Researchers postulate graphene can be transistor, superconductor simultaneously.

With the breakthroughs in quantum technologies that graphene has enabled, among countless other discoveries this physics-bending 2D carbon has warranted, researchers need only find the right pattern to truly unlock the potential of this miracle material. Supported by the landmark discovery of "twisted graphene," scientists at ETH Zurich Laboratory for Solid State Physics have added yet another remarkable thing this nanomaterial can do – superconductivity.

Roughly one year ago, a team of researchers led by Klaus Ensslin and Thomas Ihn at ETH Zurich's Laboratory for Solid State Physics was able to demonstrate that twisted graphene – more simply, two single layers of graphene slightly rotated from one another – could be used to create Josephson junctions, the fundamental building blocks for superconducting devices

You can read about what twisted graphene is and its various functions at Physics' newest twisted graphene family in the July 13, 2022, edition of Metal Tech News.

Based on this work, the researchers were able to produce the first superconducting quantum interference device, or SQUID, from twisted graphene for the purpose of demonstrating the interference of superconducting quasiparticles (a unit of energy in a crystal lattice that exhibits characteristics that can be likened to particles).

However, SQUIDs are neither new nor groundbreaking, as conventional SQUIDs are already used in medicine, geology, and even archaeology.

Due to their sensitive sensors being capable of measuring even the smallest changes in magnetic fields, SQUIDs are essentially quantum transistors with limitless possibilities.

"SQUIDs are to superconductivity what transistors are to semiconductor technology-the fundamental building blocks for more complex circuits," explained Ensslin.

Two jobs, same material

The graphene SQUIDs created by doctoral student Elías Portolés are not necessarily more sensitive than their conventional counterparts, which are usually made from aluminum.

"It's not a breakthrough for SQUID technology as such," said Ensslin. "However it does broaden graphene's application spectrum significantly. Five years ago, we were already able to show that graphene could be used to build single-electron transistors. Now we've added superconductivity."

What is truly remarkable is that graphene's behavior can be controlled in a targeted manner by "biasing" an electrode. Depending on the voltage applied, the material can be insulating, conducting, or superconducting.

"The rich spectrum of opportunities offered by solid-state physics is at our disposal," continued Ensslin.

Another interesting note is that two fundamental building blocks of a semiconductor (transistor) and a superconductor (SQUID) can now be combined in a single material. This makes it possible to build novel control operations.

"Normally, the transistor is made from silicon and the SQUID from aluminum," Ensslin added. "These are different materials requiring different processing technologies."

Yet graphene can do the job of both at the same time.

Fabrication still too hard

Discovered by the Massachusetts Institute of Technology roughly five years ago, the superconductivity of graphene is no secret. However, only a dozen or so experimental groups worldwide are exploring graphene's potential in this regard. Furthermore, even fewer are capable of converting superconducting graphene into a functioning component.

The challenge is that researchers have to carry out several delicate work steps one after the other: first, they have to align the graphene sheets at the exact right angle relative to each other. The next steps then include connecting electrodes and etching holes.

If the graphene were to be heated up, as often happens during cleanroom processing, the two layers re-align, and the twisted angle vanishes.

"The entire standard semiconductor technology has to be readjusted, making this an extremely challenging job," said Portolés.

Despite this, Ensslin is thinking one step ahead. With quite a variety of different qubit technologies currently being assessed, each with its own advantages and disadvantages, he postulates a coupling.

For the most part, this is being done by various research groups within the National Center of Competence in Quantum Science and Technology (QSIT). If the job as transistor and SQUID can be accomplished with graphene alone, it might be possible to combine the benefits as well.

"The result would be two different quantum systems on the same crystal," said Ensslin.

This would also generate new possibilities for research on superconductivity.

"With these components, we might be better able to understand how superconductivity in graphene comes about in the first place," he added. "All we know today is that there are different phases of superconductivity in the material, but we do not yet have a theoretical model to explain them."