828 research outputs found
A classical model for the negative dc conductivity of ac-driven 2D electrons near the cyclotron resonance
A classical model for {\em dc} transport of two dimensional electrons in a
perpendicular magnetic field and under strong irradiation is considered. We
demonstrate that, near the cyclotron resonance condition, and for {\em linear}
polarization of the {\em ac} field, a strong change of the diagonal component,
, of the {\em dc} conductivity occurs in the presence of a {\em weak}
nonparabolicity of the electron spectrum. Small change in the electron
effective mass due to irradiation can lead to negative , while the
Hall component of the {\em dc} conductivity remains practically unchanged.
Within the model considered, the sign of depends on the relative
orientation of the {\em dc} and {\em ac} fields, the sign of the detuning of
the {\em ac} frequency from the cyclotron resonance, and the sign of
nonparabolic term in the energy spectrum.Comment: 4 pages, 1 figur
DNA methylation-associated colonic mucosal immune and defense responses in treatment-naïve pediatric ulcerative colitis
Inflammatory bowel diseases (IBD) are emerging globally, indicating that environmental factors may be important in their pathogenesis. Colonic mucosal epigenetic changes, such as DNA methylation, can occur in response to the environment and have been implicated in IBD pathology. However, mucosal DNA methylation has not been examined in treatment-naïve patients. We studied DNA methylation in untreated, left sided colonic biopsy specimens using the Infinium HumanMethylation450 BeadChip array. We analyzed 22 control (C) patients, 15 untreated Crohn’s disease (CD) patients, and 9 untreated ulcerative colitis (UC) patients from two cohorts. Samples obtained at the time of clinical remission from two of the treatment-naïve UC patients were also included into the analysis. UC-specific gene expression was interrogated in a subset of adjacent samples (5 C and 5 UC) using the Affymetrix GeneChip PrimeView Human Gene Expression Arrays. Only treatment-naïve UC separated from control. One-hundred-and-twenty genes with significant expression change in UC (> 2-fold, P < 0.05) were associated with differentially methylated regions (DMRs). Epigenetically associated gene expression changes (including gene expression changes in the IFITM1, ITGB2, S100A9, SLPI, SAA1, and STAT3 genes) were linked to colonic mucosal immune and defense responses. These findings underscore the relationship between epigenetic changes and inflammation in pediatric treatment-naïve UC and may have potential etiologic, diagnostic, and therapeutic relevance for IBD
Nucleon-Nucleon Scattering under Spin-Isospin Reversal in Large-N_c QCD
The spin-flavor structure of certain nucleon-nucleon scattering observables
derived from the large N_c limit of QCD in the kinematical regime where
time-dependent mean-field theory is valid is discussed. In previous work, this
regime was taken to be where the external momentum was of order N_c which
precluded the study of differential cross sections in elastic scattering. Here
it is shown that the regime extends down to order N_c^{1/2} which includes the
higher end of the elastic regime. The prediction is that in the large N_c
limit, observables describable via mean-field theory are unchanged when the
spin and isospin of either nucleon are both flipped. This prediction is tested
for proton-proton and neutron-proton elastic scattering data and found to fail
badly. We argue that this failure can be traced to a lack of a clear separation
of scales between momentum of order N_c^{1/2} and N_c^1 when N_c is as small as
three. The situation is compounded by an anomalously low particle production
threshold due to approximate chiral symmetry.Comment: 5 pages, 1 figur
Поднадкостничная имплантация на нижней челюсти
В статье описан метод субпериостальной имплантации. Данная методика является альтернативой в лечении пациентов с частичной и полной адентией. Разобраны показания, этапы лечения и возможные осложнения.В статье описан метод субпериостальной имплантации. Данная методика является альтернативой в лечении пациентов с частичной и полной адентией. Разобраны показания, этапы лечения и возможные осложнения
A 7/9 - Approximation Algorithm for the Maximum Traveling Salesman Problem
We give a 7/9 - Approximation Algorithm for the Maximum Traveling Salesman
Problem.Comment: 6 figure
Microscopic nonequilibrium theory of double-barrier Josephson junctions
We study nonequilibrium charge transport in a double-barrier Josephson
junction, including nonstationary phenomena, using the time-dependent
quasiclassical Keldysh Green's function formalism. We supplement the kinetic
equations by appropriate time-dependent boundary conditions and solve the
time-dependent problem in a number of regimes. From the solutions,
current-voltage characteristics are derived. It is understood why the
quasiparticle current can show excess current as well as deficit current and
how the subgap conductance behaves as function of junction parameters. A
time-dependent nonequilibrium contribution to the distribution function is
found to cause a non-zero averaged supercurrent even in the presence of an
applied voltage. Energy relaxation due to inelastic scattering in the
interlayer has a prominent role in determining the transport properties of
double-barrier junctions. Actual inelastic scattering parameters are derived
from experiments. It is shown as an application of the microscopic model, how
the nature of the intrinsic shunt in double-barrier junctions can be explained
in terms of energy relaxation and the opening of Andreev channels.Comment: Accepted for Phys. Rev.
Hadronic physics with domain-wall valence and improved staggered sea quarks
With the advent of chiral fermion formulations, the simulation of light
valence quarks has finally become realistic for numerical simulations of
lattice QCD. The simulation of light dynamical quarks, however, remains one of
the major challenges and is still an obstacle to realistic simulations. We
attempt to meet this challenge using a hybrid combination of Asqtad sea quarks
and domain-wall valence quarks. Initial results for the proton form factor and
the nucleon axial coupling are presented.Comment: Two Talks presented at Lattice2004(spectrum), LaTex, 6 pages, 6 eps
figure
Новый подход в соединении абатмента и имплантата, особенности ортопедического этапа при использовании имплантационной системы Leone exacone
The article is described to advantages and disadvantages of cone- or screw-retained abutments. A new method of abutment fixation to implant is introduced. Due to self-locking properties of the Morse cone and to the internal hexagon's resistance to torsion movements, quality of treatment improves.В статье рассмотрены основные варианты соединения абатмента и имплантата, их недостатки и преимущества. Описана новая методика фиксации абатмента за счет самоблокирующего конуса Морзе с внутренним шестигранником
A classification of smooth embeddings of 3-manifolds in 6-space
We work in the smooth category. If there are knotted embeddings S^n\to R^m,
which often happens for 2m<3n+4, then no concrete complete description of
embeddings of n-manifolds into R^m up to isotopy was known, except for disjoint
unions of spheres. Let N be a closed connected orientable 3-manifold. Our main
result is the following description of the set Emb^6(N) of embeddings N\to R^6
up to isotopy.
The Whitney invariant W : Emb^6(N) \to H_1(N;Z) is surjective. For each u \in
H_1(N;Z) the Kreck invariant \eta_u : W^{-1}u \to Z_{d(u)} is bijective, where
d(u) is the divisibility of the projection of u to the free part of H_1(N;Z).
The group Emb^6(S^3) is isomorphic to Z (Haefliger). This group acts on
Emb^6(N) by embedded connected sum. It was proved that the orbit space of this
action maps under W bijectively to H_1(N;Z) (by Vrabec and Haefliger's
smoothing theory). The new part of our classification result is determination
of the orbits of the action. E. g. for N=RP^3 the action is free, while for
N=S^1\times S^2 we construct explicitly an embedding f : N \to R^6 such that
for each knot l:S^3\to R^6 the embedding f#l is isotopic to f.
Our proof uses new approaches involving the Kreck modified surgery theory or
the Boechat-Haefliger formula for smoothing obstruction.Comment: 32 pages, a link to http://www.springerlink.com added, to appear in
Math. Zei
Synthesis and characterization of atomically-thin graphite films on a silicon carbide substrate
This paper reports the synthesis and detailed characterization of graphite
thin films produced by thermal decomposition of the (0001) face of a 6H-SiC
wafer, demonstrating the successful growth of single crystalline films down to
approximately one graphene layer. The growth and characterization were carried
out in ultrahigh vacuum (UHV) conditions. The growth process and sample quality
were monitored by low-energy electron diffraction, and the thickness of the
sample was determined by core level x-ray photoelectron spectroscopy.
High-resolution angle-resolved photoemission spectroscopy shows constant energy
map patterns, which are very sharp and fully momentum-resolved, but nonetheless
not resolution limited. We discuss the implications of this observation in
connection with scanning electron microscopy data, as well as with previous
studies
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