1,661 research outputs found
Confining massless Dirac particles in two-dimensional curved space
Dirac particles have been notoriously difficult to confine. Implementing a
curved space Dirac equation solver based on the quantum Lattice Boltzmann
method, we show that curvature in a 2-D space can confine a portion of a
charged, mass-less Dirac fermion wave-packet. This is equivalent to a finite
probability of confining the Dirac fermion within a curved space region. We
propose a general power law expression for the probability of confinement with
respect to average spatial curvature for the studied geometry.Comment: 10 pages 8 figure
Time-resolved oxygen production by PSII: chasing chemical intermediates
AbstractPhotosystem II (PSII) produces dioxygen from water in a four-stepped process, which is driven by four quanta of light and catalysed by a Mn-cluster and tyrosine Z. Oxygen is liberated during one step, coined S3⇒S0. Chemical intermediates on the way from reversibly bound water to dioxygen have not yet been tracked, however, a break in the Arrhenius plot of the oxygen-evolving step has been taken as evidence for its existence.We scrutinised the temperature dependence of (i) UV-absorption transients attributable to the reduction of the Mn-cluster and tyrosine Z by water, and (ii) polarographic transients attributable to the release of dioxygen. Using a centrifugatable and kinetically competent Pt-electrode, we observed no deviation from a linear Arrhenius plot of oxygen release in the temperature range from −2 to 32 °C, and hence no evidence, by this approach, for a sufficiently long-lived chemical intermediate. The half-rise times of oxygen release differed between Synechocystis WT* (at 20 °C: 1.35 ms) and a point mutant (D1–D61N: 13.1 ms), and the activation energies differed between species (Spinacia oleracea, 30 kJ/mol versus Synechocystis, 41 kJ/mol) and preparations (PSII membranes, 41 kJ/mol versus core complexes, 33 kJ/mol, Synechocystis).Correction for polarographic artefacts revealed, for the first time, a temperature-dependent lag-phase of the polarographic transient (duration at 20 °C: 0.45 ms, activation energy: 31 kJ/mol), which was indicative of a short-lived intermediate. It was, however, not apparent in the UV-transients. Thus the “intermediate” was probably newly formed and transiently bound oxygen
Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment
The exciton spin dynamics are investigated both experimentally and
theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect
band gap and a type-II band alignment. The magnetic-field-induced circular
polarization of photoluminescence, , is studied as function of the
magnetic field strength and direction as well as sample temperature. The
observed nonmonotonic behaviour of these functions is provided by the interplay
of bright and dark exciton states contributing to the emission. To interpret
the experiment, we have developed a kinetic master equation model which
accounts for the dynamics of the spin states in this exciton quartet, radiative
and nonradiative recombination processes, and redistribution of excitons
between these states as result of spin relaxation. The model offers
quantitative agreement with experiment and allows us to evaluate, for the
studied structure, the heavy-hole factor, , and the spin
relaxation times of electron, s, and hole, s, bound in the exciton.Comment: 17 pages, 16 figure
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