2,026 research outputs found
Diagrammatic routes to nonlocal correlations beyond dynamical mean field theory
Strong electronic correlations pose one of the biggest challenges to solid
state theory. We review recently developed methods that address this problem by
starting with the local, eminently important correlations of dynamical mean
field theory (DMFT). On top of this, non-local correlations on all length
scales are generated through Feynman diagrams, with a local two-particle vertex
instead of the bare Coulomb interaction as a building block. With these
diagrammatic extensions of DMFT long-range charge-, magnetic-, and
superconducting fluctuations as well as (quantum) criticality can be addressed
in strongly correlated electron systems. We provide an overview of the
successes and results achieved---hitherto mainly for model Hamiltonians---and
outline future prospects for realistic material calculations.Comment: 60 pages, 42 figures, replaced by the version to be published in Rev.
Mod. Phys. 201
The soft and the hard pomerons in hadron elastic scattering at small t
We consider simple-pole descriptions of soft elastic scattering for pp, pbar
p, pi+ p, pi- p, K+ p and K- p. We work at t and s small enough for
rescatterings to be neglected, and allow for the presence of a hard pomeron.
After building and discussing an exhaustive dataset, we show that simple poles
provide an excellent description of the data in the region - 0.5 GeV^2 < t <
-0.1 GeV^2, 6 GeV<sqrt(s)< 63 GeV. We show that new form factors have to be
used, and get information on the trajectories of the soft and hard pomerons.Comment: 27 pages, 9 figures, LaTeX. A few typos fixed, and references
correcte
Evidence for the formation of magnetic moments in the cuprate superconductor HgCuBaCaCuO below seen by NQR
We report pure zero field nuclear magnetic resonance (NQR) measurements on
the optimally doped three layer high--compounds HgBaCaCuO and
HgBaCaCuO(F) with 134 K. Above two Cu NQR line pairs are
observed in the spectra corresponding to the two inequivalent Cu lattice sites.
Below the Cu NQR spectra show additional lines leading to the extreme
broadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. The
spin-lattice relaxation curves follow a triple exponential function with
coefficients depend onto the saturation time (number of saturation pulses),
whereas the spin-spin relaxation curve is described by a single exponential
function. From the spin-lattice relaxation we deduced a complete removal of the
Kramers degeneracy of the Cu quadrupole indicating that the additional lines
are due to a Zeemann splitting of the Cu lines due to the spontaneous
formation of magnetic moments within the CuO layers. Below 140 K, the spectra
are well fitted by a number of 6 Cu line pairs. From the number of
the Cu lines, the position of the lines relative to each other and the complete
removal of the Kramers degeneracy we deduced an orientation of the magnetic
moments parallel to the symmetry axis of the electric field gradient tensor
with magnitudes of the order of 1000 G. We also discuss the possible
microscopic origin of the observed internal magnetic fields.Comment: 11 pages, 12 figure
Search for Higher Flavor Multiplets in Partial Wave Analyses
The possible existence of higher multi-quark flavor multiplets of baryons is
investigated. We argue that the S-matrix should have poles with any quantum
numbers, including those which are exotic. This argument provides a novel
justification for the existence of hadrons with arbitrary exotic structure.
Though it does not constitute a proof, there are still no theoretical arguments
against exotics. We then consider KN and piN scattering. Conventional and
modified partial-wave analyses provide several sets of candidates for
correlated pairs (Theta1, Delta), each of which could label a related 27-plet.
Properties of the pairs (masses, mass orderings, spin-parity quantum numbers)
do not quite correspond to the current theoretical expectations. Decay widths
of the candidates are either wider or narrower than expected. Possible reasons
for such disagreements are briefly discussed.Comment: 12 pages, 1 figure; v2: references corrected; v3: minor changes, to
appear in Eur.Phys.J.
A possible explanation why the Theta+ is seen in some experiments and not in others
To understand the whole set of positive and null data on the
Theta+(1530)-production, we suggest the hypothesis that multiquark hadrons are
mainly generated from many-quark states, which emerge either as short-term
hadron fluctuations, or as hadron remnants in hard processes. This approach
allows us to describe both non-observation of the Theta+ in current null
experiments and peculiar features of its production in positive experiments.
Further, we are able to propose new experiments that might be decisive for the
problem of the Theta+ existence. Distributions of the Theta+ in such
experiments can give important information both on higher Fock components of
conventional hadrons and about structure and hadronization properties of hadron
remnants produced in hard processes. We also explain that description of
multiquark hadrons may require a modified form of the constituent quark model,
with quark masses and couplings being intermediate between their values for the
familiar constituent quarks and the current ones.Comment: 18 pages. Some changes in the text; experimental suggestions
collected in a special subsection, references added and refreshe
Structure and propertties of ball milled utrahigh-molecular weight Polyethylene - clay composite
In this work the composite material based on polymer matrix filled with clay is studied. The preparation of powder composition consists of mechanical activation of substances and further common ball milling of polymer and clay in a high energy planetary ball mill. The process is divided into two stages; the first stage involves crushing of clay to obtain a nanosized powder, and in the second stage preparation of powdered nanocomposite is carried out. New clay-polymer composite shows considerable increase in modulus of elasticity and a decrease in coefficient of friction
Pion Generalized Dipole Polarizabilities by Virtual Compton Scattering
We present a calculation of the cross section and the event generator of the
reaction . This reaction is sensitive to the pion
generalized dipole polarizabilities, namely, the longitudinal electric
, the transverse electric , and the magnetic
which, in the real-photon limit, reduce to the ordinary electric
and magnetic polarizabilities and , respectively.
The calculation of the cross section is done in the framework of chiral
perturbation theory at . A pion VCS event generator has been
written which is ready for implementation in GEANT simulation codes or for
independent use.Comment: 33 pages, Revtex, 15 figure
Generation and Structure of Solitary Rossby Vortices in Rotating Fluids
The formation of zonal flows and vortices in the generalized
Charney-Hasegawa-Mima equation is studied. We focus on the regime when the size
of structures is comparable to or larger than the deformation (Rossby) radius.
Numerical simulations show the formation of anticyclonic vortices in unstable
shear flows and ring-like vortices with quiescent cores and vorticity
concentrated in a ring. Physical mechanisms that lead to these phenomena and
their relevance to turbulence in planetary atmospheres are discussed.Comment: 3 pages in REVTeX, 5 postscript figures separately, submitted to
Phys. Rev.
Photoproduction of neutral pion pairs in the Coulomb field of the nucleus
The total cross section for Coulomb photoproduction of neutral pion pairs in
the reaction gamma A --> pi0 pi0 A is estimated within the effective chiral
lagrangian approach.
The amplitude of gamma gamma* --> pi0 pi0 with one off-shell photon is
calculated at O(p^6) in the momentum expansion; in addition, nuclear absorption
is taken into account.
Besides its experimental feasibility, the results of the calculation
demonstrate that the reaction gamma A --> pi0 pi0 A is a powerful source of
information on the process gamma gamma --> pi0 pi0 close to threshold.Comment: 18 pages, 6 figures, uses epsf, ioplppt.sty, iopl12.sty (included);
Added a discussion of main background from double pion photoproduction on
nucleon gamma N --> 2pi0 N via baryon resonance exchange mechanis
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