2,390 research outputs found
Signature of Spin Collective Mode in Local Tunneling Spectra of a d-wave Superconductor
We consider the influence of magnetic excitations on the local density of
states in the d-wave superconductor. The magnetic susceptibility is calculated
within the renormalized model and its influence on the quasiparticle
self-energy is considered using a minimal model originally proposed by
Polkovnikov {\it et al.}[cond-mat/0203176]. We find the local density of states
possess periodic components both along and directions
with the associated wavevectors changing in magnitude as the quasiparticle
energy is varied. Comparison with the STM experiment reveals that the
calculated LDOS modulation is inconsistent with the measured data.Comment: Two figures separately attached as .jpg file
Decision Problems For Convex Languages
In this paper we examine decision problems associated with various classes of
convex languages, studied by Ang and Brzozowski (under the name "continuous
languages"). We show that we can decide whether a given language L is prefix-,
suffix-, factor-, or subword-convex in polynomial time if L is represented by a
DFA, but that the problem is PSPACE-hard if L is represented by an NFA. In the
case that a regular language is not convex, we prove tight upper bounds on the
length of the shortest words demonstrating this fact, in terms of the number of
states of an accepting DFA. Similar results are proved for some subclasses of
convex languages: the prefix-, suffix-, factor-, and subword-closed languages,
and the prefix-, suffix-, factor-, and subword-free languages.Comment: preliminary version. This version corrected one typo in Section
2.1.1, line
Signal Transmission Across Tile Assemblies: 3D Static Tiles Simulate Active Self-Assembly by 2D Signal-Passing Tiles
The 2-Handed Assembly Model (2HAM) is a tile-based self-assembly model in
which, typically beginning from single tiles, arbitrarily large aggregations of
static tiles combine in pairs to form structures. The Signal-passing Tile
Assembly Model (STAM) is an extension of the 2HAM in which the tiles are
dynamically changing components which are able to alter their binding domains
as they bind together. For our first result, we demonstrate useful techniques
and transformations for converting an arbitrarily complex STAM tile set
into an STAM tile set where every tile has a constant, low amount of
complexity, in terms of the number and types of ``signals'' they can send, with
a trade off in scale factor.
Using these simplifications, we prove that for each temperature
there exists a 3D tile set in the 2HAM which is intrinsically universal for the
class of all 2D STAM systems at temperature (where the STAM does
not make use of the STAM's power of glue deactivation and assembly breaking, as
the tile components of the 2HAM are static and unable to change or break
bonds). This means that there is a single tile set in the 3D 2HAM which
can, for an arbitrarily complex STAM system , be configured with a
single input configuration which causes to exactly simulate at a scale
factor dependent upon . Furthermore, this simulation uses only two planes of
the third dimension. This implies that there exists a 3D tile set at
temperature in the 2HAM which is intrinsically universal for the class of
all 2D STAM systems at temperature . Moreover, we show that for each
temperature there exists an STAM tile set which is intrinsically
universal for the class of all 2D STAM systems at temperature ,
including the case where .Comment: A condensed version of this paper will appear in a special issue of
Natural Computing for papers from DNA 19. This full version contains proofs
not seen in the published versio
Axion-like particle effects on the polarization of cosmic high-energy gamma sources
Various satellite-borne missions are being planned whose goal is to measure
the polarization of a large number of gamma-ray bursts (GRBs). We show that the
polarization pattern predicted by current models of GRB emission can be
drastically modified by the existence of very light axion-like particles
(ALPs), which are present in many extensions of the Standard Model of particle
physics. Basically, the propagation of photons emitted by a GRB through cosmic
magnetic fields with a domain-like structure induces photon-ALP mixing, which
is expected to produce a strong modification of the original photon
polarization. Because of the random orientation of the magnetic field in each
domain, this effect strongly depends on the orientation of the photon line of
sight. As a consequence, photon-ALP conversion considerably broadens the
original polarization distribution. Searching for such a peculiar feature
through future high-statistics polarimetric measurements is therefore a new
opportunity to discover very light ALPs.Comment: Final version (21 pages, 8 eps figures). Matches the version
published on JCAP. Added a Section on the effects of cosmic expansion on
photon-ALP conversions. Figures modified to take into account this effect.
References updated. Conclusions unchanged
Coulomb effects in tunneling through a quantum dot stack
Tunneling through two vertically coupled quantum dots is studied by means of
a Pauli master equation model. The observation of double peaks in the
current-voltage characteristic in a recent experiment is analyzed in terms of
the tunnel coupling between the quantum dots and the coupling to the contacts.
Different regimes for the emitter chemical potential indicating different peak
scenarios in the tunneling current are discussed in detail. We show by
comparison with a density matrix approach that the interplay of coherent and
incoherent effects in the stationary current can be fully described by this
approach.Comment: 6 pages, 6 figure
Gauge coupling unification with large extra dimensions
We make a detailed study of the unification of gauge couplings in the MSSM
with large extra dimensions. We find some scenarios where unification can be
achieved (with the strong coupling constant at the Z mass within one standard
deviation of the experimental value) with both the compactification scale and
the SUSY breaking scale in the few TeV range. No enlargement of the gauge group
or particle content is needed. One particularly interesting scenario is when
the SUSY breaking scale is larger than the compactification scale, but both are
small enough to be probed at the CERN LHC. Unification in two scales scenarios
is also investigated and found to give results within the LHC.Comment: 17 pages, 3 figures, some discussions added, few additional
references included. Version to appear in Phys. Rev.
The Energy-dependent Checkerboard Patterns in Cuprate Superconductors
Motivated by the recent scanning tunneling microscopy (STM) experiments [J.
E. Hoffman {\it et al.}, Science {\bf 297}, 1148 (2002); K. McElroy {\it et
al.}, Nature (to be published)], we investigate the real space local density of
states (LDOS) induced by weak disorder in a d-wave superconductor. We first
present the energy dependent LDOS images around a single weak defect at several
energies, and then point out that the experimentally observed checkerboard
pattern in the LDOS could be understood as a result of quasiparticle
interferences by randomly distributed defects. It is also shown that the
checkerboard pattern oriented along to the Cu-O bonds at low energies
would transform to that oriented parallel to the Cu-O bonds at higher energies.
This result is consistent with the experiments.Comment: 3 pages, 3 figure
WD + MS systems as the progenitor of SNe Ia
We show the initial and final parameter space for SNe Ia in a () plane and find that the positions of some famous
recurrent novae, as well as a supersoft X-ray source (SSS), RX J0513.9-6951,
are well explained by our model. The model can also explain the space velocity
and mass of Tycho G, which is now suggested to be the companion star of Tycho's
supernova. Our study indicates that the SSS, V Sge, might be the potential
progenitor of supernovae like SN 2002ic if the delayed dynamical-instability
model due to Han & Podsiadlowski (2006) is appropriate. Following the work of
Meng, Chen & Han (2009), we found that the SD model (WD + MS) with an optically
thick wind can explain the birth rate of supernovae like SN 2006X and reproduce
the distribution of the color excess of SNe Ia. The model also predicts that at
least 75% of all SNe Ia may show a polarization signal in their spectra.Comment: 6 pages, 2 figures, accepted for publication in Astrophysics & Space
Science (Proceeding of the 4th Meeting on Hot Subdwarf Stars and Related
Objects, edited by Zhanwen Han, Simon Jeffery & Philipp Podsiadlowski
The fermi arc and fermi pocket in cuprates in a short-range diagonal stripe phase
In this paper we studied the fermi arc and the fermi pocket in cuprates in a
short-range diagonal stripe phase with wave vectors , which
reproduce with a high accuracy the positions and sizes of the fermi arc and
fermi pocket and the superstructure in cuprates observed by Meng et
al\cite{Meng}. The low-energy spectral function indicates that the fermi pocket
results from the main band and the shadow band at the fermi energy. Above the
fermi energy the shadow band gradually departs away from the main band, leaving
a fermi arc. Thus we conclude that the fermi arc and fermi pocket can be fully
attributed to the stripe phase but has nothing to do with pairing.
Incorporating a d-wave pairing potential in the stripe phase the spectral
weight in the antinodal region is removed, leaving a clean fermi pocket in the
nodal region.Comment: 5 pages, 6 figure
Enhanced expression of fibroblast growth factors and receptor FGFR-1 during vascular remodeling in chronic obstructive pulmonary disease
Important characteristics of chronic obstructive pulmonary disease (COPD)
include airway and vascular remodeling, the molecular mechanisms of which
are poorly understood. We assessed the role of fibroblast growth factors
(FGF) in pulmonary vascular remodeling by examining the expression pattern
of FGF-1, FGF-2, and the FGF receptor (FGFR-1) in peripheral area of lung
tissues from patients with COPD (FEV(1) < or = 75%; n = 15) and without
COPD (FEV(1) > or = 85%; n = 13). Immunohistochemical staining results
were evaluated by digital video image analysis as well as by manual
scoring. FGF-1 and FGFR-1 were detected in vascular smooth muscle (VSM),
airway smooth muscle, and airway epithelial cells. FGF-2 was localized in
the cytoplasm of airway epithelium and in the nuclei of airway smooth
muscle, VSM, and endothelial cells. In COPD cases, an unequivocal increase
in FGF-2 expression was observed in VSM (3-fold, P = 0.001) and
endothelium (2-fold, P = 0.007) of small pulmonary vessels with a luminal
diameter under 200 micro m. In addition, FGFR-1 levels were elevated in
the intima (1.5-fold, P = 0.05). VSM cells of large (> 200 micro m)
pulmonary vessels showed increased staining for FGF-1 (1.6-fold, P < 0.03)
and FGFR-1 (1.4-fold, P < 0.04) in COPD. Pulmonary vascular remodeling,
assessed as the ratio of alpha-smooth muscle actin staining and vascular
wall area with the lumen diameter, was increased in large vessels of
patients with COPD (P = 0.007) and was inversely correlated with FEV(1)
values (P < 0.007). Our results suggest an autocrine role of the
FGF-FGFR-1 system in the pathogenesis of COPD-associated vascular
remodeling
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