Magnetic Properties of the Metamagnet Ising Model in a three-dimensional Lattice in a Random and Uniform Field

By employing the Monte Carlo technique we study the behavior of Metamagnet Ising Model in a random field. The phase diagram is obtained by using the algorithm of Glaubr in a cubic lattice of linear size $L$ with values ranging from 16 to 42 and with periodic boundary conditions.Comment: 4 pages, 6 figure

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 $T_c$ {\em vs} transverse field $\Gamma$) we obtain shows a finite slope near $T=0$, 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 LiHo$_{0.167}$Y$_{0.833}$F$_4$, 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

Resonant x-ray scattering study on multiferroic BiMnO3

Resonant x-ray scattering is performed near the Mn K-absorption edge for an epitaxial thin film of BiMnO3. The azimuthal angle dependence of the resonant (003) peak (in monoclinic indices) is measured with different photon polarizations; for the $\sigma\to\pi'$ channel a 3-fold symmetric oscillation is observed in the intensity variation, while the $\sigma\to\sigma'$ scattering intensity remains constant. These features are accounted for in terms of the peculiar ordering of the manganese 3d orbitals in BiMnO3. It is demonstrated that the resonant peak persists up to 770 K with an anomaly around 440 K; these high and low temperatures coincide with the structural transition temperatures, seen in bulk, with and without a symmetry change, respectively. A possible relationship of the orbital order with the ferroelectricity of the system is discussed.Comment: 14 pages, 4 figure

Magnetism and Pairing in Hubbard Bilayers.

We study the Hubbard model on a bilayer with repulsive on-site interactions, $U$, in which fermions undergo both intra-plane ($t$) and inter-plane ($t_z$) hopping. This situation is what one would expect in high-temperature superconductors such as YBCO, with two adjacent CuO$_2$ planes. Magnetic and pairing properties of the system are investigated through Quantum Monte Carlo simulations for both half- and quarter-filled bands. We find that in all cases inter-planar pairing with $d_{x^2-z^2}$ symmetry is dominant over planar pairing with $d_{x^2-y^2}$ symmetry, and that for $t_z$ large enough pair formation is possible through antiferromagnetic correlations. However, another mechanism is needed to make these pairs condense into a superconducting state at lower temperatures. We identify the temperature for pair formation with the spin gap crossover temperature. [Submitted to Phys. Rev. B]Comment: 7 pages, uuencoded self-unpacking PS file with text and figures

Caracterização fenológica de cultivares de uvas de mesa do Banco de Germoplasma da Embrapa Semiárido.

Caracterizou-se a fenologia de 135 acessos de uvas de mesa pertencentes ao BAG da Embrapa Semiárido, considerando-se as médias em número de dias de oito ciclos no primeiro semestre e de seis ciclos no segundo semestre entre 2002 e 2011. Foi caracterizada a duração das principais fases fenológicas: gemas inchadas ao início de brotação (GI-IB), início de brotação à plena floração (IB-Fl), plena floração ao início de frutificação (Fl-IFr), início de frutificação ao início de maturação (IFr-IMt) e início de maturação até final de maturação (IMt-Mt), bem como o ciclo fenológico completo da poda à colheita, em dois grupos de cultivares de uvas de mesa. Os acessos foram classificados como: precoces, medianos e tardios. A variação na frequência dos acessos nas classes fenológicas foi influenciada pelas condições climáticas, como temperaturas mais altas no segundo semestre, que favoreceram a maior frequência de acessos precoces. A duração dos estádios fenológicos GI-IB, IB-Fl e Fl-IFr foram semelhantes. Observou-se maiores variações para os estádios IFr-IMt e Imt-Mt, prolongando-se a duração nas cultivares tardias, evidenciando a influência das condições climáticas do semestre do ano em que se realiza a poda na duração do ciclo fenológico de uvas de mesa cultivadas no Vale do São Francisco.bitstream/item/84655/1/BPD106.pd

Critical temperature for the two-dimensional attractive Hubbard Model

The critical temperature for the attractive Hubbard model on a square lattice is determined from the analysis of two independent quantities, the helicity modulus, $\rho_s$, and the pairing correlation function, $P_s$. These quantities have been calculated through Quantum Monte Carlo simulations for lattices up to $18\times 18$, and for several densities, in the intermediate-coupling regime. Imposing the universal-jump condition for an accurately calculated $\rho_s$, together with thorough finite-size scaling analyses (in the spirit of the phenomenological renormalization group) of $P_s$, suggests that $T_c$ is considerably higher than hitherto assumed.Comment: 5 pages, 6 figures. Accepted for publication in Phys. Rev.