Dynamical properties of a two-dimensional electron gas in a magnetic field within the composite fermion model

We investigate the response of a two-dimensional electron gas, in the fractional quantum Hall regime, to the sudden appearance of a localised charged probe using the Chern-Simons theory of composite fermions. The dynamic structure factor of the electron gas is found to have a major influence on the spectral function of the probe. In particular, there is an orthogonality catastrophe when the filling factor is an even-denominator filling fraction due to the compressibility of the state, but there is no catastrophe at odd-denominator filling factors because these states have a gap to excitations. The catastrophe is found to be more severe for composite fermions in zero effective magnetic field than it is for electrons in zero real magnetic field. Oscillations in the spectral function, arising when the composite fermions are at integer filling, have a period equal to the composite fermion cyclotron energy. We propose a tunneling experiment which directly measures the spectral function from which one could determine the composite fermion effective mass.Comment: 15 pages of REVTEX. Uses multicol package. Twoside option is default. There are 29 figures in GIF format to save spac

Use of cohesive elements in fatigue analysis

Cohesive laws describe the resistance to incipient separation of material surfaces. A cohesive finite element is formulated on the basis of a particular cohesive law. Cohesive elements are placed at the boundary between adjacent standard volume finite elements to model fatigue damage that leads to fracture at the separation of the element boundaries per the cohesive law. In this work, a cohesive model for fatigue crack initiation is taken to be the irreversible loadingunloading hysteresis that represents fatigue damage occuring due to cyclic loads leading to the initiation of small cracks. Various cohesive laws are reviewed and one is selected that incorporates a hysteretic cyclic loading that accounts for energetic dissipative mechanisms. A mathematical representation is developed based on an exponential effective load-separation cohesive relationship. A three-dimensional cohesive element is defined using this compliance relationship integrated at four points on the mid-surface of the area element. Implementation into finite element software is discussed and particular attention is applied to numerical convergence issues as the inflection point between loading and 'unloading in the cohesive law is encountered. A simple example of a displacementcontrolled fatigue test is presented in a finite element simulation. Comments are made on applications of the method to prediction of fatigue life for engineering structures such as pressure vessels and piping

Ultrafast absorption kinetics of NADH in folded and unfolded conformations

The non-radiative energy transfer is shown to occur on a ~3ps time scale for NADH in the folded form in H2O. Addition of methanol thermodynamically favours the open form, for which energy transfer does not occur

Do OB Runaway Stars Have Pulsar Companions?

We have conducted a VLA search for radio pulsars at the positions of 44 nearby OB runaway stars. The observations involved both searching images for point sources of continuum emission and a time series analysis. Our mean flux sensitivity to pulsars slower than 50 ms was 0.2 mJy. No new pulsars were found in the survey. The size of the survey, combined with the high sensitivity of the observations, sets a significant constraint on the probability, $f_p$, of a runaway OB star having an observable pulsar companion. We find $f_p \le 6.5$\% with 95\% confidence, if the general pulsar luminosity function is applicable to OB star pulsar companions. If a pulsar beaming fraction of \onethird\ is assumed, then we estimate that fewer than 20\% of runaway OB stars have neutron star companions, unless pulsed radio emission is frequently obscured by the OB stellar wind. Our result is consistent with the dynamical (or cluster) ejection model for the formation of OB runaways. The supernova ejection model is not ruled out, but is constrained by these observations to allow only a small binary survival fraction, which may be accommodated if neutron stars acquire significant natal kicks. According to Leonard, Hills and Dewey (1994), a 20\% survival fraction corresponds to a 3-d kick velocity of 420 km s$^{-1}$. This value is in close agreement with recent revisions of the pulsar velocity distribution.Comment: Submitted to the Astronomical Journal. 16 pages. Latex uses aaspp4.sty. 3 postscript figures. Address correspondence to Colin Philp ([email protected]). Revision was to replace .ps file with latex fil

Gravitational waves from binary systems in circular orbits: Convergence of a dressed multipole truncation

The gravitational radiation originating from a compact binary system in circular orbit is usually expressed as an infinite sum over radiative multipole moments. In a slow-motion approximation, each multipole moment is then expressed as a post-Newtonian expansion in powers of v/c, the ratio of the orbital velocity to the speed of light. The bare multipole truncation of the radiation consists in keeping only the leading-order term in the post-Newtonian expansion of each moment, but summing over all the multipole moments. In the case of binary systems with small mass ratios, the bare multipole series was shown in a previous paper to converge for all values v/c < 2/e, where e is the base of natural logarithms. In this paper, we extend the analysis to a dressed multipole truncation of the radiation, in which the leading-order moments are corrected with terms of relative order (v/c)^2 and (v/c)^3. We find that the dressed multipole series converges also for all values v/c < 2/e, and that it coincides (within 1%) with the numerically ``exact'' results for v/c < 0.2.Comment: 9 pages, ReVTeX, 1 postscript figur

Accessing Patient Records in Virtual Healthcare Organisations

The ARTEMIS project is developing a semantic web service based P2P interoperability infrastructure for healthcare information systems that will allow healthcare providers to securely share patient records within virtual healthcare organisations. Authorisation decisions to access patient records across organisation boundaries can be very dynamic and must occur within a strict legislative framework. In ARTEMIS we are developing a dynamic authorisation mechanism called PBAC that provides a means of contextual and process oriented access control to enforce healthcare business processes. PBAC demonstrates how healthcare providers can dynamically share patient records for care pathways across organisation boundaries

Orthogonality catastrophe in a composite fermion liquid

We discuss the emergence of an orthogonality catastrophe in the response of a composite fermion liquid as the filling factor \nu approaches 1/2m, where m=1,2,3.... A tunneling experiment is proposed in which dramatic changes in the I-V characteristic should be observable as \nu is varied. Explicit I-V characteristics calculated within the so-called Modified Random Phase Approximation, are provided for \nu=1/3 -> \nu=1/2.Comment: Latex two-column 6 pages including 5 figure

The Number of States of Two Dimensional Critical String Theory

We discuss string theory vacua which have the wrong number of spacetime dimensions, and give a crude argument that vacua with more than four large dimensions are improbable. We then turn to two dimensional vacua, which naively appear to violate Bekenstein's entropy principle. A classical analysis shows that the naive perturbative counting of states is unjustified. All excited states of the system have strong coupling singularities which prevent us from concluding that they really exist. A speculative interpretation of the classical solutions suggests only a finite number of states will be found in regions bounded by a finite area. We also argue that the vacuum degeneracy of two dimensional classical string theory is removed in quantum mechanics. The system appears to be in a Kosterlitz-Thouless phase. This leads to the conclusion that it is also improbable to have only two large spacetime dimensions in string theory. However, we note that, unlike our argument for high dimensions, our conclusions about the ground state have neglected two dimensional quantum gravitational effects, and are at best incomplete.Comment: 12 pages, harvma

Dynamics and pattern formation in invasive tumor growth

In this work, we study the in-vitro dynamics of the most malignant form of the primary brain tumor: Glioblastoma Multiforme. Typically, the growing tumor consists of the inner dense proliferating zone and the outer less dense invasive region. Experiments with different types of cells show qualitatively different behavior. Wild-type cells invade a spherically symmetric manner, but mutant cells are organized in tenuous branches. We formulate a model for this sort of growth using two coupled reaction-diffusion equations for the cell and nutrient concentrations. When the ratio of the nutrient and cell diffusion coefficients exceeds some critical value, the plane propagating front becomes unstable with respect to transversal perturbations. The instability threshold and the full phase-plane diagram in the parameter space are determined. The results are in a good agreement with experimental findings for the two types of cells.Comment: 4 pages, 4 figure