217 research outputs found
Beyond the Hubbard-I Solution with a One-Pole Self-Energy at Half-Filling with the Moment Approach: Non-Linear Effects
We have postulated a single pole for the self-energy,
, looking for the consequences on the one-particle
Green function, in the Hubbard model. We find that
satisfies the first two sum rules or moments of Nolting (Z.
Physik 255, 25 (1972)) for any values of the two unknown parameters
of . In order to find these two parameters we have used
the third and four sum rules of Nolting. turns out to be
identical to the one of Nolting (Z. Physik 225, 25 (1972)), which is beyond a
Hubbard-I solution since satisfies four sum rules. With our proposal we have
been able to obtain an expansion in powers of for the self-energy (here to
second order in ). We present numerical results at half-filling for 1- the
static spin susceptibility, vs and 2- the band narrowing
parameter, vs . The two-pole Ansatz of Nolting for the one-particle
Green function is equivalent to a single pole Ansatz for the self-energy which
remains the fundamental quantity for more elaborated calculations when, for
example, lifetime effects are included.Comment: 4 pages. 2 figures (under request). To appear in Physica A. Presented
at LAWNP'97, Canela/RS (BRAZIL
Moment Approach for the 2D Attractive Hubbard Model
We constructed the one-particle spectral functions (diagonal and
off-diagonal) which reproduce BCS for weak coupling and which take into account
the effect of correlations on superconductivity in the attractive Hubbard
model. The diagonal spectral function is composed of three peaks and the
off-diagonal one is composed of two peaks. This ansatz satisfies the sum rules
for the first six moments. Our solutions are valid for intermediate coupling,
i.e., for . Our set of analytical equations for the unknown
variables is self-consistent and has been solved numerically in lowest order of
the order parameter. As a result, we obtain that the presence of the third
band, or {\it upper Hubbard band}, strongly renormalizes the two lower bands,
making that the energy gap be -dependent while the order parameter is
pure s-wave. This shows that the order parameter and the gap are two different
quantities.Comment: 8 pages, 3 figures (2 ps files for Fig. 2) To appear in Physica
Specification and Testing of E-Commerce Agents Described by Using UIOLTSs
International audienceIn this paper we expand our work in our specification formalism UIOLTSs. We present three implementation relations and provide alternative characterizations of these relations in terms of the tests that the implementation under test successfully passes. In addition, we present the main ideas to obtain an algorithm to derive complete test suites from specifications
Rotura traumåtica del tendón del supraespinoso en un niño de seis años
Presentamos un caso de un niño de seis años que sufriĂł un traumatismo por caĂda sobre su hombro derecho, con rotura completa del tendĂłn del supraespinoso. Comprobado con Resonancia MagnĂ©tica. Se operĂł para efectuar una reinserciĂłn transĂłsea del tendĂłn mediante cirugĂa abierta. DespuĂ©s de un mes de inmovilizaciĂłn se comenzaron los ejercicios activos progresivos con recuperaciĂłn completa a los tres meses de la intervenciĂłn.A six year old boy sustained a right shoulder injury after a causal fall. Magnetic Resonance Imaging showed a complete rupture of the supraespinatus tendon. Open surgery was indicated by transosseus reinsertion of the tendon on the humeral head. After one month of shoulder immobilization, the patient physical therapy with progressive active exercises. The patient achivied complete joint motion and function three months after surgery
Two-bands superconductivity with intra- and interband pairing for synthetic superlattices
We consider a model for superconductivity in a two-band superconductor,
having an anisotropic electronic structure made of two partially overlapping
bands with a first hole-like and a second electron-like fermi surface. In this
pairing scenario, driven by the interplay between interband and
intraband pairing terms, we have solved the two gap equations at the
critical temperature and calculate and the chemical potential
as a function of the number of carriers for various values of pairing
interactions, , , and . The results show the
complexity of the physics of condensates with multiple order parameters with
the chemical potential near band edges.Comment: 6 pages, 2 figure
Ginzburg-Landau Expansion in Non-Fermi Liquid Superconductors: Effect of the Mass Renormalization Factor
We reconsider the Ginzburg-Landau expansion for the case of a non-Fermi
liquid superconductor. We obtain analytical results for the Ginzburg-Landau
functional in the critical region around the superconducting phase transition,
T <= T_c, in two special limits of the model, i.e., the spin-charge separation
case and the anomalous Fermi liquid case. For both cases, in the presence of a
mass renormalization factor, we derived the form and the specific dependence of
the coherence length, penetration depth, specific heat jump at the critical
point, and the magnetic upper critical field. For both limits the obtained
results reduce to the usual BCS results for a two dimensional s-wave
superconductor. We compare our results with recent and relevant theoretical
work. The results for a d--wave symmetry order parameter do not change
qualitatively the results presented in this paper. Only numerical factors
appear additionally in our expressions.Comment: accepted for publication in Physical Review
An alternative dimensional reduction prescription
We propose an alternative dimensional reduction prescription which in respect with Green functions corresponds to drop the extra spatial coordinate. From this, we construct the dimensionally reduced Lagrangians both for scalars and fermions, discussing bosonization and supersymmetry in the particular 2-dimensional case. We argue that our proposal is in some situations more physical, in the sense that it mantains the form of the interactions between particles, thus preserving the dynamics corresponding to the higher dimensional space.Facultad de Ciencias Exacta
P14 282. Endocarditis protésica. experiencia de 20 años
ObjetivosRevisamos la experiencia de nuestro centro en endocarditis protĂ©sica (EP).Material y mĂ©todosEntre 1990â2010, se intervienen 62 casos, que representaron el 23% de 276 casos totales de endocarditis y 2,46% de pacientes valvulares (precoces 0,58% con 20 casos, tardĂas 1,68% con 42 casos). No hubo diferencias en tipo ni localizaciĂłn de las prĂłtesis, ni en incidencia entre las dos dĂ©cadas (2,59% en 1990â2000 y 2,32% en 2000â2010). Edad media 59 años. GĂ©rmenes: S. viridans 20,9%; S. epidermidis 16,1%; S. aureus 11,6%; C. burnetii 9,6%; enterococos 8%; difteroides 6,4%; hongos 6,4%. En un 8% de los casos se encontraron gĂ©rmenes raros aislados, mientras que no se identificĂł germen en 12,9%. Se intervinieron de forma urgente el 30% de pacientes (19 casos, el 45% de formas precoces y el 23% de formas tardĂas).ResultadosLa mortalidad precoz fue del 16%, a expensas sobre todo de EP precoz (12%). La supervivencia global a 10 años fue del 50%, con diferencias entre los dos grupos (EP precoz 15%; EP tardĂa 66%). El 70% de supervivientes se encuentra actualmente en clase funcional I-II/IV.ConclusiĂłnLa EP sigue teniendo una incidencia relativamente elevada en nuestro medio. La EP precoz tiene una mortalidad muy elevada. La cirugĂa de la EP tardĂa tiene muy buenos resultados a largo plazo
Search for the Flavor-Changing Neutral Current Decay with the HERA-B Detector
We report on a search for the flavor-changing neutral current decay using events recorded with a dimuon trigger in
interactions of 920 GeV protons with nuclei by the HERA-B experiment. We find
no evidence for such decays and set a 90% confidence level upper limit on the
branching fraction .Comment: 17 pages, 4 figures (of which 1 double), paper to be submitted to
Physics Letters
Measurement of the J/Psi Production Cross Section in 920 GeV/c Fixed-Target Proton-Nucleus Interactions
The mid-rapidity (dsigma_(pN)/dy at y=0) and total sigma_(pN) production
cross sections of J/Psi mesons are measured in proton-nucleus interactions.
Data collected by the HERA-B experiment in interactions of 920 GeV/c protons
with carbon, titanium and tungsten targets are used for this analysis. The
J/Psi mesons are reconstructed by their decay into lepton pairs. The total
production cross section obtained is sigma_(pN)(J/Psi) = 663 +- 74 +- 46
nb/nucleon. In addition, our result is compared with previous measurements
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