New insights into van der Waals materials found

The study provides new insights into the interactions of layered materials with laser and electron beams.

Since graphene became available by scotch tape, a large class of van der Waals (vdW) layered materials has been researched intensively. They are served as electrodes, thermoelectrics, optoelectronics, substrates, and as precursors for 2D materials.

Now, a new study by the Penn State and SLAC National Accelerator Laboratory in California offers detailed insights into the interactions of layered materials with laser and electron beams.

For the study, scientists used a combination of ultrafast pulses of laser light that excite the atoms in a material lattice of gallium telluride, followed by exposing the lattice to an ultrafast pulse of an electron beam. This shows the lattice vibrations in real-time using electron diffraction and could lead to a better understanding of these materials.

One of the fascinating observations with regards to their work is the breaking of a law that applies to all material systems. Friedel’s Law places that in the diffraction pattern, the sets of centrosymmetric Bragg praks ought to be symmetric, result of Fourier transformation.

In this case, however, the pairs of Bragg peaks show opposite oscillating patterns. They call this phenomenon the dynamic breaking of Friedel’s Law. It is an infrequent, if not unprecedented, observation in the interactions between the beams and these materials.

Shengxi Huang, assistant professor of electrical engineering and corresponding author of a paper in ACS Nano that describes their work, said“Why do we see the breaking of Friedel’s Law?. It is because of the lattice structure of this material. In layered 2D materials, the atoms in each layer typically align very well in the vertical direction. In gallium telluride, the atomic alignment is a little bit off.”

When the laser beam shines onto the material, the heating generates the lowest-order longitudinal acoustic phonon mode, which creates a wobbling effect for the lattice. This can affect the way electrons diffract in the lattice, leading to the unique dynamic breaking of Friedel’s law.

This technique is also useful for studying phase change materials, which absorb or radiate heat during phase change. Such materials can generate the electrocaloric effect in solid-state refrigerators. This technique will also be interesting to people who study oddly structured crystals and the general 2D materials community.  

Journal Reference:
  1. Qingkai Qian et al. Coherent Lattice Wobbling and Out-of-Phase Intensity Oscillations of Friedel Pairs Observed by Ultrafast Electron Diffraction. ACS Nano Article ASAP. DOI: 10.1021/acsnano.0c02643

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