Superconducting materials are similar to the carpool lane in a congested interstate. Like commuters who ride together, electrons that pair up can bypass the regular traffic, moving through the material with zero friction.
But just as with carpools, how easily electron pairs can flow depends on a number of conditions, including the density of pairs that are moving through the material. This "superfluid stiffness," or the ease with which a current of electron pairs can flow, is a key measure of a material's superconductivity.
Physicists at MIT and Harvard University have now directly measured superfluid stiffness for the first time in "magic-angle" graphene—materials that are made from two or more atomically thin sheets of graphene twisted with respect to each other at just the right angle to enable a host of exceptional properties, including unconventional superconductivity.
This superconductivity makes magic-angle graphene a promising building block for future quantum-computing devices, but exactly how the material superconducts is not well-understood. Knowing the material's superfluid stiffness will help scientists identify the mechanism of superconductivity in magic-angle graphene.
The team's measurements suggest that magic-angle graphene's superconductivity is primarily governed by quantum geometry, which refers to the conceptual "shape" of quantum states that can exist in a given material.
The results, which are reported in the journal Nature, represent the first time scientists have directly measured superfluid stiffness in a two-dimensional material. To do so, the team developed a new experimental method which can now be used to make similar measurements of other two-dimensional superconducting materials.
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