National Institute Of Materials Physics - Romania

Statistics of level spacing and magnetization are studied for the phase diagram of the integer quantum Hall effect in a two-dimensional finite lattice model with Anderson disorder.

We calculate the quantum states corresponding to the drifting and channeled classical orbits in a two-dimensional electron gas (2DEG) with strong magnetic and electric modulations along one spatial direction, x. The channeled states carry high, concentrated currents along the y-axis, and are confined in an effective potential well. The quantum and the classical states are compared. (C) 1998 Elsevier Science B.V. All rights reserved.

The transport properties of a rectangular mesoscopic plaquette in the presence of a perpendicular magnetic field are studied in a tight-binding model with randomly distributed traps. The longitudinal and Hall resistances are calculated in the four-probe Landauer-Buttiker formalism which accounts automatically both for the quantum coherence and the trapping-induced localization. The localized character of eigenvectors and the specific aspect of the density of states at a given magnetic flux are correlated with the behaviour of the mentioned resistances as function of the Fermi energy. The Hall insulator and quantum Hall regimes are evidenced. The magnetic field dependence of the configurational averages of the longitudinal and Hal; resistance is studied in a purely quantum-mechanical approach. Both, negative and positive magnetoresistances are found.

The spectrum and the eigenstates of a finite two-dimensional tight-binding electronic system, with Dirichlet boundary conditions, in a magnetic field and with an external linear potential are studied. The eigenstates show an equipotential character, and may cross the plaquette in the direction perpendicular to the electric field. When leads are added to the plaquette, the channels carrying the current may be shortcut by equipotentials, resulting in additional plateaus situated between the usual integer-quantum-Hall-effect plateaus. This ideals confirmed by a numerical calculation within the four-terminal Landauer-Buttiker approach.

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.