Showing papers by "Jožef Stefan Institute published in 1999"

Accurate Density Functional with Correct Formal Properties: A Step Beyond the Generalized Gradient Approximation

Abstract: We approximate the exchange-correlation energy of density functional theory as a controlled extrapolation from the slowly varying limit. While generalized gradient approximations (GGA's) require only the local density and its first gradient as input, our meta-GGA also requires the orbital kinetic energy density. Its exchange energy component recovers the fourth-order gradient expansion, while its correlation energy is free of self-interaction error. Molecular atomization energies and metal surface energies are significantly improved over GGA, while lattice constants are little changed.

Spherical random-bond-random-field model of relaxor ferroelectrics

Abstract: A model of relaxor ferroelectrics based on the interacting polar clusters picture has been formulated. The electric dipole moment of a nanosized polar domain is allowed a large number of discrete orientations and its length is assumed to fluctuate in a broad interval. Introducing a set of quasicontinuous order parameter fields and imposing a global spherical constraint, the spherical random-bond--random-field (SRBRF) model is written down and its static properties are investigated. It is found that for weak random fields the scaled third-order nonlinear susceptibility ${a}_{3}={\ensuremath{\chi}}_{3}/{\ensuremath{\chi}}_{1}^{4}$ shows a nearly divergent behavior in the spherical glass phase, but there is no such anomaly in a random-field frustrated ferroelectric state. The probability distribution of local cluster polarization is calculated and its relation to the quadrupole perturbed NMR line shape of ${}^{93}\mathrm{Nb}$ in PMN is discussed. The fact that the observed line shape is Gaussian at all temperatures provides strong support to the SRBRF model.

Text-learning and related intelligent agents: a survey

Abstract: In surveying current research in the development of text-learning intelligent agents, the author focuses on three key criteria: what representation the particular application uses for documents, how it selects features, and what learning algorithm it uses. She then describes Personal WebWatcher, a content-based intelligent agent that uses text-learning for user-customized Web browsing.

PVD coatings as an environmentally clean alternative to electroplating and electroless processes

Abstract: Today the development of clean technologies in all spheres of industrial manufacturing is an essential task, not only for material and metal finishing but also for plasma surface engineering. Among the most critical group of technologies which needs to be replaced by alternative technologies are processes used to produce functional galvanic and decorative coatings. The electroplating of finishes, such as hard chromium, cadmium and nickel in metal finishing is today recognized as a major source of environmental pollution in every country. Therefore wet bath technologies have started to lose favour compared with high performance dry coating methods such as physical vapour deposition (PVD), plasma-assisted chemical vapour deposition, chemical vapour deposition and thermal spraying. Among these techniques, the results obtained with PVD coatings in metal cutting and forming in the last 15 years show the most promising solution of the complicated situation in which galvanic coatings seemed to be technologically and economically irreplaceable. In this paper the general situation in this field is shown. Already today it is possible to replace efficiently some of the galvanic processes in specific cases (e.g. Cr, Ni, Cd, Zn, Au). It is important to point out that PVD is considered to be a technique which can provide not only metallic, but also alloyed and ceramic coatings with a virtually unlimited range of chemical composition and therefore controlled protective, mechanical and wear-resistant properties. Entering into competition with galvanic coatings the manufacturers of PVD coaters were confronted with new requirements: a huge quantity of substrates of the same size, to be chemically and plasma cleaned and then coated at the highest possible deposition rate. For industrial mass production one can therefore use only large PVD batch systems or in-line coaters. The alternative for today's low price galvanic coatings is therefore dry and clean PVD technologies, fully supported by legislation on environmental protection. The economics depend directly on the substrate type and the quantity. The first positive results on the replacement of electrodeposited nickel on aluminium substrates and hard chrome on soft iron are also reported here. A soldering test was made on a sputtered nickel layer. Wear and corrosion tests were performed with iron cores, coated with PVD CrN coating. All tests were made in the Slovenian automotive industry. Results show that for a large number of substrates PVD clean technology is already economically competitive with galvanic coatings.

Quasiparticle relaxation dynamics in superconductors with different gap structures: Theory and experiments on YBa 2 Cu 3 O 7 − δ

Abstract: Photoexcited quasiparticle relaxation dynamics are investigated in a ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ superconductor as a function of doping \ensuremath{\delta} and temperature $T$ using ultrafast time-resolved optical spectroscopy. A model calculation is presented that describes the temperature dependence of the photoinduced quasiparticle population ${n}_{\mathrm{pe}},$ photoinduced transmission $\ensuremath{\Delta}\mathcal{T}/\mathcal{T},$ and relaxation time \ensuremath{\tau} for three different superconducting gaps: (i) a temperature-dependent collective gap such that $\mathit{\ensuremath{\Delta}}(T)\ensuremath{\rightarrow}0$ as $\stackrel{\ensuremath{\rightarrow}}{T}{T}_{c},$ (ii) a temperature-independent gap, which might arise for the case of a superconductor with preformed pairs, and (iii) an anisotropic (e.g., $d$-wave) gap with nodes. Comparison of the theory with data of photoinduced transmission $|\ensuremath{\Delta}\mathcal{T}/\mathcal{T}|,$ reflection $|\ensuremath{\Delta}\mathcal{R}/\mathcal{R}|,$ and quasiparticle recombination time \ensuremath{\tau} in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ over a very wide range of doping $(0.1l\ensuremath{\delta}l0.48)$ is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping $(\ensuremath{\delta}l0.1)$ and a temperature-independent isotropic gap in underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}(0.15l\ensuremath{\delta}l0.48).$ A pure $d$-wave gap was found to be inconsistent with the data.

Local Polarization Distribution and Edwards-Anderson Order Parameter of Relaxor Ferroelectrics

Abstract: The temperature dependence of the Edwards-Anderson order parameter ${q}_{\mathrm{EA}}$ and the local polarization distribution function $W(\stackrel{\ensuremath{\rightarrow}}{p})$ have been determined in a PMN single crystal via 2D ${}^{93}\mathrm{Nb}$ NMR. A glasslike freezing of reorientable polar clusters occurs in the temperature range of the diffuse relaxor transition, whereas the NMR spectra corresponding to pinned nanodomains do not change with temperature. The obtained form of $W(\stackrel{\ensuremath{\rightarrow}}{p})$ as well as the temperature dependence of ${q}_{\mathrm{EA}}$ and the nonlinear dielectric susceptibility can be well described by a newly proposed spherical random bond--random field model of relaxor ferroelectrics.

Crystal structure of MHC class II-associated p41 Ii fragment bound to cathepsin L reveals the structural basis for differentiation between cathepsins L and S.

Abstract: The lysosomal cysteine proteases cathepsins S and L play crucial roles in the degradation of the invariant chain during maturation of MHC class II molecules and antigen processing. The p41 form of the invariant chain includes a fragment which specifically inhibits cathepsin L but not S. The crystal structure of the p41 fragment, a homologue of the thyroglobulin type-1 domains, has been determined at 2.0 A resolution in complex with cathepsin L. The structure of the p41 fragment demonstrates a novel fold, consisting of two subdomains, each stabilized by disulfide bridges. The first subdomain is an alpha-helix-beta-strand arrangement, whereas the second subdomain has a predominantly beta-strand arrangement. The wedge shape and three-loop arrangement of the p41 fragment bound to the active site cleft of cathepsin L are reminiscent of the inhibitory edge of cystatins, thus demonstrating the first example of convergent evolution observed in cysteine protease inhibitors. However, the different fold of the p41 fragment results in additional contacts with the top of the R-domain of the enzymes, which defines the specificity-determining S2 and S1' substrate-binding sites. This enables inhibitors based on the thyroglobulin type-1 domain fold, in contrast to the rather non-selective cystatins, to exhibit specificity for their target enzymes.

Porewater Distribution and Benthic Flux Measurements of Mercury and Methylmercury in the Gulf of Trieste (Northern Adriatic Sea)

Abstract: The Gulf of Trieste is one of the most mercury-contaminated areas in the Mediterranean Sea. It is characterized by high mercury inputs from the Isonzo River whose tributary, the Idrijca River, drains the mercury mining area of Idrija in Slovenia where extraction activity has taken place for nearly 500 years. This appears, therefore, to be one of the most suitable sites for studying processes that affect Hg cycling in the marine environment and for determining whether sediments might act as secondary sources of mercury species in the water column. Porewater seasonal distributions of total dissolved Hg (HgT) and methylmercury (MeHg) were investigated. Usingin situbenthic chambers it was possible to determine benthic fluxes of HgTand MeHg at the water–sediment interface throughout the year. Benthic fluxes were also compared with diffusive fluxes calculated from porewater profiles. The results indicate that, following hypoxic conditions which occurred in late summer in the sea-bottom layer, highest benthic effluxes and porewater concentrations of Hg and MeHg appeared during autumn and winter. This was probably due to the transition from rapid sulphate reduction in late summer to cooler temperatures, higher oxygenation of the bottom water layer, and lower microbial activity which is well suited for Hg transformation, accumulation and flux. A tentative budget based on benthic flux measurements indicates that 75% of HgTis buried in the sediment whereas 25% of HgT, approximately 23% in methylated form, is annually recycled and released at the water–sediment interface.

Superconducting Gap Δ c , the Pseudogap Δ p , and Pair Fluctuations above T c in Overdoped Y 1 − x Ca x Ba 2 Cu 3 O 7 − δ from Femtosecond Time-Domain Spectroscopy

Abstract: The gap structure in ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals is investigated by real-time measurements of the quasiparticle (QP) relaxation dynamics with femtosecond optical spectroscopy. We find evidence for the coexistence of two distinct gaps. One is a $T$-independent pseudogap ${\ensuremath{\Delta}}_{p}$ and the other is a $T$-dependent gap ${\ensuremath{\Delta}}_{c}(T)$ with a BCS-like $T$ dependence. From QP relaxation time measurements above ${T}_{c}$ we ascertain that fluctuations associated with the latter are consistent with time-dependent Ginzburg-Landau theory and are distinct from the pseudogap whose presence is apparent well above ${T}_{c}$ for all $x$.

Single Particle and Collective Excitations in the One-Dimensional Charge Density Wave Solid K 0.3 MoO 3 Probed in Real Time by Femtosecond Spectroscopy

Abstract: Ultrafast transient reflectivity changes caused by collective and single particle excitations in the quasi-one-dimensional charge-density wave (CDW) semiconductor ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3}$ are investigated with optical pump-probe spectroscopy. The temperature dependence of nonequilibrium single particle excitations across the CDW gap and their recombination dynamics are reported for the first time. In addition, amplitude mode reflectivity oscillations are observed in real time. A $T$-dependent overdamped response is also observed which is attributed to relaxation of the phason mode.

Electric-field–temperature phase diagram of the relaxor ferroelectric lanthanum-modified lead zirconate titanate

Abstract: Transition lines between various phases in the electric-field--temperature phase diagram of 9/65/35 lanthanum-modified lead zirconate titanate ceramics were determined by measurements of the temperature and electric-field-dependent dielectric constant. Above a critical field ${(E}_{C})$ the dc bias electric field induces a transition from the relaxor (R) to the long-range ferroelectric (FE) phase. In the temperature direction of the approach to the FE phase the R-FE transition line was determined from the field-cooled--field-heated dielectric susceptibilities, while depolarization temperatures were obtained from the field-cooled--zero-field-heated dielectric susceptibilities. A considerably large shift was found for the above two R-FE transition lines demonstrating the strong impact of the electric field on the stability of the FE phase with increasing temperature. It was found that below ${E}_{C}$ ergodicity is broken due to the divergence of the longest relaxation time at the freezing temperature ${T}_{0}=259 \mathrm{K}.$ Hence the system exhibits a transition line between the ergodic (ER) and nonergodic (NR) relaxor state. In the dc bias field direction of the approach to the FE phase, the temperature dependence of ${E}_{C},$ i.e., the transition lines between ER or NR and FE phases were studied by measurements of the complex dielectric constant as a function of a dc bias field at several fixed temperatures. The experimental results are compared with the results of a spherical random bond-random field model of relaxor ferroelectrics.

Slow dynamics and ergodicity breaking in a lanthanum-modified lead zirconate titanate relaxor system

Abstract: The freezing of the dynamic process in a 9/65/35 lanthanum lead zirconate-titanate (PLZT) ceramics has been investigated by measurements of the frequency-dependent complex dielectric constant and the quasistatic field-cooled (FC) and zero-field-cooled (ZFC) dielectric susceptibilities. It was found that the aging process is responsible for the difference in temperature variations of the FC static dielectric constant and the static dielectric constant determined in the dynamic ZFC experiment. Analysis of the complex dielectric susceptibility by a temperature-frequency plot has revealed that for an aged PLZT sample the ergodicity is broken due to the divergence of the longest relaxation time in the vicinity of 249 K, i.e., the temperature where the ferroelectric phase can also be induced by applying sufficiently high electric field. However, the bulk of the distribution of relaxation times was found to remain finite even below the freezing temperature. It is shown that the behavior of the relaxation spectrum and the splitting between the field-cooled and zero-field-cooled dielectric constants in PLZT relaxor is qualitatively similar to what was observed in the lead magnesium niobate (PMN) relaxor and is reminiscent of the nonergodic behavior reported in various spin glasses. Moreover, the temperature dependence of the third order nonlinear susceptibility indicates a glassy rather than ferroelectric multidomain nature of the nonergodic relaxor state in both PMN and PLZT systems.

Comparisons between MCNP, EGS4 and experiment for clinical electron beams

Abstract: Understanding the limitations of Monte Carlo codes is essential in order to avoid systematic errors in simulations, and to suggest further improvement of the codes. MCNP and EGS4, Monte Carlo codes commonly used in medical physics, were compared and evaluated against electron depth dose data and experimental backscatter results obtained using clinical radiotherapy beams. Different physical models and algorithms used in the codes give significantly different depth dose curves and electron backscattering factors. The default version of MCNP calculates electron depth dose curves which are too penetrating. The MCNP results agree better with experiment if the ITS-style energy-indexing algorithm is used. EGS4 underpredicts electron backscattering for high-Z materials. The results slightly improve if optimal PRESTA-I parameters are used. MCNP simulates backscattering well even for high-Z materials. To conclude the comparison, a timing study was performed. EGS4 is generally faster than MCNP and use of a large number of scoring voxels dramatically slows down the MCNP calculation. However, use of a large number of geometry voxels in MCNP only slightly affects the speed of the calculation.

Use of Functional Electrical Stimulation in the Lower Extremities of Incomplete Spinal Cord Injured Patients

Abstract: After a program of therapeutic electrical stimulation, 3 groups of incomplete spinal cord injured (SCI) patients were identified, those in whom an improvement of both voluntary and stimulated muscle force was observed, those with an increase in stimulation response only, and patients in whom no effect of electrical stimulation training could be recorded. As it is difficult to predict the outcome of the electrical stimulation rehabilitation process, a diagnostic procedure was developed to predict soon after accidents which incomplete SCI patients are candidates for permanent use of a functional electrical stimulation (FES) orthotic aid. The candidates for chronic use of FES are patients with weak ankle dorsiflexors and sufficiently strong knee extensors. These patients are equipped with a single channel peroneal stimulator augmenting dorsiflexion and knee and hip flexion in a total lower limb flexion response. By applying FES to the ankle plantar flexors, the swing phase of walking can be significantly shortened and faster walking obtained.

Biaxial torus around nematic point defects.

Abstract: We study the biaxial structure of both line and point defects in a nematic liquid crystal confined within a capillary tube whose lateral boundary enforces homeotropic anchoring. According to Landau--de Gennes theory the local order in the material is described by a second-order tensor $\mathbf{Q},$ which encompasses both uniaxial and biaxial states. Our study is both analytical and numerical. We show that the core of a line defect with topological charge $M=1$ is uniaxial in the axial direction. At the lateral boundary, the uniaxial ordering along the radial direction is reached in two qualitatively different ways, depending on the sign of the order parameter on the axis. The point defects with charge $M=\ifmmode\pm\else\textpm\fi{}1$ exhibit a uniaxial ring in the plane orthogonal to the cylinder axis. This ring is in turn surrounded by a torus on which the degree of biaxiality attains its maximum. The typical lengths that characterize the structure of these defects depend both on the cylinder radius and the biaxial correlation length. It seems that the core of the point defect does not depend on the far nematic director field in the bulk limit.

Cysteine-scanning mutagenesis of an eukaryotic pore-forming toxin from sea anemone: topology in lipid membranes.

Abstract: Equinatoxin II is a cysteineless pore-forming protein from the sea anemone Actinia equina It readily creates pores in membranes containing sphingomyelin Its topology when bound in lipid membranes has been studied using cysteine-scanning mutagenesis At approximately every tenth residue, a cysteine was introduced Nineteen single cysteine mutants were produced in Escherichia coli and purified The accessibility of the thiol groups in lipid-embedded cysteine mutants was studied by reaction with biotin maleimide Most of the mutants were modified, except those with cysteines at positions 105 and 114 Mutants R144C and S160C were modified only at high concentrations of the probe Similar results were obtained if membrane-bound biotinylated mutants were tested for avidin binding, but in this case three more mutants gave a negative result: S1C, S13C and K43C Furthermore, mutants S1C, S13C, K20C, K43C and S95C reacted with biotin only after insertion into the lipid, suggesting that they were involved in major conformational changes occurring upon membrane binding These results were further confirmed by labeling the mutants with acrylodan, a polarity-sensitive fluorescent probe When labeled mutants were combined with vesicles, the following mutants exhibited blue-shifts, indicating the transfer of acrylodan into a hydrophobic environment: S13C, K20C, S105C, S114C, R120C, R144C and S160C The overall results suggest that at least two regions are embedded within the lipid membrane: the N-terminal 13‐20 region, probably forming an amphiphilic helix, and the tryptophan-rich 105‐120 region Arg144, Ser160 and residues nearby could be involved in making contacts with lipid headgroups The association with the membrane appears to be unique and different from that of bacterial pore-forming proteins and therefore equinatoxin II may serve as a model for eukaryotic channel-forming toxins

Experiments in Predicting Biodegradability

Abstract: We present a novel application of inductive logic programming (ILP) in the area of quantitative structure-activity relationships (QSARs). The activity we want to predict is the biodegradability of chemical compounds in water. In particular, the target variable is the half-life in water for aerobic aqueous biodegradation. Structural descriptions of chemicals in terms of atoms and bonds are derived from the chemicals' SMILES encodings. Definition of substructures are used as background knowledge. Predicting biodegradability is essentially a regression problem, but we also consider a discretized version of the target variable. We thus employ a number of relational classification and regression methods on the relational representation and compare these to propositional methods applied to different propositionalisations of the problem. Some expert comments on the induced theories are also given.