Science society APS honors CASUS researcher Tobias Dornheim for his work in plasma physics

It was a question that had bothered physics for decades: How do interacting electrons behave under extreme conditions such as massive pressures and extraordinary heat? Between 2015 and 2017, scientists from Germany, the U.S., and the U.K. finally devised a satisfactory solution to this knowledge gap in quantum mechanics. For this achievement, the researchers involved, including Dr. Tobias Dornheim from the Center for Advanced Systems Understanding (CASUS) at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), will now receive the American Physical Society’s (APS) John Dawson Award for Excellence in Plasma Physics Research 2021. The award is to be presented at the Annual Meeting of the APS Division of Plasma Physics in November 2021.

The presented solution to the so-called fermion sign problem is actually more of a smart workaround. “We have developed and cleverly combined different, complementary simulation methods,” explains Dornheim. “The individual methods allow, each in different parameter ranges, the relatively fast and straightforward calculation of the thermodynamic properties of interacting electrons in warm dense matter. Overall, our predictions for the electron behavior across all relevant conditions were now more accurate than ever before.”

Warm dense matter refers to an exotic state of matter at extreme pressure and moderate to high temperatures, occurring, for example, at the core of stars and planets or when solids are exposed to high-intensity lasers. In addition to Dornheim, Prof. William Matthew Colwyn Foulkes from the Imperial College London, Travis Sjostrom (Ph.D.) from the United States’ Los Alamos National Laboratory, Fionn Malone (Ph.D.) from the software company QC Ware Corp, Dr. Tim Schoof from the German Electron Synchrotron (DESY) in Hamburg, Dr. Simon Groth from McKinsey & Company and Prof. Michael Bonitz from the University of Kiel will receive the award from the physics specialist society, which is endowed with a total of 5,000 US dollars.

At the microscopic level, the behavior of electrons is governed by the laws of quantum mechanics, which requires solving complex mathematical equations. An essential model for describing electron properties is the so-called homogeneous electron gas. Among other things, this model is important for understanding phenomena such as conduction electrons in solids or superconductivity, i.e., electrical conduction without resistance. It also provides the basis for the so-called density functional theory (DFT) to describe a many-electron system. DFT is currently considered one of the most important simulation methods in physics and chemistry. In industry, it is used to calculate material properties.

From low to high temperatures

Prior to the publications of the scientists honored, accurate simulations of the electron gas were limited to electrons within the low-temperature range. For warm dense matter – ten thousand times warmer than room temperature and up to a hundred times denser than ordinary solids – it was only possible to work with rough and sometimes hardly verifiable approximations. With the award winners’ exact simulations, the era of stopgap solutions came to an end, and research has since progressed rapidly.

The new simulation methods belong to the group of Quantum Monte Carlo (QMC) methods. Tobias Dornheim has developed the permutation blocking path integral Monte Carlo (PB-PIMC) method, which allows energies to be calculated over a very large parameter range. In parallel, Simon Groth and Tim Schoof, with Michael Bonitz as group leader, have developed a second QMC method, configuration path integral Monte Carlo (CPIMC), which covers a complementary parameter range.

Fionn Malone and William Matthew Colwyn Foulkes contributed a third QMC method similar to the CPIMC method. Dornheim and Groth subsequently combined these methods so that they could be used not only for a finite number of parameters but also for infinite system sizes (“finite-size correction”). Travis Sjostrom also made an important contribution to this work. Finally, the group around Dornheim and Groth succeeded in finding the appropriate exchange-correlation functional for density functional theory, which has meanwhile been implemented in the corresponding standard library libxc and is actively used by researchers worldwide.

As a postdoctoral researcher at CASUS, Dornheim has contributed to the improved understanding of warm dense matter in recent years through his creative approaches. For example, the physicist developed new approaches to the dynamical structure factor and the linear as well as the nonlinear density response of the electron gas. In addition, a new algorithm co-developed by Dornheim allows the so-called local field correction to be calculated very precisely, which decisively improves the modeling and interpretation of results in X-ray experiments. In accordance with the CASUS mission to open up the possibilities of digitization for science, the neural network trained by Dornheim and his colleagues can calculate the local field correction on a simple laptop. Previously, a high-performance computer had to be used for this task.

Tobias Dornheim will present his current research results at the APS Plasma Physics Division meeting in November. Due to the SARS-CoV-2 pandemic, the event incorporates virtual formats for some of the program items. Thus, the meeting can be followed virtually in its entirety. Speakers who will not attend the meeting in person will have their presentations pre-recorded and streamed to the meeting. All contributions can also be accessed later on the website (only with prior conference registration).

63rd Annual Meeting of the APS Division of Plasma Physics
November 8-12, 2021, Pittsburgh, Pennsylvania (USA)
The exact time of Dornheim’s lecture “Effective Static Approximation: A Fast and Reliable Tool for Warm-Dense Matter Theory” is yet to be announced.

APS award announcement

About the Center for Advanced Systems Understanding

CASUS was founded 2019 in Görlitz/Germany and pursues data-intensive interdisciplinary systems research in such diverse disciplines as earth system research, systems biology or materials research. The goal of CASUS is to create digital images of complex systems of unprecedented fidelity to reality with innovative methods from mathematics, theoretical systems research, simulations as well as data and computer science to give answers to urgent societal questions. Partners are the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Helmholtz Centre for Environmental Research in Leipzig (UFZ), the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden (MPI-CBG), the Technical University of Dresden (TUD) and the University of Wrocław. CASUS is funded by the Federal Ministry of Education and Research (BMBF) and the Saxon State Ministry for Science, Culture and Tourism.

About the Helmholtz-Zentrum Dresden-Rossendorf

The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) performs – as an independent German research center – research in the fields of energy, health, and matter. We focus on answering the following questions:
• How can energy and resources be utilized in an efficient, safe, and sustainable way?
• How can malignant tumors be more precisely visualized, characterized, and more effectively treated?
• How do matter and materials behave under the influence of strong fields and in smallest dimensions?
To help answer these research questions, HZDR operates large-scale facilities, which are also used by visiting researchers: the Ion Beam Center, the High-Magnetic Field Laboratory Dresden, and the ELBE Center for High-Power Radiation Sources. HZDR is a member of the Helmholtz Association and has six sites (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Schenefeld near Hamburg) with almost 1,400 members of staff, of whom about 500 are scientists, including 170 Ph.D. candidates.