34 research outputs found
Intriguing electron correlation effects in the photoionization of metallic quantum--dot nanorings
We report detailed results on ionization in metallic quantum--dot (QD)
nanorings described by the extended Hubbard model at half filling obtained by
exact numerical diagonalization. In spite of very strong electron correlations,
the ionization spectra are astonishingly scarce. We attribute this scarcity to
a hidden quasi--symmetry, generalizing thereby similar results on optical
absorption recently reported [I. Baldea and L. S. Cederbaum, \prb {\bf 75},
125323 (2007); {\bf 77}, 165339 (2008)]. Numerical results indicate that this
hidden quasi--symmetry of the extended Hubbard model does not evolve into a
true (hidden) symmetry but remains a quasi--symmetry in the case of the
restricted Hubbard model as well. Based on the observation on the number of
significant ionization signals per each spatial symmetry, we claim the
existence of a one--to--one map between the relevant ionization signals of the
correlated half-filled nanorings and the one-hole and two-hole--one-particle
processes possible in the noninteracting case. Similar to the case of optical
absorption, numerous avoided crossings (anticrossings) are present in the
ionization spectra, which often involve more than two states. The present
results demonstrate that ionization could be a useful tool to study electron
correlations in metallic QD--nanoarrays, providing information that is
complementary to optical absorption
Review of biorthogonal coupled cluster representations for electronic excitation
Single reference coupled-cluster (CC) methods for electronic excitation are
based on a biorthogonal representation (bCC) of the (shifted) Hamiltonian in
terms of excited CC states, also referred to as correlated excited (CE) states,
and an associated set of states biorthogonal to the CE states, the latter being
essentially configuration interaction (CI) configurations. The bCC
representation generates a non-hermitian secular matrix, the eigenvalues
representing excitation energies, while the corresponding spectral intensities
are to be derived from both the left and right eigenvectors. Using the
perspective of the bCC representation, a systematic and comprehensive analysis
of the excited-state CC methods is given, extending and generalizing previous
such studies. Here, the essential topics are the truncation error
characteristics and the separability properties, the latter being crucial for
designing size-consistent approximation schemes. Based on the general order
relations for the bCC secular matrix and the (left and right) eigenvector
matrices, formulas for the perturbation-theoretical (PT) order of the
truncation errors (TEO) are derived for energies, transition moments, and
property matrix elements of arbitrary excitation classes and truncation levels.
In the analysis of the separability properties of the transition moments, the
decisive role of the so-called dual ground state is revealed. Due to the use of
CE states the bCC approach can be compared to so-called intermediate state
representation (ISR) methods based exclusively on suitably orthonormalized CE
states. As the present analysis shows, the bCC approach has decisive advantages
over the conventional CI treatment, but also distinctly weaker TEO and
separability properties in comparison with a full (and hermitian) ISR method
DMFC as Battery-Extender in solar-boat application
For special applications Direct Methanol Fuel Cells (DMFC) are close to commercial application or already commercialized today. However for the step from laboratory to a broader market of fuel cells, a significant cost reduction, as well as a lifetime and power density improvement of the systems is needed. The Goals of the BZ-BattExt Project should be reached by applying new knowledge in alternative materials, improved operation strategies and enhanced sub systems.
In the project a 100 W DMFC compact system as battery extender was successfully developed and operated. The reduction of the number of components and the simplification of the system led to a high reduction in price, weight and volume. The feasibility of a micro-DMFC system was evaluated which enables a minimised system periphery due to an improved
System Architecture. For this, alternative materials and functional components were developed and investigated leading to new membranes with reduced water and methanol permeation allowing a low air stoich operation and higher system efficiency. Gas diffusion layers of various compositions were tested and optimised materials were selected. New sealing materials with good methanol stability and optimized processibility in commercial production
Processes were developed and the MEA preparation was adapted to the new materials. The use of a simple, cost-effective way of stack production was demonstrated for DMFC use. Using this new components and materials, coupled with the enhanced subsystem architectures and enhanced operation strategies, the build up and start-up of an improved micro DMFC System was achieved. The technical feasibility of the Results was investigated in the real application. The micro DMFC System was used as a battery range extender in a 6m solar boat. The DMFC fuel cell system serves as a basis for an efficient, compact and cost effective alternative for battery- or battery-extender systems and can fulfil a broad variety of applications
BZ-BattExt â DMFC as Battery-Extender in solar-boat application
For special applications Direct Methanol Fuel Cells (DMFC) are close to commercial application or already commercialized today. However for the step from laboratory to a broader market of fuel cells, a significant cost reduction, as well as an improvement in life time and power density of the systems is needed. These items were the focus of the project BZ-BattExt, to be reached by new knowledge in alternative materials, operation strategies as also the realisation of enhanced sub systems. This project is funded by the German Federal Ministry of Education and Research in the program of Micro fuel cells. In the project the feasibility of a micro-DMFC system is evaluated which enables a minimised system periphery due to an improved system architecture. For this, alternative materials and functional components were developed and investigated. New membranes with reduced water and methanol permeation allow operation at low air stoichiometry and favourable system efficiency. Gas diffusion layers of various compositions were tested and optimised material was selected. New sealing materials with good methanol stability and optimized processibility in commercial production process were developed. MEA preparation was adapted to the new materials. The use of a simple, cost-effective way of stack production was demonstrated for DMFC use. The investigation and construction of enhanced subsystems and operation strategies, which enable and optimise the use of new components and materials, as also the realisation of the micro-DMFC system is a focus of the project. The technical feasibility of the results is investigated in the application, which means it is tested as battery extender of a solar boat. The DMFC fuel cell system serves as a basis for an efficient, compact and cost effective alternative for battery- or battery-extender systems and can fulfil a broad variety of applications
Finite-temperature second-order many-body perturbation theory revisited
We present an algebraic, nondiagrammatic derivation of finite-temperature second-order many-body perturbation theory [FT-MBPT(2)], using techniques and concepts accessible to theoretical chemical physicists. We give explicit expressions not just for the grand potential but particularly for the mean energy of an interacting many-electron system. The framework presented is suitable for computing the energy of a finite or infinite system in contact with a heat and particle bath at finite temperature and chemical potential. FT-MBPT(2) may be applied if the system, at zero temperature, may be described using standard (i.e., zero-temperature) second-order many-body perturbation theory [ZT-MBPT(2)] for the energy. We point out that in such a situation, FT-MBPT(2) reproduces, in the zero-temperature limit, the energy computed within ZT-MBPT(2). In other words, the difficulty that has been referred to as the KohnâLuttinger conundrum, does not occur. We comment, in this context, on a ârenormalizationâ scheme recently proposed by Hirata and He
Fuel Cell System for Solar Boat Battery-Extender - BZ-BattExt -
For special applications Direct Methanol Fuel Cells (DMFC) are close to commercial application or already commercialized today. However for the step from laboratory to a broader market of fuel cells, significant cost reduction, as also an improvement in life time and power density of the systems is needed. These items were the focus of the project BZ-BattExt, to be reached by new knowledge in alternative materials, operation strategies as also the realisation of enhanced sub systems. This project is funded by the German Federal Ministry of Education and Research in the program of Micro fuel cells. In the project the feasibility of a micro-DMFC-system is evaluated which enables a minimised system periphery due to an improved system architecture. For this, alternative materials and functional components were developed and investigated. New membranes with reduced water and methanol permeation allow operation at low air stoichiometry, favourable for system efficiency. Gas diffusion layers of various compositions were tested and optimised material selected. New sealing materials with good methanol stability and optimized processibility in commercial production process were developed. MEA preparation was adapted to the new materials. The use of a simple, cost-effective way of stack production was demonstrated for DMFC use. The investigation and construction of enhanced subsystems and operation strategies, which enable and optimise the use of new components and materials, as also the realisation of the micro-DMFC-system is a focus of the project. The technical feasibility of the results is investigated application oriented, which means it is tested as battery extender of a solar boat. The fuel cell system serves as a basis for an efficient, compact and cost effective micro-DMFC-System as alternative for battery- or battery-extender systems which covers a broad variety of applications
Core Relaxation Effects in Molecular Photoionization
Ionization of K-shell or, more generally, of deep inner-shell electrons in atoms and molecules is accompanied by a considerable rearrangement of the valence (outer-shell) electrons in response to the reduced shielding of the nuclear attraction.(1) This adjustment of the valence electrons, referred to as electronic relaxation, leads to a significant energy lowering of the final ionic state relative to a state where the valence electron distribution of the initial state is maintained (âfrozenâ). The magnitude of this relaxation energy scales with the number of valence electrons. In the case of the K-shell ionization of second-row atoms (Z = 3â10), for example, the relaxation energies (in eV) are approximately given by Î ^R(Z) = 3.1 (Z â 2.2). In a molecular environment the corresponding relaxation energies are typically 2â3 eV larger than the values for the free atom. Relaxation not only plays a role in the ionic core but also affects the motion of the outgoing photoelectron. The relaxation of the valence electrons, being essentially a contraction of the valence charge distribution, quite effectively screens the inner-shell hole potential experienced by the photoelectron. This means that the potential of the relaxed ionic core is less attractive than its unrelaxed (frozen) counterpart. As a consequence, resonances in the photoionization cross section will appear at higher energy for a relaxed core than for an unrelaxed (frozen) core. Concomitantly with the shift to higher energy, the resonance peaks will be lowered and broadened as a result of relaxation