Giuseppe Pastori Parravicini

Defect chemistry and dielectric properties of Yb3+:CaTiO3 perovskite

The defect chemistry of Yb3+:CaTiO3 solid solutions has been investigated both theoretically and experimentally. Three different incorporation mechanisms with similar solution energy were predicted for Yb3+ by atomistic simulation: (i) Ca site substitution with Ca vacancy compensation; (ii) Ti site substitution with O vacancy compensation; (iii) simultaneous substitution at both Ca and Ti sites with self-compensation. X-ray diffraction and scanning electron microscopy results strongly support the possibility to realize the above defect chemistries in CaTiO3 by changing the Ca∕Ti ratio to force Yb3+ on the Ca site (Ca∕Ti 1), or on both sites (Ca∕Ti=1) according to the calculations. The temperature dependence of the relative dielectric constant (102–105Hz) of ceramics corresponding to predominant Yb substitution either at the Ca site or the Ti site is qualitatively similar to that of undoped CaTiO3. The Curie-Weiss temperature is shifted to more negative values in comparison to CaTiO3,...

Current distribution and conductance quantization in the integer quantum Hall regime

Charge transport of a two-dimensional electron gas in the presence of a magnetic field is studied by means of the Keldysh–Green function formalism and the tight-binding method. We evaluate the spatial distributions of persistent (equilibrium) and transport (nonequilibrium) currents, and give a vivid picture of their profiles. In the quantum Hall regime, we find exact conductance quantization both for persistent currents and for transport currents, even in the presence of impurity scattering centres and moderate disorder.

Physical and numerical aspects in Lanczos and modified Lanczos calculations

Abstract The use of recurrence relations in Lanczos procedure is often limited in practice by loss of numerical stability. In many applications this fact severely limits the numerical reliability of the results and hence the interpretation of the physical models proposed. In this paper we focus on the effects of finite precision arithmetic in the Lanczos algorithm and examine some methods for assessing and testing the reliability of the tridiagonal matrix generated. In addition to the residual vector test we propose an initial vector test, and an orthogonalization test to assess the quality of the eigenvalues. We also implement the use of multiple precision to obtain a significant improvement in the tridiagonal chain transformation. As a final resort, we consider the modified Lanczos procedure to obtain one by one, energies and wavefunctions of a system, within any desired energy range and with any desired accuracy.

Incommensurate potentials : analytic and numerical progress

Incommensurate potentials present quite peculiar physical properties due to the deterministic break of translational symmetry; among others the authors mention generalized metal-insulator transitions, mobility edges and the singular behaviour of localization length. The authors stress here how these aspects can be fruitfully described by the joint use of the renormalization procedure and an appropriate envelope function formalism. The application to some significant examples clearly reveals that their approach is well suited to treat more complex and realistic incommensurate potentials.

Electronic interactions in quantum dots within the Keldsyh formalism

Within the Keldysh formalism, we consider electron transport through single-level quantum dots, and revisit the proper self-energy diagram up to quadratic terms in the Coulomb interaction. Convolutions of Greens functions in real energy domain are analyzed in terms of retarded and advanced, lesser and greater, time-ordered and anti-time-ordered components. By virtue of the multipole expansion of the Fermi functions and other elaborations, we provide elegant and fully analytic expressions for the various components of the bubble diagram, with promise of deeper insight on the many-body effects in biased devices.

Augmented space formalism and renormalization for ground and excited states of magnetic systems

The augmented space formalism has been used so far for the study of disordered crystals. We stress here that the topological structure of the augmented space of a binary alloy is formally equivalent to the set of configurations of an Ising system. In particular in both cases a very useful binary representation for labelling the vectors of the space can be done. Starting from this observation we have developed a very efficient description of the Ising hamiltonian, and we have shown that it can be succesfully applied both in the direct diagonalization approach and in conjunction with the renormalization scheme. Within this last case we have proposed a generalization of the techniques commonly used in the literature, obtaining very good results.