This three-dimensional image of electrons on the surface of a Weyl semi-metal was produced by scanning tunneling microscopy. Image: Yazdani et al., Princeton University.
This three-dimensional image of electrons on the surface of a Weyl semi-metal was produced by scanning tunneling microscopy. Image: Yazdani et al., Princeton University.

Researchers at Princeton University have observed a bizarre behavior in a strange new crystal that could hold the key for future electronic technologies. In contrast to most materials, where electrons travel over the surface, the electrons in these new materials sink into the depths of the crystal through special conductive channels.

"It is like these electrons go down a rabbit hole and show up on the opposite surface," said Ali Yazdani, professor of physics at Princeton University. "You don't find anything else like this in other materials." The finding is reported in a paper in Science.

Yazdani and his colleagues discovered this odd behavior while studying electrons in a crystal made of layers of tantalum and arsenic. The material, called a Weyl semi-metal, behaves like both a metal, which conducts electrons, and an insulator, which blocks them. A better understanding of these and other ‘topological’ materials could someday lead to new, faster electronic devices.

The team's experimental results suggest that the surface electrons plunge into the crystal only when traveling at a certain speed and direction called the Weyl momentum, said Yazdani. "It is as if you have an electron on one surface, and it is cruising along, and when it hits some special value of momentum, it sinks into the crystal and appears on the opposite surface," he explained.

These special values of momentum, also called Weyl points, can be thought of as portals through which the electrons depart from the surface and are conducted to the opposing surface. Theory predicts that the points come in pairs, so that a departing electron will make the return trip through the partner point.

The team were inspired to explore the behavior of these electrons by research published last year in Science by another Princeton team and separately by two independent groups, which revealed that electrons in Weyl semi-metals are quite unusual. For example, experiments implied that while most surface electrons create a wave pattern that resembles the spreading rings that ripple out when a stone is thrown into a pond, the surface electrons in the new materials should only make half circles, which were given the name ‘Fermi arcs’.

To get a closer look at the patterns of electron flow in Weyl semi-metals, postdoctoral researcher Hiroyuki Inoue and graduate student András Gyenis in Yazdani's lab, with help from graduate student Seong Woo Oh, used a highly sensitive instrument called a scanning tunneling microscope. This is one of the only tools that can observe electron waves on a crystal surface. They obtained the tantalum arsenide crystals from graduate student Shan Jiang and assistant professor Ni Ni at the University of California, Los Angeles.

The results were puzzling. "Some of the interference patterns that we expected to see were missing," Yazdani said.

To help explain the phenomenon, Yazdani consulted Andrei Bernevig, associate professor of physics at Princeton, who is an expert in the theory of topological materials and whose group was involved in the first predictions of Weyl semi-metals in a 2015 paper published in Physical Review X. Bernevig, with help from postdoctoral researchers Jian Li and Zhijun Wang, realized that the observed pattern made sense if the electrons in these unusual materials were sinking into the bulk of the crystal.

"Nobody had predicted that there would be signals of this type of transport from a scanning tunneling microscope, so it came as a bit of a surprise," said Bernevig. The next step, he adds, is to look for this behavior in other crystals.

This story is adapted from material from Princeton University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.