X-ray tracking: Electrons mapped

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  • Published: Dec 1, 2013
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: X-ray tracking: Electrons mapped

Electronic tracker

Crystals with rock salt structure. Upper crystal: sodium chloride (NaCl) with blue balls for Na+ ions and green balls for Cl- ions. Lower crystal: lithium hydride (LiH) with small blue balls for Li0.5+ ions and white balls for H0.5- ions. The grey-shaded plane indicates the sectional views. (Credit: MBI)

Femtosecond X-ray pulses can be used to track the movements of electrons under a strong electric field. Proof of principle with a crystal of lithium hydride provides important insights into electronic behaviour revealing, for instance, how the interactions between electrons decisively influence the direction in which they move.

Ionic crystals, by definition, contain a regular arrangement of positively and negatively charged ions. But, as with most rules there are exceptions. Take the cubic structure of lithium hydride (LiH), which consist of lithium (Li) and hydrogen (H) atoms wherein the charges are not distributed in the manner that one would expect to see in a more conventional ionic solid such as sodium chloride. Indeed, chemists see LiH as have "half" a positive charge on the lithium and "half" a negative on the hydrogen as its bonding sits half way between a fully covalent form and a completely ionic structure.

Bonding on the fence

As such, LiH has some intriguing properties. Of course, the electric forces are balanced in the crystal so that the spatial arrangement of the atoms nudges the solid to an energy minimum. However, if an electric field is applied to the material its electrons are set in motion, with a strong influence on the characteristics of this motion established by the spatial correlations among the electrons. Theorists have looked long and hard at electron correlations but their work has lacked experimental evidence to underpin it.

Now, a research team at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, in Berlin, Germany, and the Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Hydrogen and Energy, EMPA, in Dübendorf, Switzerland have addressed electron correlations by following ultrafast electron motions in space and time, so allowing them to produce a three-dimensional map of the electron distribution in lithium hydride. They set the electrons in motion using short bursts of light - 50 femtosecond short - to generate a strong electric field within the material. 100 femtosecond X-ray pulses are then scattered from the ‘excited’ crystal to give the team an image of the moment by moment distribution of the electrons.

Writing in the journal Physical Review Letters, Vincent Juvé, Marcel Holtz, Flavio Zamponi, Michael Woerner, Thomas Elsaesser and Andreas Borgschulte reveal the pattern of transient electron distributions. The data show an extremely rapid and unanticipated shift of electronic charge from the half-charged lithium to the half charged hydrogen ions over a distance of just one-fifth of a nanometre.

This unexpected result suggests that lithium hydride becomes more ionic in character under an applied electric field, which contrasts starkly with the behaviour of its chemical cousins lithium or sodium borohydride, LiBH4 or NaBH4. Of course, the electric field due to the optical pulse reverses its direction every 1.3 femtoseconds so the electrons are driven back and forth between the two sites very quickly, 1 percent the speed of light, in effect. When there is no optical pulse the electrons revert to their original distribution.

Mechanistic distribution

The team has established a qualitative mechanism for the unexpected electron movements that they observed wherein the electric field of the light pulse accelerates the electrons in such a way that they are more uniformly distributed over the unit cell. Li has initially more electrons with the consequence of a loss of electrons during the optical pulse.

Because of the small electron number in LiH, the team explains that all of the electrons contribute to this effect, making the electron distribution very sensitive to correlation effects. They suggest that this mechanism is supported by the theoretical calculations of the electron distribution.

Of course, as with many optical and electronic effects ultimately, the work may one day have implications for the high-speed control of properties of this or related materials and hints at potential in ultrafast switching for a future generation of electronics devices.

Related Links

Phys Rev Lett, 2013; 111 (21): "Field-Driven Dynamics of Correlated Electrons in LiH and NaBH4 Revealed by Femtosecond X-Ray Diffraction"

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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