Showing papers in "Acta Physica Polonica A in 2006"

Modeling of Semiconductor Nanostructures with nextnano 3

Abstract: nextnano 3 is a simulation tool that aims at providing global insight into the basic physical properties of realistic three-dimensional mesoscopic semiconductor structures. It focuses on quantum mechanical properties such as the global electronic structure, optical properties, and the efiects of electric and magnetic flelds for virtually any geometry and combination of semiconducting materials. For the calculation of the carrier dynamics a drift-difiusion model based on a quantum-mechanically calculated density is employed. In this paper we present an overview of the capabilities of nextnano 3 and discuss some of the main equations that are implemented into the code. As examples, we flrst discuss the strain tensor components and the piezoelectric efiect associated with a compressively strained InAs layer for difierent growth directions, secondly, we calculate self-consistently the quantum mechanical electron density of a Double Gate MOSFET, then we compare the intersubband transitions in a multi-quantum well structure that have been obtained with a single-band efiective mass approach and with an 8-band k ¢ p model, and flnally, we calculate the energy spectrum of a structure in a uniform magnetic fleld.

The Numerical Solution of the Imaginary-Axis Eliashberg Equations

Abstract: In the paper, we solve the imaginary-axis Eliashberg equations. We calculate numerically self-consistently the superconducting order function, the wave function renormalization factor, and the energy shift function as a function of the Matsubara frequency. We consider difierent values of the average number of the electrons per lattice site. Additionally, we study the temperature dependence of the order function and the wave function renormalization factor. The possible extension of the Eliashberg theory to the case of the high-TC superconductors was also brie∞y discussed.

The Ion-Matter Interaction with Swift Heavy Ions in the Light of Inelastic Thermal Spike Model

Abstract: A description of the inelastic thermal spike model is presented in order to correlate the energy deposited by swift heavy ions to the nanometric matter transformation induced in inorganic metallic and insulating materials. Knowing that insulator is more sensitive than metallic material and that amorphous material is in general more sensitive than a crystalline one, it appears evident that the electron–phonon coupling constant g plays a key role. It will be shown that in metallic material we are able to describe different phenomena with the same value of g: for example, track formation with defect annealing or sputtering of atoms. In insulators the emphasis is made on results obtained for amorphizable materials like SiO2 quartz and for non-amorphizable ionic crystals like CaF2. Assuming that tracks result from a transient thermal process, a quantitative development of the model is proposed using the electron–atom mean free path λ (inversely proportional to the square root of g) as a free parameter. With this parameter it is possible to quantitatively describe track radii in a wide range of ion velocities — whatever the bonding character of the crystal is — assuming specific criteria: tracks may result from a rapid quenching of a cylinder of matter in which the energy deposited on the lattice has overcome either the energy necessary to reach a quasi-molten phase in the case of amorphizable materials or the vaporization energy in the case of non-amorphizable materials. The evolution of the λ parameter of the considered insulator decreases versus the band gap energy. In this model, velocity effect, and a link between track formation and sputtering of atoms is established for amorphizable insulators while open questions appear for ionic crystals.

Refractive indices, order parameter and optical transmittance studies of a nematic liquid crystal mixture

Abstract: Measurements of ordinary refractive index, birefringence, density, and order parameter were made on a technologically important nematogen and the data obtained for its nematic and isotropic phase were reported. A modified wedge method was used for the measurement of the birefringence (delta n). The nematic-isotropic phase transition temperature matches very well as is exhibited from the value of refractive indices and densities obtained using different techniques. The optical anisotropy and density data were used to determine the order parameter and principal polarizability of the nematic mixture using the Vuks approach and their temperature dependence was discussed. The macroscopic order parameter was obtained and compared with microscopic order parameter. These two values agree very well.

Influence of Isomorphic Atom Substitution on Lattice Anisotropy of Thallium Dichalcogenide Layered Mixed Crystals

Abstract: Compositional variation of the lattice parameters of TlBX2-type (B = Ga or In and X = S or Se) mixed crystals with monoclinic structure were studied by X-ray diffraction technique. The lattice anisotropy (c/b) of these mixed crystals changes linearly with substitution of the atoms located both at the centers and the vertices of the corresponding BX4 tetrahedra. A brief survey of the important features of the effect of isomorphic atom substitution on the lattice anisotropy and the unit cell volume of TlBX2type mixed crystals with layered structure was presented.

Studies of the Magnetite Nanoparticles by Means of Mössbauer Spectroscopy

Abstract: The magnetite nanoparticles were prepared by modified Massart’s method in water and in alcohol. The influence of the condition of preparation on the properties of magnetite nanoparticles were investigated by Mossbauer spectroscopy. The size of the particles were determined by transmission electron microscopy. It was shown that the particles size in the alcoholic reaction is smaller than in aqueous reaction. Moreover, the increase in the reaction time improves the stoichiometry of magnetite nanoparticles.

Dielectric and X-ray Studies of Eleventh and Twelfth Members of Two Isothiocyanato Mesogenic Compounds

Abstract: Results of the dielectric and X-ray studies of the long-alkyl-chain members (n = 11, 12) of two homologous series: nBT (4-n-alkyl-4 0 -isothiocyanato-biphenyl) and nCHBT (4-trans-4 0 -n-alkyl-cyclohexyl-isothiocyanato-benzene) in their liquid crystalline phases are presented. These compounds exhibit difierent polymorphisms: the nematic phase for nCHBTs and the smectic E and smectic A phases for nBTs. The dielectric measurements were performed in a wide frequency range of 1 kHz{3 GHz. The layer thickness in the smectic E and smectic A phases and the orthorhombic unit cell parameters of the smectic E phase were determined using the small angle X-ray difiraction method. In the smectic E phase the layer-thickness-to-molecular-length ratios are found to be close to 1. The corresponding ratios observed for the smectic A phase of nBTs are considerably higher (… 1:24), which indicates that an alternating head-to-tail arrangement of molecules in the layers is favored. The rotational dynamics of molecules in the smectic E phases is discussed in relation to their packing in a unit cell.

Correlation Functions of Impedance and Scattering Matrix Elements in Chaotic Absorbing Cavities

Abstract: Wave scattering in chaotic systems with a uniform energy loss (absorption) is considered. Within the random matrix approach we calculate exactly the energy correlation functions of different matrix elements of impedance or scattering matrices for systems with preserved or broken time-reversal symmetry. The obtained results are valid at any number of arbitrary open scattering channels and arbitrary absorption. Elastic enhancement factors (defined through the ratio of the corresponding variance in reflection to that in transmission) are also discussed.

Measurement and Analysis of the Positron Annihilation Lifetime Spectra for Mesoporous Silica

Abstract: Positron annihilation lifetime spectroscopy as a technique of porousmaterials investigation is still under development. Both measurement con-ditions and lifetime spectra analysis methods have to be adapted to therequirements of positron porosimetry. If complex positron annihilation life-time spectra of porous materials are processed, numerical methods of life-time spectra analysis have to be used with a particular care. Among variousmethods of positron annihilation lifetime spectra analysis available, the mostpopular ones were tested implemented in programs MELT and LT. It wasfound, from the simulated spectra study, that a large number of counts ina spectrum is needed to obtain reliable results of analysis. Even then onlyan approximate solution is available. Determination of the approximationnature helps in a further interpretation of the results and in positron an-nihilation lifetime spectrometer setting adequate to the problem. For thispurpose, a few complex spectra obtained from positron beam facility weretested. The results show a negligible in°uence of resolution function on longlifetime components, while the precise estimation of the background level iscrucial. Also some distortions caused by imperfection of numerical analysismethods are pointed out.PACS numbers: 78.70.Bj, 36.10.Dr, 61.43.Gt, 81.07.{b, 41.75.Fr

Comparative Study of Orientational Order of Some Liquid Crystals from Various Homologous Series

Abstract: The long-range orientational order of liquid crystals from homologous series of 4-n-alkyl-4′-cyanobiphenyl, 4-n-alkoxy-4′-cyanobiphenyl, trans-4-n-alkyl(4′-cyano-phenyl)-hexane, and 4-(trans-4′-n-alkylcyclohexyl)-isothiocyanato-benzene was studied by means of classical methods of optical spectroscopy: absorption of the polarized light, fluorescence depolarization, and Raman scattering depolarization. The absorption, emission, and Raman scattering spectra of linearly polarized light were recorded as a function of temperature in the whole range of the mesophase. On the basis of these spectra the order parameters 〈P2〉 and 〈P4〉 as well as the orientational distribution function were determined. The results obtained for members of various series with the same number of carbon atom in the alkyl chain were compared. It was found that the orientational order of liquid crystal molecules depends on the structure of the rigid core, on the kind of the terminal group as well as on the alkyl chain length. The odd–even effect for the order parameters, explained as the alteration of interaction between alkyl chains, was observed.

Distinguishing Prostate Cancer from Hyperplasia

Abstract: The prostate gland is the most common site of neoplastic disorders inmen. The knowledge of the pathogenesis of benign prostate hyperplasia andprostate cancer is still under investigation. X-ray °uorescence analysis usingsynchrotron radiation seems to be an appropriate technique to study theelemental composition of biological tissues. The measurements presented inthis paper were carried out on the L-beam line at the HASYLAB, DESY(Germany). The observed concentrations of Fe, Cu, Mn, and Zn are signifl-cantly higher in cancerous tissues, as compared to normal and hyperplasticones. Our results reveal also the existence of the two types of hyperplasia.PACS numbers: 87.64.{t

Photoluminescence and Electron Paramagnetic Resonance Studies of Bulk GaN Doped with Gadolinium

Abstract: Theoretical predictions of possible room temperature ferromagnetism in GaN diluted with manganese strongly motivated studies of nitrides based diluted magnetic semiconductors (DMS) with transition metals or rare-earths. Over the last years, above room temperature ferromagnetism for MBE grown GaN layers weakly doped with Gd has been reported by different researchers [for example see ref.1]. The estimated average value of magnetic moment per Gd ion has been obtained as high as 500 times that of Gd atomic value. It has been argued that the observed enhancement of the average magnetic moment resulted from strong contribution of GaN host lattice to macroscopic magnetization of the whole material [1]. The origin of such behavior remains still puzzling and more studies of GaN:Gd are required to clarify this issue. In this paper we present photoluminescence (PL) and electron paramagnetic resonance (EPR) experiments on strain free GaN bulk crystals of wurtzite structure doped with gadolinium. The samples were grown from the solution of nitrogen in liquid gallium under high (1.5GPa) pressure (HP) of N2 and at elevated temperatures of about 1500 0 C. Doping with Gd was obtained by adding this element to the solution. Both PL and EPR techniques are well known as a powerful experimental tools to study electronic and magnetic properties of transition metals and rare-earth in semiconductors, and they have not been exploited up to now for GaN:Gd system. In EPR experiment sharp absorption lines were detected, which could be attributed to Gd 3+ (4f 7 ) state. Identification of the lines was possible by comparison with other semiconductors doped with Gd. The observed Dysonian shape of the lines originated from presence of high concentration of free electrons in measured GaN. However, the estimated value of free electron concentration was definitively lower than in undoped HP GaN, what indicated purification role of Gd from unintentional donors (presumably oxygen). PL studies revealed intensive spectra in two energy range: close to GaN band gap (3.13.6 eV) and in the “red” range of 1.6-1.8 eV. Band-to band transitions, typical for GaN doped with free electron concentration at a high 10 18 cm -3 level, were clearly seen. They were followed by several sharp peaks in direction of lower energy. In the “red” range sharp structure with main line at 1.77eV energy was detected. The FWHM of this line was as small as 5meV. This structure was related to Gd 3+ (4f 7 ) state. Judging from EPR and PL results Gd dopant was present in the studied samples. However, no ferromagnetic behavior of the samples was observed by means of EPR. This creates some doubts if ferromagnetism observed by others was really related to GaGdN system.

Bound Water Structure on the Surfaces of Usnea antarctica as Observed by NMR and Sorption Isotherm

Abstract: Antarctic lichens are extremophilic organisms very resistant on desiccation stress and cold [1–3]. They perform active photosynthesis below 0◦C [4, 5] and below ice nucleation of their cellular fluids [6]. An important mechanism of freezing resistance is the ice crystallite growth in extracellular spaces accompanied by simultaneous drastic decrease in hydration of intracellular spaces, thus the explanation of the molecular mechanisms of drought resistance and the mechanisms of cold resistance may be common. During acute desiccation stress lichen thallus is covered by single or less than monolayer of water [7], may hydrate from gaseous phase to the level allowing the initiation of photosynthesis [7, 8], and reversibly dehydrates below the two-dimensional percolation threshold of water [7]. Therefore, the knowledge on bound water structure (organization) may help in understanding of molecular mechanisms of the metabolic activity recovery during rehydration. ∗corresponding author; e-mail: hubert.haranczyk@uj.edu.pl

The Effect of Water Accessible Paramagnetic Ions on Subcellular Structures Formed in Developing Wheat Photosynthetic Membranes as Observed by NMR and by Sorption Isotherm

Abstract: The rehydration from the gaseous phase of the developing native or EDTA-washed from unbound and loosely bound paramagnetic ions wheat thylakoid membrane lyophilizate was investigated using hydration kinetics, sorption isotherm, and high power proton relaxometry. Hydration time courses are single exponential for all target humidities. The sorption isotherm is well fitted by the Dent model, with the mass of water saturating primary binding sites equal to ∆M/m0 = 0.024 and 0.017 for native and EDTA-washed membranes, respectively. Proton free induction decays distinguish: (i) a Gaussian component, S0, coming from protons of solid matrix of lyophilizate; (ii) a Gaussian component, S1, from water bound to the primary water binding sites in proximity of water accessible paramagnetic ions; (iii) an exponentially decaying contribution, L1, from water tightly bound to lyophilizate surface; and (iv) exponentially decaying loosely bound water pool, L2. Sorption isotherm fitted to NMR data shows a significant contribution of water “sealed” in membrane structures (∆Ms/m0 = 0.052 for native and 0.061 for EDTA-washed developing membranes, respectively).

Investigation of Starch Hydration by 2D Time Domain NMR

Abstract: Proton exchange between spin groups of the solid matrix of hydrated granular potato starch and water was studied using the 2D time domain NMR. The proton spin–spin relaxation time T2, and spin–lattice relaxation time T1 (selective and non-selective pulse sequences) were measured at room temperature. The observed spin relaxation results were analysed for exchange assuming a two-site exchange model (between water and solid matrix of starch). In this analysis we determined the intrinsic spin–lattice relaxation time for water protons (49 ms) and solid starch matrix protons (172 ms), as well as the water–starch magnetization exchange rate (86 s−1).

Aspects of the Scattering and Impedance Properties of Chaotic Microwave Cavities

Abstract: We consider the statistics of the impedance Z of a chaotic microwave cavity coupled to a single port. We remove the non-universal effects of the coupling from the experimental Z data using the radiation impedance obtained directly from the experiments. We thus obtain the normalized impedance whose probability density function is predicted to be universal in that it depends only on the loss (quality factor) of the cavity. We find that impedance fluctuations decrease with increasing loss. The results apply to scattering measurements on any wave chaotic system.

Mössbauer Studies of Cu(II) Ions Interaction with the Non-Heme Iron and Cytochrome b559in a Chlamydomonas reinhardtii PSI Minus Mutant

Abstract: Mossbauer spectroscopy was applied, for the first time, to study the interaction of copper ions with the non-heme iron and the heme iron of cytochrome b(559) in photosystem II thylakoids isolated from a Chlamydomonas reinhardtii photosystem I minus mutant. We showed that copper ions oxidize the heme iron and change its low spin state into a high spin state. This is probably due to deprotonation of the histidine coordinating the heme. We also found that copper preserves the non-heme iron in a low spin ferrous state, enhancing the covalence of iron bonds as compared to the untreated sample. We Suggest that a, disruption of hydrogen bonds stabilizing the quinone-iron complex by Cu2+ is the mechanism responsible for a new arrangement of the binding site of the non-heme iron leading to its more "tense" structure. Such a diamagnetic state of the non-heme iron induced by copper results in a magnetic decoupling of iron from the primary quinone acceptor. These results indicate that Cu does not cause removal of the non-heme iron from its binding site. The observed Cu2+ action on the non-heme iron and cytochrome b559 is similar to that previously observed for alpha-tocopherol quinone.

Electron Orderings of Half-Filled Extended Hubbard Models with Spin- and Charge-Exchange Interaction

Abstract: The electron orderings of the half-filled extended Hubbard models (i) with anisotropic spin-exchange interactions (the t−U−J‖−J⊥ model) and (ii) with charge-exchange interaction (the t−U−I model) are discussed. In particular, we present new results concerning the phase diagrams of the model (i) in the case of uniaxial spin exchange (J‖ 6= 0) for d-dimensional hypercubic lattices (1 ≤ d ≤ ∞) and conclude about the basic effects of the particular spinand charge-exchange interaction terms (J‖, J⊥, and I) in both models.

Positron Annihilation in Steel Burnished by Vibratory Shot Peening

Abstract: ing time of treatment. The samples were investigated by nondestructive positron annihilation techniques: angular correlations of two-quantum annihilation radiation and positron annihilation lifetime spectroscopy. To determine residual stresses present under burnished surface the same samples were studied by destructive Davidenkov method. Change of absolute value of the weighted average of residual stresses over positron range in the series of the samples is in good agreement with change of S=W ratio obtained by angular correlations of two-quantum annihilation radiation. Both parameters increase during flrst 10 minutes of shot peening and then stabilize. Positron annihilation lifetime spectra allowed to identify two types of structural defects: smaller ones like vacancies or dislocations and bigger | probably clusters of vacancies. Increase in shot peening time causes reduction of positron trapping rate and lifetime rise in bigger defects.

The structure of dendrimers with charged terminal groups: Monte Carlo simulations

Abstract: Taking into account the full Coulomb potential and the excluded volume interactions, properties of dendrimers with generations g = 5, 6 with charged terminal groups and counterions in an athermal solvent are examined by lattice Monte Carlo simulations. The study treats counterions explicitly and focuses on the local structure of the systems inspected by pair correlation functions gab that provide information on distributions of monomers, terminal groups and ions in space at various temperatures T ∗. Special emphasis is placed on counterions and their role they play in causing conformational changes of the molecules. The simulations show that counterions penetrate the interior of the dendrimers, and there is a major increase in their concentration there as T ∗ decreases. Some of them condense onto the terminal groups and a reduction in the mean effective charge 〈Q〉 of the dendrimers appears. Within the range of temperatures where the condensation (as a function of T ∗) is sharp the molecules weakly swell up when compared to their size at the other temperatures. This kind of behaviour is also reflected by the distributions of monomers and terminal groups.

Influence of Positron Irradiation on Positronium Formation in n-Alkanes

Abstract: -positronium component in the lifetime spec-trum depends on the concentration of trapped excess electrons, but also onconcentration of radicals and ions acting as electron scavengers. The re-sults indicate the existence of two kinds of electron traps; those which arenot emptied by thermal treatment disappear after several days of positronirradiation.PACS numbers: 36.10.Dr, 61.80.Fe, 78.70.Bj

Hydrolytic Degradation of Poly(L-Lactide-co-Glycolide) Studied by Positron Annihilation Lifetime Spectroscopy and Other Techniques

Abstract: Changes of the poly(L-lactide-co-glycolide) structure as a function of degradation time in phosphate-buffered saline for 7 weeks were investigated by gel permeation chromatography, differential scanning calorimetry, nuclear magnetic resonance (H NMR), and positron annihilation lifetime spectroscopy. Surface properties as wettability by sessile drop and topography by atomic force microscopy were also characterized. Chain-scission of polyester bonds in hydrolysis reaction causes a quite uniform decrease in molecular weight, and finally results in an increase in semicrystallinity. Molecular composition of the copolymer and water contact angle do not change considerably during degradation time. Atomic force microscopy studies suggest that the copolymer degrades by “in bulk” mechanism. The average size of the molecular-level free volume holes declines considerably after one week of degradation and remains constant till the sixth week of degradation. The free volume fraction decreases as a function of degradation time.

Effects of Vibronic Dynamics of Cu(H2O)6Complexes and Electron Spin Relaxation in Temperature Dependence of EPR Linewidth in Diamagnetic Tutton Salt Single Crystals

Abstract: EPR linewidth of Cu in the Tutton salt crystals weakly depends on temperature at low temperatures and then it rapidly grows above 60 K. We present detailed results of measurements and analysis for Cu in K2Zn(SO4)2 · 6H2O, K2Zn(SO4)2 · 6D2O, (NH4)2Mg(SO4)2 · 6H2O and Cs2Zn(SO4)2 ·6H2O in a temperature range of 4.2–300 K and compare them with already published electron spin–lattice relaxation data. The relaxation contributes weakly to the linewidth which is dominated by molecular dynamics and grows exponentially with temperature. To describe this we are discussing the influence of jumps between two sites of Cu complexes in a slow motion region where the sites are differently thermally populated. This case has not been considered so far. We have derived appropriate expressions describing the contribution of jumps to the linewidth which allows the determination of the jump rate and energy difference δA,B between the two sites being two Jahn–Teller distorted configurations of the vibronic Cu(H2O)6 complexes. The jump rate 1/τ strongly depends on temperature and reaches 10 s−1 at room temperature, whereas the δA,B varies from 117 cm−1 for K2Zn(SO4)2 · 6D2O to 422 cm−1 for Cs2Zn(SO4)2 · 6H2O. The comparison with vibronic level splitting, which varies in the range of 67–102 cm−1, indicates that the reorientation mechanism involves phonon induced tunnelling via excited vibronic levels. These reorientations do not contribute, however, to the spin–lattice relaxation which is governed by ordinary two-phonon relaxation processes in the whole temperature range. Thus, the reorientations and spin relaxation are two independent phenomena contributing to the total linewidth.

Approximations by Graphs and Emergence of Global Structures

Abstract: We study approximations of billiard systems by lattice graphs. It is demonstrated that under natural assumptions the graph wave functions approximate solutions of the Schrodinger equation with energy rescaled by the billiard dimension. As an example, we analyze a Sinai billiard with attached leads. The results illustrate emergence of global structures in large quantum graphs and offer interesting comparisons with patterns observed in complex networks of a different nature.

Negative Magnetisation and Absence of Superconductivity in RFe4Al8(R=Lu, Yb) Compounds

Abstract: We report researches for superconductivity and the effect of negative magnetisation in the RFe4Al8 (R = Lu, Yb) compounds by means of magnetic measurements and microwave absorption. Contrary to the earlier reports we do not confirm the existence of any traces of superconductivity in these compounds. Instead of the superconductivity and the Meissner effect, the YbFe4Al8 compound exhibits the effect of negative magnetisation, whereas the LuFe4Al8 compound shows an exact antiferromagnetic behaviour.

Studies of the Magnetic Axis Switching in Magnetite

Abstract: The influence of the external magnetic field on magnetic and crystallographic structure of magnetite was studied. We have confirmed, by means of vibrating sample magnetometer measurements, that external magnetic field can switch the easy magnetic axis to the new crystallographic direction. We have also proved by the direct observation of the crystalline structure that this phenomenon is strictly related to the crystallographic structure relaxation.

Investigating mechanisms of radiation-induced DNA damage using low-energy photons

Abstract: Central to any mechanistic biophysical model of radiation damage to DNA is the relationship between the amount and distribution of energy deposited in the DNA helix and the subsequent production of DNA damage. It is now clear from a number of studies that the minimum energy required to produce bond breaks in DNA is significantly lower than might be expected. For example, some biophysical models have assumed that it takes several 10s of eV to produce a double-strand break in DNA. However, using low-energy photons, we have shown that energy depositions as low as 7 eV can induce double-strand break and that this is enhanced when the DNA is hydrated, showing that free radical damage is also important.

The interaction of water with glycine: A combined inelastic neutron scattering and Raman spectra studies

Abstract: The vibrational dynamics of water around glycine was investigated by using Raman spectroscopy and inelastic neutron scattering. Experiments of deuterated glycine versus deuterium were performed as comparison. The study shows that for glycine, the exchange of proton{deuteron on the active NH + side was easy, whereas there was hardly exchange on the CH2 side. Comparing difierent proportion of glycine vs. water molecules we obtained that the presence of water hardly changes the main features of glycine illustrating its hydrophobic character. The intralayer hydrogen bonds of glycine crystal are di‐cult to be replaced due to its stronger bond than water.

Structure and Relative Stability of Sin(n=10-16) Clusters

Abstract: Ab initio molecular dynamics simulated annealing technique coupled with density functional theory in the local density approximation implemented in Spanish initiative for electronic simulations with thousands of atoms method is employed to search the ground state geometries of silicon clusters containing 10–16 atoms. We found a number of new isomers which are not previously reported. The atoms in all these clusters exhibit pronounced preference for residing on the surface. The binding energies increase while the highest occupied–lowest unoccupied molecular orbital gap generally decreases with the increase in clusters size.