National Institute Of Materials Physics - Romania

Publications

873 articles found

831. Electronic structure of (001) AlAs-InAs-GaAs multilayer structures

Authors: Velasco, VR; Fernandez-Alvarez, L; Aldea, A; Vlaev, S; Monsivais, G; Garcia-Moliner, F

Published: SEP 3 1998, SURFACE SCIENCE, 412-13, 404, DOI: 10.1016/S0039-6028(98)00458-0

The electronic structure of different (001) (AlAs)(m)(InAs)(1)(GaAs)(m) multilayer structures is studied. We have thus seen the influence of the relative thicknesses of the constituent materials and of the positions of the InAs principal layers in the multilayer structures on the energy eigenstates and on their spatial localization. The calculations are based on an sp(3)s* empirical tight-binding model and on the surface Green function matching method. (C) 1998 Elsevier Science B.V. All rights reserved.

832. Current voltage characteristics of alpha-quartz

Authors: Enculescu, I; Iliescu, B

Published: JUN 1998, EUROPEAN PHYSICAL JOURNAL-APPLIED PHYSICS, 2, 207, DOI: 10.1051/epjap:1998185

Dc conductivity measurements for Z-cut natural quartz plates, at different temperatures and values of the applied electric field were made. The evolution of I-V characteristics shapes is studied for electrodiffusion times up to 100 h (in interstitial alkali ions transport region). The non-ohmic behavior is discussed and is considered to be due to the space charge produced by the uncompensated Al3+ ions substitutional for Si4+.

833. QHE, magnetoresistance and disordered transport on 2D mesoscopic plaquettes

Authors: Aldea, A; Gartner, P; Nita, M

Published: JAN 1998, PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 205, 356, DOI: 10.1002/(SICI)1521-3951(199801)205:1<351::AID-PSSB351>3.3.CO;2-Z

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.

834. Ionic space charge limited currents in natural quartz crystal

Authors: Enculescu, I; Iliescu, B

Published: 1998, FIFTH CONFERENCE ON OPTICS (ROMOPTO '97), PTS 1 AND 2, 3405, 266, DOI: 10.1117/12.312762

Arrhenius plots and current-voltage characteristics were studied for natural quartz samples, in different conditions of air-sweeping time, of temperature, of electrodes (with or without any source of positive ions). Some observations about the influence of the ionic space charge limited current are made. Some dependencies of ionic current versus voltage were found.

835. Specific plateaus of the quantum Hall effect induced by an applied bias

Authors: Aldea, A; Gartner, P; Manolescu, A; Nita, M

Published: MAY 15 1997, PHYSICAL REVIEW B, 55, 13392, DOI: 10.1103/PhysRevB.55.R13389

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.

836. Dynamics of the dauphine twins in quartz crystal up to the transition point

Authors: Iliescu, B; Enculescu, I; Chirila, R

Published: 1997, FERROELECTRICS, 190, 124, DOI: 10.1080/00150199708014103

The paper presents two original methods for obtaining electrical mins up to the transition point in quartz crystal: a) thermal treatment in a temperature gradient of 10-15 degree/cm at about 500 degrees C and b) applying an electric field of 1-4kV/cm at 500 degrees C. The twinning time dependence on the temperature. the dynamic of twin domain formation as a function of directions. thickness, temperature gradient, for +5X resonator plates are discussed. The influence of the Dauphine twins on the performances of piezoelectric resonators: resonant frequency, dynamic inductance and equivalent dynamic capacity was previously reported [1]. This paper presents the influence of twinning area on the frequency spectrum of a resonator plate of +5X-cut and the changing of the frequency-temperature characteristic when the resonator plate becomes twinned.

837. Electrical conductivity of quartz crystals

Authors: Enculescu, I; Iliescu, B

Published: 1997, CRYSTAL RESEARCH AND TECHNOLOGY, 32, 891, DOI: 10.1002/crat.2170320702

838. Hall effect and magnetoresistance in four-probe tight-binding mesoscopic rings

Authors: Gartner, P; Aldea, A

Published: MAR 1996, ZEITSCHRIFT FUR PHYSIK B-CONDENSED MATTER, 99, 373, DOI: 10.1007/s002570050050

The Hall effect and magnetoresistance of a mesoscopic system having a four-probe ring geometry are calculated in a tight-binding model, either ordered or disordered, as function of magnetic flux and Fermi energy. The changes induced in the spectrum by the magnetic field affect differently the Aharonov-Bohm (AB) oscillating pattern of the two resistances: after the gap reaches the Fermi energy, the Hall effect remains bounded while the magnetoresistance increases exponentially with the sample size. The results are shown to depend significantly on the number and geometry of the probes.

839. On the determination of birefringence of liquid crystals by the interference method

Authors: Moldovan, R; Tintaru, M; Frunza, S; Beica, T; Polosan, S

Published: 1996, CRYSTAL RESEARCH AND TECHNOLOGY, 31, 955, DOI: 10.1002/crat.2170310724

A new method of processing the experimental data in the interference method for the determination of birefringence of liquid crystals which allows obtaining results in good agreement with those from prism or wedge methods, is presented.

840. A NOTE ON OFF-CENTER DIFFUSION

Authors: DESPA, F; APOSTOL, M

Published: APR 1995, SOLID STATE COMMUNICATIONS, 94, 155, DOI: 10.1016/0038-1098(94)00855-8

Within a one-dimensional model of off-centre diffusion it is shown that the diffusion coefficient depends on the number of off-centre sites and on their orientation relative to the host lattice.