A Monte Carlo study of the anisotropic N=3 Ashkin-Teller model

The phase diagram of an asymmetric N = 3 Ashkin-Teller model is obtained by a numerical analysis which combines Monte Carlo renormalization group and reweighting techniques. Present results reveal several differences with those obtained by mean-field calculations and a Hamiltonian approach. In particular, we found Ising critical exponents along a line where Goldschmidt has located the Kosterlitz-Thouless multicritical point. On the other hand, we did find nonuniversal exponents along another transition line. Symmetry breaking in this case is very similar to the N = 2 case, since the symmetries associated to only two of the Ising variables are broken. However, for large values of the coupling constant ratio XW = W/K, when the only broken symmetry is of a hidden variable, we detected first-order phase transitions giving evidence supporting the existence of a multicritical point, as suggested by Goldschmidt, but in a different region of the phase diagram. © 2002 Elsevier Science B.V. All rights reserved.

Dielectric properties of Sr0.75Ba0.25Nb 2O6 thin films: Bias field, frequency and temperature dependence

Ferroelectric strontium barium niobate solid solutions had received great attention due to their excellent pyroelectric, electrooptic and photorefractive properties. Furthermore, they usually also present very interesting phase transition characteristics. In this work, polycrystalline single phase Sr 0.75 Ba 0.25 Nb 2 O 6 thin films were prepared by a hybrid chemical method and deposited on Pt/Ti/SiO 2 /Si substrates. The temperature dependence of dielectric constant was measured at different frequencies and bias field levels. The presence of two dielectric dispersion regions with relaxor characteristics was observed at distinct temperature ranges, corresponding to the ferro-paraelectric and to a structural phase transition at low temperatures, respectively. A specific dielectric dispersion region, associated with an incommensurate superstructure frequently observed in bulk samples, was not observed in this films probably due to their small grain sizes. © 2002 Taylor & Francis.

Dilatometria de sólidos com resolução subatômica

The main goal of this bachelor work is the study of the physical properties of solids by thermal expansion coefficient measurements, which allows the exploration of various aspects of phase transitions. In this study, some theoretical aspects were focused, for example the first and second order phase transitions, Landau theory for phase transition and the volumetric thermal expansion coefficient. Furthermore, on the origin of the thermal expansion from the Lennard-Jones potential, that contains the anharmonic terms, which give rise to the phononic thermal expansion in solids

Magnetic-size effects in ultrathin layers

We study N-layer samples (N ≤ 10) for the Heisenberg model, with ferro- and antiferromagnetic exchange couplings, using a modified version of the Onsager reaction field approximation. The present scheme includes short-range spin-spin correlations, and allows for layer-dependent order parameters when free surface boundary conditions are imposed. The limits N = 1 (two dimensions) and N → ∞ (three dimensions) can be solved analytically, while systems with several layers have to be numerically calculated. We found no indication of a phase transition at finite temperature up to the sizes investigated (N = 10), the layered systems behaving essentially as two-dimensional. A phase transition is only obtained for the three-dimensional limit. © 1993.

Investigação das propriedades de transporte da fase ferroelétrica de Mott-Hubbard em condutores moleculares

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Red Mud from Brazil: Thermal Behavior and Physical Properties

The main constituents of red mud produced in Aluminio city (S.P., Brazil) are iron, aluminum, and silicon oxides. It has been determined that the average particle diameter for this red mud is between 0.05 and 0.002 mm. It is observed that a decrease in the percentage of smaller particles occurs at temperatures greater than 400 degrees C. This observation corresponds with the thermal analysis and X-ray diffraction (XRD) data, which illustrate the phase transition of goethite to hematite. A 10% mass loss is observed in the thermal analysis patterns due to the hydroxide-oxide phase transitions of iron (primary phase transition) and aluminum (to a lesser extent). The disappearance and appearance of the different phases of iron and aluminum confirms the decomposition reactions proposed by the thermal analysis data. This Brazilian red mud has been classified as mesoporous at all temperatures except between 400 and 500 degrees C where the classification changes to micro/mesoporous.

Temperature-Dependence of Cationic Lipid Bilayer Intermixing: Possible Role of Interdigitation

In this work, we investigated the properties of a fusogenic cationic lipid, diC14-amidine, and show that this lipid possesses per se the capacity to adopt either an interdigitated structure (below and around its transition temperature) or a lamellar structure (above the transition temperature). To provide experimental evidence of this lipid bilayer organization, phospholipids spin-labeled at different positions of the hydrocarbon chain were incorporated into the membrane and their electron spin resonance (ESR) spectra were recorded at different temperatures. For comparison, similar experiments were performed with dimyristoyl phosphatidylcholine, a zwitterionic lipid (DMPC) which adopts a bilayer organization over a broad temperature range. Lipid mixing between diC14-amidine and asolectin liposomes was more efficient below (10-15 degrees C) than above the transition temperature (above 25 degrees C). This temperature-dependent "fusogenic" activity of diC14-amidine liposomes is opposite to what has been observed so far for peptides or virus-induced fusion. Altogether, our data suggest that interdigitatiori is a highly fusogenic state and that interdigitation-mediated fusion occurs via an unusual temperature-dependent mechanism that remains to be deciphered.

Bose-Einstein Condensation of Magnons in NiCl2-4SC(NH2)(2)

A Bose-Einstein condensation (BEC) has been observed in magnetic insulators in the last decade. The condensed bosons are magnons associated with an ordered magnetic phase induced by a magnetic field. We review the experiments in the spin-gap compound NiCl2-4SC(NH2)(2), in which the formation of BEC occurs by applying a magnetic field at low temperatures. This is a contribution to the celebration of the 50th anniversary of the Solid State and Low Temperature Laboratory of the University of So Paulo, where this compound was first magnetically characterized.

Quantum-Critical Spin Dynamics in Quasi-One-Dimensional Antiferromagnets

By means of nuclear spin-lattice relaxation rate T-1(-1), we follow the spin dynamics as a function of the applied magnetic field in two gapped quasi-one-dimensional quantum antiferromagnets: the anisotropic spin-chain system NiCl2-4SC(NH2)(2) and the spin-ladder system (C5H12N)(2)CuBr4. In both systems, spin excitations are confirmed to evolve from magnons in the gapped state to spinons in the gapless Tomonaga-Luttinger-liquid state. In between, T-1(-1) exhibits a pronounced, continuous variation, which is shown to scale in accordance with quantum criticality. We extract the critical exponent for T-1(-1), compare it to the theory, and show that this behavior is identical in both studied systems, thus demonstrating the universality of quantum-critical behavior.

Dissipation effects in random transverse-field Ising chains

We study the effects of Ohmic, super-Ohmic, and sub-Ohmic dissipation on the zero-temperature quantum phase transition in the random transverse-field Ising chain by means of an (asymptotically exact) analytical strong-disorder renormalization-group approach. We find that Ohmic damping destabilizes the infinite-randomness critical point and the associated quantum Griffiths singularities of the dissipationless system. The quantum dynamics of large magnetic clusters freezes completely, which destroys the sharp phase transition by smearing. The effects of sub-Ohmic dissipation are similar and also lead to a smeared transition. In contrast, super-Ohmic damping is an irrelevant perturbation; the critical behavior is thus identical to that of the dissipationless system. We discuss the resulting phase diagrams, the behavior of various observables, and the implications to higher dimensions and experiments.

Percolation transition in quantum Ising and rotor models with sub-Ohmic dissipation

We investigate the influence of sub-Ohmic dissipation on randomly diluted quantum Ising and rotor models. The dissipation causes the quantum dynamics of sufficiently large percolation clusters to freeze completely. As a result, the zero-temperature quantum phase transition across the lattice percolation threshold separates an unusual super-paramagnetic cluster phase from an inhomogeneous ferromagnetic phase. We determine the low-temperature thermodynamic behavior in both phases, which is dominated by large frozen and slowly fluctuating percolation clusters. We relate our results to the smeared transition scenario for disordered quantum phase transitions, and we compare the cases of sub-Ohmic, Ohmic, and super-Ohmic dissipation.

Quantum spherical model with competing interactions

We analyse the phase diagram of a quantum mean spherical model in terms of the temperature T, a quantum parameter g, and the ratio p = -J(2)/J(1) where J(1) > 0 refers to ferromagnetic interactions between first-neighbour sites along the d directions of a hypercubic lattice, and J(2) < 0 is associated with competing anti ferromagnetic interactions between second neighbours along m <= d directions. We regain a number of known results for the classical version of this model, including the topology of the critical line in the g = 0 space, with a Lifshitz point at p = 1/4, for d > 2, and closed-form expressions for the decay of the pair correlations in one dimension. In the T = 0 phase diagram, there is a critical border, g(c) = g(c) (p) for d >= 2, with a singularity at the Lifshitz point if d < (m + 4)/2. We also establish upper and lower critical dimensions, and analyse the quantum critical behavior in the neighborhood of p = 1/4. 2012 (C) Elsevier B.V. All rights reserved.

Alzheimer random walk model: Two previously overlooked diffusion regimes

A non-Markovian one-dimensional random walk model is studied with emphasis on the phase-diagram, showing all the diffusion regimes, along with the exactly determined critical lines. The model, known as the Alzheimer walk, is endowed with memory-controlled diffusion, responsible for the model's long-range correlations, and is characterized by a rich variety of diffusive regimes. The importance of this model is that superdiffusion arises due not to memory per se, but rather also due to loss of memory. The recently reported numerically and analytically estimated values for the Hurst exponent are hereby reviewed. We report the finding of two, previously overlooked, phases, namely, evanescent log-periodic diffusion and log-periodic diffusion with escape, both with Hurst exponent H = 1/2. In the former, the log-periodicity gets damped, whereas in the latter the first moment diverges. These phases further enrich the already intricate phase diagram. The results are discussed in the context of phase transitions, aging phenomena, and symmetry breaking.

Complex mixed state of the Pauli-limited superconductor CeCoIn5

Magnetization measurements were performed on CeCoIn5 at temperatures down to 20 mK and magnetic fields up to 17 T applied along different crystallographic orientations. For field configurations nearly parallel to the ab plane (theta less than or similar to 40 degrees and T <= 50 mK), we have found an intriguing vortex dynamics regime revealed by a hysteretic and metastable anomalous peak effect (APE), which gives evidence of surface barrier effects enhanced by antiferromagnetic fluctuations in the mixed state of CeCoIn5. Furthermore, we have observed crossover features in the torque and magnetization traces at fields below H-c2, which are consistent with vortices lattice phase transitions and with the anomalies speculated to be the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state in CeCoIn5. All of the above features were found to be dramatically perturbed in Ce0.98Gd0.02CoIn5.

Influence of Ba-substitution on the structural and ferroelectric properties of Pb1-xBaxZr0.40Ti0.60O3 ceramic materials

This work presents a comprehensive study about the influence of Ba-substitution on the structural and ferroelectric properties of Pb1-xBaxZr0.40Ti0.60O3 (PBZT) ceramic system. Pb1-xBaxZr0.40Ti0.60O3 ceramic samples were then prepared by solid state reaction method and characterized as a function of composition and temperature by X-ray diffraction (XRD) and impedance spectroscopy techniques. The dielectric measurements show that the substitution of Pb2+ for Ba2+ ions leads to a diffuse behavior of the dielectric permittivity curves for all samples and that only the x = 0.50 sample presents a typical relaxor behavior. In good agreement with dielectric measurements, the structural phase transition study showed a phase transition from a tetragonal structure with P4mm space group to a cubic structure with Pm-3m space group for all samples, except for the x = 0.50 sample were a cubic structure was observed in the complete temperature interval measured.