National Institute Of Materials Physics - Romania

Publications

860 articles found

821. 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.

822. 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.

823. 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.

824. Electrical conductivity of quartz crystals

Authors: Enculescu, I; Iliescu, B

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

825. 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.

826. 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.

827. 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.

828. IR SPECTROSCOPIC MEASUREMENTS OF CRYSTALLINITY AND PHASE PURITY OF ZSM-5 SAMPLES

Authors: FRUNZA, L; DINCA, V; VAGAON, M; CRISAN, D; FLORESCU, I; MANOIU, D; CRISAN, M; RAGAN, N

Published: MAR 1993, REVUE ROUMAINE DE CHIMIE, 38, 246, DOI:

The use of IR spectroscopic method to choose the best batches of ZSM-5 zeolites. is discussed. The crystallinity and the particle size were estimated from IR spectra and by XRD and electron microscopy measurements.

829. HOPPING CONDUCTIVITY OF THE FIBONACCI-CHAIN QUASI-CRYSTAL - COMMENT

Authors: DULEA, M; ALDEA, A

Published: OCT 1 1992, PHYSICAL REVIEW B, 46, 8643, DOI: 10.1103/PhysRevB.46.8642

This Comment refers to a recent publication on the ac hopping conduction of the Fibonacci chain [Phys. Rev. B 43, 1183 (1991)]. The renormalization procedure proposed by its authors gives a correct formula for the conductivity [Eq. (15)]. Nevertheless, their conclusion regarding the vanishing of the dc conductivity disagrees with the low-frequency asymptotics previously derived by us for the same model, and also disagrees with the general result given by the resistance-network analogy of the Miller-Abrahams rate equations. Their result assumes the factorization of the frequency in Eq. (15), which we prove is not true. We show also that their numerical curves, which apparently support their conclusion, are strongly influenced by errors in the low-frequency range.

830. CRYSTALLINE PHASES IN SUPERCONDUCTING BI-SR-CA-CU2-O-X CERAMIC

Authors: POPESCU, M; ALDICA, G; SARBU, C; PAUSESCU, P; CRUCEANU, E; VASILIU, F; APOSTOL, M

Published: DEC 16 1990, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 122, 643, DOI: 10.1002/pssa.2211220225

The crystalline phases in the superconducting ceramic Bi:Sr:Ca:Cu2:O(x) exhibiting either low superconducting transition (almost-equal-to 80 K) or both, low and high superconducting transitions (almost-equal-to 110 K) are identified by X-ray and electron diffraction. Two types of modulated structures are observed and their relation to T(c) is discussed within the frame of the oxygen-displacive mode theory.