18,462 research outputs found
Disordered two-dimensional superconductors: roles of temperature and interaction strength
We have considered the half-filled disordered attractive Hubbard model on a
square lattice, in which the on-site attraction is switched off on a fraction
of sites, while keeping a finite on the remaining ones. Through Quantum
Monte Carlo (QMC) simulations for several values of and , and for system
sizes ranging from to , we have calculated the
configurational averages of the equal-time pair structure factor , and,
for a more restricted set of variables, the helicity modulus, , as
functions of temperature. Two finite-size scaling {\it ansatze} for have
been used, one for zero-temperature and the other for finite temperatures. We
have found that the system sustains superconductivity in the ground state up to
a critical impurity concentration, , which increases with , at least up
to U=4 (in units of the hopping energy). Also, the normalized zero-temperature
gap as a function of shows a maximum near , for . Analyses of the helicity modulus and of the pair structure factor
led to the determination of the critical temperature as a function of , for
4 and 6: they also show maxima near , with the highest
increasing with in this range. We argue that, overall, the observed
behavior results from both the breakdown of CDW-superconductivity degeneracy
and the fact that free sites tend to "push" electrons towards attractive sites,
the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe
Ising Spin Glass in a Transverse Magnetic Field
We study the three-dimensional quantum Ising spin glass in a transverse
magnetic field following the evolution of the bond probability distribution
under Renormalisation Group transformations. The phase diagram (critical
temperature {\em vs} transverse field ) we obtain shows a finite
slope near , in contrast with the infinite slope for the pure case. Our
results compare very well with the experimental data recently obtained for the
dipolar Ising spin glass LiHoYF, in a transverse field.
This indicates that this system is more apropriately described by a model with
short range interactions than by an equivalent Sherrington-Kirkpatrick model in
a transverse field.Comment: 7 pages, RevTeX3, Nota Cientifica PUC-Rio 23/9
Fluctuating local moments, itinerant electrons and the magnetocaloric effect: the compositional hypersensitivity of FeRh
We describe an ab-initio Disordered Local Moment Theory for materials with
quenched static compositional disorder traversing first order magnetic phase
transitions. It accounts quantitatively for metamagnetic changes and the
magnetocaloric effect. For perfect stoichiometric B2-ordered FeRh, we calculate
the transition temperature of the ferromagnetic-antiferromagnetic transition to
be 495K and a maximum isothermal entropy change in 2 Tesla of J~K~kg. A large (40\%) component of is
electronic. The transition results from a fine balance of competing electronic
effects which is disturbed by small compositional changes - e.g. swapping just
2\% Fe of `defects' onto the Rh sublattice makes drop by 290K. This
hypersensitivity explains the narrow compositional range of the transition and
impurity doping effects.Comment: 11 pages, 4 figure
Magnetic exchange mechanism for electronic gap opening in graphene
We show within a local self-consistent mean-field treatment that a random
distribution of magnetic adatoms can open a robust gap in the electronic
spectrum of graphene. The electronic gap results from the interplay between the
nature of the graphene sublattice structure and the exchange interaction
between adatoms.The size of the gap depends on the strength of the exchange
interaction between carriers and localized spins and can be controlled by both
temperature and external magnetic field. Furthermore, we show that an external
magnetic field creates an imbalance of spin-up and spin-down carriers at the
Fermi level, making doped graphene suitable for spin injection and other
spintronic applications.Comment: 5 pages, 5 figure
Vegetação espontânea e áreas de solo descoberto em pastagens de capim-buffel sob diferentes ofertas de biomassa no período seco na região Semiárida de Pernambuco.
Efeitos de Caracteres Físicos e Químicos Sobre o Teor de Óleo de Frutos de Pupunha.
O objetivo deste estudo foi avaliar a relação entre variáveis físicas e químicas de frutos de pupunha, apontando os efeitos diretos e indiretos de variáveis explicativas sobre a variável principal, teor de óleo
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