Ultrafast Electron-Phonon Interactions

When illuminated by the synchrotron light, nickel emits x-rays itself due to the decay of valence electrons. The number of emitted photons reduces when increasing the temperature from room temperature to 900 °C. (Source: HZB)

How fast can a magnet switch its orien­tation and what are the micro­scopic mechanisms at play? These questions are of first importance for the development of data storage and computer chips. Now, a Helmholtz-Zentrum Berlin HZB team has for the first time been able to experi­mentally assess the principal micro­scopic process of ultra-fast magnetism. The methodology developed for this purpose can also be used to inves­tigate inter­actions between spins and lattice oscillations in graphene, super­conductors or other quantum materials.

Interactions between electrons and phonons are regarded as the microscopic driving force behind ultrafast magne­tization or demagne­tization processes. However, it was not possible until now to observe such ultrafast processes in detail due to the absence of suitable methods. Now, a team headed by Alexander Föhlisch has developed an original method to determine experi­mentally for the first time the electron-phonon driven spin-flip scattering rate in two model systems: ferro­magnetic Nickel and nonmagnetic copper.

They used X-ray emission spectro­scopy (XES) at BESSY II to do this. X-rays excited core electrons in the samples (Ni or Cu) to create core-holes, which were then filled by the decay of valence electrons. This decay results in the emission of light, which can then be detected and analyzed. The samples were measured at different tempera­tures to observe the effects of lattice vibrations increasing from room tempera­ture to 900 degrees Celsius.

As the tempera­ture increased, ferro­magnetic nickel showed a strong decrease in emissions. This observation fits well with the theoretical simulation of processes in the electronic band structure of nickel after excitations: by increasing the tempera­ture and thus, the phonon population, the rate of scattering between electrons and phonons increases. Scattered electrons are no more available for decay, which results in a waning of the light emission. As expected, in the case of dia­magnetic copper, the lattice vibrations had hardly any influence on the measured emissions.

“We believe that our article is of high interest not only to specialists in the fields of magnetism, elec­tronic properties of solids and X-ray emission spectro­scopy, but also to a broader readership curious about the latest develop­ments in this dynamic field of research,” says Régis Decker, post­doctoral scientist in the Föhlisch team. The method can also be used for the analysis of ultrafast spin flip processes in novel quantum materials such as graphene, super­conductors or topo­logical insulators. (Source: HZB)

Reference: R. Decker et al.: Measuring the atomic spin-flip scattering rate by x-ray emission spectroscopy, Sci. Rep. 9, 8977 (2019); DOI: 10.1038/s41598-019-45242-8

Link: Institute for Methods and Instrumentation for Synchrotron Radiation Research FG-ISRR, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, Germany

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