3,022 research outputs found
A van Hemmen-Kondo model for disordered strongly correlated electron systems
We present here a theoretical model in order to describe the competition
between the Kondo effect and the spin glass behavior. The spin glass part of
the starting Hamiltonian contains Ising spins with an intersite exchange
interaction given by the local van Hemmen model, while the Kondo effect is
described as usual by the intrasite exchange . We obtain, for large
values, a Kondo phase and, for smaller values, a succession, with
decreasingComment: 14 pages, 4 figures, accepted for publication in Phys. Rev.
An AGN Identification for 3EG J2006-2321
We present a multiwavelength analysis of the high-energy gamma-ray source 3EG
J2006-2321. The flux of this source above 100 MeV is shown to be variable on
time scales of days and months. Optical observations and careful examination of
archived radio data indicate that its radio counterpart is PMN J2005-2310, a
flat-spectrum radio quasar with a 5-GHz flux density of 260 mJy. Study of the
optical counterpart indicates a redshift of 0.833 and variable linear
polarization. No X-ray source has been detected near the position of PMN
J2005-2310, but an X-ray upper limit is derived from ROSAT data. This upper
limit provides for a spectral energy distribution with global characteristics
similar to those of known gamma-ray blazars. Taken together, these data
indicate that 3EG J2006-2321, listed as unidentified in the 3rd EGRET Catalog,
is a member of the blazar class of AGN. The 5-GHz radio flux density of this
blazar is the lowest of the 68 EGRET-detected AGN. The fact that EGRET has
detected such a source has implications for unidentified EGRET sources,
particularly those at high latitudes (), many of which may be
blazars.Comment: 22 pages, 6 figures. To appear in ApJ v569 n1, 10 April 200
Influence of Pyrolysis Parameters on the Performance of CMSM
Carbon hollow fiber membranes have been prepared by pyrolysis of a P84/S-PEEK blend. Proximate analysis of the precursor was performed using thermogravimetry (TGA), and a carbon yield of approximately 40% can be obtained. This study aimed at understanding the influence of pyrolysis parameters—end temperature, quenching effect, and soaking time—on the membrane properties. Permeation experiments were performed with N2, He, and CO2. Scanning electron microscopy (SEM) has been done for all carbon hollow fibers. The highest permeances were obtained for the membrane submitted to an end temperature of 750°C and the highest ideal selectivities for an end temperature of 700°C. In both cases, the membranes were quenched to room temperatur
Spin Glass and ferromagnetism in disordered Cerium compounds
The competition between spin glass, ferromagnetism and Kondo effect is
analysed here in a Kondo lattice model with an inter-site random coupling
between the localized magnetic moments given by a generalization of
the Mattis model which represents an interpolation between ferromagnetism and a
highly disordered spin glass. Functional integral techniques with Grassmann
fields have been used to obtain the partition function. The static
approximation and the replica symmetric ansatz have also been used. The
solution of the problem is presented as a phase diagram giving {\it
versus} where is the temperature, and are the
strengths of the intrasite Kondo and the intersite random couplings,
respectively. If is small, when temperature is decreased, there is a
second order transition from a paramagnetic to a spin glass phase. For lower
, a first order transition appears between the spin glass phase and a
region where there are Mattis states which are thermodynamically equivalent to
the ferromagnetism. For very low , the Mattis states become stable. On
the other hand, it is found as solution a Kondo state for large
values. These results can improve the theoretical description of the well known
experimental phase diagram of .Comment: 17 pages, 5 figures, accepted Phys. Rev.
Comparação de resultados antes e depois da implantação das normas nbr para extrativos totais e lignina insolúvel.
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