Three-dimensional image using scanning tunneling electron microscopy of electrons on the surface of a Weyl semi-metal, a kind of crystal with unusual conducting and insulating properties. Credit: Yazdani et al., Princeton University.
Princeton Uni researchers have observed a bizarre behavior in a strange new topological crystal material that could hold the key for faster electronics. Unlike most materials in which electrons travel on the surface, in these new materials the electrons 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 Prof. Ali Yazdani, . “You don’t find anything else like this in other materials.”
Yazdani and his colleagues discovered the odd behavior while studying electrons in a crystal made of layers of tantalum and arsenic. The material, called a Weyl semi-metal, behaves both like a metal, which conducts electrons, and an insulator, which blocks them. The experimental results suggest that the surface electrons plunge into the crystal only when traveling at a certain speed and direction of travel 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 said. These values of momentum: Weyl points, can be thought of as portals where the electrons can depart from the surface and be conducted to the opposing surface. The theory predicts the points come in pairs, so a departing electron will make the return trip through the partner point.
Previous 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 make only a half circle, earning them the name “Fermi arcs.”
To get a more direct look at the patterns Yazdani’s lab used a highly sensitive scanning tunneling microscope, one of the few tools that can observe electron waves on a crystal surface. The results were puzzling. “Some of the interference patterns that we expected to see were missing,” Yazdani said.
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. http://www.eurekalert.org/pub_releases/2016-03/pu-dtr031016.php
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