Mixing of d(x)(-y)(2)(2) and d(xy) superconducting states for different filling and temperature

We investigate the solution of the gap equation for mixed order parameter symmetry states as a function of filling using a two-dimensional tight-binding model incorporating second-neighbor hopping for tetragonal and orthorhombic lattice, the principal (major) component of the order parameter is taken to be of the d(x2-y2) type, As suggested in several investigations the minor component of the order parameter is taken to be of the d(xy) type. Both the permissible mixing angles 0 and pi/2 between the two components are considered. As a function of filling pronounced maxima of d(x2-y2) order parameter is accompanied by minima of the d(xy) order parameter. At fixed filling. The temperature dependence of the two components of the order parameter is also studied in all cases. The variation of critical temperature T, with filling is also studied and T-c is found to increase with second-neighbor hopping. (C) 2001 Elsevier B.V. B.V. All rights reserved.

q-Analog of the gap equation for cuprates

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Cation-oxygen interaction and oxygen stability in CaCu3Ti4O12 and CdCu3Ti4O12 lattices

The cubic perovskite related material CaCu3Ti4O12 has attracted a great deal of attention due to the high values of the static dielectric constant, of order 104, approximately constant in the temperature range 100-600 K. The substitution of Ca by Cd results in a similar temperature dependence but a static dielectric constant more than one order of magnitude lower. A theoretical electronic structure study is performed on CaCu3Ti4O12 (CCTO) and CdCu3Ti4O12 (CdCTO) using a tight binding with overlap method. Although the calculations are performed in a paramagnetic configuration, excellent agreement with experiment was found for the calculated band gap of CCTO. In spite of the fact that the band structures of both systems look practically the same, a significant difference is found in the calculated bond strength of Ca-O and Cd-O pairs, driven by the presence of Ti, with Ca-O interaction in CCTO loosened with respect to Cd-O interaction in the cadmium compound. It is suggested that O vacancies are more easily formed in CCTO, this being related to the lower electronegativity of Ca as compared to Cd. The formation of oxygen vacancies could be the origin of the difference in the static dielectric constant of the two compounds.

Extended states in disordered doped polyacetylene chains

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Disordered distribution of defects in polythiophene

In this work we study the electronic structure associated to a disordered distribution of bipolarons in polythiophene. The polymer chain is modelled by a tight-binding Hamiltonian with explicit treatment of electron-phonon coupling and the elastic energy of the sigma framework. The model also includes the electrostatic interaction due to the counterions. The density of states of the disordered system is obtained by the use of the Negative Factor Counting technique. Our results show that ion-induced conformational disorder can account for the closure of the gap and that the states around the Fermi level are extended. © 1993.

Generalized simulated annealing: Application to silicon clusters

We have compared the recently introduced generalized simulated annealing (GSA) with conventional simulated annealing (CSA). GSA was tested as a tool to obtain the ground-state geometry of molecules. We have used selected silicon clusters (Sin, n=4-7,10) as test cases. Total energies were calculated through tight-binding molecular dynamics. We have found that the replacement of Boltzmann statistics (CSA) by Tsallis's statistics (GSA) has the potential to speed up optimizations with no loss of accuracy. Next, we applied the GSA method to study the ground-state geometry of a 20-atom silicon cluster. We found an original geometry, apparently lower in energy than those previously described in the literature.

Temperature dependence of structural and electronic properties of the spin-liquid candidate κ-(BEDT-TTF) 2Cu 2(CN) 3

We investigate the effect that the temperature dependence of the crystal structure of a two-dimensional organic charge-transfer salt has on the low-energy Hamiltonian representation of the electronic structure. For that, we determine the crystal structure of κ-(BEDT-TTF) 2Cu 2(CN) 3 for a series of temperatures between T=5 and 300 K by single crystal X-ray diffraction and analyze the evolution of the electronic structure with temperature by using density functional theory and tight binding methods. We find a considerable temperature dependence of the corresponding triangular lattice Hubbard Hamiltonian parameters. We conclude that even in the absence of a change of symmetry, the temperature dependence of quantities like frustration and interaction strength can be significant and should be taken into account. © 2012 American Physical Society.

The effects of water molecules on the electronic and structural properties of peptide nanotubes

The self-assembly of short amino acid chains appears to be one of the most promising strategies for the fabrication of nanostructures. Their solubility in water and the possibility of chemical modification by targeting the amino or carboxyl terminus give peptide-based nanostructures several advantages over carbon nanotube nanostructures. However, because these systems are synthesized in aqueous solution, a deeper understanding is needed on the effects of water especially with respect to the electronic, structural and transport properties. In this work, the electronic properties of l-diphenylalanine nanotubes (FF-NTs) have been studied using the Self-Consistent Charge Density-Functional-based Tight-Binding method augmented with dispersion interaction. The presence of water molecules in the central hydrophilic channel and their interaction with the nanostructures are addressed. We demonstrate that the presence of water leads to significant changes in the electronic properties of these systems decreasing the band gap which can lead to an increase in the hopping probability and the conductivity. © the Owner Societies 2013.

Cálculo de funções de Wannier eletrônicas para aplicações em ciência dos materiais

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Cálculos de estrutura eletrônica de materiais mediante combinação linear de orbitais atômicos

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Mechanical properties and fracture dynamics of silicene membranes

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Fullerenes generated from porous structures

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Local order and magnetic field effects on the electronic properties of disordered binary alloys in the quantum site percolation limit

Electronic properties of disordered binary alloys are studied via the calculation of the average Density of States (DOS) in two and three dimensions. We propose a new approximate scheme that allows for the inclusion of local order effects in finite geometries and extrapolates the behavior of infinite systems following finite-size scaling ideas. We particularly investigate the limit of the Quantum Site Percolation regime described by a tight-binding Hamiltonian. This limit was chosen to probe the role of short range order (SRO) properties under extreme conditions. The method is numerically highly efficient and asymptotically exact in important limits, predicting the correct DOS structure as a function of the SRO parameters. Magnetic field effects can also be included in our model to study the interplay of local order and the shifted quantum interference driven by the field. The average DOS is highly sensitive to changes in the SRO properties and striking effects are observed when a magnetic field is applied near the segregated regime. The new effects observed are twofold: there is a reduction of the band width and the formation of a gap in the middle of the band, both as a consequence of destructive interference of electronic paths and the loss of coherence for particular values of the magnetic field. The above phenomena are periodic in the magnetic flux. For other limits that imply strong localization, the magnetic field produces minor changes in the structure of the average DOS. © World Scientific Publishing Company.

Phase transition from a dx2-y2 to a dx2-y2+dxy superconductor

We study the phase transition from a dx2-y2 to a dx2-y2+dxy superconductor using the tight-binding model of two-dimensional cuprates. As the temperature is lowered past the critical temperature Tc, first a dx2-y2 superconducting phase is created. With further reduction of temperature, the dx2-y2+dxy phase is created at temperature T=Tc1. We study the temperature dependencies of the order parameter, specific heat, and spin susceptibility in these mixed-angular-momentum states on a square lattice and on a lattice with orthorhombic distortion. The above-mentioned phase transitions are identified by two jumps in specific heat at Tc and Tc1. ©1999 The American Physical Society.

Ordered phases in the extended Hubbard model

A novel method to probe the diverse phases for the extended Hubbard model (EHM), including the correlated hopping term, is presented. We extend an effective medium approach [1] to a bipartite lattice, allowing for charge- and/or spin-ordered phases. We calculate the necessary correlation functions to build the EHM phase diagram.