Understand the physics of new metals

Researchers from the Paul Scherrer Institute PSI and the Brookhaven National Laboratory (BNL), working as part of an international team, have developed a new method of complex X-ray studies that will provide a better understanding of so-called correlated metals. These materials could prove useful for practical applications in fields such as superconductivity, data processing and quantum computers. Today, researchers present their work in the journal Physical examination 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 strongly interact with each other. The movement of an electron in a correlated material leads to a complex and coordinated reaction of 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 which are used in medicine, for example, in magnetic resonance imaging. 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 multitude of fascinating phenomena,” says Thorsten Schmitt, head of the Spectroscopy of New Materials group at PSI: “However, understanding and exploiting the complex behavior behind these phenomena remains a major challenge. Schmitt and his research group tackle this task with a method in which they use the intense and extremely precise X-rays of the Swiss light source SLS at PSI. This modern technique, which has been developed at PSI in recent years, is called Resonant X-ray Inelastic Scattering, or RIXS for short.

X-rays excite electrons

With RIXS, soft X-rays are scattered over a sample. The incident x-ray beam is tuned in such a way that it elevates electrons from a lower electron orbital to an upper one, which means that special resonances are excited. It unbalances the system. Various electrodynamic processes bring it back to the ground state. Some of the excess energy is again emitted in the form of X-rays. The spectrum of this inelastically scattered radiation provides information on the underlying processes and therefore on the electronic structure of the material.

“In recent years, RIXS has become a powerful experimental tool for deciphering the complexity of correlated materials,” explains Schmitt. When used to study correlated insulators in particular, it works very well. So far, however, the method has failed to probe the correlated metals. Its failure was due to the difficulty of interpreting the extremely complicated spectra caused by many different electrodynamic processes during scattering. “In this regard, 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 United States and now at Humboldt University in Berlin. “Calculating RIXS results for correlated metals is difficult because you have to deal with multiple electron orbitals, large bandwidths, and a large number of electronic interactions at the same time,” says Gilmore. Correlated isolators are easier to manipulate because fewer orbitals are involved; this allows model calculations 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 other electrons.

To test the calculation, PSI researchers experimented with a substance that BNL scientist Jonathan Pelliciari had studied in detail as part of his doctoral thesis at PSI: barium-iron-arsenide. If you add a specific amount of potassium atoms to the material, it becomes superconducting. It belongs to a class of unconventional high-temperature iron-based superconductors that should provide a better understanding of the phenomenon. “Until now, the interpretation of RIXS measurements on such complex materials has been primarily guided by intuition. Now, these RIXS calculations give us experimenters a framework that allows for a more practical interpretation of the results. Our RIXS measurements at PSI on barium-iron-arsenide are in excellent agreement with the calculated profiles, ”says Pelliciari.

Combination of experience and theory

In their experiments, the researchers studied the 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 for materials made up of several elements,” explains Schmitt. The well-tuned x-ray beam causes an internal electron in the iron atom to rise from the ground state at the central level to the higher energy valence band, which is only partially occupied. This initial excitation of the central electron can cause further secondary excitations and trigger many complex decay processes that ultimately manifest in the spectral structures of satellites. (See graphic.)

Since the contributions of the many reactions are sometimes small and close together, it is difficult to know what processes actually took place in the experience. Here the combination of experience and theory helps. “If you have no theoretical support for difficult experiments, you cannot understand the processes, ie physics, in detail,” says Schmitt. The same goes for theory: “Often you don’t know which theories are realistic until you can compare them to an experiment. Advances in understanding come when experience and theory come together. for the interpretation of spectroscopic experiments on correlated metals. “

The international team has published its work in the journal Physical examination X.