6,125 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
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
Fluctuating "order parameter" for a quantum chaotic system with partially broken time-reversal symmetry
The functional defined as the squared modulus of the spatial average of the
wave function squared, plays the role of an ``order parameter'' for the
transition between Hamiltonian ensembles with orthogonal and unitary symmetry.
Upon breaking time-reversal symmetry, the order parameter crosses over from one
to zero. We compute its distribution in the crossover regime and find that it
has large fluctuations around the ensemble average. These fluctuations imply
long-range spatial correlations in the eigenfunction and non-Gaussian
perturbations of eigenvalues, in precise agreement with results by Fal'ko and
Efetov and by Taniguchi, Hashimoto, Simons, and Altshuler. As a third
implication of the order-parameter fluctuations we find correlations in the
response of an eigenvalue to independent perturbations of the system.Comment: 4 pages, REVTeX-3.0, 1 figure. Reference added to Y. V. Fyodorov and
A. D. Mirlin, Phys. Rev. B 51, 13403 (1995
Ehrenfest-time dependence of weak localization in open quantum dots
Semiclassical theory predicts that the weak localization correction to the
conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time
exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys.
Rev. B {\bf 54}, 14424 (1996)]. We report numerical simulations of weak
localization in the open quantum kicked rotator that confirm this prediction.
Our results disagree with the `effective random matrix theory' of transport
through ballistic chaotic cavities.Comment: 4 pages, 2 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
Crossover from weak localization to weak antilocalization in a disordered microbridge
We calculate the weak localization correction in the double crossover to
broken time-reversal and spin-rotational symmetry for a disordered microbridge
or a short disordered wire using a scattering-matrix approach. Whereas the
correction has universal limiting values in the three basic symmetry classes,
the functional form of the magnetoconductance is affected by eventual
non-homogeneities in the microbridge.Comment: 5 pages, RevTeX; 2 figure
Finite size effects and localization properties of disordered quantum wires with chiral symmetry
Finite size effects in the localization properties of disordered quantum
wires are analyzed through conductance calculations. Disorder is induced by
introducing vacancies at random positions in the wire and thus preserving the
chiral symmetry. For quasi one-dimensional geometries and low concentration of
vacancies, an exponential decay of the mean conductance with the wire length is
obtained even at the center of the energy band. For wide wires, finite size
effects cause the conductance to decay following a non-pure exponential law. We
propose an analytical formula for the mean conductance that reproduces
accurately the numerical data for both geometries. However, when the
concentration of vacancies increases above a critical value, a transition
towards the suppression of the conductance occurs.
This is a signature of the presence of ultra-localized states trapped in
finite regions of the sample.Comment: 5 figures, revtex
Measuring the value of life: Exploring a new method for deriving the monetary value of a QALY
Economic evaluations of new health technologies now typically produce an incremental cost per Quality Adjusted Life Year (QALY) value. The QALY is a measure of health benefit that combines length of life with quality of life, where quality of life is assessed on a scale where zero represents a health state equivalent to being dead and one represents full health. The challenge for decision makers, such as the Treasury, is to determine the appropriate size of the healthcare budget. Bodies such as the National Institute for Health and Clinical Excellent (NICE) in the U.K. must then determine how much they can afford to pay for a gain of one QALY, while operating under this fixed budget. While there is no fixed cost-effectiveness threshold and each intervention is assessed on a case by case basis, under normal circumstances the threshold will not be below £20,000 and not above £30,000 per QALY.
Recent research has sought to determine the monetary value individuals place on a QALY to inform the size of the healthcare budget and the level of the cost-effectiveness threshold. This research has predominantly used Willingness to Pay (WTP) approaches. However, WTP has a number of known problems, most notably its insensitivity to scope. In this paper we present an alternative approach to estimating the monetary value of a QALY (MVQ), which is based upon a Time Trade Off (TTO) exercise of income with health held constant at perfect health. We present the methods and theory underlying this experimental approach and some results from an online feasibility study in the Netherlands
The impact of losses in income due to ill health: does the EQ-5D reflect lost earnings?
Two key questions in the context of UK health policy are: do the published preference indices for EQ-5D reflect the impact of lost earnings? Are we currently implicitly including indirect costs in our analyses? It is crucial to investigate whether or not individuals take into account any possible impact of lost income in health state valuation exercises. If respondents do consider income effects, and these considerations change valuations, then these effects would need to be excluded both under the current NICE reference case, or where productivity costs are included in the numerator to avoid double counting. This study adapts the study design used to generate population value sets for EQ-5D, as first used in the Measurement and Valuation of Health (MVH) Study, and carries out valuations of hypothetical EQ-5D states using Time Trade Off (TTO) exercises through an online survey administered in the Netherlands. Furthermore, this study uses a number of different TTO questions to explore the impact of losses in income on the valuation of hypothetical health states, and to determine the relationship between income and health.EQ-5D; time trade-off; health-related loss of income
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