we reformulate the parameters in
the canonical model to better approximate the physics addressed in the grand
canonical model, calculations for observables converge. Next we turn to the
issue of bimodality in the probability distribution of the largest fragment in
both canonical and grand canonical ensembles. We demonstrate that this
distribution is very closely related to average multiplicities. The
relationship of the bimodal distribution to phase transition is discussed.
|
Vortex proliferation in the Berezinskii-Kosterlitz-Thouless regime on a
two-dimensional lattice of Bose-Einstein condensates | We observe the proliferation of vortices in the
Berezinskii-Kosterlitz-Thouless regime on a two-dimensional array of
Josephson-coupled Bose-Einstein condensates. As long as the Josephson
(tunneling) energy J exceeds the thermal energy T, the array is vortex-free.
With decreasing J/T, vortices appear in the system in ever greater numbers. We
confirm thermal activation as the vortex formation mechanism and obtain
information on the size of bound vortex pairs as J/T is varied.
|
The core binary fractions of star clusters from realistic simulations | We investigate the evolution of binary fractions in star clusters using
N-body models of up to 100000 stars. Primordial binary frequencies in these
models range from 5% to 50%. Simulations are performed with the NBODY4 code and
include a full mass spectrum of stars, stellar evolution, binary evolution and
the tidal field of the Galaxy. We find that the overall binary fraction of a
cluster almost always remains close to the primordial value, except at late
times when a cluster is near dissolution. A critical exception occurs in the
central regions where we observe a marked increase in binary fraction with time
-- a simulation starting with 100000 stars and 5% binaries reached a core
binary frequency as high as 40% at the end of the core-collapse phase
(occurring at 16 Gyr with ~20000 stars remaining). Binaries are destroyed in
the core by a variety of processes as a cluster evolves, but the combination of
mass-segregation and creation of new binaries in exchange interactions produces
the observed increase in relative number. We also find that binaries are cycled
into and out of cluster cores in a manner that is analogous to convection in
stars. For models of 100000 stars we show that the evolution of the core-radius
up to the end of the initial phase of core-collapse is not affected by the
exact value of the primordial binary frequency (for frequencies of 10% or
less). We discuss the ramifications of our results for the likely primordial
binary content of globular clusters.
|
Approaching the Heisenberg limit in an atom laser | We present experimental and theoretical results showing the improved beam
quality and reduced divergence of an atom laser produced by an optical Raman
transition, compared to one produced by an RF transition. We show that Raman
outcoupling can eliminate the diverging lens effect that the condensate has on
the outcoupled atoms. This substantially improves the beam quality of the atom
laser, and the improvement may be greater than a factor of ten for experiments
with tight trapping potentials. We show that Raman outcoupling can produce atom
lasers whose quality is only limited by the wavefunction shape of the
condensate that produces them, typically a factor of 1.3 above the Heisenberg
limit.
|
A practical Seedless Infrared-Safe Cone jet algorithm | Current cone jet algorithms, widely used at hadron colliders, take event
particles as seeds in an iterative search for stable cones. A longstanding
infrared (IR) unsafety issue in such algorithms is often assumed to be solvable
by adding extra `midpoint' seeds, but actually is just postponed to one order
higher in the coupling. A proper solution is to switch to an exact seedless
cone algorithm, one that provably identifies all stable cones. The only
existing approach takes N 2^N time to find jets among N particles, making it
unusable at hadron level. This can be reduced to N^2 ln(N) time, leading to
code (SISCone) whose speed is similar to that of public midpoint
implementations. Monte Carlo tests provide a strong cross-check of an
analytical proof of the IR safety of the new algorithm, and the absence of any
'R_{sep}' issue implies a good practical correspondence between parton and
hadron levels. Relative to a midpoint cone, the use of an IR safe seedless
algorithm leads to modest changes for inclusive jet spectra, mostly through
reduced sensitivity to the underlying event, and significant changes for some
multi-jet observables.
|
Isospin breaking in the yield of heavy meson pairs in e+e- annihilation
near threshold | We revisit the problem of interplay between the strong and the Coulomb
interaction in the charged-to-neutral yield ratio for $B {\bar B}$ and $D {\bar
D}$ pairs near their respective thresholds in $e^+e^-$ annihilation. We
consider here a realistic situation with a resonant interaction in the isospin
I=0 channel and a nonresonant strong scattering amplitude in the I=1 state. We
find that the yield ratio has a smooth behavior depending on the scattering
phase in the I=1 channel. The same approach is also applicable to the $K {\bar
K}$ production at the $\phi(1020)$ resonance, where the Coulomb effect in the
charged-to-neutral yield ratio is generally sensitive to the scattering phases
in both the isoscalar and the isovector channels. Furthermore, we apply the
same approach to the treatment of the effect of the isotopic mass difference
between the charged and neutral mesons and argue that the strong-scattering
effects generally result in a modification to the pure kinematical effect of
this mass difference.
|
QED x QCD Resummation and Shower/ME Matching for LHC Physics | We present the theory of QED x QCD resummation and its interplay with
shower/matrix element matching in precision LHC physics scenarios. We
illustrate the theory using single heavy gauge boson production at hadron
colliders.
|
On the number of topological types occurring in a parametrized family of
arrangements | Let ${\mathcal S}(\R)$ be an o-minimal structure over $\R$, $T \subset
\R^{k_1+k_2+\ell}$ a closed definable set, and $$ \displaylines{\pi_1:
\R^{k_1+k_2+\ell}\to \R^{k_1 + k_2}, \pi_2: \R^{k_1+k_2+\ell}\to \R^{\ell}, \
\pi_3: \R^{k_1 + k_2} \to \R^{k_2}} $$ the projection maps.
For any collection ${\mathcal A} = \{A_1,...,A_n\}$ of subsets of
$\R^{k_1+k_2}$, and $\z \in \R^{k_2}$, let $\A_\z$ denote the collection of
subsets of $\R^{k_1}$, $\{A_{1,\z},..., A_{n,\z}\}$, where $A_{i,\z} = A_i \cap
\pi_3^{-1}(\z), 1 \leq i \leq n$. We prove that there exists a constant $C =
C(T) > 0,$ such that for any family ${\mathcal A} = \{A_1,...,A_n\}$ of
definable sets, where each $A_i = \pi_1(T \cap \pi_2^{-1}(\y_i))$, for some
$\y_i \in \R^{\ell}$, the number of distinct stable homotopy types of $\A_\z,
\z \in \R^{k_2}$, is bounded by $ \displaystyle{C \cdot n^{(k_1+1)k_2},} $
while the number of distinct homotopy types is bounded by $ \displaystyle{C
\cdot n^{(k_1+3)k_2}.} $ This generalizes to the general o-minimal setting,
bounds of the same type proved in \cite{BV} for semi-algebraic and
semi-Pfaffian families. One main technical tool used in the proof of the above
results, is a topological comparison theorem which might be of independent
interest in the study of arrangements.
|
Generalized Twistor Transform And Dualities, With A New Description of
Particles With Spin, Beyond Free and Massless | A generalized twistor transform for spinning particles in 3+1 dimensions is
constructed that beautifully unifies many types of spinning systems by mapping
them to the same twistor, thus predicting an infinite set of duality relations
among spinning systems with different Hamiltonians. Usual 1T-physics is not
equipped to explain the duality relationships and unification between these
systems. We use 2T-physics in 4+2 dimensions to uncover new properties of
twistors, and expect that our approach will prove to be useful for practical
applications as well as for a deeper understanding of fundamental physics.
Unexpected structures for a new description of spinning particles emerge. A
unifying symmetry SU(2,3) that includes conformal symmetry SU(2,2)=SO(4,2) in
the massless case, turns out to be a fundamental property underlying the
dualities of a large set of spinning systems, including those that occur in
high spin theories. This may lead to new forms of string theory backgrounds as
well as to new methods for studying various corners of M theory. In this paper
we present the main concepts, and in a companion paper we give other details.
|
Remnant evolution after a carbon-oxygen white dwarf merger | We systematically explore the evolution of the merger of two carbon-oxygen
(CO) white dwarfs. The dynamical evolution of a 0.9 Msun + 0.6 Msun CO white
dwarf merger is followed by a three-dimensional SPH simulation. We use an
elaborate prescription in which artificial viscosity is essentially absent,
unless a shock is detected, and a much larger number of SPH particles than
earlier calculations. Based on this simulation, we suggest that the central
region of the merger remnant can, once it has reached quasi-static equilibrium,
be approximated as a differentially rotating CO star, which consists of a
slowly rotating cold core and a rapidly rotating hot envelope surrounded by a
centrifugally supported disc. We construct a model of the CO remnant that
mimics the results of the SPH simulation using a one-dimensional hydrodynamic
stellar evolution code and then follow its secular evolution. The stellar
evolution models indicate that the growth of the cold core is controlled by
neutrino cooling at the interface between the core and the hot envelope, and
that carbon ignition in the envelope can be avoided despite high effective
accretion rates. This result suggests that the assumption of forced accretion
of cold matter that was adopted in previous studies of the evolution of double
CO white dwarf merger remnants may not be appropriate. Our results imply that
at least some products of double CO white dwarfs merger may be considered good
candidates for the progenitors of Type Ia supernovae. In this case, the
characteristic time delay between the initial dynamical merger and the eventual
explosion would be ~10^5 yr. (Abridged).
|
Direct Theorems in the Theory of Approximation of the Banach Space
Vectors by Entire Vectors of Exponential Type | For an arbitrary operator A on a Banach space X which is a generator of
C_0-group with certain growth condition at the infinity, the direct theorems on
connection between the smoothness degree of a vector $x\in X$ with respect to
the operator A, the order of convergence to zero of the best approximation of x
by exponential type entire vectors for the operator A, and the k-module of
continuity are given. Obtained results allows to acquire Jackson-type
inequalities in many classic spaces of periodic functions and weighted $L_p$
spaces.
|
Parametrized Post-Newtonian Expansion of Chern-Simons Gravity | We investigate the weak-field, post-Newtonian expansion to the solution of
the field equations in Chern-Simons gravity with a perfect fluid source. In
particular, we study the mapping of this solution to the parameterized
post-Newtonian formalism to 1 PN order in the metric. We find that the PPN
parameters of Chern-Simons gravity are identical to those of general
relativity, with the exception of the inclusion of a new term that is
proportional to the Chern-Simons coupling parameter and the curl of the PPN
vector potentials. We also find that the new term is naturally enhanced by the
non-linearity of spacetime and we provide a physical interpretation for it. By
mapping this correction to the gravito-electro-magnetic framework, we study the
corrections that this new term introduces to the acceleration of point
particles and the frame-dragging effect in gyroscopic precession. We find that
the Chern-Simons correction to these classical predictions could be used by
current and future experiments to place bounds on intrinsic parameters of
Chern-Simons gravity and, thus, string theory.
|
Scaling of Resistance and Electron Mean Free Path of Single-Walled
Carbon Nanotubes | We present an experimental investigation on the scaling of resistance in
individual single walled carbon nanotube devices with channel lengths that vary
four orders of magnitude on the same sample. The electron mean free path is
obtained from the linear scaling of resistance with length at various
temperatures. The low temperature mean free path is determined by impurity
scattering, while at high temperature the mean free path decreases with
increasing temperature, indicating that it is limited by electron-phonon
scattering. An unusually long mean free path at room temperature has been
experimentally confirmed. Exponentially increasing resistance with length at
extremely long length scales suggests anomalous localization effects.
|