2,191 research outputs found
A generating functional approach to the Hubbard model
The method of generating functional is generalized to the case of strongly
correlated systems, and applied to the Hubbard model. For the electronic
Green's function constructed for Hubbard operators, an equation using
variational derivatives with respect to the fluctuating fields has been derived
and its multiplicative form has been determined. Corrections for the electronic
self-energy are calculated up to the second order with respect to the parameter
W/U (W width of the band), and a mean field type approximation was formulated,
including both charge and spin static fluctuations. The equations for the
Bose-like Green's functions have been derived, describing the collective modes:
the magnons and doublons. The properties of the poles of the doublon Green's
functions depend on electronic filling. The investigation of the special case
n=1 demonstrates that the doublon Green's function has a soft mode at the wave
vector Q=(pi,pi,...), indicating possible instability of the uniform
paramagnetic phase relatively to the two sublattices charge ordering. However
this instability should compete with an instability to antiferromagnetic
ordering.Comment: 31 pages, 7 figures, to be published in Eur. Phys. J.
Rate of steady-state reconnection in an incompressible plasma
The reconnection rate is obtained for the simplest case of 2D symmetric
reconnection in an incompressible plasma. In the short note (Erkaev et al.,
Phys. Rev. Lett.,84, 1455 (2000)), the reconnection rate is found by matching
the outer Petschek solution and the inner diffusion region solution. Here the
details of the numerical simulation of the diffusion region are presented and
the asymptotic procedure which is used for deriving the reconnection rate is
described. The reconnection rate is obtained as a decreasing function of the
diffusion region length. For a sufficiently large diffusion region scale, the
reconnection rate becomes close to that obtained in the Sweet-Parker solution
with the inverse square root dependence on the magnetic Reynolds number,
determined for the global size of the current sheet. On the other hand, for a
small diffusion region length scale, the reconnection rate turns out to be very
similar to that obtained in the Petschek model with a logarithmic dependence on
the magnetic Reynolds number. This means that the Petschek regime seems to be
possible only in the case of a strongly localized conductivity corresponding to
a small scale of the diffusion region.Comment: 11 pages, 3 figure
Saturn's aurora in the January 2004 events
International audienceDifferences in the solar wind interaction with the magnetosphere of Saturn relative to the Earth result from the decrease in the solar wind plasma density and magnetic field strength with distance from the Sun, and from the change in the average angle of the IMF at Saturn's orbit. Other reasons are related to Saturn's rapid rotation and internal magnetospheric plasma sources. Moreover, the IMF structure observed by Cassini in 2003?2004 during the approach to Saturn is consistent with corotating interaction regions (CIRs) existing during the declining phase of the solar cycle. Two cases on 16 and 26 January 2004 are considered when disturbances in the solar wind passed Cassini and then Saturn. After the solar wind shock encountered the kronian magnetosphere, the auroral oval became brighter (especially at dawn) with a reduced radius. In these cases the auroral power was anti-correlated with the radius of the oval. Possible mechanisms responsible for such unexpected behavior are presented and discussed in detail
On Possible Measurement of Gravitational Interaction Parameters on Board a Satellite
The recently suggested SEE (Satellite Energy Exchange) method of measuring
the gravitational constant , possible equivalence principle violation
(measured by the E\"{o}tv\"{o}s parameter ) and the hypothetic 5th force
parameters and on board a drag-free Earth's satellite is
discussed and further developed. Various particle trajectories near a heavy
ball are numerically simulated. Some basic sources of error are analysed. The
measurement procedure is modelled by noise insertion to a ``true''
trajectory. It is concluded that the present knowledge of (for
m) and can be improved by at least two orders of
magnitude.Comment: (only two misprints on title page) 7 page
The dissimilarity map and representation theory of
We give another proof that -dissimilarity vectors of weighted trees are
points on the tropical Grassmanian, as conjectured by Cools, and proved by
Giraldo in response to a question of Sturmfels and Pachter. We accomplish this
by relating -dissimilarity vectors to the representation theory of Comment: 11 pages, 8 figure
A few notes about the strigol´niki heresy
RésuméQuelques notes à propos de l’hérésie des « strigol´niki ».En mettant l’Église en accusation, les strigol´niki ne faisaient qu’exprimer les états d’âme du peuple chrétien, qui craignait que les sacrements administrés par des clercs indignes se révélassent inefficaces. La volée de critiques adressée au clergé témoigne à sa manière du succès de la catéchisation et du degré de dévouement à leur sacerdoce qu’on exigeait désormais des pasteurs. Il semble que le noyau de l’hérésie, autour duquel sont venues s’agglutiner des pratiques traditionnelles de piété et des croyances populaires, ait été la protestation contre la simonie entendue au sens large, c’est-à -dire comme toute espèce de transgression liée à l’ordination des prêtres. Les strigol´niki ont, semble-t-il, partagé ces caractéristiques, ainsi qu’une dévotion particulière pour l’Évangile, avec les Patarini de Milan.AbstractWhile accusing the Church, the strigol´niki actually expressed the mood of the flock, who feared that sacraments administered by unworthy clerics might be ineffective. The stream of criticism addressed to clerics testifies to the success of catechization and to the higher level of dedication required from the pastors. It seems that the heretic nucleus around which elements of traditional forms of piety and national belief conglomerated was protest against simony, broadly understood as any kind of infringement pertaining to the ordainment of priests. Alongside with especial reverence for the Gospel, these features of the strigol´niki movement appear similar to the Milanese Patarini’s
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