Miguel Angelo Cavalheiro Gusmao

Specific heat and magnetic order in LaMnO/sub 3+[delta]/

Magnetic and specific-heat measurements are performed in three different samples of LaMnO_{3+\delta}, with \delta=0.11, 0.15 and 0.26, presenting important disorder effects, such as carrier localization, due to high amounts of La and Mn vacancies. For the samples with \delta =0.11 and 0.15, magnetic measurements show signatures of a two-step transition: as the temperature is lowered, the system enters a ferromagnetic phase followed by a disorder-induced cluster-glass state. Spin-wave-like contributions and an unexpected large linear term are observed in the specific heat as a function of temperature. In the sample with the highest vacancy content, \delta=0.26, the disorder is sufficient to suppress even short-range ferromagnetic order and yield a spin-glass-like state.

Scaling beyond 3D XY in the fluctuation conductivity of YBa2Cu3O7−δ

Abstract Systematic measurements of the in-plane fluctuation magnetoconductivity in a YBa 2 Cu 3 O 7−δ single crystal are presented. Fields up to 500 mT were applied either parallel or perpendicular to the Cu–O planes. The data reveal the occurrence of Gaussian-fluctuation regimes far from the transition. Closer to T c , the results clearly show the occurrence of a genuine critical regime, where the exponent is consistent with predictions for the 3D XY universality class with model- E dynamics. Still closer to T c , evidence is found for a fluctuation regime beyond 3D XY . This new scaling can be interpreted as revealing an ultimate first-order character of the superconducting transition in YBa 2 Cu 3 O 7−δ .

Effect of band filling in the Kondo lattice : a mean-field approach

The usual Kondo lattice, including an antiferromagnetic exchange interaction between nearest-neighboring localized spins, is treated here in a mean-field scheme that introduces two mean-field parameters: one associated with the local Kondo effect, and the other related to the magnetic correlations between localized spins. Phases with short-range magnetic correlations or coexistence between those and the Kondo effect are obtained. By varying the number of electrons in the conduction band, we notice that the Kondo effect tends to be suppressed away from half filling, while magnetic correlations can survive if the Heisenberg coupling is strong enough. An enhanced linear coefficient of the specific heat is obtained at low temperatures in the metallic state.

Bias dependence of magnetoresistance in Fe–Al2O3 granular thin films

This paper reports on the magnetotransport behavior of Fe–Al2O3 granular thin films when the injected dc current is varied. The electric resistance as a function of temperature, magnetoresistance, and the current vs applied bias potential measurements were used to characterize the samples. It was found that the transport mechanism which best describes the electronic properties of these samples is variable range hopping. Non-Ohmic behavior was observed and is claimed as responsible for the great modification of the electronic characteristics of the system as a function of the applied bias potential. Inversion of the tunneling magnetoresistance is observed for applied bias potential greater than 3 V. Such inverted magnetoresistance comes from the activation of low resistivity tunneling paths that are promoted by increasing the bias potential. An expression is proposed to describe the magnetoresistance behavior.

New investigation of the magnetic structure of CoNb2O6 columbite

The structural and magnetic properties of the CoNb2O6 compound have been investigated with a particular interest in their low-dimensional magnetic behavior which is characterized by the presence of weakly interacting ferromagnetic chains. We investigate this cobalt niobate by combining magnetic measurements; x-ray and neutron diffraction (ND). The ND was carried out on powder samples at different temperatures above (20 K) and below the ordering temperature (2.4, 1.8, and 1.4 K). The compound exhibits an orthorhombic crystal structure of Pbcn symmetry, typical of a columbite structure. Magnetic ordering at 2.5 K is found with the propagation vector (0, 0.4, 0), in agreement with earlier studies. However, at lower temperatures the present investigation shows the coexistence of two different magnetic phases: the propagation vector (0.5, 0.5, 0) is found to be necessary to refine the ND measurements at both 1.8 and 1.4 K, in addition to (0, 0.5, 0) which was the only one reported in earlier works.

ELECTRONIC AND PHONONIC STATES OF THE HOLSTEIN-HUBBARD DIMER OF VARIABLE LENGTH

We consider a model Hamiltonian for a dimer of length a including all the electronic one- and two-body terms consistent with a single orbital per site, a free Einstein phonon term for a frequency V, and an electronphonon coupling g0 of the Holstein type. The bare electronic interaction parameters were evaluated in terms of Wannier functions built from Gaussian atomic orbitals. An effective polaronic Hamiltonian was obtained by an unrestricted displaced-oscillator transformation, followed by evaluation of the phononic terms over a squeezedphonon variational wave function. For the cases of quarter-filled and half-filled orbitals, and over a range of dimer length values, the ground state for given g0 and V was identified by simultaneously and independently optimizing the orbital shape, the phonon displacement, and the squeezing effect strength. As a varies, we generally find discontinuous changes of both electronic and phononic states, accompanied by an appreciable renormalization of the effective electronic interactions across the transitions, due to the equilibrium shape of the wave functions strongly depending on the phononic regime and on the type of ground state. @S0163-1829~98!06035-4#

Competition between magnetic order and Kondo effect in cerium compounds

We present a mean-field analysis of the competition between magnetic order and Kondo effect in a Kondo-lattice model usually employed to discuss properties of certain cerium compounds. A phase diagram is obtained showing an antiferromagnetic phase and a Kondo-compensated regime, in agreement with the Doniach diagram. A general discussion of the mean-field approach is also presented.

X-ray diffraction and magnetic susceptibility measurements for FexNi1−xTa2O6

The trirutile family of the tapiolite-like compound FexNi1−xTa2O6 was investigated by means of x-ray powder diffraction (XRD) and magnetic susceptibility measurements. From Rietveld refinement it is demonstrated that the family is a homogeneous solid solution obeying Vegards law. Magnetic susceptibility curves exhibit typical signatures of low-dimensional systems, with broadened maxima due to short range correlations immediately above the antiferromagnetic transition temperature TN. The T versus x phase diagram presents a minimum that suggests bicritical behaviour.

Model calculation of the interaction terms and ground states of the extended Hubbard model on a dimer

Abstract We consider the complete extended Hubbard Hamiltonian on a dimer, including all one- and two-body terms. The energy eigenvalues and eigenvectors and various correlation functions have been obtained in explicit form. Introducing model Wannier orbitals we have evaluated the interaction parameters as functions of the dimer length and orbital shape. Minimization of the lowest-energy eigenvalue yields the equilibrium shape for different fillings.

Critical exponents of the disorder-driven superfluid-insulator transition in one-dimensional Bose-Einstein condensates

Centro Brasileiro de Pesquisas F´isicas, Rio de Janeiro, RJ, Brazil(Dated: November 17, 2011)We investigate the nature of the superfluid-insulator quantum phase transition driven by disorderfor non-interacting ultracold atoms on one-dimensional lattices. We consider two different cases:Anderson-type disorder, with local energies randomly distributed, and pseudo-disorder due to apotential incommensurate with the lattice, which is usually called the Aubry-Andr´emodel. A scalinganalysis of numerical data for the superfluid fraction for different lattice sizes allows us to determinequantum critical exponents characterizing the disorder-driven superfluid-insulator transition. Wealso briefly discuss the effect of interactions close to the non-interacting quantum critical point ofthe Aubry-Andr´e model.