Excitation of robust materials
Physics team observed extremely fast electronic changes in real time in a special material class

Date:
July 7, 2020
Source:
Kiel University
Summary:
So-called topological materials have special electronic properties,
which are very robust against external perturbations. In tungsten
ditelluride such a topologically protected state can be ''broken
up'' using special laser pulses within picoseconds and thus change
its properties. This could be a key requirement for realising
extremely fast, optoelectronic switches. For the first time,
physicists observed changes to the electronic properties of this
material in experiments in real-time.



FULL STORY ==========================================================================
In physics, they are currently the subject of intensive research;
in electronics, they could enable completely new functions. So-called topological materials are characterised by special electronic properties,
which are also very robust against external perturbations. This material
group also includes tungsten ditelluride. In this material, such a topologically protected state can be "broken up" using special laser
pulses within a few trillionths of a second ("picoseconds") and thus
change its properties. This could be a key requirement for realising
extremely fast, optoelectronic switches. For the first time physicists
at Kiel University (CAU), in cooperation with researchers at the Max
Planck Institute for Chemical Physics of Solids (MPI-CPfS) in Dresden,
Tsinghua University in Beijing and Shanghai Tech University, have been
able to observe changes to the electronic properties of this material
in experiments in real-time. Using laser pulses, they put the atoms in
a sample of tungsten ditelluride into a state of controlled excitation,
and were able to follow the resulting changes in the electronic properties "live" with high- precision measurements. They published their results
recently in the scientific journal Nature Communications.


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"If these laser-induced changes can be reversed again, we essentially
have a switch that can be activated optically, and which can change
between different electronic states," explained Michael Bauer, professor
of solid state physics at the CAU. Such a switching process has already
been predicted by another study, in which researchers from the USA were recently able to directly observe the atomic movements in tungsten
ditelluride. In their study, the physicists from the Institute of
Experimental and Applied Physics at the CAU now focused on the behaviour
of the electrons, and how the electronic properties in the same material
can be altered using laser pulses.

Weyl semimetals with unusual electronic properties "Some of the
electrons in tungsten ditelluride are highly mobile, so they are
excellent information carriers for electronic applications. This is due
to the fact that they behave like so-called Weyl fermions," said doctoral researcher Petra Hein to explain the unusual properties of the material,
also known as a Weyl semimetal. Weyl fermions are massless particles with special properties and have previously only been observed indirectly as "quasi-particles" in solids like tungsten ditelluride. "For the first
time, we were now able to make the changes in the areas of the electronic structure visible, in which these Weyl properties are exhibited."
Excitations of the material changes its electronic properties To capture
the barely-visible changes in the electronic properties a highly-
sensitive experimental design, extremely precise measurements and an
extensive analysis of the data obtained were required. During the past
years the Kiel research team was able to develop such an experiment with
the necessary long- term stability. With the generated laser pulses they
put the atoms inside a sample of tungsten ditelluride into a state of vibrational excitation.

Different overlapping vibrational excitations arose, which in turn
changed the electronic properties of the material. "One of these atomic vibrations was known to change the electronic Weyl properties. We wanted
to find out exactly what this change looks like," said Hein to describe
one of the key goals of the study.

Series of snapshots shows how properties change In order to observe this specific process, the research team irradiated the material with a second
laser pulse after a few picoseconds. This released electrons from the
sample, which allowed drawing conclusions about the electronic structure
of the material -- the method is known as "time-resolved photoelectron spectroscopy." "Due to the short exposure time of only 0.1 picoseconds,
we get a snapshot of the electronic state of the material. We can combine
many of these individual images into a film and thereby observe how the material reacts to the excitation by the first laser pulse," said Dr
Stephan Jauernik to explain the measurement method.

Recording a single data set on the extremely short modification process typically took one week. The Kiel research team evaluated a large number
of such data sets using a newly developed analytical approach and was
thus able to visualize the changes in the electronic Weyl properties of tungsten ditelluride.

Extremely short switching processes conceivable "Our results demonstrate
the sensitive and highly-selective interplay between the vibrations
of the atoms of the solid and the unusual electronic properties of
tungsten ditelluride," summarised Bauer. Follow-up research aims
at investigating whether such electronic switching processes can be
triggered even faster -- directly by the irradiating laser pulse -- as
has already been theoretically predicted for other topological materials.


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


========================================================================== Journal Reference:
1. Petra Hein, Stephan Jauernik, Hermann Erk, Lexian Yang, Yanpeng
Qi, Yan
Sun, Claudia Felser, Michael Bauer. Mode-resolved reciprocal space
mapping of electron-phonon interaction in the Weyl semimetal
candidate Td-WTe2. Nature Communications, 2020; 11 (1) DOI:
10.1038/s41467-020- 16076-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200707113324.htm

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