Physicists at RIKEN have developed an electronic device that hosts unusual states of matter, which could one day be useful for quantum computation.
When a material exists as an ultrathin layer—a mere one or a few atoms
thick—it has totally different properties from thicker samples of the
same material. That's because confining electrons to a 2D plane gives
rise to exotic states. Because of their flat dimensions and their broad
compatibility with existing semiconductor technologies, such 2D
materials are promising for harnessing new phenomenon in electronic
devices.
These states include quantum spin Hall insulators, which conduct
electricity along their edges but are electrically insulating in
their interiors. Such systems when coupled with superconductivity
have been proposed as a route toward engineering topological
superconducting states that have potential application in future
topological quantum computers.
Now, Michael Randle at the RIKEN Advanced Device Laboratory, along
with co-workers from RIKEN and Fujitsu, have created a 2D Josephson
junction with active components entirely from a material known to be
a quantum spin Hall insulator. The work is published in the journal
Advanced Materials.
A Josephson junction is generally made by sandwiching a material
between two elemental superconductors. In contrast, Randle and team
fabricated their device from a single crystal of monolayer 2D
tungsten telluride, which had previously been shown to exhibit both
a superconducting state and a quantum spin Hall insulator one.
"We fabricated the junction entirely from monolayer tungsten
telluride," says Randle. "We did this by exploiting its ability to
be tuned into and out of the superconducting state using
electrostatic gating."
The team used thin layers of palladium to connect to the sides of a
tungsten telluride layer surrounded and protected by boron nitride.
They were able to observe an interference pattern when they measured
the sample's magnetic response, which is characteristic of a
Josephson junction with 2D superconducting leads.
While this study provides a framework for understanding complex
superconductivity in 2D systems, further work is required to clearly
identify the more exotic physics the systems promise. The challenge
is that tungsten telluride is difficult to process into devices due
to the rapid oxidization within minutes of its surface under ambient
conditions, which requires all fabrication to be performed in an
inert environment.
"The next step involves the implementation of ultraflat
pre-patterned gate structures by using, for example,
chemical–mechanical polishing," explains Randle. "If this is
achieved, we hope to form Josephson junctions with precisely
tailored geometries and to use our cutting-edge microwave resonator
experiment techniques to observe and investigate the exciting
topological nature of the devices."
