Coincidence Helps Expand Cornerstone of Physics

Jülich/Garching, 2 May 2019 – Atomic nuclei and electrons in solids influence each other’s motion – and they do so not only in rare exceptional cases, as previously believed. Scientists from Forschungszentrum Jülich and Technische Universität München (TUM) made this discovery while conducting measurements at Heinz Maier-Leibnitz Zentrum in Garching. The discovery was possible thanks to a student experiment conducted in 2015. The effect could be useful for data processing or for lossless transmission of electric current.

Jülich physicist Dr. Astrid Schneidewind and her colleagues spent years trying to understand deviations in the scattering pattern of neutrons that ought not to exist. In the end, they reached the limits of one of the cornerstones of physics: the Born–Oppenheimer approximation, a concept that had been accepted for more than 90 years.

The assumption, dating from the year 1927, is frequently used for tasks such as simplifying the calculation of multi-particle systems. The approximation assumes that the motion of atomic nuclei and electrons in solids can be considered separate from each other because the particles’ masses are very different. To illustrate: if an electron was the size of a grain of sand, then an atomic nucleus, for example that of iron, would be the size of a medicine ball – its motion would therefore be slower and more sluggish.

Few exceptions previously known

In the 1980s, researchers found some materials to which this approximation does not apply, i.e. where the sluggish activity of the medicine balls does indeed have an influence on the considerably faster grains of sand.

"Until now, it was assumed that these materials are absolute exceptions for which there is a logical explanation," says Schneidewind. "They are special cases where the lattice vibrations of the atomic nuclei, referred to as phonons, have the same energy values as the potential energy changes of the electrons in the shell."

Coincidental discovery

The researchers found something surprising in a compound called CeAuAl3, however: unexpected energy states of electrons and phonons. The scientists owe their discovery somewhat to luck. Schneidewind, who is responsible for the PANDA triple-axis spectrometer at Garching’s Heinz Maier-Leibnitz Zentrum, required a sample for a student experiment using neutrons. At the same time, her colleague and TUM scientist Christian Franz succeeded in growing a crystal of the compound for the first time. Various researchers had already investigated the substance in powder form, but none had noticed any abnormalities.

Motivated by investigations of similar substances, Schneidewind simply placed the crystal in the PANDA spectrometer overnight for the experiment. The surprise was all the greater for the physicist’s colleague Dr. Petr Čermák, a postdoc at Forschungszentrum Jülich at the time and also responsible for PANDA, when he and the students saw the measurement results the next morning: couplings between the motions of the atomic nuclei and the electrons were visible that, according to the Born–Oppenheimer approximation, ought not to exist. In-depth measurements by the team confirmed the first results: the interaction between lattice vibrations and electrons leads to new energy states of the electrons, despite the fact that – unlike in all the previous special cases – a different kind of phonon is involved.

Applications in data processing and superconductivity

"We were able to show for the first time that there must be many more materials than previously believed that have such couplings between electrons and their atomic nuclei in solids," explains Christian Pfleiderer, Professor of Topology of Correlated Systems at TUM, who had worked with his colleagues on the interpretation of the measurement results. "At the same time, this opens up a large range of potential forms of electronic order and functionalities stemming from such couplings."
Dr. Petr Čermák, who now works at Charles University in Prague, adds: "This surprising coupling between atomic nucleus and shell opens up many potential applications, for example for data processing." These materials are also expected to play a major role in understanding superconductivity.

Original publication: Magnetoelastic hybrid excitations in non-centrosymmetric CeAuAl3
Čermák P, Schneidewind A, Liu B, Koza M M, Franz C, Schönmann R, Sobolev O, Pfleiderer C
PNAS (published ahead of print March 20, 2019), DOI: 10.1073/pnas.1819664116

Further Information:

Jülich Centre for Neutron Science (JCNS)

Heinz Maier-Leibnitz-Zentrum (MLZ)

Contact:

Dr. Astrid Schneidewind
Jülich Centre for Neutron Science, branch office at MLZ
Tel.: 089 289-14749
E-Mail: a.schneidewind@fz-juelich.de

Press contact:

Dr. Regine Panknin, press officer
Forschungszentrum Jülich
Tel.: 02461 61-9054
E-Mail: r.panknin@fz-juelich.de

Angela Wenzik, science journalist
Forschungszentrum Jülich
Tel: 02461 61-6048
E-Mail: a.wenzik@fz-juelich.de