5,005 research outputs found
Possible indicators for low dimensional superconductivity in the quasi-1D carbide Sc3CoC4
The transition metal carbide Sc3CoC4 consists of a quasi-one-dimensional (1D)
structure with [CoC4]_{\inft} polyanionic chains embedded in a scandium
matrix. At ambient temperatures Sc3CoC4 displays metallic behavior. At lower
temperatures, however, charge density wave formation has been observed around
143K which is followed by a structural phase transition at 72K. Below T^onset_c
= 4.5K the polycrystalline sample becomes superconductive. From Hc1(0) and
Hc2(0) values we could estimate the London penetration depth ({\lambda}_L ~=
9750 Angstroem) and the Ginsburg-Landau (GL) coherence length ({\xi}_GL ~= 187
Angstroem). The resulting GL-parameter ({\kappa} ~= 52) classifies Sc3CoC4 as a
type II superconductor. Here we compare the puzzling superconducting features
of Sc3CoC4, such as the unusual temperature dependence i) of the specific heat
anomaly and ii) of the upper critical field H_c2(T) at T_c, and iii) the
magnetic hysteresis curve, with various related low dimensional
superconductors: e.g., the quasi-1D superconductor (SN)_x or the 2D
transition-metal dichalcogenides. Our results identify Sc3CoC4 as a new
candidate for a quasi-1D superconductor.Comment: 4 pages, 5 figure
Gauge symmetric delta(1232) couplings and the radiative muon capture in hydrogen
Using the difference between the gauge symmetric and standard pi-N-delta
couplings, a contact pi-pi-N-N term, quadratic in the pi-N-delta coupling, is
explicitly constructed. Besides, a contribution from the delta excitation
mechanism to the photon spectrum for the radiative muon capture in hydrogen is
derived from the gauge symmetric pi-N-delta and gamma-N-delta couplings. It is
shown for the photon spectrum, studied recently experimentally, that the new
spectrum is for the photon momentums k > 60 MeV by 4-10 % smaller than the one
obtained from standardly used couplings with the on-shell deltas.Comment: 9 pages, 3 figure
Generalized polarizabilities and the spin-averaged amplitude in virtual Compton scattering off the nucleon
We discuss the low-energy behavior of the spin-averaged amplitude of virtual
Compton scattering (VCS) off a nucleon.
Based on gauge invariance, Lorentz invariance and the discrete symmetries, it
is shown that to first order in the frequency of the final real photon only two
generalized polarizabilities appear.
Different low-energy expansion schemes are discussed and put into
perspective.Comment: 13 pages, 1 postscript figure, Revtex using eps
Virtual Compton scattering off the nucleon at low energies
We investigate the low-energy behavior of the four-point Green's function
describing virtual Compton scattering off the nucleon.
Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive
the leading terms of an expansion of the operator in the four-momenta and
of the initial and final photon, respectively.
The model-independent result is expressed in terms of the electromagnetic
form factors of the free nucleon, i.e., on-shell information which one obtains
from electron-nucleon scattering experiments.
Model-dependent terms appear in the operator at ,
whereas the orders and are
contained in the low-energy theorem for , i.e., no new
parameters appear.
We discuss the leading terms of the matrix element and comment on the use of
on-shell equivalent electromagnetic vertices in the calculation of ``Born
terms'' for virtual Compton scattering.Comment: 26 pages, RevTex, to appear in Phys. Rev.
Virtual Compton Scattering off the Nucleon in Chiral Perturbation Theory
We investigate the spin-independent part of the virtual Compton scattering
(VCS) amplitude off the nucleon within the framework of chiral perturbation
theory. We perform a consistent calculation to third order in external momenta
according to Weinberg's power counting. With this calculation we can determine
the second- and fourth-order structure-dependent coefficients of the general
low-energy expansion of the spin-averaged VCS amplitude based on gauge
invariance, crossing symmetry and the discrete symmetries. We discuss the
kinematical regime to which our calculation can be applied and compare our
expansion with the multipole expansion by Guichon, Liu and Thomas. We establish
the connection of our calculation with the generalized polarizabilities of the
nucleon where it is possible.Comment: 26 pages, 2 Postscript figures, RevTex using epsfi
Structure analysis of the virtual Compton scattering amplitude at low energies
We analyze virtual Compton scattering off the nucleon at low energies in a
covariant, model-independent formalism.
We define a set of invariant functions which, once the irregular nucleon pole
terms have been subtracted in a gauge-invariant fashion, is free of poles and
kinematical zeros.
The covariant treatment naturally allows one to implement the constraints due
to Lorentz and gauge invariance, crossing symmetry, and the discrete
symmetries.
In particular, when applied to the reaction,
charge-conjugation symmetry in combination with nucleon crossing generates four
relations among the ten originally proposed generalized polarizabilities of the
nucleon.Comment: 19 pages, LaTeX2e/RevTeX, no figures, original sections IV.-VI.
removed, to be discussed in a separate publication, none of the conclusions
change
Where Nanophotonics and Microfluidics Meet
A new generation of photonic devices has recently emerged that relies on using geometries of
sub-wavelength microstructures within a high refractive index contrast materials system. These
geometries are used to confine and manipulate light within very small volumes. High optical field
densities can be obtained within such structures, and these in turn can amplify optical
nonlinearities. Moreover, many of these structures, as for example photonic crystals and slotted
waveguides, can be engineered for the efficient localization of light within the low-index regions of
high index contrast microstructures. When such structures are back-filled nonlinear polymers or
liquids, devices can be tuned and novel phenomena can be observed. In particular, such devices
are very interesting when constructed from silicon on insulator (SOI) material in which the optical
waveguide also serves as a transparent electrical contact. Here we show examples of the design,
fabrication and testing of optical microstructures in which the electro-optic (χ2) and photorefractive
(χ3) nonlinearities are used for electro-optic tuning, frequency mixing, optical
rectification, and high-speed switching of light
The Threshold Pion-Photoproduction of Nucleons In The Chiral Quark Model
In this paper, we show that the low energy theorem (LET) of the threshold
pion-photoproduction can be fully recovered in the quark model. An essential
result of this investigation is that the quark-pion operators are obtained from
the effective chiral Lagrangian, and the low energy theorem does not require
the constraints on the internal structures of the nucleon. The pseudoscalar
quark-pion coupling generates an additional term at order only
in the isospin amplitude . The role of the transitions between the
nucleon and the resonance and P-wave baryons are also discussed,
we find that the leading contributions to the isospin amplitudes at
are from the transition between the P-wave baryons and the nucleon and the
charge radius of the nucleon. The leading contribution from the P-wave baryons
only affects the neutral pion production, and improve the agreement with data
significantly. The transition between the resonance and the
nucleon only gives an order corrections to
Nucleon-nucleon bremsstrahlung: An example of the impossibility of measuring off-shell amplitudes
For nearly fifty years theoretical and experimental efforts in
nucleon-nucleon bremsstrahlung (NN) have been devoted to measuring
off-shell amplitudes and distinguishing among various NN potentials on the
basis of their off-shell behavior. New experiments are underway, designed
specifically to attain kinematics further off shell than in the past, and thus
to be more sensitive to the off-shell behavior. This letter shows that,
contrary to these expectations, and due to the invariance of the S-matrix under
transformations of the fields, the off-shell NN amplitude is as a matter of
principle an unmeasurable quantity in NN.Comment: 9 pages, Latex, using RevTeX; Minor wording changes, title changed,
version to be published in Phys. Rev. Letter
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