CECAM Report

PROGRAM CEP

Organizers: Tim O. Wehling (Bremen), Thomas Frauenheim (Bremen), Silke Biermann (Palaiseau Cedex), Johannes Lischner (London), Nikolay Prokofiev (Amherst, Massachusetts), Malte Schüler (Bremen), Andrew Millis (New York)

Venue: University of Bremen, Bremen Center for Computational Materials Science (BCCMS), Germany, 8th until 12th of October 2018

Sponsors: University of Bremen (BCCMS), Psi-k, DFG, CECAM

Electrons in real materials are subject to Coulomb interaction among each other. This interaction is long-ranged, gives rise to correlation effects, and often poses fundamental problems in ab initio simulations of real materials. A nowadays commonly used ab initio approach for strongly correlated materials is the augmentation of density functional (DFT) based methods with many-body treatments of the Hubbard model, such as the combination of DFT and dynamical mean field theory [1]. This approach, however, neglects all correlation effects stemming from non-local Coulomb interaction, since the Hubbard model only includes the on-site part of the interaction. Combining more sophisticated diagrammatic ab initio methods (GW) with methods and models which contain the long-range contributions of the interaction (EDMFT) [2] alleviates these problems in part but comes with the introduction of further approximations. Assessing the quality of such approaches is currently hampered by the fact that even the extended Hubbard model, which is the minimal many-body model capturing explicitly non-local interactions, is at best partially understood. Continue reading Report: CECAM / PSI-K Workshop Bremen on Correlated electron physics beyond the Hubbard model →

CECAM Report

Program GASC

Organizers: Thomas Frauenheim (Bremen), Peter Déak (Bremen), Klaus Irmscher (Berlin), Susanne Siebentritt (Luxembourg),  Joel B. Varley (Livermore, California)

Venue: University of Bremen, Bremen Center for Computational Materials Science (BCCMS), Germany, 8th until 12th of October 2018

Sponsors: University of Bremen (BCCMS), Psi-k, DFG

Defect engineering in micro/optoelectronics and in photovoltaics has immensely profited from electronic structure calculations. In the past two decades, local and semi-local approximations of density functional theory were the workhorses of theoretical studies but, by now, it has become clear that they do not allow a sufficiently accurate and reliable prediction of defect properties in wide band gap materials. While ab initio energy methods for calculating the total energy are struggling with the system sizes necessary for defect modeling, semi-empirical methods using various corrections or hybrid functionals are being applied for the purpose. While the theoretical background of these methods and their relation to each other is by now more or less understood, the transferability of the semi-empirical parameters and the overall predictive power is still unclear. Progress requires further systematic testing and comparison of the various methods, as well as validation against experiments. For that, accurate measurement data on defects are needed on a set of materials, which are structurally or compositionally related. Gallium based semiconductors, like GaN, Ga2O3 and CuInxGa1-xSe(S)2 chalcogenides (CIGS) offer a good possibility for testing theory and are interesting also experimentally due to their versatile applications.

While there are still open defect-related questions in the much studied blue LED-material GaN, very few defects could be positively identified as yet in CIGS solar cell materials, while the research on the potential power semiconductor and UV transparent electrode Ga2O3 has barely started. Following the successful workshops on Gallium Oxide and Related Materials, held in Kyoto (Japan) in 2015, and in Parma (Italy) in 2017, as well as several workshops on chalcogenide photovoltaic materials, e.g., Symposium V at the E-MRS Spring meeting 2016, the CECAM-workshop in Bremen 2018 will focus on bringing together experimentalist interested in gallium oxide and Ga-based chalocgenides with theorists who are active in the field. A friendly and stimulating environment will facilitate discussions, adding impetus to both the development of practically applicable theoretical methods and to progress in the defect engineering of these materials.

CECAM Qubit_Abstract book

ProgramQubits

Organizers: Adam Gali (Budapest), Thomas Frauenheim (Bremen), Jörg Wrachtrup (Stuttgart)

Venue: University of Bremen, Bremen Center for Computational Materials Science (BCCMS), Germany, 9th until 13th of July 2018

Sponsors: University of Bremen (BCCMS), Psi-k, DFG

https://www.bccms.uni-bremen.de/veranstaltungen/2018/cecam-qubit/

Point defects acting as color centers in solids may realize single photon source and quantum bits that can be harnessed in quantum information processing and nanoscale sensor applications which may revolutionize the info-communication technology, biological research and therapy. The leading contender is the nitrogenvacancy center in diamond which may be considered as a robust quantum tool. Several quantum algorithms and protocols for sensing have been already demonstrated by this center. However, researchers face many materials science problems in order to maintain the favorable intrinsic properties of this color center that can be perturbed by other defects either in bulk or at the surface of diamond that is difficult to resolve because of its chemical hardness and the concurrent stability of carbon allotropes.

Recently, theory-driven search for alternative materials could identify other quantum bit candidates in technologically mature wide band gap semiconductors, particularly silicon carbide, that have been recently demonstrated in experiments. However, the knowledge about these color centers is scarce and only the tight collaboration of experimental and atomistic simulation researchers would lead to a rapid progress in the field. The proposed workshop aims at bringing together world-leading experts in all these fields to improve interdisciplinary cooperation overcoming traditional boundaries between scientific disciplines.

Organizers:   Thomas Frauenheim (University of Bremen, Germany)

Oleg Prezhdo (University of Southern California, Los Angeles, US)

Christoph Lienau (University of Oldenburg, Germany)

Chiyung Yam (Computational Science Research Center, Beijing, China)

Location:       University of Bremen, Germany,

9^th until 13^th of October 2017

1. State-of-the-Art Summary

Advances of time-resolved experimental techniques, needed for a detailed understanding of charge carrier dynamics as they occur in real time, require matching progress in theoretical approaches. Applications to novel, emerging nanoscale materials, which ultimately lead to faster, more efficient and miniaturized devices, pose multiple theoretical challenges. Modeling time-resolved experimental data becomes a major goal of a theorist.

The proposed workshop became a forum to brainstorm ideas about solutions to important computational problems, and identify new directions for time-dependant electronic structure method development and challenging applications. In this way, we have been able to create an exchange mechanism to unite a core of developers in an interactive environment, in order to initiate design of a new generation software tools for quantum modelling of realistic complex systems and nanostructures in electronic ground and excited states. The delivery of this technology to a broad community will facilitate breakthroughs on high-impact materials science problems.
Continue reading Scientific report regarding the CECAM Workshop: “Charge carrier dynamics in nanostructures: optoelectronic and photo-stimulated processes” →

Organizers:   Thomas Frauenheim (University of Bremen)

Qiang Cui (University of Wisconsin, Madison, USA)

Bob Hamers, University of Wisconsin, Madison, USA)

Joel Pedersen, University of Wisconsin, Madison, USA)

Location:       University of Bremen, Germany,

12^th June until 16^th June 2017

1. Summary

The workshop “Tackling Complexity of the Nano/Bio Interface – Computational and Experimental Approaches” was held at the University of Bremen, Germany from June 12^th to 16^th 2017. In total, 72 participants from Belgium, Brazil, Finland, Ireland, Luxemburg, The Netherlands, Portugal, Slovenia, Spain, Sweden, Switzerland, Russia, France, Germany, UK and US attended the workshop.

The programme consisted of 28 invited lectures, one poster session presenting 27 posters and many events (reception / conference dinner) to allow for informal exchange. The lectures were scheduled to last 40 min, including 5-10 min discussion time. In addition to this extended time for discussion, the chairpersons were instructed to introduce the subject of the session and to actively participate in the discussion. This “Gordon-conference-style” was essential to guarantee a vivid discussion. The organizers ensured that well-established scientists acted as invited speakers and chairpersons. Continue reading Scientific Report regarding the CECAM Workshop: “Tackling complexity of the nano/bio interface – computational and experimental approaches” →

Organizers:

Thomas Frauenheim (University of Bremen, Germany)

Oleg Prezhdo (University of Southern California, L. A., US)

Sheng Meng (Institute of Physics, CAS Beijing, China)

Johannes Lischner (Imperial College London, UK)

Location:       University of Bremen, Germany,

10^th until 14^th of October 2016

1. Summary

There is enormous interest in understanding and controlling photo-induced charge transfer and chemical reactions for energy storage. These can be due either to water splitting and carbon dioxide reduction or by electron-hole pair separation at hybrid chromophore- or hybrid polymer-solid interfaces in photovoltaic devices, stimulating an increasing number of experimental and theoretical studies. Computational atomistic studies of experimental realistic setups require models that include an inorganic semiconductor nanostructure, acting as a catalyst and organic molecules in solvents. In photovoltaic applications, e.g. one has to consider multi-component systems, involving several chromophores tuned to absorb different wavelengths of light, an acceptor that removes an electron from the chromophores and creates separated electron-hole pairs, as well as electron and hole conducting media. Such models already may involve hundreds to thousands of atoms, extending far beyond the limits of any ab initio calculations. Furthermore, the non-equilibrium processes involved in the photo-induced charge separation and transport require explicit time domain modelling. Relevant processes occur on ultrafast time-scales and in most cases cannot be described by rate expressions. Charge separation, Auger-type energy exchange between electrons and holes, generation of additional charges by Auger mechanisms, energy losses to heat due to charge-phonon interactions, charge and energy transfer, and electron-hole recombination occur in parallel and competition requiring significant efforts in method development and clarification of multiple conceptual problems.
Continue reading Scientific Report regarding the CECAM Workshop: “Computational insight into photo-induced processes at interfaces” →

Organizers:

Thomas Heine, University of Leipzig (Germany)

Yandong Ma, Jacobs University Bremen (Germany)

Tim Wehling, University of Bremen (Germany)

Young-Hee Lee (Institute for Basic Science, Suwon (Korea)

Thomas Frauenheim, University of Bremen (Germany)

Location:

University of Bremen, Germany, 27^th June until 1^st of July 2016

I. Summary

The workshop “Tailor-made 2D-materials and functional devices” was held at the University of Bremen, Germany from June 27^th to 1^st 2016. In total, 76 participants from Belgium, Brazil, Finland, Ireland, Luxemburg, The Netherlands, Portugal, Slovenia, Spain, Sweden, Switzerland, Denmark, France, Germany, UK and US attended the workshop.

The programme consisted of 30 invited lectures, one poster session presenting 37 posters and many events (reception / conference dinner) to allow for informal exchange. The lectures were scheduled to last 40 min, including 5-10 min discussion time. In addition to this extended time for discussion, the chairpersons were instructed to introduce the subject of the session and to actively participate in the discussion. This “Gordon-conference-style” was essential to guarantee a vivid discussion. The organizers ensured that well-established scientists acted as invited speakers and chairpersons.

Concerning the poster session, we accepted only 37 posters to allow for an intense exchange of ideas at each single poster. Here, we encouraged in particular the young scientists to ask questions. The participation of PhD students was supported by partly covering local accommodation costs.

Due to the compact organization and accommodation in one hotel only all participants stayed together for the whole time of the conference, which additionally enforced the scientific discussion which was mandatory since scientists from various separated fields, i.e. advanced quantum chemistry, many-body perturbation theory, DFT and beyond, scanning probe techniques, optical spectroscopy, ARPES, etc. were attending the meeting to merge ideas and formulate a common goal for future directions and collaborations of theory groups with experimental groups.

Financial support from the DFG, Psi-k Network, and the German CECAM node multi-scale modelling from first principles, cecam-mm1p.de and the University Bremen is gratefully acknowledged.

Continue reading Scientific Report regarding the CECAM Workshop: “Tailor-made 2D-materials and functional devices” →