National Institute Of Materials Physics - Romania

Publications

5975 articles found

5451. PERIODIC STRUCTURES IN A TWISTED NEMATIC LIQUID-CRYSTAL CELL UNDER DC EXCITATION

Authors: FRUNZA, S; MOLDOVAN, R; BEICA, T; GIURGEA, M; STOENESCU, DN

Published: NOV 1 1992, EUROPHYSICS LETTERS, 20, 411, DOI: 10.1209/0295-5075/20/5/005

Some new results concerning the appearance of a periodic structure in a pi/2-twist cell filled with MBBA under d.c. excitation are reported. This structure consists of rectilinear domains whose orientation varies with the applied voltage around the normal to the molecular orientation in the middle of the cell and this variation covers a large range of angles even for fixed polarity of the excitation. When the polarity is changed, the sense of the domain orientation is also changed.

5452. ANCHORING ENERGY AND EASY DIRECTION OF NONUNIFORM SURFACES

Authors: BARBERO, G; BEICA, T; ALEXEIONESCU, AL; MOLDOVAN, R

Published: NOV 1992, JOURNAL DE PHYSIQUE II, 2, 2024, DOI:

The effect of a periodic distribution of easy directions on the surface macroscopic properties of nematic samples are considered. Our analysis shows that the effective surface energy of the sample contains an interference term coming from the two different easy axis. This allows to introduce an effective uniform easy direction, depending on the two easy directions and containing an interference term too, as suggested a few years ago by some researchers.

5453. TEXTURE IN RIETVELD REFINEMENT

Authors: POPA, NC

Published: OCT 1 1992, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 25, 616, DOI: 10.1107/S0021889892004795

A texture model suitable for use in Rietveld-refinement programs is proposed. It is based on the series expansion of the pole distribution function in symmetrized harmonics. The model was tested on a textured plate sample of Al2O3.

5454. 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.

5455. A METHOD FOR MEASURING THE NEUTRON LIFETIME WITH STORED ULTRACOLD NEUTRONS

Authors: STOICA, AD

Published: SEP 1 1992, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 320, 607, DOI: 10.1016/0168-9002(92)90959-8

A method for measuring the neutron lifetime is proposed, using simultaneous detection of electrons and upscattered neutrons generated during the ultracold neutron storage.

5456. MAKING DIODE-LASERS LONGER TO BOOST POWER

Authors: PETRESCUPRAHOVA, IB

Published: SEP 1992, LASER FOCUS WORLD, 28, 63, DOI:

5457. MAGNETIC-PROPERTIES OF XGD2O3(1-X)[3B2O3PBO] GLASSES

Authors: ARDELEAN, I; BURZO, E; MITULESCUUNGUR, D; SIMON, S

Published: SEP 1992, JOURNAL OF NON-CRYSTALLINE SOLIDS, 146, 260, DOI: 10.1016/S0022-3093(05)80499-5

Results of magnetic measurements performed on xGd2O3.(1-x)[3B2O3.PbO] glasses, in the temperature range 4.2-300 K are reported. For samples with x > 2 mol% Gd2O3, a downward curvature of the reciprocal susceptibilities, at T < 30 K, is observed. At higher temperatures, a Curie-Weiss behaviour is shown. The paramagnetic Curie temperatures for glasses having x > 2 mol% Gd2O3 are negative and increase in absolute magnitude when increasing the Gd2O3 content. The magnetic behaviour of Gd2O3-B2O3-PbO glasses are analyzed in terms of a random distribution of gadolinium ions in glass matrix.

5458. SPECTROSCOPY, FLUORESCENCE MECHANISMS AND ENERGY TRANSFERS IN CADMIUM CHLORIDE DOUBLY DOPED WITH COPPER AND MANGANESE

Authors: VODA, M; GHIORDANESCU, V; PEDRINI, C

Published: AUG 24 1992, JOURNAL OF PHYSICS-CONDENSED MATTER, 4, 7152, DOI: 10.1088/0953-8984/4/34/014

We have observed and identified the optical excitation and emission spectra of Mn2+ ions in the doubly doped CdCl2:Mn:CU system between 77 and 300 K. A comparison is made with the singly doped CdCl2 system. The temperature dependence of the absorption strength is explained by vibrational electronic interaction, and the quantum efficiency is close to unity for low-Mn2+-concentration crystals. In CdCl2 the 1E(g) state of Cu+ lies in the conduction band. By excitation in the main absorption band, 1A1g --> 1E(g), of Cu+ in CdCl2:Mn:Cu crystals an energy transfer to Mn2+ ions takes place through intermediate metastable states (excimer-like species) which form in the lattice near the Cu+ centres. Because these states thermally ionize, the energy transfer rate decreases when the temperature increases and above 100 K the photoionization of Cu+ centres becomes dominant.

5459. ROTATIONAL EFFECTS IN MOLECULAR-CRYSTALS

Authors: FILOTI, G; TURCANU, CN

Published: AUG 17 1992, JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY-LETTERS, 165, 375, DOI: 10.1007/BF02164430

The Mossbauer spectrum of ferric oxinate exhibits an asymmetric quadrupole doublet. The modification of the dynamic Mossbauer parameters (to the equal values for each line) following the irradiation, was explained by the internal oxine ligands rotation inside of the octahedral coordination.

5460. THE TIME-DEPENDENCE OF CURRENTS IN MISIM STRUCTURES WITH ZNS - MN ACTIVE LAYERS

Authors: GOLDENBLUM, A; OPREA, A

Published: AUG 16 1992, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 132, 395, DOI: 10.1002/pssa.2211320213

The shapes of current versus time curves in ZnS : Mn MISIM structures are studied in dependence on voltage, temperature, and Mn concentration. The experimental curves compare well with those computed on the basis of a tunneling model which takes into account the presence of the internal field due to bulk ionized centres and the filling level of the interface states.