Superconducting junction made from a single 2D material promises to harness strange new physics

RIKEN physicists have developed an electronic device that hosts unusual states of matter, which could one day be useful for quantum computing.

When a material exists as an ultrathin layer – only one or a few atoms thick – it has completely different properties than thicker samples of the same material. Indeed, confining electrons in a 2D plane gives rise to exotic states. Due to their flat dimensions and broad compatibility with existing semiconductor technologies, these 2D materials hold promise for exploiting new phenomena in electronic devices.

These states include quantum spin Hall insulators, which conduct electricity along their edges but are electrically insulating inside. Such systems, when coupled with superconductivity, have been proposed as a path toward engineering topological superconducting states that could have application in future topological quantum computers.

Now, Michael Randle of the RIKEN Advanced Device Laboratory, along with colleagues at 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 elementary superconductors. In contrast, Randle and his team made their device from a single-layer 2D tungsten telluride single crystal, which had previously been shown to exhibit both a superconducting state and a quantum spin Hall insulator state.

“We made the junction entirely from single-layer tungsten telluride,” says Randle. “We did this by exploiting its ability to move in and out of the superconducting state using electrostatic triggering.”

The team used thin layers of palladium to connect alongside a layer of tungsten telluride surrounded and protected by boron nitride. They were able to observe an interference pattern when they measured the magnetic response of the sample, characteristic of a Josephson junction with 2D superconducting conductors.

While this study provides a framework for understanding complex superconductivity in 2D systems, further work is needed to clearly identify the more exotic physics that the systems promise. The challenge is that tungsten telluride is difficult to make into devices due to the rapid oxidation of its surface within minutes under ambient conditions, requiring all fabrication to be done in an inert environment.

“The next step involves the implementation of pre-patterned ultra-flat grid structures using, for example, chemical-mechanical polishing,” says Randle. “If this is achieved, we hope to form Josephson junctions with precisely matched geometries and use our cutting-edge microwave resonator experimental techniques to observe and study the exciting topological nature of the devices.”