2,758 research outputs found
QCD effective action with dressing functions - consistency checks in the perturbative regime
In a previous paper, we presented solution to the Slavnov--Taylor identity
for the QCD effective action, and argued that the action terms containing
(anti)ghost fields are unique. These terms have the same form as those in the
classical action, but the gluon and (anti)ghost effective fields are convoluted
with gluon and ghost dressing functions G_A and G_c, the latter containing
perturbative and nonperturbative effects (but not including the soliton-like
vacuum effects). In the present work we show how the perturbative QCD (pQCD)
can be incorporated into the framework of this action, and we present explicit
one-loop pQCD expressions for G_A and G_c. We then go on to check the
consistency of the obtained results by considering an antighost
Dyson--Schwinger equation (DSE). By solving the relations that result from the
Legendre transformation leading to the effective action, we obtain the
effective fields as power expansions of sources. We check explicitly that the
aforementioned one-loop functions G_A and G_c fulfil the antighost DSE at the
linear source level. We further explicitly check that these one-loop G_A and
G_c have the regularization-scale and momentum dependence consistent with the
antighost DSE at the quadratic source level. These checks suggest that the the
effective action with dressing functions represents a consistent framework for
treating QCD, at least at the one-loop level.Comment: 17 pages, revtex4; dimensional regularization used instead of
Pauli-Villars, the check of identity in the linear-in-sources Dyson-Schwinger
equation now includes the finite part; conclusions unchanged; to appear in
Phys.Rev.
An approach to solve Slavnov-Taylor identities in nonsupersymmetric non-Abelian gauge theories
We present a way to solve Slavnov--Taylor identities in a general
nonsupersymmetric theory. The solution can be parametrized by a limited number
of functions of spacetime coordinates, so that all the effective fields are
dressed by these functions via integral convolution. The solution restricts the
ghost part of the effective action and gives predictions for the physical part
of the effective action.Comment: revised version, section 3 is enlarged, 24 pages, Latex2e, no
figures, version accepted by Phys. Rev.
Diffraction 2000: New Scaling Laws in Shadow Dynamics
New scaling structure for the shadow corrections in elastic scattering from
deuteron at high energies has been presented and discussed. It is shown that
this structure corresponds to the experimental data on
proton(antiproton)-deuteron total cross sections. The effect of weakening for
the inelastic screening at superhigh energies has been theoretically predicted.Comment: LaTex2e, espcrc2.sty, 2 figures, Contribution to the Workshop
"Diffraction 2000", Cetraro, Ialy, Sept. 2-7, 2000, to be published in
proceedings of the Worksho
Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2
We analyze the qualitative features in the transition spectra of a model
superconductor with multiple energy gaps, using a simple extension of the
Mattis-Bardeen expression for probes with case I and case II coherence factors.
At temperature T = 0, the far infrared absorption edge is, as expected,
determined by the smallest gap. However, the large thermal background may mask
this edge at finite temperatures and instead the secondary absorption edges
found at Delta_i+Delta_j may become most prominent. At finite T, if certain
interband matrix elements are large, there may also be absorption peaks at the
gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of
sample quality on the measured spectra and the possible relation of these
predictions to the recent infrared absorption measurement on MgB_2
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Imaging of buried 3D magnetic rolled-up nanomembranes
Increasing performance and enabling novel functionalities of microelectronic devices, such as three-dimensional (3D) on-chip architectures in optics, electronics, and magnetics, calls for new approaches in both fabrication and characterization. Up to now, 3D magnetic architectures had mainly been studied by integral means without providing insight into local magnetic microstructures that determine the device performance. We prove a concept that allows for imaging magnetic domain patterns in buried 3D objects, for example, magnetic tubular architectures with multiple windings. The approach is based on utilizing the shadow contrast in transmission X-ray magnetic circular dichroism (XMCD) photoemission electron microscopy and correlating the observed 2D projection of the 3D magnetic domains with simulated XMCD patterns. That way, we are not only able to assess magnetic states but also monitor the field-driven evolution of the magnetic domain patterns in individual windings of buried magnetic rolled-up nanomembranes
Assessing students in community settings: the role of peer evaluation
The assessment of students in community settings faces unique difficulties. Since students are usually posted in small groups in different community settings and since the learning (largely) takes place outside the classroom, assessing student performance becomes an intrinsically complex endeavor. In this article, the proposition is made and tested that peers may be used to accurately assess particular aspects of performance, in particular those which need extensive and close observation. Examples are: Effort displayed while working in a community, quality of the interaction with that community, display of leadership, and subject-matter contributions
Penetration depth anisotropy in two-band superconductors
The anisotropy of the London penetration depth is evaluated for two-band
superconductors with arbitrary inter- and intra-band scattering times. If one
of the bands is clean and the other is dirty in the absence of inter-band
scattering, the anisotropy is dominated by the Fermi surface of the clean band
and is weakly temperature dependent. The inter-band scattering also suppress
the temperature dependence of the anisotropy
Perturbation theories for the S=1/2 spin ladder with four-spin ring exchange
The isotropic S=1/2 antiferromagnetic spin ladder with additional four-spin
ring exchange is studied perturbatively in the strong coupling regime with the
help of cluster expansion technique, and by means of bosonization in the weak
coupling limit. It is found that a sufficiently large strength of ring exchange
leads to a second-order phase transition, and the shape of the boundary in the
vicinity of the known exact transition point is obtained. The critical exponent
for the gap is found to be , in agreement both with exact results
available for the dimer line and with the bosonization analysis. The phase
emerging for high values of the ring exchange is argued to be gapped and
spontaneously dimerized. The results for the transition line from strong
coupling and from weak coupling match with each other naturally.Comment: 8 pages, 4 figures, some minor changes in text and reference
Mesoscopic interplay of superconductivity and ferromagnetism in ultra-small metallic grains
We review the effects of electron-electron interactions on the ground-state
spin and the transport properties of ultra-small chaotic metallic grains. Our
studies are based on an effective Hamiltonian that combines a superconducting
BCS-like term and a ferromagnetic Stoner-like term. Such terms originate in
pairing and spin exchange correlations, respectively. This description is valid
in the limit of a large dimensionless Thouless conductance. We present the
ground-state phase diagram in the fluctuation-dominated regime where the
single-particle mean level spacing is comparable to the bulk BCS pairing gap.
This phase diagram contains a regime in which pairing and spin exchange
correlations coexist in the ground-state wave function. We discuss the
calculation of the tunneling conductance for an almost-isolated grain in the
Coulomb-blockade regime, and present measurable signatures of the competition
between superconductivity and ferromagnetism in the mesoscopic fluctuations of
the conductance.Comment: 6 pages, 3 figures, To be published in the proceedings of the NATO
Advance Research Workshop "Recent Advances in Nonlinear Dynamics and Complex
System Physics.
Spin and energy transfer in nanocrystals without transport of charge
We describe a mechanism of spin transfer between individual quantum dots that
does not require tunneling. Incident circularly-polarized photons create
inter-band excitons with non-zero electron spin in the first quantum dot. When
the quantum-dot pair is properly designed, this excitation can be transferred
to the neighboring dot via the Coulomb interaction with either {\it
conservation} or {\it flipping} of the electron spin. The second dot can
radiate circularly-polarized photons at lower energy. Selection rules for spin
transfer are determined by the resonant conditions and by the strong spin-orbit
interaction in the valence band of nanocrystals. Coulomb-induced energy and
spin transfer in pairs and chains of dots can become very efficient under
resonant conditions. The electron can preserve its spin orientation even in
randomly-oriented nanocrystals.Comment: 13 pages, 3 figure
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