4,000 research outputs found
Carrier multiplication yields in PbS and PbSe nanocrystals measured by transient photoluminescence
We report here an assessment of carrier multiplication (CM) yields in PbSe
and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton
dynamics in transient photoluminescence decays. Interest in CM, the generation
of more than one electron and hole in a semiconductor after absorption of one
photon, has renewed in recent years because of reports suggesting greatly
increased efficiencies in nanocrystalline materials compared to the bulk form,
in which CM was otherwise too weak to be of consequence in photovoltaic energy
conversion devices. In our PbSe and PbS NC samples, however, we estimate using
transient photoluminescence that at most 0.25 additional e-h pairs are
generated per photon even at energies hv > 5Eg, instead of the much higher
values reported in the literature. We argue by comparing NC CM estimates and
reported bulk values on an absolute energy basis, which we justify as
appropriate on physical grounds, that the data reported thus far are
inconclusive with respect to the importance of nanoscale-specific phenomena in
the CM process.Comment: 10 pages, 7 figure
Coordinate Singularities in Harmonically-sliced Cosmologies
Harmonic slicing has in recent years become a standard way of prescribing the
lapse function in numerical simulations of general relativity. However, as was
first noticed by Alcubierre (1997), numerical solutions generated using this
slicing condition can show pathological behaviour. In this paper, analytic and
numerical methods are used to examine harmonic slicings of Kasner and Gowdy
cosmological spacetimes. It is shown that in general the slicings are prevented
from covering the whole of the spacetimes by the appearance of coordinate
singularities. As well as limiting the maximum running times of numerical
simulations, the coordinate singularities can lead to features being produced
in numerically evolved solutions which must be distinguished from genuine
physical effects.Comment: 21 pages, REVTeX, 5 figure
General approach for studying first-order phase transitions at low temperatures
By combining different ideas, a general and efficient protocol to deal with
discontinuous phase transitions at low temperatures is proposed. For small
's, it is possible to derive a generic analytic expression for appropriate
order parameters, whose coefficients are obtained from simple simulations. Once
in such regimes simulations by standard algorithms are not reliable, an
enhanced tempering method, the parallel tempering -- accurate for small and
intermediate system sizes with rather low computational cost -- is used.
Finally, from finite size analysis, one can obtain the thermodynamic limit. The
procedure is illustrated for four distinct models, demonstrating its power,
e.g., to locate coexistence lines and the phases density at the coexistence.Comment: 5 page
Non-Hermitian oscillator Hamiltonian and su(1,1): a way towards generalizations
The family of metric operators, constructed by Musumbu {\sl et al} (2007 {\sl
J. Phys. A: Math. Theor.} {\bf 40} F75), for a harmonic oscillator Hamiltonian
augmented by a non-Hermitian -symmetric part, is re-examined in the
light of an su(1,1) approach. An alternative derivation, only relying on
properties of su(1,1) generators, is proposed. Being independent of the
realization considered for the latter, it opens the way towards the
construction of generalized non-Hermitian (not necessarily -symmetric)
oscillator Hamiltonians related by similarity to Hermitian ones. Some examples
of them are reviewed.Comment: 11 pages, no figure; changes in title and in paragraphs 3 and 5;
final published versio
Lifshitz-type formulas for graphene and single-wall carbon nanotubes: van der Waals and Casimir interations
Lifshitz-type formulas are obtained for the van der Waals and Casimir
interaction between graphene and a material plate, graphene and an atom or a
molecule, and between a single-wall carbon nanotube and a plate. The reflection
properties of electromagnetic oscillations on graphene are governed by the
specific boundary conditions imposed on the infinitely thin positively charged
plasma sheet, carrying a continuous fluid with some mass and charge density.
The obtained formulas are applied to graphene interacting with Au and Si
plates, to hydrogen atoms and molecules interacting with graphene, and to
single-wall carbon nanotubes interacting with Au and Si plates. The
generalizations to more complicated carbon nanostructures are discussed.Comment: 11 pages, 5 figures, 2 tables; to appear in Phys. Rev. B; misprints
in Eqs.(33) and (34) are correcte
Moyal products -- a new perspective on quasi-hermitian quantum mechanics
The rationale for introducing non-hermitian Hamiltonians and other
observables is reviewed and open issues identified. We present a new approach
based on Moyal products to compute the metric for quasi-hermitian systems. This
approach is not only an efficient method of computation, but also suggests a
new perspective on quasi-hermitian quantum mechanics which invites further
exploration. In particular, we present some first results which link the Berry
connection and curvature to non-perturbative properties and the metric.Comment: 14 pages. Submitted to J Phys A special issue on The Physics of
Non-Hermitian Operator
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