Publications

A quantum kinetics of the Bose-Einstein condensation in the self-consistent (s.c.) Hartree-Fock-Bogoliubov (HFB) model of the interacting Bose gas is formulated and numerically solved for the example of excitons scattering with a thermal bath of acoustic phonons. The theory describes the condensation in real time starting from a nonequilibrium initial state towards the equilibrium HFB solution. The s.c. changes of the spectrum are automatically incorporated in the scattering terms.

Based on the general Lindhard theory describing the partitioning of particles' energy in materials between ionisation and displacements, approximate analytical solutions have been derived, for media containing one and more atomic species, for particles identical and different from the medium ones. Particular cases, and the limits of these equations at very high energies are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

Two new phases Th4Fe13Sn5 and ThFe0.22Sn2 have been studied by Fe-57 and Sn-119 Mossbauer spectroscopy. The Fe-57 Mossbauer spectra of Th4Fe13Sn5, measured at room temperature and 4.2 Ii without and with applied magnetic field up to 5.5 T, exhibit four sextets. Our measurements prove that all four Ft: sub-lattices are collinear. The Sn-119 Mossbauer spectra of Th4Fe13Sn5 show two sextets with different B-eff, reflecting the influence of ferromagnetic nn iron atoms. The results obtained from full-potential linear augmented plane wave (LAPW) calculations are in agreement with the experimental data and confirm the assignment of Mossbauer spectra, The Mossbauer data for the ThFe0.22Sn compound prove the existence of two different sites both for Fe and Sn, all having icosahedral co-ordination. (C) 2001 Elsevier Science B.V, All rights reserved.

A high-frequency (95 GHz) electron paramagnetic resonance (EPR) study is reported on single crystals of the planar tetranuclear complex Fe-4(OCH3)(6)(dpm)(6) (where Hdpm = dipivaloylmethane), which has been previously shown to present typical single-molecule magnet behavior. The spectra, all originating from the S = 5 ground state, possess quasi-axial symmetry along the normal to the plane defined by the four Fe(m) ions. The measured spectra are shown to belong to three different structural variations of the compound, resulting from disorder in the ligands around two of the Fe(III) ions. Accurate values could be obtained for the second- and fourth-order crystal field parameters related to the parallel EPR spectra, while the other parameters could be determined only for the dominant species. The separation between individual lines is decreasing and vanishing with increasing temperature. This effect is attributed to the contribution of fast relaxing excited states, whose population is varying with temperature.

A novel approach to model diffraction line shifts caused by elastic residual or applied stresses in textured polycrystals is proposed. The model yields the complete strain and stress tensors as a function of crystallite orientation, as well as the average values of the macroscopic strain and stress tensors. It is particularly suitable for implementation in Rietveld refinement programs. The requirements on refinable parameters for all crystal Laue classes are given. The effects of sample symmetry are also included and the conditions for strain invariance to both the sample symmetries (texture and stress/strain) are discussed.

The effects of the incorporation of small amounts of Sn, Mn and Sm (0.5 at.%) on the structure and transport properties of glassy As2Se3 have been investigated. The metals influence both the disordered network structure of the glass matrix and the transport properties in accordance with the chemical properties of the doping elements. On metal addition, the electrical conductivity increases while the photoconductivity decreases. For Sm-modified glass, the position of the characteristic photoconduction band shifts toward higher photon energy. For Sn, the band shift is almost negligible and, for the addition of Mn, a photoresponse was not detected.

The photoluminescence spectra of epilayers of AlGaAs having various compositions were measured. The samples were anodically oxidised in a controlled acid-glycol-water solution (AGW) and the image was recorded optically. The photoluminescence shift is related to the change in the colour of the anodic oxide grown on the AlGaAs/GaAs heterostructure. We propose a calibration method for the colour of the anodic oxide formed under prescribed conditions, which has potential technological application. (C) 2001 Elsevier Science B.V. All rights reserved.

PbS films were obtained on glass substrates using the chemical bath deposition (CBD) method. The infrared (IR) photosensitivity increases of about 1000 times when a reducing agent (hydroxylamine hydrochloride NH2OHHCl) is added in the deposition bath. A further enhancement was tried by adding small amounts of antimony chloride (SbCl3) solution in the deposition bath. The result was just opposite. It was found that the photosensitivity decreases with increasing the Sb salt content. The changes in PbS photoconjuctive properties are correlated with the changes in film morphology due to the two compounds added in the bath - the reducing agent and the Sb salt. (C) 2001 Elsevier Science B.V. All rights reserved.

Photoconductive properties of some sol-gel deposited. ferroelectrie thin films are investigated in the ultraviolet (UV) domain. The investigated materials are: Bi4Ti3O12 (BiT), SrBi2Ta2O9 (SBT) and Pb(Zr0.45Ti0.55)O-3 (PZT). It was found that all these materials exhibit pronounced photoconductivity in the 250-500 nm domain. The best current responsivity was obtained for Bi4Ti3O12 (BiT) films. (C) 2001 Elsevier Science B.V. All rights reserved.

Photodetectors (PDs) based on three separate confinement-multiple quantum well (SCH-MQW) structures (with 2, 4 and 8 QWs) and sis graded-index (GRIN) SCH-MQW structures were studied for two types of devices: guided wave and non-guided wave devices. Non-guided wave devices were processed and the capacitance and the responsivity, in the wavelength range of 750-990 nm, were determined for various bias voltages. Guided wave PDs were simulated using the transfer matrix method for optical mode calculation and the minimum length of the detectors was calculated for absorbing 99% of the incident light. The junction capacitance of the guided wave PDs was calculated from the measured capacitance of non-guided wave devices and the minimum size of the devices, values of 215, 101, and 52 fF were obtained for 2, 4 and 8 QW SCH-MQW structures. Laser diodes were also fabricated from the three different SCH-MQW structures. Threshold current densities (J(th)) of 737. 755 and 1210 A cm(-2) were measured for 2, 4 and 8 QWs, respectively. Further improvement was achieved using GRINSCH-MQW structures and LDs with 4 QWs exhibited 33% decrease in J(th) and 22% increase in slope efficiency. (C) 2001 Elsevier Science B.V. All rights reserved.