Representative stock image of a quantum computer. Bartlomiej Wroblewski/iStock
In a global first, researchers at the Massachusetts Insitute of Technology (MIT) measured superfluid stiffness – the ease with which a current of electron pairs can flow through “magic-angle” graphene. This can help unlock its potential as a superconductive material as well as a building block for quantum computers in the future, a press release said.
Since its discovery just a few decades ago, graphene has rapidly become a material of choice for many applications. Made from a single sheet of graphite no more than an atom thick, the material is preferred due to its qualities, such as its ability to conduct heat and electricity while demonstrating superior strength and durability.
More recently, researchers found that when two sheets of graphene are stacked on top of each other at a precise ‘magic angle,’ the material assumes a twisted structure but also gains additional properties like superconductivity.
What is superfluid stiffness?
In this new structure known as magic-angle twisted bilayer graphene, or MATBG for short, electron pairs do not repel each other like they conventionally do in everyday materials. Instead, they form a superfluid, allowing them to flow through the material effortlessly.
How easily these electron pairs flow depends on multiple conditions and is referred to as superfluid stiffness by the scientific community. Knowing the superfluid stiffness of MATBG can help scientists better understand why the material behaves like a superconductor.
“Superfluid stiffness refers to how easy it is to get these particles to move to drive superconductivity,” explained Joel Wang, a research scientist at MIT who was involved in the research.
To measure superfluid stiffness, scientists place the material in a microwave resonator where the oscillation of an electric signal is measured at microwave frequencies; when superconducting material is placed in this device, it changes the resonance frequency related to superfluid stiffness.
However, the device has always been used for large and thick materials. MATBG, on the other hand, is microscopically thin.
Artist’s rendering of graphene. Credit: imaginima/iStock
Superfluid stiffness of magic-angle twisted bilayer graphene
“Compared to MATBG, the typical superconductor that is probed using resonators is 10 to 100 times thicker and larger in area,” explained Wang in the press release.
To measure it in MATBG, the researchers had to attach it to an extremely delicate material and make a lossless contact. The team sought help from MIT’s Will Oliver, who has been working on developing delicate two-dimensional materials for building quantum computers in the future.
The researchers used aluminum, a material regularly used by the Oliver group in their quantum experiments, and the material used to make the resonator device. They connected aluminum leads to the MATBG structure and then linked it to the resonator.
After sending microwave signals through the resonator, the team measured the kinetic inductance of the material, which can be converted to arrive at superfluid stiffness values. Surprisingly, the researchers found that the value was much larger than predicted.
“We saw a tenfold increase in superfluid stiffness compared to conventional expectations, with a temperature dependence consistent with what the theory of quantum geometry predicts,” added Miuko Tanaka, a former post-doctoral fellow at MIT, in the press release.
The researchers attribute this to the quantum geometry of the material. The findings pave the way for the material to be used to build quantum computers in the future.
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The research findings were published in the journal Nature.
