Higher Order Effects in the Dielectric Constant of Percolative Metal-Insulator Systems above the Critical Point

The dielectric constant of a conductor-insulator mixture shows a pronounced maximum above the critical volume concentration. Further experimental evidence is presented as well as a theoretical consideration based on a phenomenological equation. Explicit expressions are given for the position of the maximum in terms of scaling parameters and the (complex) conductances of the conductor and insulator. In order to fit some of the data, a volume fraction dependent expression for the conductivity of the more highly conductive component is introduced.Comment: 4 pages, Latex, 4 postscript (*.epsi) files submitted to Phys Rev.

Distributed Approximation of Minimum Routing Cost Trees

We study the NP-hard problem of approximating a Minimum Routing Cost Spanning Tree in the message passing model with limited bandwidth (CONGEST model). In this problem one tries to find a spanning tree of a graph $G$ over $n$ nodes that minimizes the sum of distances between all pairs of nodes. In the considered model every node can transmit a different (but short) message to each of its neighbors in each synchronous round. We provide a randomized $(2+\epsilon)$-approximation with runtime $O(D+\frac{\log n}{\epsilon})$ for unweighted graphs. Here, $D$ is the diameter of $G$. This improves over both, the (expected) approximation factor $O(\log n)$ and the runtime $O(D\log^2 n)$ of the best previously known algorithm. Due to stating our results in a very general way, we also derive an (optimal) runtime of $O(D)$ when considering $O(\log n)$-approximations as done by the best previously known algorithm. In addition we derive a deterministic $2$-approximation

Griffiths-McCoy Singularities in the Random Transverse-Field Ising Spin Chain

We consider the paramagnetic phase of the random transverse-field Ising spin chain and study the dynamical properties by numerical methods and scaling considerations. We extend our previous work [Phys. Rev. B 57, 11404 (1998)] to new quantities, such as the non-linear susceptibility, higher excitations and the energy-density autocorrelation function. We show that in the Griffiths phase all the above quantities exhibit power-law singularities and the corresponding critical exponents, which vary with the distance from the critical point, can be related to the dynamical exponent z, the latter being the positive root of [(J/h)^{1/z}]_av=1. Particularly, whereas the average spin autocorrelation function in imaginary time decays as [G]_av(t)~t^{-1/z}, the average energy-density autocorrelations decay with another exponent as [G^e]_av(t)~t^{-2-1/z}.Comment: 8 pages RevTeX, 8 eps-figures include

Ambient Isotopic Meshing of Implicit Algebraic Surface with Singularities

A complete method is proposed to compute a certified, or ambient isotopic, meshing for an implicit algebraic surface with singularities. By certified, we mean a meshing with correct topology and any given geometric precision. We propose a symbolic-numeric method to compute a certified meshing for the surface inside a box containing singularities and use a modified Plantinga-Vegter marching cube method to compute a certified meshing for the surface inside a box without singularities. Nontrivial examples are given to show the effectiveness of the algorithm. To our knowledge, this is the first method to compute a certified meshing for surfaces with singularities.Comment: 34 pages, 17 Postscript figure

Properties of the random field Ising model in a transverse magnetic field

We consider the effect of a random longitudinal field on the Ising model in a transverse magnetic field. For spatial dimension $d > 2$, there is at low strength of randomness and transverse field, a phase with true long range order which is destroyed at higher values of the randomness or transverse field. The properties of the quantum phase transition at zero temperature are controlled by a fixed point with no quantum fluctuations. This fixed point also controls the classical finite temperature phase transition in this model. Many critical properties of the quantum transition are therefore identical to those of the classical transition. In particular, we argue that the dynamical scaling is activated, i.e, the logarithm of the diverging time scale rises as a power of the diverging length scale

Atom trapping and two-dimensional Bose-Einstein condensates in field-induced adiabatic potentials

We discuss a method to create two-dimensional traps as well as atomic shell, or bubble, states for a Bose-Einstein condensate initially prepared in a conventional magnetic trap. The scheme relies on the use of time-dependent, radio frequency-induced adiabatic potentials. These are shown to form a versatile and robust tool to generate novel trapping potentials. Our shell states take the form of thin, highly stable matter-wave bubbles and can serve as stepping-stones to prepare atoms in highly-excited trap eigenstates or to study `collapse and revival phenomena'. Their creation requires gravitational effects to be compensated by applying additional optical dipole potentials. However, in our scheme gravitation can also be exploited to provide a route to two-dimensional atom trapping. We demonstrate the loading process for such a trap and examine experimental conditions under which a 2D condensate may be prepared.Comment: 16 pages, 10 figure

Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned YBa2Cu3O7\rm YBa_2Cu_3O_7

The complex resistivity $\hat{\rho}(\omega)$ of the vortex lattice in an untwinned crystal of 93-K $\rm YBa_2Cu_3O_7$ has been measured at frequencies $\omega/2\pi$ from 100 kHz to 20 MHz in a 2-Tesla field $\bf H\parallel c$, using a 4-probe RF transmission technique that enables continuous measurements versus $\omega$ and temperature $T$. As $T$ is increased, the inductance ${\cal L}_s(\omega) ={\rm Im} \hat{\rho}(\omega)/ \omega$ increases steeply to a cusp at the melting temperature $T_m$, and then undergoes a steep collapse consistent with vanishing of the shear modulus $c_{66}$. We discuss in detail the separation of the vortex-lattice inductance from the `volume' inductance, and other skin-depth effects. To analyze the spectra, we consider a weakly disordered lattice with a low pin density. Close fits are obtained to $\rho_1(\omega)$ over 2 decades in $\omega$. Values of the pinning parameter $\kappa$ and shear modulus $c_{66}$ obtained show that $c_{66}$ collapses by over 4 decades at $T_m$, whereas $\kappa$ remains finite.Comment: 11 pages, 8 figures, Phys. Rev. B, in pres

Alaraajojen lihasten spastisuus ennen ja jälkeen avustetun polkuharjoittelun

Opinnäytetyön tavoitteena oli kerätä tietoa aivoverenkiertohäiriötä, selkäydinvauriota sekä CP-vammaa sairastavien neurologisten asiakkaiden spastisten alaraajojen lihasten spastisuuden aiheuttaman lihasaktivaation mahdollisesta muutoksesta ennen ja jälkeen avustetulla polkulaitteella suoritetun polkuharjoituksen. Tarkoituksena oli tuottaa tutkittua tietoa kyseisen terapiamuodon vaikutuksesta edellä mainittuja oireyhtymiä sairastavien kuntoutuksessa. Toimeksiantaja voi hyödyntää tuloksia suunnitellessaan ja arvioidessaan neurologisten asiakkaiden kuntoutuksessa käytettäviä terapiamuotoja. Lisäksi tarkoituksena oli tuottaa fysioterapia-alalle tietoa terapiamuodon vaikutuksesta alaraajojen spastisuuteen. Työn tekijät syvensivät työn kautta omaa ammattitaitoaan tulevaa ammattia varten. Opinnäytetyömme tutkimusongelmana oli miten polkulaitteella suoritettu 20 minuutin avustettu polkuliike vaikuttaa aivoverenkiertohäiriötä, selkäydinvauriota sekä CP-vammaa sairastavien neurologisten asiakkaiden spastisuuden aiheuttamaan alaraajojen lihasaktivaatioon. Opinnäytetyö toteutettiin tapaustutkimuksena, johon osallistui viisi tutkimushenkilöä. Tutkimuksen aineisto kerättiin määrällisin menetelmin, joita olivat elektromyografia (EMG), Modified Modified Ashworth Scale (MMAS) sekä kysymyslomake. EMG ja MMAS mittaukset suoritettiin yhtäaikaisesta ennen polkuharjoitusta ja sen jälkeen. Mittareilla saadut tulokset analysoitiin MegaWin-ohjelmalla ja Microsoft Excel-taulukkolaskentaohjelmalla. Tulokset on esitetty numeerisessa ja graafisessa muodossa. Tutkimuksesta saatujen tulosten mukaan spastisuuden aiheuttama lihasaktivaatio väheni polkuharjoittelun jälkeen jokaisessa mitatussa lihaksessa EMG- ja MMAS -mittareilla mitattuna. Myös kysymyslomakkeella saatujen tulosten mukaan polkuharjoittelun vaikutukset spastisuuteen ovat positiivisia. Näin ollen tutkimustulosten perusteella avustetulla polkuharjoittelulla oli lihasten spastisuutta alentava vaikutus. Pienen tutkimusjoukon johdosta tuloksia ei voi kuitenkaan yleistää, mutta ne ovat suuntaa-antavia.The aim of this thesis is to gather information on possible changes in the spasticity of the lower limb muscles before and after assisted cycling exercise in clients with stroke, spinal cord injury and cerebral palsy. The purpose of this thesis is to produce information about the effects of the assisted cycling exercise in rehabilitation with clients suffering from the above mentioned injuries. The commissioner, Kemijärven Fysikaalinen Hoitolaitos Ky, can benefit from the achieved results while planning the rehabilitation of neurological clients. The authors’ purpose is to generate knowledge on the effects of assisted cycling exercise in spasticity of the lower limb muscles for physiotherapy field to use. The authors benefit from the thesis by obtaining their own expertise for the upcoming profession. The research problem of this thesis was to discover how the 20-minute assisted cycling exercise affects the spasticity of the lower limbs muscles in clients with stroke, spinal cord injury and cerebral palsy. This thesis is a case study in which participated five study subjects. The research data was gathered with the following quantitative methods: Electromyography (EMG), Modified Modified Ashworth Scale (MMAS) and questionnaire. EMG and MMAS were administrated simultaneously before and after assisted cycling exercise. The results were analysed with MegaWin-program and Microsoft Excel Spreadsheet. The results are displayed in numerical and graphical form. The results of this thesis show that after the assisted cycling exercise the muscle activation caused by spasticity, previously measured by EMG and MMAS, was reduced in every tested muscle. According to results from the questionnaire the effects of assisted cycling exercise was also positive. Therefore, it could be said that assisted cycling exercise reduces the spasticity in lower limb muscles. Due to the limited amount of participant in the study group, the results cannot be generalised, nevertheless, they can be used as directional information

Causal explanation for observed superluminal behavior of microwave propagation in free space

In this paper we present a theoretical analysis of an experiment by Mugnai and collaborators where superluminal behavior was observed in the propagation of microwaves. We suggest that what was observed can be well approximated by the motion of a superluminal X wave. Furthermore the experimental results are also explained by the so called scissor effect which occurs with the convergence of pairs of signals coming from opposite points of an annular region of the mirror and forming an interference peak on the intersection axis traveling at superluminal speed. We clarify some misunderstandings concerning this kind of electromagnetic wave propagation in vacuum.Comment: 9 pages, 3 figures, accepted for publication in Physics Letters