1,334 research outputs found
Compressibility of a two-dimensional extended Hubbard model
The compressibility of an extended Hubbard model is investigated by the
Roth's two-pole approximation. Using the factorization procedure proposed by
Beenen and Edwards, superconductivity with singlet -wave pairing
is also considered. Within this framework, the effects of hybridization
and Coulomb interaction on the compressibility are studied carefully. It
has been found that the compressibility diverges and then it becomes negative
near the half-filling. Within Roth's method, it has been verified that an
important contribution for the negative compressibility comes from the
spin-correlation term present in Roth's band shift. This
correlation function plays an important role due to its high doping dependence.
Also, its effects in the band shift and consequently in the compressibility are
pronounced near the half-filling. The numerical results show that the
hybridization acts in the sense of suppressing the negative compressibility
near half-filling. Finally, the possibility of a connection between the
negative compressibility and the phase separation is also discussed.Comment: 3 pages, 1 figure, accepted for publication in Physica
Role of Hybridization in the Superconducting Properties of an Extended Hubbard Model: a Detailed Numerical Study
The Roth's two-pole approximation has been used by the present authors to
study the effects of the hybridization in the superconducting properties of a
strongly correlated electron system. The model used is the extended Hubbard
model which includes the hybridization, the -band and a narrow
-band. The present work is an extension a previous reference [Intern. Journ.
of Modern Phys. B, Vol. 18 No. 2 (2004) 241]. Nevertheless, some important
correlation functions necessary to estimate the Roth's band shift, are included
together with the temperature and the Coulomb interaction to describe
the superconductivity. The superconducting order parameter of a cuprate system,
is obtained following Beenen and Edwards formalism. Here, we investigate in
detail the change of the order parameter associated to temperature, Coulomb
interaction and Roth's band shift effects on superconductivity. The phase
diagram with versus the total occupation numbers , shows the
difference respect to the previous work.Comment: 4 pages, 1 figure, accept to be published in Physica
Turmalinas en la pegmatita granÃtica de Li-Cs-Ta del Grupo Namacotche, Mozambique: cristalquÃmica y origen
The field work, backscattered electron images and detailed microanalyses of three generations of tourmaline from the Namacotche LCT pegmatites allows de distinction between the compositional magmatic and hydrothermal tourmalines. The generation 1 occurs in the outer intermediate zone of the pegmatite. It consists of zoned crystals with an oscillatory inner core of foitite and schorl, an outer core of schorl and an Fe-rich fluor-elbaite rim. Unzoned Fe-rich fluor-elbaite crystals occur in the inner intermediate zone of the pegmatite. All the crystals are derived by fractionation of a (Al, Li, B)-rich pegmatite melt. However, the rim of zoned crystals and some compositions of unzoned crystals show evidence of hydrothermal fluids, as they plot outside the fractionation trends. The zoned fluor-elbaite crystals of the generation 2 are from the inner intermediate zone of the pegmatite. They have a pink core and a green rim. The rim has higher YFe2+, Na, F contents YFe2+/(YFe2++Licalc.) value and lower Si, YAl, Licalc. and X-site vacancy contents, X-vacancy/(Na+X-vacancy) value than the core. Both zones are hydrothermal. The rim is an overgrowth. The fluor-elbaite gemmy crystals of the generation 3 occur in sheared breccia blasts and clasts with a cookeite matrix. They depend mainly on the fluid-rich hydrothermal environment of low temperatures (280-150ºC). Some from the cycle a may result from the dissolution of magmatic tourmaline crystals of the generation 1 from the sheared outer and inner intermediate zones of the pegmatite due to reaction with late fluids in chemical disequilibrium, followed by growth of tourmaline with low temperature hydrothermal fluids. The evolution from the cycle a to the cycle b and to the cycle c of the generation 3 implies that the hydrothermal reacting fluids were undergoing fractionation and becoming richer in Li and poorer in Fe2+ during the late hydrothermal crystallization of the pegmatites.El trabajo de campo, las imágenes de electrones retrodispersados y microanálisis detallados de tres generaciones de turmalina de las pegmatitas LCT de Namacotche permiten distinguir entre las turmalinas magmáticas y las hidrotermales. La generación 1 aparece en la zona intermedia exterior de la pegmatita. Consiste en cristales zonados con un núcleo interno oscilante de foitita y esquisto, un núcleo externo de esquisto y un borde de fluor-elbaita rico en Fe. En la zona intermedia interior de la pegmatita aparecen cristales no zonados de fluor-elbaita ricos en Fe. Todos los cristales derivan del fraccionamiento de un fundido pegmatÃtico rico en Al, Li y B. Sin embargo, el borde de los cristales zonados y algunas composiciones de cristales no zonados muestran evidencias de fluidos hidrotermales, ya que trazan fuera de las tendencias de fraccionamiento. Los cristales zonados de fluor-elbaita de la generación 2 proceden de la zona intermedia interna de la pegmatita. Tienen un núcleo rosa y un borde verde. El borde tiene mayor contenido en YFe2+, Na, F (YFe2+/(YFe2++Licalc.)) y menor contenido en Si, YAl, Licalc. y vacantes X, vacantes X/(Na+X-vacantes) que el núcleo. Ambas zonas son hidrotermales. El borde es un sobrecrecimiento. Los cristales geminados de fluor-elbaita de la generación 3 aparecen en clastos de brechas cizalladas y en clastos con matriz de cookeita. Dependen principalmente del ambiente hidrotermal rico en fluidos de bajas temperaturas (280-150ºC). Algunos del ciclo a pueden ser el resultado de la disolución de cristales magmáticos de turmalina de la generación 1 de las zonas cizalladas externa e interna intermedia de la pegmatita debido a la reacción con fluidos tardÃos en desequilibrio quÃmico, seguida del crecimiento de la turmalina con fluidos hidrotermales de baja temperatura. La evolución del ciclo a al ciclo b y al ciclo c de la generación 3 implica que los fluidos hidrotermales reaccionantes estaban sufriendo fraccionamiento y haciéndose más ricos en Li y más pobres en Fe2+ durante la cristalización hidrotermal tardÃa de las pegmatitas
Polynomial Realization of and Fusion Rules at Exceptional Values of
Representations of the algebra are constructed in the space of
polynomials of real (complex) variable for . The spin addition rule
based on eigenvalues of Casimir operator is illustrated on few simplest cases
and conjecture for general case is formulated
A Vector Supersymmetry in Noncommutative U(1) Gauge Theory with the Slavnov Term
We consider noncommutative U(1) gauge theory with the additional term,
involving a scalar field lambda, introduced by Slavnov in order to cure the
infrared problem. we show that this theory, with an appropriate space-like
axial gauge-fixing, wxhibits a linear vector supersymmetry similar to the one
present in the 2-dimensional BF model. This vector supersymmetry implies that
all loop corrections are independent of the -vertex and thereby
explains why Slavnov found a finite model for the same gauge-fixing.Comment: 18 pages, 3 figures; v2 Acknowledgments adde
Continuity of Optimal Control Costs and its application to Weak KAM Theory
We prove continuity of certain cost functions arising from optimal control of
affine control systems. We give sharp sufficient conditions for this
continuity. As an application, we prove a version of weak KAM theorem and
consider the Aubry-Mather problems corresponding to these systems.Comment: 23 pages, 1 figures, added explanations in the proofs of the main
theorem and the exampl
On the use of a single site approximation to describe correlation in pure metals
The magnetic properties of pure transition-like metals are discussed within
the single site approximation, to take into account the electron correlation.
The metal is described by two hybridized bands one of which includes the
Coulomb correlation. Our results indicate that ferromagnetism follows from
adequate values of the correlation and hybridization.Comment: 2 pages and 2 figure
On the consistency of the three-dimensional noncommutative supersymmetric Yang-Mills theory
We study the one-loop quantum corrections to the U(N) noncommutative
supersymmetric Yang-Mills theory in three spacetime dimensions (NCSYM). We
show that the cancellation of the dangerous UV/IR infrared divergences only
takes place in the fundamental representation of the gauge group. Furthermore,
in the one-loop approximation, the would be subleading UV and UV/IR infrared
divergences are shown to vanish.Comment: 8 pages and 2 figure
Exchange couplings in the magnetic molecular cluster Mn12Ac
The magnetic properties of the molecular cluster Mn12Ac are due to the four
Mn3+ ions which have spins S=3/2 and the eight Mn4+ ions with spins S=2. These
spins are coupled by superexchange mechanism. We determine the four exchange
couplings assuming a Heisenberg-type interaction between the ions. We use exact
diagonalization of the spin Hamiltonian by a Lanczos algorithm and we adjust
the couplings to reproduce the magnetization curve of Mn12Ac. We also impose
the constraint of reproducing a gap of 35K between a S=10 ground state and a
first excited state with S=9. We predict that there is an excited level with
S=8 at 37K above the ground state, only slightly above the S=9 excited state
which lies at 35K and the next excited state is a S=9 multiplet at 67K above
the S=10 ground state.Comment: 15 pages, 6 figures, submitted to Phys Rev B, corrected a misTeX:
values of J1, J2 have changed, refs update
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