23 research outputs found
Implementation and testing of Lanczos-based algorithms for Random-Phase Approximation eigenproblems
No full textThe treatment of the Random-Phase Approximation Hamiltonians, encountered in different frameworks, like time-dependent density functional theory or Bethe-Salpeter equation, is complicated by their non-Hermicity. Compared to their Hermitian Hamiltonian counterparts, computational methods for the treatment of non-Hermitian Hamiltonians are often less efficient and less stable, sometimes leading to the breakdown of the method. Recently [Gruning et al. Nano Lett. 8 (2009) 28201, we have identified that such Hamiltonians are usually pseudo-Hermitian. Exploiting this property, we have implemented an algorithm of the Lanczos type for Random-Phase Approximation Hamiltonians that benefits from the same stability and computational load as its Hermitian counterpart, and applied it to the study of the optical response of carbon nanotubes. We present here the related theoretical grounds and technical details, and study the performance of the algorithm for the calculation of the optical absorption of a molecule within the Bethe-Salpeter equation framework. (C) 2011 Elsevier B.V. All rights reserved.</p
Exciton-Plasmon States in Nanoscale Materials: Breakdown of the Tamm-Dancoff Approximation
No full textWithin the Tamm-Dancoff approximation, ab initio approaches describe excitons as packets of electron-hole pairs propagating only forward in time. However, we show that in nanoscale materials excitons and plasmons hybridize, creating exciton-plasmon states where the electron-hole pairs oscillate back and forth in time. Then, as exemplified by the trans-azobenzene molecule and the carbon nanotubes, the Tamm-Dancoff approximation yields errors larger than the accuracy claimed in ab initio calculations. Instead, we propose a general and efficient approach that avoids the Tamm-Dancoff approximation, correctly describes excitons, plasmons, and exciton-plasmon states, and provides a good agreement with experimental results.</p
Effect of spatial nonlocality on the density functional band gap
No full textWe show that for a large class of exchange-correlation functionals the local exchange-correlation potential obtained within an optimized effective potential severely underestimates the band gap. On the other hand, the corresponding nonlocal potential obtained from a generalized Kohn-Sham scheme provides a much better description of the band gap, in good agreement with experiments. These results strongly indicate that a local exchange-correlation potential, however good the exchange-correlation approximation, cannot capture the delicate interplay between correlation effects and spatial localization in the KS band structure, unless the (cumbersome) contribution from the derivative discontinuity of the exchange-correlation energy functional is considered.</p
Quasiparticle calculations of the electronic properties of ZrO2 and HfO2 polymorphs and their interface with Si
No full textQuasiparticle calculations are performed to investigate the electronic band structures of various polymorphs of Hf and Zr oxides. The corrections with respect to density-functional-theory results are found to depend only weakly on the crystal structure. Based on these bulk calculations as well as those for bulk Si, the effect of quasiparticle corrections is also investigated for the band offsets at the interface between these oxides and Si assuming that the lineup of the potential at the interface is reproduced correctly within density-functional theory. On the one hand, the valence-band offsets are practically unchanged with a correction of a few tenths of electron volts. On the other hand, conduction-band offsets are raised by 1.3-1.5 eV. When applied to existing calculations for the offsets at the density-functional-theory level, our quasiparticle corrections provide results in good agreement with the experiment.</p
Crownwall Failure Analysis through Finite Element Method
Get PDFSeveral failures of recurved concrete crownwalls have been observed in recent years. This work aims to get a better insight within the processes underlying the loading phase of these structures due to non-breaking wave impulsive loading conditions and to identify the dominant failure modes. The investigation is carried out through an offline one-way coupling of computational fluid dynamics (CFD) generated wave pressure time series and a time-varying structural Finite Element Analysis. The recent failure of the Civitavecchia (Italy) recurved parapet is adopted as an explanatory case study. Modal analysis aimed to identify the main modal parameters such as natural frequencies, modal masses and modal shapes is firstly performed to comprehensively describe the dynamic response of the investigated structure. Following, the CFD generated pressure field time-series is applied to linear and non-linear finite element model, the developed maximum stresses and the development of cracks are properly captured in both models. Three non-linear analyses are performed in order to investigate the performance of the crownwall concrete class. Starting with higher quality concrete class, it is decreased until the formation of cracks is reached under the action of the same regular wave condition. It is indeed shown that the concrete quality plays a dominant role for the survivability of the structure, even allowing the design of a recurved concrete parapet without reinforcing steel bars.Coastal Engineerin
Density functionals from many-body perturbation theory: The band gap for semiconductors and insulators
Get PDFTheoretically the Kohn-Sham band gap differs from the exact quasiparticle energy gap by the derivative discontinuity of the exchange-correlation functional. In practice for semiconductors and insulators the band gap calculated within any local or semilocal density approximations underestimates severely the experimental energy gap. On the other hand, calculations with an "exact" exchange potential derived from many-body perturbation theory via the optimized effective potential suggest that improving the exchange-correlation potential approximation can yield a reasonable agreement between the Kohn-Sham band gap and the experimental gap. The results in this work show that this is not the case. In fact, we add to the exact exchange the correlation that corresponds to the dynamical (random phase approximation) screening in the GW approximation. This accurate exchange-correlation potential provides band structures similar to the local density approximation with the corresponding derivative discontinuity that contributes 30%-50% to the energy gap. Our self-consistent results confirm substantially the results for Si and other semiconductors obtained perturbatively [R. W. Godby , Phys. Rev. B 36, 6497 (1987)] and extend the conclusion to LiF and Ar, a wide-gap insulator and a noble-gas solid. (c) 2006 American Institute of Physics.</p
Optical properties of periodic systems within the current-current response framework: pitfalls and remedies
Get PDFWe compare the optical absorption of extended systems using the density-density and current-current linear response functions calculated within many-body perturbation theory. The two approaches are formally equivalent for a finite momentum q of the external perturbation. At q=0, however, the equivalence is maintained only if a small q expansion of the density-density response function is used. Moreover, in practical calculations, this equivalence can be lost if one naively extends the strategies usually employed in the density-based approach to the current-based approach. Specifically, we discuss the use of a smearing parameter or of the quasiparticle lifetimes to describe the finite width of the spectral peaks and the inclusion of electron-hole interaction. In those instances, we show that the incorrect definition of the velocity operator and the violation of the conductivity sum rule introduce unphysical features in the optical absorption spectra of three paradigmatic systems: silicon (semiconductor), copper (metal) and lithium fluoride (insulator). We then demonstrate how to correctly introduce lifetime effects and electron-hole interactions within the current-based approach
Curved concrete crownwalls on vertical breakwaters: Finite Element Analysis
No full textCrownwalls are often placed on top of vertical composite breakwaters to reduce overtopping. Adding a seawards facing overhang at the top of crownwalls has successfully reduced overtopping even further without increasing the freeboard of the crownwall. Crownwalls with a fully curved (FC) face are most often used in seawalls where they are subjected to wave load by breaking waves. However, it has recently become of interest to explore the possibility of using the fully curved shape for vertical breakwater crownwalls instead of a crownwall with a recurve (R). This thesis aimed to investigate further the wave loading by non-breaking waves and the dynamic response of a FC crownwall on a vertical breakwater through an offline one-way coupling of a CFD-generated pressure-time series and a FEM model in Diana FEA. The results were then compared to those of other studies on the wave load and dynamic response of a R crownwall. Pressure-time series from CFD numerical simulations of three wave states (W5, W6 and W7) were analysed to determine how the structure was loaded. The analysis of the pressure-time series showed that for the smallest wave state, W5, the pressure distribution at the moment of maximum total force was trapezoidal. However, for the larger wave states, W6 and W7, there was a pressure peak at the top of the curve of the crownwall due to the C-CI phenomenon. Waves with impulsive pressure and force impacts were found in all wave states, and the pressure and force impulses were much less variable than the maximum pressure and force values. Dynamic nonlinear analysis showed the fully curved crownwall cracks under wave load by wave states 6 and 7. Cracking occurs at the centre of the curve, where the largest tensile stresses are located. Comparing the results to those of the recurved crownwall, it was concluded that the fully curved crownwall was less favourable than the recurved crownwall. For the same wave states, the FC crownwall was subjected to larger wave forces than the R crownwall, which led to a larger bending moment exerted on the structure. The FC crownwall is also larger than the R crownwall, with the same freeboard, and therefore requires more use of concrete. Due to the shape of the FC crownwall, the maximum tensile stresses are 2.5-3 times higher than those in the R crownwall. More use of steel reinforcement bars is therefore also needed. It would, however, be interesting to investigate further FC crownwalls on vertical breakwaters with different geometries, e.g. radius of the curve, to see if their design can be improved. The applicability of the extended Goda method for calculating static wave load on recurved crownwalls was also investigated for the FC crownwall. It was found that for waves with wave steepness less than 6% and vertical wave velocity less than 2.4 m/s, the calculation method could be used for a preliminary design. However, the dynamic response of the structure must be addressed in the final design, e.g. with a dynamic analysis or the DAF method.Civil Engineering | Structural Engineering | Hydraulic Structure
La paradoja de la destrucción organizante
Get PDFThe author’s intention is to take Winnicott’s paradoxical ideas on aggression and des truction and combine them with the notions on the use of the object. Thus, the author intends to reflect upon moments that are essentiallydynamic, both of psychic structuring and of transference work and production. They are constitutive elements which determine ideas of loss and substitution included in “the use of the object”. Their high degree of abstraction enables us to place them in view of symbolisation as a nuclear part of neurosis. At the same time, for Winnicott, the ideas of health and sickness dependon how the “other one” responds to aggression and destruction. For want of, there stems the violence of helplessness, which leaves the person without symbolic and structuring resources. The author tries to approach Freud’s and Winnicott’s ideas on “destruction” in its vital function of “cutting out” (possession for Winnicott, seizure in Freud) and also assesses Freud’s idea on the death impulse as enabling life. This piece of work gives a special importance to Winnicott’s idea that the motion will or won’t be destructive depending on the answer received by the “other one”. This strongly reverts a geneticist view. It is an explanation on thefunction of the “other one” (his unconscious drive as “other one”) in his course to the impulse.Se propone tomar las ideas de Winnicott, inmersas en formulaciones paradojales, acerca de la agresión y destrucción, reunidas con sus nociones acerca del uso del objeto para pensarlos momentos esencialmente dinámicos, tanto de la estructuración psíquica como del trabajo y producción transferencial. Se trata de elementos constitutivos que determinan nociones de pérdida y sustitución contenidas en el \u27uso del objeto\u27 con un importante nivel de abstracción que nos permite ubicarlos en la perspectiva de la simbolización como elemento nuclear de la neurosis. A su vez Winnicott hace depender la noción de salud o enfermedad de la respuesta del otro a la agresión y la destructividad .Allí radica, en su defecto, la violencia del desamparo que deja al sujeto inerme para disponibilidades simbólicas y estructurantes. Se realizan algunos acercamientos conceptuales entre Freud y Winnicott en lo relativo a la función vital de corte de la \u27destrucción\u27,(posesión en Winnicott, apoderamiento en Freud), así como la valoración de la idea freudiana de la pulsión de muerte habilitando la vida. Se jerarquiza la idea winnicottiana acerca de que la moción será destructiva o no dependiendo de la respuesta del otro. Lo cual revierte fuertemente toda perspectiva genetista. Constituye una explicitación de la función del otro (ahora sí su deseo inconsciente como Otro) en el derrotero pulsional. Finalmente, con un breve rastreo del texto sobre la Tendencia antisocial se objetivan articulaciones teórico-clínicas a propósito de una pequeña con conductas de robo que recrea en transferencia
