New insights into van der Waals materials found

Date:
July 6, 2020
Source:
Penn State
Summary:
Layered van der Waals materials are of high interest for electronic
and photonic applications, according to researchers who provide
new insights into the interactions of layered materials with laser
and electron beams.



FULL STORY ========================================================================== Layered van der Waals materials are of high interest for electronic and photonic applications, according to researchers at Penn State and SLAC
National Accelerator Laboratory, in California, who provide new insights
into the interactions of layered materials with laser and electron beams.


========================================================================== Two-dimensional van der Waals materials are composed of strongly bonded
layers of molecules with weak bonding between the layers.

The researchers 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.

"This is a quite unique technique," said Shengxi Huang, assistant
professor of electrical engineering and corresponding author of a paper in
ACS Nano that describes their work. "The purpose is to understand fully
the lattice vibrations, including in-plane and out-of-plane." One of
the interesting observations in their work is the breaking of a law
that applies to all material systems. Friedel's Law posits that in the diffraction pattern, the pairs of centrosymmetric Bragg peaks should be symmetric, directly resulting from 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 a very
rare if not unprecedented observation in the interactions between the
beams and these materials.

"Why do we see the breaking of Friedel's Law?" she said. "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.


========================================================================== Story Source: Materials provided by Penn_State. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Qingkai Qian, Xiaozhe Shen, Duan Luo, Lanxin Jia, Michael Kozina,
Renkai
Li, Ming-Fu Lin, Alexander H. Reid, Stephen Weathersby, Suji Park,
Jie Yang, Yu Zhou, Kunyan Zhang, Xijie Wang, Shengxi Huang. Coherent
Lattice Wobbling and Out-of-Phase Intensity Oscillations of Friedel
Pairs Observed by Ultrafast Electron Diffraction. ACS Nano, 2020;
DOI: 10.1021/ acsnano.0c02643 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200706140911.htm

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