Showing papers by "Helsinki University of Technology published in 1987"

Quantized vortices in superfluid He-3

Abstract: The first measurements on vortices in rotating superfluid $^{3}\mathrm{He}$ have been conducted in the Low Temperature Laboratory at Helsinki University of Technology during the past five years. These experiments have revealed unique vortex phenomena that are not observed in any other known superfluids. In this review, the concept of broken symmetry is applied to investigate the quantized vortex lines in superfluid $^{3}\mathrm{He}$. In the superfluid $A$ phase, vorticity can be supported by a continuous winding of the order parameter; this gives rise to continuous "coreless" vortices with two flow quanta. Novel vortices with a half-integer number of circulation quanta may also exist in $^{3}\mathrm{He}$-$A$ due to a combined symmetry of the superfluid state. In the superfluid $B$ phase, the vortices have a complicated core structure. The vortex-core matter is ferromagnetic and superfluid, and it displays broken parity. The ferromagnetism of the core is observed in NMR experiments due to a gyromagnetic effect. The calculated core structures exhibit an experimentally observed first-order phase transition. This vortex-core transition in rotating $^{3}\mathrm{He}$-$B$ may be understood in terms of a change in the topology for flaring-out of the vortex singularity into higher dimensions; the topological identification further suggests that the phase transition manifests a spontaneous bifurcation of vorticity---involving half-quantum vortices in $^{3}\mathrm{He}$-$B$. These recent advances of interest in quantum liquids are also of general relevance to a wide range of fields beyond low-temperature physics.

Neuromagnetic responses of the human auditory cortex to on- and offsets of noise bursts.

Abstract: Cortical sources of neuromagnetic responses to noise bursts were compared in 7 healthy humans. The earliest response, P40m, peaking about 40 ms after the stimulus onset, was followed by a prominent deflection in the opposite direction at about 100 ms (N100m) and by another peak at 200 ms (P200m). A sustained field, seen near the end of the 400- to 550-ms stimuli, ended with an off-response of the same polarity as N100m. All deflections could be explained by cortical activity within the Sylvian fissure. The source of P200m was anterior to other sources of the on-response, except P40m. When noise bursts, or pauses of equal duration in a continuous noise were presented to the subject, striking similarities were found between the 100-ms deflections of the on- and off-responses: both peaked at about the same latency; their estimated sources were close to each other in the supratemporal plane and their amplitudes depended in a similar way on the interstimulus interval (1.1-8.8 s). However, only the on-response ...

Convergence of dynamic iteration methods for initial value problems

Abstract: In VLSI-simulation there has recently been interest in an iterative technique called the waveform relaxation method. In this paper we set up a theoretical framework to analyze the convergence of such methods. We restrict the discussion to linear systems and do not consider the effects of time discretization, but assume that the initial value problems are solved exactly.

Profiling multilayer structures with monoenergetic positrons.

Abstract: Variable-energy (0--25 keV) positron stopping and annihilation behavior is studied in a multilayer structure, which has subsequent (\ensuremath{\sim}3000-A\r{}-thick) ZnS and ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ layers on a glass substrate. Direct information on positron slowing-down properties is obtained. The positron implantation profile is shown to possess the shape of a derivative of a Gaussian function, in contrast to the more commonly used exponential profile. The mean positron penetration depth varies with incident positron energy E (in keV) as z\ifmmode\bar\else\textasciimacron\fi{}=[4.0(3) \ensuremath{\mu}g/${\mathrm{cm}}^{2}$](E${)}^{1.62(5)}$. The feasibility of the present technique for depth profiling of heterogeneous samples is considered. The accuracy of determining the positions of the interfaces is typically less than 100 A\r{} in the present system. The mobility of positrons in ZnS and ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ layers is observed to be very low. This corresponds to positron trapping into structural defects with a relatively high concentration.

Extinction Behavior of Dry Snow in the 18-to 90-GHz Range

Abstract: The extinction properties of several dry snow types were examined in the 18-to 90-GHz range. The snow types ranged from newly fallen snow to refrozen snow, and the density and mean grain size varied from 0.17 to 0.39 g/cm3 and from 0.2 to 1.6 mm, respectively. From measurements of the transmission loss as a function of sample thickness at a temperature of -15°C, the extinction coefficient and the surface scattering loss (due to surface roughness at the front and back surfaces of the snow slab) were determined for each snow type. The experimental values were compared against theoretical results computed according to the strong fluctuation theory. In general, good agreement with the experimental data was obtained at 18, 35, and 60 GHz when the grain size used in the theoretical calculations was chosen to be slightly smaller than the observed snow-particle size. However, the extinction coefficient of large-grained refrozen snow as predicted by the strong fluctuation theory is much larger at 90 GHz than the values determined experimentally. The attenuation in snow was observed to increase only slightly with increasing temperature in the -35 to -1°C range.

Adaptive, associative, and self-organizing functions in neural computing.

Abstract: This paper contains an attempt to describe certain adaptive and cooperative functions encountered in neural networks. The approach is a compromise between biological accuracy and mathematical clarity. Two types of differential equation seem to describe the basic effects underlying the formation of these functions: the equation for the electrical activity of the neuron and the adaptation equation that describes changes in its input connectivities. Various phenomena and operations are derivable from them: clustering of activity in a laterally interconnected network; adaptive formation of feature detectors; the autoassociative memory function; and self-organized formation of ordered sensory maps. The discussion tends to reason what functions are readily amenable to analytical modeling and which phenomena seem to ensue from the more complex interactions that take place in the brain.

Enzymatic hydrolysis of cellulose: is the current theory of the mechanisms of hydrolysis valid?

Abstract: (1987). Enzymatic Hydrolysis of Cellulose: is the Current Theory of the Mechanisms of Hydrolysis Valid? Critical Reviews in Biotechnology: Vol. 5, No. 1, pp. 67-87.

Responses of the human auditory cortex to vowel onset after fricative consonants.

Abstract: Neuromagnetic responses to different auditory stimuli (noise bursts and short speech stimuli) were mapped over both hemispheres of seven healthy subjects. The results indicate that a particular acoustic feature of speech, vowel onset after voice-less fricative consonants, evokes a prominent response in the human supratemporal auditory cortex. Although the observed response seems to be specific to acoustic rather than phonetic characteristics of the stimuli, it might reflect feature detection essential for further speech processing.

Quantum theory of post-collision interaction in inner-shell photoionization: Final-state interaction between two continuum electrons.

Abstract: In inner-shell photoionization followed by Auger decay, the Auger electron initially screens the ionic Coulomb field ``seen'' by the photoelectron. This screening phenomenon affects the post-collision interaction that shifts the Auger energy and distorts the line shape. The analytical Auger electron line-shape formula which we derived previously on the basis of asymptotic Coulomb wave functions is now modified to account for the screening effect. The ionic charge seen by the photoelectron is reinterpreted on the basis of asymptotic properties of the continuum wave function pertaining to two outgoing electrons. It is shown that this procedure is consistent with semiclassical models which account for the time required for the Auger electron to overtake the photoelectron. Our modified Coulomb line shapes are found to agree perfectly with the semiclassical line shapes for photon excess energies ${E}_{\mathrm{exc}\mathrm{\ensuremath{\gtrsim}}({\ensuremath{\Gamma}}_{i}{\ensuremath{\varepsilon}}_{A}{)}^{1/2}}$, where ${\ensuremath{\Gamma}}_{i}$ is the width of the initial hole state and ${\ensuremath{\varepsilon}}_{A}$, the Auger electron energy. Agreement between the present theory and recent synchrotron-radiation measurements consequently becomes excellent in this range of photon energies as well.

Comparison of cutting performance of ion-plated NbN, ZrN, TiN and (Ti, Al)N coatings

Abstract: Three binary nitrides of the refractory materials titanium, zirconium and niobium and ternary (Ti, Al)N as well as quaternary (Ti, Al, V)N coatings were deposited by reactive triode ion plating on powder metallurgically produced high-speed steel (PM-HSS) inserts. The coated tools were then tested by dry turning of hardened and tempered AISI 4140 steel. The effect of the deposition parameters on coating properties was studied in detail. The results showed that NbN coating does not seem to improve the wear resistance in the turning of steel. Titanium-based nitride films such as TiN and (Ti, Al)N, however, appeared very suitable, as they showed increasing cutting performance and finer structures with increasing substrate current density. ZrN, however, showed the opposite effect. The best turning properties were achieved with a substrate bias of −100 V. Increasing thickness improved cutting performance almost linearly in the range from 1 to 5 μm. Substrate polishing before the deposition of the TiN layer had a strong effect, almost corresponding to doubling of the coating thickness. For example, a thickness increase from 3 to 5 μm yielded a performance improvement which could also be achieved simply by polishing the substrates carefully before coating. The three best coating materials in decreasing order of performance were ZrN, (Ti, Al)N and TiN. The differences between the best coating materials were generally not large.

Electron and positron energy levels in solids.

Abstract: The self-consistent electron densities and the corresponding positron states are calculated for several metals and semiconductors in the local-density approximation of the density-functional formalism. The calculations are performed with the linear-muffin-tin-orbital band-structure method. The emphasis of this work is on the energy levels of the delocalized positron and the electron chemical potential which are now calculated with respect to the same potential reference. These energies determine quantities such as the positron and positronium work functions and the deformation potentials which are important parameters in slow-positron-beam experiments. The theoretical results are compared to values extracted from experiments.

Effects on DC SQUID characteristics of damping of input coil resonances

Abstract: The possibility of improving dc SQUID performance by damping the input circuit resonances caused by parasitic capacitances is studied experimentally A high-quality dc SQUID was coupled to a first-order axial gradiometer built for neuromagnetic research, and a resistor-capacitor shunt was connected in parallel with the input coil of the SQUID Ten different RC shunts were studied with the SQUID operating in a flux-locked loop, carefully shielded against external disturbances It was found that increasing the shunt resistance resulted in smoother flux-voltage characteristics and smaller noise At best, the minimum obtainable equivalent flux noise level was one-fourth that for the unshunted SQUID The noise level is a function of the shunt resistance R/sub s/ only, except for shunt capacitance values bringing the low-frequency resonance of the input coil close to the flux modulation frequency At a constant bias current level, where the amplitude of the flux-voltage characteristics is at maximum, the equivalent flux noise varies as R/sub s//sup -07/ The results agree reasonably well with recently published predictions based on numerical simulations where the whole input circuit with parasitic capacitances was taken into account

Age-size structure in populations with quiescence

Abstract: An analysis is given of a mathematical model for the continuous age-size distribution of a population that has both normal (growing and reproducing) and quiescent individuals. Individuals can transit back and forth from one state to the other. The theory of positive operator semigroups is used to show that under general assumptions about individual behavior the age-size distribution of the population converges to a stable distribution. The features of the model are illustrated by several examples. The examples are designed to minimize technicalities, yet still reveal the interesting stability phenomena that occur when structure is combined with quiescence. For these examples the extinction or asynchronous exponential growth of the population is analyzed in terms of the parameters effecting transition between the normal and quiescent states. The continuous model is compared with an analogous discrete model.

Order parameter of superfluid 3He-B near surfaces.

Abstract: The order parameter of superfluid $^{3}\mathit{B}$ is calculated near surfaces with the quasiclassical theory and a thin-dirty-layer surface-scattering model. The surface structures depend on the temperature, the surface roughness, the Fermi-liquid parameters, and the mass superflow tangential to the surface. All these are studied with special emphasis on the roughness and the superflow. A numerical technique of solving the quasiclassical equations is discussed.

Sets of convergence and stability regions

Abstract: This paper continues the authors' study of the convergence of dynamic iteration methods for large systems of linear initial value problems. We ask for convergence on [0, ∞) and show how the convergence can be reduced to a graphical test relating the splitting of the matrix to the stability properties of the discretization method.

Positron diffusion in Mo: The role of epithermal positrons.

Abstract: Positrons at keV energies, incident on Mo(111) are observed to escape the sample prior to thermalization. Analysis of positron back-diffusion data shows that an excess contribution of ``hot'' positronium (Ps) seriously affects positron mobility measurements. Suppressing the ``hot'' Ps fraction the positron diffusion coefficient is shown to vary as ${T}^{\mathrm{\ensuremath{-}}0.50(4)}$. The positron implantation profile possesses a shape of a derivative of a Gaussian function. The implications of these observations to related experiments are discussed.

Quantum Theory of Post-Collision Interaction in Inner-Shell Photoionization

Abstract: We have performed fully quantum-mechanical and relativistic calculations of the post-collision-interaction effect in x-ray-induced argonK−L 2 L 3(1 D) Auger and xenonL 2−L 3 N 4(J=3) Coster-Kronig-electron emission. The Dirac-Fock computations include a complete integration over intermediate one-hole continuum states in the lowest-order expression of the resonant double-photoionization cross section. The results are in excellent agreement with synchrotron-radiation measurements of the post-collision-interaction shifts. We have also made nonrelativistic Hartree-Fock test calculations of the argonK−L 2 L 3(1 D) and xenonL 3−M 4 M 5(1 G 4) Auger-electron line shapes. The quantum-mechanical results are compared with rigorous semiclassical calculations which have been made without using the stationary-phase approximation. The results of this computational analysis are interpreted in terms of an analytical line-shape formula based on asymptotic Coulomb wave functions. As a consequence the most salient features of the post-collision interaction in inner-shell photoionization are explained.

An abstract delay-differential equation modelling size dependent cell growth and division

Abstract: A two-phase model for the growth of a single cell population structured by size is formulated and analysed. The model takes the form of a delay-differential equation in a Banach space. Using positivity arguments, we describe the spectrum of the infinitesimal generator of the semigroup associated with solutions. Under a certain condition on the growth rate of individual cells the semigroup is compact after finite time. This enables us to determine the ultimate behavior of solutions and prove the existence of a stable size distribution.

An efficient Schottky-varactor frequency multiplier at millimeter waves. Part I: Doubler

Abstract: The efficiency of millimeter wave doublers with a wide tunable bandwidth was studied. The efficiency depends on the varactor parameters and the embedding impedances seen by the diode at fundamental and harmonic frequencies. Millimeter wave doublers were simulated with a nonlinear analysis program to find optimum embedding impedances for a given diode. Also the sensitivity of the efficiency to various diode and circuit parameters was evaluated. A scaled model was constructed in order to experimentally optimize the impedances. For experimental verification a doubler from 40–58 GHz to 80–116 GHz was constructed. The highest efficiency measured was 45% at 94 GHz with 5 mW input power. The highest efficiency obtained with 20 mW input power was 38%.