Scientists at RIKEN have devised an electronic device housing unconventional states of matter, potentially holding significance for quantum computation in the future.
A schematic image showing a Josephson junction (central section) made from a single layer of tungsten telluride. The red spheres are electron with spin up, while the blue ones have spin down. Image Credit: 2023 RIKEN Advanced Device Laboratory
When a substance takes on an ultrathin form, just one or a few atoms thick, it demonstrates distinct properties compared to thicker counterparts of the same material. This phenomenon arises because confining electrons to a two-dimensional plane results in the emergence of exotic states.
The promise of 2D materials lies in their flat dimensions and compatibility with established semiconductor technologies, making them attractive for exploring novel phenomena in electronic devices.
These states encompass quantum spin Hall insulators, capable of conducting electricity along their edges while remaining electrically insulating in their interiors. When coupled with superconductivity, such systems have been proposed as a potential avenue for engineering topological superconducting states with applications in future topological quantum computers.
In a recent development, Michael Randle and collaborators from the RIKEN Advanced Device Laboratory, in collaboration with Fujitsu, have successfully crafted a 2D Josephson junction with active components entirely composed of a material recognized as a quantum spin Hall insulator.
Traditionally, a Josephson junction involves sandwiching a material between two elemental superconductors. In contrast, Randle and his team manufactured their device using a single crystal of monolayer 2D tungsten telluride, a material previously demonstrated to exhibit both a superconducting state and a quantum spin Hall insulator state.
We fabricated the junction entirely from monolayer tungsten telluride. We did this by exploiting its ability to be tuned into and out of the superconducting state using electrostatic gating.
Michael Randle, Advanced Device Laboratory, RIKEN
The researchers utilized thin palladium layers to establish connections on the sides of a tungsten telluride layer, which was enveloped and safeguarded by boron nitride. During their measurements of the sample’s magnetic response, they observed an interference pattern, a distinctive feature of a Josephson junction with 2D superconducting leads.
While this investigation lays the groundwork for comprehending intricate superconductivity in 2D systems, additional research is imperative to distinctly elucidate the more unconventional physics that these systems hold promise for.
The complexity arises from the challenging nature of processing tungsten telluride into devices, primarily due to its rapid oxidation within minutes when exposed to ambient conditions. Consequently, all fabrication procedures must be conducted in an inert environment.
The next step involves the implementation of ultraflat pre-patterned gate structures by using, for example, chemical–mechanical polishing. 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.
Michael Randle, Advanced Device Laboratory, RIKEN
Journal Reference:
Randle, M. D., et al. (2023) Gate-Defined Josephson Weak-Links in Monolayer WTe2. Advanced Materials. https://doi.org/10.1002/adma.202301683