1,262 research outputs found
Spiral Spin Order and Transport Anisotropy in Underdoped Cuprates
We discuss the spiral spin density wave model and its application to explain
properties of underdoped LaSrCuO. We argue that the spiral
picture is theoretically well justified in the context of the extended
model, and then show that it can explain a number of observed features, such as
the location and symmetry of the incommensurate peaks in elastic neutron
scattering, as well as the in-plane resistivity anisotropy. A consistent
description of the low doping region (below 10% or so) emerges from the spiral
formulation, in which the holes show no tendency towards any type of charge
order and the physics is purely spin driven.Comment: 6 pages, 3 figures; Proceedings of the International Workshop on
Effective Models for Low-Dimensional Strongly Correlated Systems, September
2005, Peyresq, Franc
Decay Rate of a Positronium. Review of Theory and Experiment
The present status of theoretical and experimental investigations of the
decay rate of a positronium is considered. The increasing interest to this
problem has been caused by the disagreement of the calculated value of
and the recent series of precise experiments. The necessity
of new calculations on the basis of the quantum field methods in bound state
theory is pointed out with taking into account the dependence of the
interaction kernel on relative energies.Comment: LaTeX file, 9 pp., Preprint IFUNAM FT-93-01
Energy Levels Of Hydrogen-Like Atomsand Fundamental Constants
The present review includes the description of theoretical methods for the
investigations of the spectra of hydrogen-like systems. Various versions of the
quasipotential approach and the method of the effective Dirac equation are
considered. The new methods, which have been developed in the eighties, are
described. These are the method for the investigation of the spectra by means
of the quasipotential equation with the relativistic reduced mass and the
method for a selection of the logarithmic corrections by means of the
renormalization group equation. The special attention is given to the
construction of a perturbation theory and the selection of graphs, whereof the
contributions of different orders of , the fine structure constant, to
the energy of the fine and hyperfine splitting in a positronium, a muonium and
a hydrogen atom could be calculated.
In the second part of this article the comparison of the experimental results
and the theoretical results concerning the wide range of topics is produced.
They are the fine and hyperfine splitting in the hydrogenic systems, the Lamb
shift and the anomalous magnetic moments of an electron and a muon. Also, the
problem of the precision determination of a numerical value of the fine
structure constant, connected with the above topics, is discussed.Comment: LaTeX file, 68 pp. (figures are available on request
The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy
We present further arguments that the Hipparcos parallaxes for some of the
clusters and associations represented in the Hipparcos catalog should be used
with caution in the study of the Galactic structure. It has been already shown
that the discrepancy between the Hipparcos and ground based parallaxes for
several clusters including the Pleiades, Coma Ber and NGC 6231 can be resolved
by recomputing the Hipparcos astrometric solutions with an improved algorithm
diminishing correlated errors in the attitude parameters. Here we present new
parallaxes obtained with this algorithm for another group of stars with
discrepant data - the galactic cluster Cr 121. The original Hipparcos
parallaxes led de Zeeuw et al. to conclude that Cr 121 and the surrounding
association of OB stars form a relatively compact and coherent moving group at
a distance of 550 -- 600 pc. Our corrected parallaxes reveal a different
spatial distribution of young stellar populace in this area. Both the cluster
Cr 121 and the extended OB association are considerably more distant (750 --
1000 pc), and the latter has a large depth probably extending beyond 1 kpc.
Therefore, not only are the recalculated parallaxes in complete agreement with
the photometric uvbybeta parallaxes, but the structure of the field they reveal
is no longer in discrepancy with that found by the photometric method.Comment: 7 pages, 1 figure, ApJL accepte
Relativistic peculiarities at stepped surfaces: surprising energetics and unexpected diffusion patterns
We revive intriguing, yet still unexplained, experimental results of Ehrlich
and co-workers [ Phys. Rev. Lett. 77 1334 (1996); Phys. Rev. Lett. 67 2509
(1991)] who have observed, that 5d adatoms distributed on (111) surface islands
of 5d metals favor the adsorption at the cluster's edge rather than at the
cluster's interior, which lies in contrast with the behavior of 4d and 3d
elements. Our state of the art ab initio calculations demonstrate that such
behavior is a direct consequence of the relativity of 5d metals.Comment: 5 pages, 5 figure
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