Psi-k - Ab initio (from electronic structure) calculation of complex processes in materials


Theory and Modeling of Electronic Excitations in Nanoscience

1) Title: Theory and Modeling of Electronic Excitations in Nanoscience

Purpose:
We aim to organize a three-day conference devoted to the theory and modeling of electronic excitations in nanostructured materials, in order to assess the latest achievements and the perspectives for first-principles theoretical methods in this field. The workshop will offer the opportunity to discuss open questions, to compare the drawbacks and advantages of different approaches, and to present pioneering applications to real materials of current experimental interest. In particular, we plan to give emphasis to recent successful simulations in the field of nanoscience (nanotubes, quantum dots/wells, superlattices, surfaces, molecular electronics, bio-electronics and interdisciplinary fields like medical applications) as well as to theoretical advances related, for instance, to the efficient and accurate solution of the Bethe-Salpeter equation for optical spectra.

The Organizers are:

Dr. Olivia Pulci () (corresponding organizer)
Dipartimento di Fisica, Universita' di Roma Tor Vergata, Via della
Ricerca Scientifica 1, 00133 Roma, Italy

Dr. Pablo Garcia-Gonzalez ()
Departamento de Fisica de la Materia Condensada,
Universidad Autonoma de Madrid, E-28049 Cantoblanco, Spain

Dr. Maurizia Palummo ()
Dipartimento di Fisica, Universita' di Roma Tor Vergata,
Via della Ricerca Scientifica 1, 00133 Roma, Italy

Dr. Arno Schindlmayr ()
Institut fuer Festkoerperforschung,
Forschungzentrum Juelich, 52425 Juelich, Germany

Dr. Nathalie Vast ()
Laboratoire des Solides Irradies,
Ecole Polytechnique, 91128 Palaiseau, France

Prof. J. R. Rehr ()
Department of Physics, Box 351560 University of Washington
Seattle, WA 98195-1560

2.
The importance of nanoscience for the future development of electronics, computing, mechanics and medical science is now fully recognized all over the world. Nanoscience is a field in continuous expansion thanks to promising interdisciplinary applications, whose functionality is not principally determined by pure material properties but by the integration of specially designed nanostructured components. In order to simulate the performance of these components, it is required to develop theoretical methods that allow the study of complex systems within a fully ab initio scheme, with the aim of understanding, projecting and inventing new materials, new applications, and new systems. In many cases, the interaction between light and matter is directly related to the working principle of nanoscale devices, especially in optoelectronics and in biological systems. In other cases, irradiation is used as a convenient tool for nondestructive material characterization in technological applications. The ability to invent new functionalities for nanoscale systems and advanced materials, and of designing new devices for specific applications, thus depends heavily on our understanding of excitations under irradiation by light, electron beams or modern photon sources. Density-functional theory (DFT) is a powerful tool for ab initio electronic-structure calculations and gives valuable information about the structural arrangement of nanoscale systems, but its applicability is, in principle, limited to the electronic ground state. As a consequence, when excited states are involved, this approach typically shows significant deviations from experiments. DFT can hence only be considered as a first step in a quantitative theoretical calculation. A more accurate description is afforded by many-body perturbation theory (MBPT) or, alternatively, time-dependent density-functional theory (TDDFT), both of which have witnessed impressive advances in recent years. The two approaches are complementary and follow different philosophies, but both open, in principle, a pathway to the exact optical spectra. In MBPT, the calculation of response functions requires a solution of the Bethe-Salpeter equation, which explicitly contains the dynamics of electron-hole pairs, created through absorption of a photon, in the form of appropriate Feynman diagrams. The resulting spectra are very accurate and serve as a benchmark for other computational schemes, but the explicit diagrammatic approach makes the method numerically expensive and effectively restricts it to simple systems. An important challenge is, therefore, to develop approximations that boost the efficiency of MBPT without sacrificing its quantitative accuracy.

On the other hand, TDDFT deals directly with global density-fluctuations resulting from external perturbations. This implicit approach is much more computationally efficient, but it is difficult to incorporate dynamic exchange-correlation effects. Much of the recent progress has resulted from a combination of these two complementary methods and from a comparison of their different strengths and weaknesses. Although a number or fundamental questions remain unanswered, the theoretical foundations are well established, and it is now possible to treat actual nanostructured materials, making contact with experimental studies and also directly with technologically relevant electronic, dynamic and optical processes. Nevertheless, the prospect of future applications to more complex systems is directly connected to the exploitation of high-performance supercomputers and to the development of more efficient and possibly parallel algorithms.

In this context, the proposed workshop is intended to provide a forum to discuss advances in the theoretical and computational treatment of optical excitations. We aim to concentrate on TDDFT and the solution of the Bethe-Salpeter equation within MBPT, which are able to describe neutral electronic excitations and have emerged as the most promising approaches with an increasing number of application in different fields.

Our motivation to organize the workshop at this time is:
i)to offer the possibility of discussions between researchers with different experiences and points of view,
ii)to assess the latest achievements and the perspectives of these theoretical approaches, to discuss their drawbacks and their advantages;
iii)to attract young researchers to this developing field in order to increase the number of scientists involved in this kind of activity, and to foster links between scattered research groups working on first-principles electronic excitations.

The progress in the theoretical treatment of electronic excitations is reflected in the healthy growth of this workshop series and in the increasing number of participants. The proposed meeting follows the previous workshops on 'Excited Electrons in Molecules, Solids and Atoms' (Lyon, 1997), 'Spectroscopy of Electronic Excitations in Materials' (Valladolid, 1998), 'Calculation of Electronic Excitations in Finite and Infinite Systems' (Lyon, 1999) and 'Excited States and Electronic Spectra' (Lyon, 2000), all organized by Giovanni Onida, Angel Rubio, and Lucia Reining.

Under the present European organizers, the series continued with the CECAM/Ψk workshop on 'Ab-initio Theoretical Approaches to the Electronic Structure and Optical Spectra of Materials' (Lyon, 2002) and with the forthcoming 'Ab-Initio Electron-Excitations Theory: Towards Systems of Biological Interest' to be held in San Sebastian in September 2003. These workshops have already become the central meeting of the theoretical/computational spectroscopy community, and we aim to continue this series in order to work towards the establishment of a strong community of scientists interested in applications of MBPT and/or TDDFT in nanoscience and related fields.

3. Tentative list of speakers:

4.
The format of the workshop will be similar to the previous successful workshops listed in section 2, since this has been proven the best way to trigger discussions between participants and provide an informal environment.

Each day of the conference will be divided into a morning session
(9:00--12.40) and an afternoon session (15:30--20:00).
Invited talks are allocated up to 40 minutes plus 20 minutes reserved for the discussion. In addition, up to 90 minutes will be reserved for one round-table discussion.

We intend to invite speakers with recognized experience in the theory and models of electronic excitations in nanomaterials with a particular interest to leading experts of many-body theory and time-dependent DFT. Also one experimentalist will participate in the conference and give a general talk as an overview of the state of the art, perspectives and new possible future applications of nanostructures.

The main audience will consist of graduate students and postdocs that will actively participate through the inclusion of short contributed talks and poster session with brief oral introductions. We feel that this is a very important point, because young researchers should be actively involved in the formation of a strong scientific community and the development of promising new approaches in our field, which are the major aims of this workshop.

The number of participants will be limited to 70-80, in order to ensure a maximum interaction between scientists. Moreover the conference, the accommodation and the meals will be in the same place; this will ensure strong contacts and continuous discussions between the young and the senior scientists.

5.
Due to the emphasis on high-level research, we do not plan specific tutorials on excited-state calculations. Nonetheless, the ample break time between sessions will help to all students and postdocs to share their current research with senior scientists. Finally, the heads of research groups will be invited to advertise job and fellowship opportunities as well as other relevant conferences on their respective fields of interest.

6.
We aim to encourage the participation of young researchers. For this reason we expect that all funding from the Ψk Network will be used to provide financial support for PhD students or postdocs. As minimal condition to receive a bursary we will require their active contribution in the form of a talk or a poster presentation. We hope to offer 14 bursaries of 550 Euro: 290 Euro for living allowance, (the full board accommodation cost is 72 Euro per day) plus 260 Euro for travel expenses. Therefore we apply for a total amount of 7700 Euro.

7.
We will apply also to local institutions (University of Rome Tor Vergata,Italian National Institute of Material Science INFM, Basilicata local government) to get funds for invited speakers. Thanks to the co-organization with an American scientist (Prof. John Rehr) we will ask some funding from U.S.A. institutions (DOE, NSF) to sponsor the participation of American researchers.

8.
We will hold the workshop on 18-21 September 2004 in the Hotel "VILLA DEL MARE" in Acquafredda di Maratea, Basilicata, Italy. This hotel (http://www.costadimaratea.com/villadelmare/), situated between Sapri and Maratea, about 200 Km South of Naples has a long experience in hosting congresses and scientific meetings and will offer the opportunity of an intense contact between the participants in an informal atmosphere.

9.
Prof. J. R. Rehr at the University of Washington, coordinator of FEFF Project (http://leonardo.phys.washington.edu/feff) and CSMN Project (http://www.phys.washington.edu/~cmsn/CRTs/ESESRF) has accepted to be co-organizer of the conference. We believe that this initiative will boost also the collaboration with researchers in the USA, who work at the forefront of nanoscience, and exchange expertise and knowledge between Europe and USA.