National Institute Of Materials Physics - Romania

We have investigated Ga0.47In0.53As/Al0.48In0.52As quantum well structures at magnetic fields close to the filling factor 1. At the critical current. a bistable and abrupt switching (transition width less than 4 ppm with respect to the filling factor) between quantum Hall conduction and dissipative conduction was observed at filling factors 1 < v < 1.5. As confirmed by Hartree-Fock calculations. we attribute this switching to a feedback effect between the current-dependent population of different spin levels and the exchange-enhanced spin gap, The effect is accompanied by a ferromagnetic memory of the spin polarization when changing the external magnetic field. (C) 2002 Elsevier Science B.V. All rights reserved.

Pyroelectric properties of triglycine sulfate (TGS) thick films, separately doped with L and D alanine were investigated. Internal bias field of about 1 kV/cm, induced by the two dopants, stabilize the polarization in the opposite direction on the ferroelectric axis. Pyroelectric current (under constant stress) was recorded with a computer controlled Keithley 6517 electrometer, crossing up and down the Curie point. A reverse external electric field was applied on doped materials during heating, crossing up the Curie point. It is shown that the pyroelectric coefficient can be increased about four times at room temperature under un optimized DC electric field applied on the pyroelectric wafer.

The possibility of obtaining copper ferrite through the thermal decomposition of the two polynuclear coordination compounds: (NH4)(8) [Fe2Cu(C2O4)(8)](I) and [Fe2Cu(C2O4)(2) (OH)(4)].4H(2)O (II) was considered. The polynuclear compounds were characterized by various physical chemical techniques, e. g., IR, UV-VIS, EPR, Mossbauer spectra, thermal analysis and magnetic measurements. The final products obtained after thermal decomposition of the complex compounds were analysed by X-ray diffraction. A mixture of tetragonal CuFe2O4, alpha-Fe2O3 and CuO is generated from the thermolysis of compound I, while a clean tetragonal CuFe2O4 with saturation magnetization of 26.89 emu g(-1) is obtained from compound II.

In high temperature superconductors (HTS) the coherence length along non-c axis directions is longer. This feature can be useful when designing electronics devices based on HTS. Therefore growth and characterization of non-c axis oriented thin HTS films is of great interest. In this paper we present a short review of our data regarding (119) Bi-2223 thin films grown by MOCVD on (100) NdGaO3 and (110) SrTiO3. The emphasis is made on improvement and control of the quality of the films by the "two-temperature" technological approach and/or use of the vicinal substrates. Phase and morphology evolution for different processing conditions, substrate's type and off-angle are presented. The highest critical temperatures of T-c0=67.2 K and T-c0=74 K for the "single-" and "two-" temperature routes were obtained on vicinal SrTiO3 with the off-angle of 20degrees. A higher off-angle promoted the formation of a specific step-like morphology with lower roughness. For the films grown on flat substrates the morphology was of mountain-range shape. Surface morphology as a result of two types of growth mechanisms (two-dimensional (2D), assisted by a so-called "twin"-growth and step-flow growth) for the (119) Bi-2223 films are discussed.

Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a Subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm(-1), observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm(-1) up-shifted replicas i.e. the bands at 178 and 190 cm(-1). A different behavior of these bands in the anti-Stokes side was observed. (C) 2002 Elsevier Science Ltd. All rights reserved.

This work presents some results obtained by confining octylcyanobiphenyl (8CB) to the pores of two SBA-15 type host materials: a silica SBA-15 (pores of 10.2 nm) and an aluminum containing AlSBA-15 (pores of 7.5 nm). The LC molecules were loaded inside the pores from a solution in acetone, in a percentage higher than 70% from the maximum possible loading. It was observed that the 8CB molecules confined to molecular sieves may preserve the phase transitions characteristic for the bulk LC provided that the confining pores are sufficiently large, as offered by SBA materials. However, present guest-host interactions play an important role in these phase transitions. Such a behavior is at variance with the 8CB confined to Controlled Porous Glass with the same pore dimensions, for which the bulk-like phase transitions can be always observed.

Molecular dynamics of a nematic liquid crystal, 4n-octylcyanobiphenyl, confined to two types of SBA-type nanoporous molecular sieves were studied by broadband dielectric spectroscopy in order to obtain information about the liquid crystal-surface interactions. The temperature interval of the dielectric measurements was large, covering all the states of the bulk liquid crystal. It is found that there are several relaxation processes covering the large frequency range. A bulk-like relaxation comes from the liquid crystal molecules located in the center of the pores, which rather do not feel the effect of the pore walls. Another relaxation comes from the molecules located in the surface layer. The latter process is much slower than the bulk-like relaxation. The differences observed between the dynamic behavior of the liquid crystal confined to the two types of nanoporous materials are related to pore size, pore shape and liquid crystal-surface interactions.

The electropolymerizations of 2-aminohippuric acid methyl ester and 2-aminobenzoic acid methyl ester monomers in phosphate buffer (pH = 6.5) were studied. The formation of thin films at the platinum electrode surface is observed as main product and the electrosynthesis process of the two substituted polyanilines shows the voltammetric characteristics of the polyaniline film electrosynthesis. The necessary charge to deposit 0.05 mum poly(2-aminohippuric acid methyl ester) or poly(2-aminobenzoic acid methyl ester) films is about 60 mC/cm(2). The polymer films were investigated by electrochemical impedance spectroscopy. On the basis of the obtained results we propose the equivalent circuit for these polymer films.

The paper presents a syntheses of the last data concerning the optimal parameters of the deposition process to obtain the best dynamic quality HTS thin films in connection with the most precise dynamic measurement method known to date. The paper makes a comparison between the performances achieved by the classical dynamical surface resistance nondestructive measurement techniques of the HTS laser ablated thin films and the quasi-optical high performances one.

Iron acetate of composition [Fe3O(H3COO)(6)(H2O3)][AuCl4].6H(2)O (1) was synthesized and investigated by X-ray diffraction analysis and Mossbauer spectroscopy. The [Fe3O(CH3COO)(6)(H2O)(3)](+) complex cation has a structure typical for mu(3)-O bridged trinuclear ferric compounds with iron atoms lying at the vertices of a regular triangle with an oxygen atom at the center. The iron atoms are each coordinated by 4 oxygen atoms of the four bridging carboxylic groups, the mu(3)-O bridging atom, and the coordinated water molecule in the trans-position to the tatter. In the trinuclear cation, the Fe(Ill) ions are coupled by antiferromagnetic exchange interactions with the exchange parameter J = -29.0 cm(-1) (HDVV model for D-3h symmetry). The specific role of the solvate water molecules in structure formation is discussed.