5,397 research outputs found
Maurer-Cartan moduli and models for function spaces
We set up a formalism of Maurer-Cartan moduli sets for L-infinity algebras
and associated twistings based on the closed model category structure on formal
differential graded algebras (a.k.a. differential graded coalgebras). Among
other things this formalism allows us to give a compact and manifestly homotopy
invariant treatment of Chevalley-Eilenberg and Harrison cohomology. We apply
the developed technology to construct rational homotopy models for function
spaces.Comment: 22 pages. This version, which will appear in Advances in Mathematics,
contains various technical corrections and updated bibliograph
Angular distribution of photoluminescence as a probe of Bose Condensation of trapped excitons
Recent experiments on two-dimensional exciton systems have shown the excitons
collect in shallow in-plane traps. We find that Bose condensation in a trap
results in a dramatic change of the exciton photoluminescence (PL) angular
distribution. The long-range coherence of the condensed state gives rise to a
sharply focussed peak of radiation in the direction normal to the plane. By
comparing the PL profile with and without Bose Condensation we provide a simple
diagnostic for the existence of a Bose condensate. The PL peak has strong
temperature dependence due to the thermal order parameter phase fluctuations
across the system. The angular PL distribution can also be used for imaging
vortices in the trapped condensate. Vortex phase spatial variation leads to
destructive interference of PL radiation in certain directions, creating nodes
in the PL distribution that imprint the vortex configuration.Comment: 4 pages, 3 figure
Codeword stabilized quantum codes: algorithm and structure
The codeword stabilized ("CWS") quantum codes formalism presents a unifying
approach to both additive and nonadditive quantum error-correcting codes
(arXiv:0708.1021). This formalism reduces the problem of constructing such
quantum codes to finding a binary classical code correcting an error pattern
induced by a graph state. Finding such a classical code can be very difficult.
Here, we consider an algorithm which maps the search for CWS codes to a problem
of identifying maximum cliques in a graph. While solving this problem is in
general very hard, we prove three structure theorems which reduce the search
space, specifying certain admissible and optimal ((n,K,d)) additive codes. In
particular, we find there does not exist any ((7,3,3)) CWS code though the
linear programming bound does not rule it out. The complexity of the CWS search
algorithm is compared with the contrasting method introduced by Aggarwal and
Calderbank (arXiv:cs/0610159).Comment: 11 pages, 1 figur
Ring-Like Solitons in Plasmonic Fiber Waveguide Composed of Metal-Dielectric Multilayers
We design a plasmonic fiber waveguide (PFW) composed of coaxial cylindrical
metal-dielectric multilayers in nanoscale, and constitute the corresponding
dynamical equations describing the modes of propagation in the PFW with the
Kerr nonlinearity in the dielectric layers. The physics is connected to the
discrete matrix nonlinear Schr\"{o}dinger equations, from which the highly
confined ring-like solitons in scale of subwavelength are found both for the
visible light and the near-infrared light in the self-defocusing condition.
Moreover, the confinement could be further improved when increasing the
intensity of the input light due to the cylindrical symmetry of the PFW, which
means both the width and the radius of the ring are reduced.Comment: 4 figures, submitte
Statistical Properties of Strings
We investigate numerically the configurational statistics of strings. The
algorithm models an ensemble of global cosmic strings, or equivalently
vortices in superfluid He. We use a new method which avoids the
specification of boundary conditions on the lattice. We therefore do not have
the artificial distinction between short and long string loops or a `second
phase' in the string network statistics associated with strings winding around
a toroidal lattice. Our lattice is also tetrahedral, which avoids ambiguities
associated with the cubic lattices of previous work. We find that the
percentage of infinite string is somewhat lower than on cubic lattices, 63\%
instead of 80\%. We also investigate the Hagedorn transition, at which infinite
strings percolate, controlling the string density by rendering one of the
equilibrium states more probable. We measure the percolation threshold, the
critical exponent associated with the divergence of a suitably defined
susceptibility of the string loops, and that associated with the divergence of
the correlation length.Comment: 20 pages, 8 figures (uuencoded) appended, DAMTP-94-56, SUSX-TP-94-7
MOCVD of hard metallurgical coatings: Examples in the Cr–C–N system
All individual phases of the ternary Cr–C–N system including stable and metastable ones can be deposited at low temperature by metalorganic chemical vapor deposition (MOCVD). These growth processes are mainly based on the use of bis(benzene)chromium as chromium source and various co-reactives. Then, from a good control of the reactive gas phase, it is possible to combine these MOCVD processes to grow in the same reactor protective coatings designed with a complex architecture based on polyphased, nanostructured or multilayer structure which exhibit enhanced properties. These deposition processes are described and the main features of the coatings are discussed
Dosimetric validation of a magnetic resonance image gated radiotherapy system using a motion phantom and radiochromic film.
PurposeMagnetic resonance image (MRI) guided radiotherapy enables gating directly on the target position. We present an evaluation of an MRI-guided radiotherapy system's gating performance using an MRI-compatible respiratory motion phantom and radiochromic film. Our evaluation is geared toward validation of our institution's clinical gating protocol which involves planning to a target volume formed by expanding 5 mm about the gross tumor volume (GTV) and gating based on a 3 mm window about the GTV.MethodsThe motion phantom consisted of a target rod containing high-contrast target inserts which moved in the superior-inferior direction inside a body structure containing background contrast material. The target rod was equipped with a radiochromic film insert. Treatment plans were generated for a 3 cm diameter spherical planning target volume, and delivered to the phantom at rest and in motion with and without gating. Both sinusoidal trajectories and tumor trajectories measured during MRI-guided treatments were used. Similarity of the gated dose distribution to the planned, motion-frozen, distribution was quantified using the gamma technique.ResultsWithout gating, gamma pass rates using 4%/3 mm criteria were 22-59% depending on motion trajectory. Using our clinical standard of repeated breath holds and a gating window of 3 mm with 10% target allowed outside the gating boundary, the gamma pass rate was 97.8% with 3%/3 mm gamma criteria. Using a 3 mm window and 10% allowed excursion, all of the patient tumor motion trajectories at actual speed resulting in at least 95% gamma pass rate at 4%/3 mm.ConclusionsOur results suggest that the device can be used to compensate respiratory motion using a 3 mm gating margin and 10% allowed excursion results in conjunction with repeated breath holds. Full clinical validation requires a comprehensive evaluation of tracking performance in actual patient images, outside the scope of this study
Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells
A microscopic theory for the generation and propagation of coherent LA
phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum
well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is
driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump
laser and is treated theoretically using the density matrix formalism. We use
realistic wurzite bandstructures taking valence-band mixing and strain-induced
piezo- electric fields into account. In addition, the many-body Coulomb
ineraction is treated in the screened time-dependent Hartree-Fock
approximation. We find that under typical experimental conditions, our
microscopic theory can be simplified and mapped onto a loaded string problem
which can be easily solved.Comment: 20 pages, 17 figure
Acceleration of Enterococcus faecalis Biofilm Formation by Aggregation Substance Expression in an Ex Vivo Model of Cardiac Valve Colonization
Infectious endocarditis involves formation of a microbial biofilm in vivo. Enterococcus faecalis Aggregation Substance (Asc10) protein enhances the severity of experimental endocarditis, where it has been implicated in formation of large vegetations and in microbial persistence during infection. In the current study, we developed an ex vivo porcine heart valve adherence model to study the initial interactions between Asc10+ and Asc10− E. faecalis and valve tissue, and to examine formation of E. faecalis biofilms on a relevant tissue surface. Scanning electron microscopy of the infected valve tissue provided evidence for biofilm formation, including growing masses of bacterial cells and the increasing presence of exopolymeric matrix over time; accumulation of adherent biofilm populations on the cardiac valve surfaces during the first 2–4 h of incubation was over 10-fold higher than was observed on abiotic membranes incubated in the same culture medium. Asc10 expression accelerated biofilm formation via aggregation between E. faecalis cells; the results also suggested that in vivo adherence to host tissue and biofilm development by E. faecalis can proceed by Asc10-dependent or Asc10-independent pathways. Mutations in either of two Asc10 subdomains previously implicated in endocarditis virulence reduced levels of adherent bacterial populations in the ex vivo system. Interference with the molecular interactions involved in adherence and initiation of biofilm development in vivo with specific inhibitory compounds could lead to more effective treatment of infectious endocarditis
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