Researchers at the Paul Scherrer PSI Institute and the Brookhaven National Laboratory (BNL), working on an international team, have developed a new method for complex X-ray studies that will help better understand so-called correlated metals. These materials could be useful for practical applications in areas such as superconductivity, data processing, and quantum computers. Today the researchers present their work in the journal Physical check X.
In substances such as silicon or aluminum, the mutual repulsion of electrons hardly affects the properties of the material. This is not the case with so-called correlated materials, in which electrons interact strongly with each other. The movement of one electron in a correlated material leads to a complex and coordinated reaction of the other electrons. It is precisely these coupled processes that make these correlated materials so promising for practical applications and, at the same time, so complicated to understand.
Highly correlated materials are candidates for new high-temperature superconductors, which can conduct electricity without loss and are used in medicine, for example, on MRI images. They could also be used to build electronic components, or even quantum computers, with which data can be processed and stored more efficiently.
“Highly correlated materials present a host of fascinating phenomena,” says Thorsten Schmitt, head of PSI’s New Materials Spectroscopy Group: “Still, it remains an important challenge to understand and exploit the complex behavior behind of these phenomena “. Schmitt and his research group tackle this task with the help of a method for which they use intense and extremely accurate X-ray radiation from the Swiss SLS light source to PSI. This modern technique, which has been further developed at PSI in recent years, is called resonant inelastic X-ray scattering or, ultimately, RIXS.
X-rays excite electrons
With RIXS, soft X-rays scatter from a sample. The incident X-ray beam is adjusted so that it raises electrons from a lower orbital of electrons to an upper orbital, which means that special resonances are excited. This unbalances the system. Various electrodynamic processes lead it back to the ground state. Part of the excess energy is emitted again as X-ray light. The spectrum of this inelastically dispersed radiation provides information about the underlying processes and therefore about the electronic structure of the material.
“In recent years, RIXS has become a powerful experimental tool for deciphering the complexity of correlated materials,” Schmitt explains. When used to investigate in particular correlated insulators, it works very well. So far, however, the method has not been successful in the correlated metal probe. Its failure was due to the difficulty of interpreting the extremely complicated spectra caused by many different electrodynamic processes during scattering. “In this sense, collaboration with theorists is essential,” explains Schmitt, “because they can simulate the processes observed in the experiment.”
Correlated metal calculations
It is a specialty of theoretical physicist Keith Gilmore, formerly of the Brookhaven National Laboratory (BNL) in the US and now at Humboldt University in Berlin. “Calculating RIXS results for correlated metals is difficult because you have to handle multiple electron orbitals, large bandwidths, and a large number of electronic interactions at once,” says Gilmore. Correlated insulators are easier to handle because fewer orbitals are involved; this allows model calculations to be made that explicitly include all electrons. To be precise, Gilmore explains, “In our new method of describing RIXS processes, we now combine the contributions that come from the excitation of one electron with the coordinated reaction of all the other electrons.”
To test the calculation, PSI researchers experimented with a substance that BNL scientist Jonathan Pelliciari had researched in detail as part of his doctoral dissertation at PSI: barium-iron-arsenide. If you add a specific amount of potassium atoms to the material, it will become a superconductor. It belongs to a class of unconventional high-temperature iron-based superconductors that are expected to provide a better understanding of the phenomenon. “Until now, the interpretation of RIXS measurements on such complex materials has been guided mainly by intuition. Now, these RIXS calculations offer experimenters a framework that allows a more practical interpretation of the results. Our RIXS measurements in PSI on barium-iron-arsenide are in excellent agreement with the calculated profiles, ”says Pelliciari.
Combination of experiment and theory
In their experiments, researchers investigated physics around the iron atom. “One of the advantages of RIXS is that you can focus on a specific component and examine it in detail to find materials that consist of several elements,” says Schmitt. The well-tuned X-ray beam causes an internal electron in the iron atom to rise from the ground state at the nucleus level to the highest energy valence band, which is only partially occupied. This initial excitation of the nucleus electron can cause new secondary excitations and trigger many complicated decay processes that eventually manifest in spectral satellite structures. (See chart.)
Because the contributions of the many reactions are sometimes small and close to each other, it is difficult to find out what processes actually took place in the experiment. Here the combination of experiment and theory helps. “If you don’t have theoretical support for difficult experiments, you won’t be able to understand the processes in detail, that is, physics,” Schmitt says. The same goes for theory: “You often don’t know which theories are realistic until you can compare them to an experiment. Progress in understanding occurs when experiment and theory are combined. This descriptive method can therefore become a reference for the interpretation of spectroscopic experiments on correlated metals. “
The international team has published its work in the journal Physical check X.
Text: Barbara Vonarburg
Physicists observe the division of an electron within a solid
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As for PSI
The Paul Scherrer PSI Institute develops, builds and operates large complex research facilities and makes them available to the national and international research community. The institute’s main research priorities are materials and materials, energy and the environment, and human health. PSI is committed to the training of future generations. Therefore, approximately a quarter of our staff are postdoctoral, postgraduate or apprentice. In total, PSI employs 2100 people, being the leading research institute in Switzerland. The annual budget amounts to approximately 400 million francs. PSI is part of the ETH domain, with the other members the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Materials Science Laboratories) and technology) and WSL (Swiss Federal Institute for Forestry, Snow and Landscape Research).
Dr. Thorsten Schmitt
Head of the new materials spectroscopy group
Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland
Phone: +41 56 310 37 62, email: [email protected] [German, English]
Dr. Keith Gilmore
Institute of Physics and IRIS Adlershof, Theoretical Solid State Physics
In the Great Wind Tunnel 2, 12489 Berlin, Germany
Phone: +49 30 2093 66370, email: [email protected]
Dr. Jonathan Pelliciari
Brookhaven National Laboratory, National Synchrotron Light Source II
PO Box 5000, Upton, NY 11973-5000, USA
Phone: 001 631 344 6223, email: [email protected]
Description of inelastic X-ray scattering resonating in correlated metals
Keith Gilmore, Jonathan Pelliciari, Yaobo Huang, Joshua J. Kas, Marcus Dantz, Vladimir N. Strocov, Shigeru Kasahara, Yuji Matsuda, Tanmoy Das, Takasada Shibauchi and Thorsten Schmitt.
Physical check X, 19.07.2021
DOI: 10.1103 / PhysRevX.11.031013