17,642 research outputs found
Optical second harmonic generation from Wannier excitons
Excitonic effects in the linear optical response of semiconductors are
well-known and the subject of countless experimental and theoretical studies.
For the technologically important second order nonlinear response, however,
description of excitonic effects has proved to be difficult. In this work, a
simplified three-band Wannier exciton model of cubic semiconductors is applied
and a closed form expression for the complex second harmonic response function
including broadening is derived. Our calculated spectra are found to be in
excellent agreement with the measured response near the band edge. In addition,
a very substantial enhancement of the nonlinear response is predicted for the
transparency region
Conditional expectations associated with quantum states
An extension of the conditional expectations (those under a given subalgebra
of events and not the simple ones under a single event) from the classical to
the quantum case is presented. In the classical case, the conditional
expectations always exist; in the quantum case, however, they exist only if a
certain weak compatibility criterion is satisfied. This compatibility criterion
was introduced among others in a recent paper by the author. Then,
state-independent conditional expectations and quantum Markov processes are
studied. A classical Markov process is a probability measure, together with a
system of random variables, satisfying the Markov property and can equivalently
be described by a system of Markovian kernels (often forming a semigroup). This
equivalence is partly extended to quantum probabilities. It is shown that a
dynamical (semi)group can be derived from a given system of quantum observables
satisfying the Markov property, and the group generators are studied. The
results are presented in the framework of Jordan operator algebras, and a very
general type of observables (including the usual real-valued observables or
self-adjoint operators) is considered.Comment: 10 pages, the original publication is available at http://www.aip.or
Bubble coalescence in breathing DNA: Two vicious walkers in opposite potentials
We investigate the coalescence of two DNA-bubbles initially located at weak
segments and separated by a more stable barrier region in a designed construct
of double-stranded DNA. The characteristic time for bubble coalescence and the
corresponding distribution are derived, as well as the distribution of
coalescence positions along the barrier. Below the melting temperature, we find
a Kramers-type barrier crossing behaviour, while at high temperatures, the
bubble corners perform drift-diffusion towards coalescence. The results are
obtained by mapping the bubble dynamics on the problem of two vicious walkers
in opposite potentials.Comment: 7 pages, 4 figure
Volume-energy correlations in the slow degrees of freedom of computer-simulated phospholipid membranes
Constant-pressure molecular-dynamics simulations of phospholipid membranes in
the fluid phase reveal strong correlations between equilibrium fluctuations of
volume and energy on the nanosecond time-scale. The existence of strong
volume-energy correlations was previously deduced indirectly by Heimburg from
experiments focusing on the phase transition between the fluid and the ordered
gel phases. The correlations, which are reported here for three different
membranes (DMPC, DMPS-Na, and DMPSH), have volume-energy correlation
coefficients ranging from 0.81 to 0.89. The DMPC membrane was studied at two
temperatures showing that the correlation coefficient increases as the phase
transition is approached
Einstein-Weyl structures corresponding to diagonal K\"ahler Bianchi IX metrics
We analyse in a systematic way the four dimensionnal Einstein-Weyl spaces
equipped with a diagonal K\"ahler Bianchi IX metric. In particular, we show
that the subclass of Einstein-Weyl structures with a constant conformal scalar
curvature is the one with a conformally scalar flat - but not necessarily
scalar flat - metric ; we exhibit its 3-parameter distance and Weyl one-form.
This extends previous analysis of Pedersen, Swann and Madsen , limited to the
scalar flat, antiself-dual case. We also check that, in agreement with a
theorem of Derdzinski, the most general conformally Einstein metric in the
family of biaxial K\"ahler Bianchi IX metrics is an extremal metric of Calabi,
conformal to Carter's metric, thanks to Chave and Valent's results.Comment: 15 pages, Latex file, minor modifications, to be published in Class.
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Integration of hydrothermal liquefaction and carbon capture and storage for the production of advanced liquid biofuels with negative CO2 emissions
The technical and economic feasibility to deliver sustainable liquid biocrude
through hydrothermal liquefaction (HTL) while enabling negative carbon dioxide
emissions is evaluated in this paper, looking into the potential of the process
in the context of negative emission technologies (NETs) for climate change
mitigation. In the HTL process, a gas phase consisting mainly of carbon dioxide
is obtained as a side product driving a potential for the implementation of
carbon capture and storage in the process (BECCS) that has not been explored
yet in the existing literature and is undertaken in this study. To this end,
the process is divided in a standard HTL base and a carbon capture add-on,
having forestry residues as feedstock. The Selexol technology is adapted in a
novel scheme to simultaneously separate the CO2 from the HTL gas and recover
the excess hydrogen for biocrude upgrading. The cost evaluation indicates that
the additional cost of the carbon capture can be compensated by revenues from
the excess process heat and the European carbon allowance market. The impact in
the MFSP of the HTL base case ranges from -7% to 3%, with -15% in the most
favorable scenario, with a GHG emissions reduction potential of 102-113%
compared to the fossil baseline. These results show that the implementation of
CCS in the HTL process is a promising alternative from technical, economic and
environmental perspective in future scenarios in which advanced liquid biofuels
and NETs are expected to play a role in the decarbonization of the energy
system
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