A flight of fancy: Electron pair emissions

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  • Published: Feb 15, 2014
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: A flight of fancy: Electron pair emissions

Incidents and accidents

Measuring electron pair emission measurements is an experimentally difficult task, not least because it requires access to a synchrotron light source. Now, researchers have developed a new way to measure the emission of electron pairs directly by using much more common time-of-flight spectrometry. Courtesy of Huth et al

Measuring electron pair emission is an experimentally difficult task, not least because it requires access to a synchrotron light source. Now, researchers have developed a new way to measure the emission of electron pairs directly by using a unique combination of a new laboratory light source with time-of-flight spectrometry.

Researchers at the Max Planck Institute of Microstructure Physics and the Martin-Luther-University Halle-Wittenberg, Germany have made a surprising discovery about electron behaviour by doubling up their instrumentation. Incident light or a particle beam can liberate electrons in pairs from a material of interest in the process of "electron pair emission." The phenomenon is exploited using synchrotron light sources, of which there are several around the world all of them in high demand and so are not always accessible to researchers hoping to uncover the fundamental properties of solid with a view to designing novel materials for a wide range of technological applications.

Until now there was no bench-top instrumentation that could be used to study materials using photon-excited electron pair emission. Michael Huth, Cheng-Tien Chiang, Andreas Trützschler, Frank Schumann, Jürgen Kirschner and Wolf Widdra writing in the journal Applied Physics Letters explain how they have remedied this situation and side-stepped the synchrotron. The team has demonstrated that they can measure the emission of electron pairs directly by combining two state-of-the-art time-of-flight spectrometers (Themis 1000 SPECS).

The teeth of the Hydra upon you

"Einstein received the Nobel Prize for his explanation of the photoelectric effect, which was published in 1905. Einstein considered the possibility that the photon energy can be transferred to more than one electron," explains MPI post-doctoral researcher Michael Huth. "The existence of this process provides direct access to the electron correlation strength."

The new approach hinges on the fact that an electron pair can be excited by a single photon from a light source and each electron can be detected by a spectrometer. Huth explains that developing a suitable setup was a complicated process requiring "a significant effort." Nevertheless, the team developed a double spectrometer system with an almost comical photoemission chamber sporting numerous leads, an observation that led the team to nickname the instrument the "Hydra".

In their proof-of-principle investigation, the team looked at the relatively simple nickel oxide (NiO), which according to theory, should display strong electron correlation effects. While measuring the energy distribution, they were surprised to discover that in contrast to the metal, the energy sum of the electron pair shows no prominent features. The significance of this effect was not lost on Huth and his colleagues. "Our observation is that metals and nickel oxide behave very differently," Huth said."“This implies that our technique allows us to quantify the electron correlation strength."

Quantification, that's the name of the game

The quantification of the electron correlation strength in a solid material is an important development because it reveals the potential for the material to display (or not) useful properties, such as superconductivity, metal-insulator transitions and long-range magnetic ordering. "Our experimental data will guide theory toward understanding the fundamental properties of solids, and one day help to design novel functional materials," Huth adds.

Now, that the instrumentation is in place and they need no synchrotron, the team can begin investigating a wide range of materials in order to help them gain a more complete picture of electron correlation. The investigations will be assisted by running experiments at different photon energies. "We also plan to optimize the efficiency and stability of our new setup for ongoing experiments," Huth says.

"The Holy Grail would be to investigate 'Cooper pairs' in superconductors where the interaction between the electrons plays a key role in the phenomenon of zero resitivity," Huth told SpectroscopyNOW.

Related Links

Appl Phys Lett Mat, 2014, 104, 061602: "Electron pair emission detected by time-of-flight spectrometers: Recent progress"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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