2,583 research outputs found
Thermodynamic basis of the concept of "recombination resistances"
The concept of "recombination resistance" introduced by Shockley and Read
(Phys. Rev. 87, 835 (1952)) is discussed within the framework of the
thermodynamics of irreversible processes ruled by the principle of the minimum
rate of entropy production. It is shown that the affinities of recombination
processes represent "voltages" in a thermodynamic Ohm-like law where the net
rates of recombinations represent the "currents". The quantities thus found
allow for the definition of the "dissipated power" which is to be related to
the rate of entropy production of the recombination processes dealt with.Comment: Submitted to Phys. Rev.
From SICs and MUBs to Eddington
This is a survey of some very old knowledge about Mutually Unbiased Bases
(MUB) and Symmetric Informationally Complete POVMs (SIC). In prime dimensions
the former are closely tied to an elliptic normal curve symmetric under the
Heisenberg group, while the latter are believed to be orbits under the
Heisenberg group in all dimensions. In dimensions 3 and 4 the SICs are
understandable in terms of elliptic curves, but a general statement escapes us.
The geometry of the SICs in 3 and 4 dimensions is discussed in some detail.Comment: 12 pages; from the Festschrift for Tony Sudber
Analytical calculation of the excess current in the OTBK theory
We present an analytical derivation of the excess current in Josephson
junctions within the Octavio-Tinkham-Blonder-Klapwijk theory for both symmetric
and asymmetric barrier strengths. We confirm the result found numerically by
Flensberg et al. for equal barriers [Phys. Rev. B 38, 8707 (1988)], including
the prediction of negative excess current for low transparencies, and we
generalize it for differing barriers. Our analytical formulae provide for
convenient fitting of experimental data, also in the less studied, but
practically relevant case of the barrier asymmetry.Comment: 13 pages, 3 figures, submitted to Superconductor Science and
Technolog
Collective Singlet Excitations and Evolution of Raman Spectral Weights in the 2D Spin Dimer Compound SrCu2(BO3)2
We present a Raman light scattering study of the two-dimensional quantum spin
system SrCu2(BO3)2 and show that the magnetic excitation spectrum has a rich
structure, including several well-defined bound state modes at low temperature,
and a scattering continuum and quasielastic light scattering contributions at
high temperature. The key to the understanding of the unique features of
SrCu2(BO3)2 is the presence of strong interactions between well-localized
triplet excitations in the network of orthogonal spin dimers realized in this
compound. Based on our analysis of the Heisenberg model relevant for this
material, we argue that the collective excitations involving two and
three-particle singlet bound states have large binding energies and are
observed as well-defined peaks in the Raman spectrum.Comment: 5 pages, 2 figures. Revised version, to appear in Phys. Rev. Lett.
(2000
Cohomogeneity one manifolds and selfmaps of nontrivial degree
We construct natural selfmaps of compact cohomgeneity one manifolds with
finite Weyl group and compute their degrees and Lefschetz numbers. On manifolds
with simple cohomology rings this yields in certain cases relations between the
order of the Weyl group and the Euler characteristic of a principal orbit. We
apply our construction to the compact Lie group SU(3) where we extend identity
and transposition to an infinite family of selfmaps of every odd degree. The
compositions of these selfmaps with the power maps realize all possible degrees
of selfmaps of SU(3).Comment: v2, v3: minor improvement
Field screening of biofumigant species for the reduction of potato cyst nematodes (Globodera sp.)
Summary Potato cyst nematodes (PCN) Globodera pallida and G. rostochiensis are the most problematic potato pests in the UK. Management of PCN is often reliant on use of nematicides; however, pesticide registration and approval is becoming increasingly stringent and the likelihood of nematicides being retracted from commercial use more probable. Biofumigation represents a potential alternative. The process involves growing a green manure for maceration and incorporation into soil whereby intracellular glucosinolate (GSL) molecules are hydrolyzed to pesticidal volatiles such as isothiocyanate gases. The experiment detailed in this paper represents one of the first studies to investigate biofumigant species and blends for efficacy against PCN in-field, and for varietal suitability to UK conditions. Statistical differences (P<0.001) in PCN egg viability were found between the untreated control and biofumigant treatments, but no difference between biofumigant treatments was recorded. Overall, biofumigation was shown to reduce encysted egg viability by approximately 42%. Additionally, fresh (P=0.008) and dry weight (P=0.044) biomass was found to be variable between biofumigant species and blends. All biofumigants were found to be suitable for UK conditions when grown between September and November
How should we measure psychological resilience in sport performers?
Psychological resilience is important in sport because athletes must constantly withstand a wide range of pressures to attain and sustain high performance. To advance psychologistsĂąâŹâą understanding of this area, there exists an urgent need to develop a sport-specific measure of resilience. The purpose of this paper is to review psychometric issues in resilience research and to discuss the implications for sport psychology. Drawing on the wider general psychology literature to inform the discussion, the narrative is divided into three main sections relating to resilience and its assessment: adversity, positive adaptation, and protective factors. The first section reviews the different ways that adversity has been measured and considers the potential problems of using items with varying degrees of controllability and risk. The second section discusses the different approaches to assessing positive adaptation and examines the issue of circularity pervasive in resilience research. The final section explores the various issues related to the assessment of protective factors drawing directly from current measures of resilience in other psychology sub-disciplines. The commentary concludes with key recommendations for sport psychology researchers seeking to develop a measure of psychological resilience in athletes
Links between topography, wind, deflation, lakes and dust: The case of the Bodélé Depression, Chad
The Bodélé Depression, Chad is the planet's largest single source of dust. Deflation from the Bodélé could be seen as a simple coincidence of two key prerequisites: strong surface winds and a large source of suitable sediment. But here we hypothesise that long term links between topography, winds, deflation and dust ensure the maintenance of the dust source such that these two apparently coincidental key ingredients are connected by land-atmosphere processes with topography acting as the overall controlling agent. We use a variety of observational and numerical techniques, including a regional climate model, to show that: 1) contemporary deflation from the Bodélé is delineated by topography and a surface wind stress maximum; 2) the Tibesti and Ennedi mountains play a key role in the generation of the erosive winds in the form of the Bodélé Low Level Jet (LLJ); 3) enhanced deflation from a stronger Bodélé LLJ during drier phases, for example, the Last Glacial Maximum, was probably sufficient to create the shallow lake in which diatoms lived during wetter phases, such as the Holocene pluvial. Winds may therefore have helped to create the depression in which erodible diatom material accumulated. Instead of a simple coincidence of nature, dust from the world's largest source may result from the operation of long term processes on paleo timescales which have led to ideal conditions for dust generation in the world's largest dust source. Similar processes plausibly operate in other dust hotspots in topographic depressions
The tunnel magnetoresistance in chains of quantum dots weakly coupled to external leads
We analyze numerically the spin-dependent transport through coherent chains
of three coupled quantum dots weakly connected to external magnetic leads. In
particular, using the diagrammatic technique on the Keldysh contour, we
calculate the conductance, shot noise and tunnel magnetoresistance (TMR) in the
sequential and cotunneling regimes. We show that transport characteristics
greatly depend on the strength of the interdot Coulomb correlations, which
determines the spacial distribution of electron wave function in the chain.
When the correlations are relatively strong, depending on the transport regime,
we find both negative TMR as well as TMR enhanced above the Julliere value,
accompanied with negative differential conductance (NDC) and super-Poissonian
shot noise. This nontrivial behavior of tunnel magnetoresistance is associated
with selection rules that govern tunneling processes and various high-spin
states of the chain that are relevant for transport. For weak interdot
correlations, on the other hand, the TMR is always positive and not larger than
the Julliere TMR, although super-Poissonian shot noise and NDC can still be
observed
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