8,245 research outputs found
Probability of Reflection by a Random Laser
A theory is presented (and supported by numerical simulations) for
phase-coherent reflection of light by a disordered medium which either absorbs
or amplifies radiation. The distribution of reflection eigenvalues is shown to
be the Laguerre ensemble of random-matrix theory. The statistical fluctuations
of the albedo (the ratio of reflected and incident power) are computed for
arbitrary ratio of sample thickness, mean free path, and absorption or
amplification length. On approaching the laser threshold all moments of the
distribution of the albedo diverge. Its modal value remains finite, however,
and acquires an anomalous dependence on the illuminated surface area.Comment: 8 pages (revtex), 3 figures, to appear in Phys.Rev.Let
Weyl node with random vector potential
We study Weyl semimetals in the presence of generic disorder, consisting of a
random vector potential as well as a random scalar potential. We derive
renormalization group flow equations to second order in the disorder strength.
These flow equations predict a disorder-induced phase transition between a
pseudo-ballistic weak-disorder phase and a diffusive strong-disorder phase for
sufficiently strong random scalar potential or for a pure three-component
random vector potential. We verify these predictions using a numerical study of
the density of states near the Weyl point and of quantum transport properties
at the Weyl point. In contrast, for a pure single-component random vector
potential the diffusive strong-disorder phase is absent.Comment: published version with minor change
Time-delay matrix, midgap spectral peak, and thermopower of an Andreev billiard
We derive the statistics of the time-delay matrix (energy derivative of the
scattering matrix) in an ensemble of superconducting quantum dots with chaotic
scattering (Andreev billiards), coupled ballistically to conducting modes
(electron-hole modes in a normal metal or Majorana edge modes in a
superconductor). As a first application we calculate the density of states
at the Fermi level. The ensemble average
deviates
from the bulk value by an amount depending on the
Altland-Zirnbauer symmetry indices . The divergent average for
in symmetry class D (, ) originates from the
mid-gap spectral peak of a closed quantum dot, but now no longer depends on the
presence or absence of a Majorana zero-mode. As a second application we
calculate the probability distribution of the thermopower, contrasting the
difference for paired and unpaired Majorana edge modes.Comment: 13 pages, 6 figure
Rectification of displacement currents in an adiabatic electron pump
Rectification of ac displacement currents generated by periodic variation of
two independent gate voltages of a quantum dot can lead to a dc voltage linear
in the frequency. The presence of this rectified displacement current could
account for the magnetic field symmetry observed in a recent measurement on an
adiabatic quantum electron pump by Switkes et al. [Science 283, 1905 (1999)].Comment: 2 pages, RevTeX; 1 figur
Participatory Scenario Generation: Communicating Usability Issues in Product Design through User Involvement in Scenario Generation\ud
Scenarios have proven to be a valuable tool in evaluating and communicating usability issues in consumer product design. Scenarios are explicit descriptions of hypothetical use situations. Realistic scenarios can serve as a valuable frame of reference to evaluate design solutions with regard to usability. To be able to achieve this required level of realism, involving users in scenario generation is essential. In this presentation we discuss how and where users can be involved in a scenario based product design process by means of examples of design projects that were executed by master students Industrial Design Engineering of the University of Twente. \ud
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We distinguish direct and indirect scenario generation. In direct scenario generation the user is actively involved in a participatory scenario generation session: the scenarios are created together with users. Indirect scenario generation is an approach in which scenarios are created by designers based on common analysis techniques like observations and interviews. These scenarios are then offered to users for confirmation. Both types of user involvement in scenario generation can be aimed at either current use scenarios which describe the current situation or future use scenarios which include a new product design. \ud
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The examples show that all strategies can be applied successfully to create realistic scenarios. Which strategy to choose depends among others upon risks and privacy issues, occurrence of infrequent events and availability of users. Furthermore, the variety of approaches shows that there is still a lot to explore with regard to benefits and limitations of the many techniques that can be applied in generating scenarios for consumer product design. We hope to contribute to this field by means of the research in our group and the work of students in the SBPD course\u
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