Atomic-scale superconducting window paves the way for advanced new quantum materials

Superconductors are materials without any electrical resistance and usually require extremely low temperatures to achieve the desired properties. They are used in a wide range of fields, from medical applications to a central role in quantum computers. Superconductivity is caused by specially linked pairs of electrons known as Cooper pairs. Until now, the appearance of Cooper pairs has been measured indirectly, macroscopically, but a new technique developed by researchers at Aalto University and Oak Ridge National Laboratory in the US can detect their appearance with atomic-level precision.

The experiments were conducted by Wonhee Ko and Petro Maksymovych of Oak Ridge National Laboratory with theoretical support from Professor Jose Lado of Aalto University. Electrons can cross energy barriers via quantum tunneling, jumping through space from one system to another in a way that cannot be explained by classical physics. For example, if an electron pairs up with another electron at the junction of a metal and a superconductor, it can form a Cooper pair that enters the superconductor while also "kicking" the other particle back into the metal, a process known as Andreev reflection. The researchers detect Cooper pairs by looking for these Andreev reflections.

To do this, they measured the current between an atomically sharp metal tip and a superconductor, and how the current depended on the separation between the tip and the superconductor. This allowed them to detect the amount of Andreev reflection returning to the superconductor while maintaining an imaging resolution comparable to that of a single atom. The results of the experiment were in perfect agreement with Rado's theoretical model.

This experimental detection of Cooper pairs at the atomic scale provides an entirely new approach to understanding quantum materials. For the first time, the researchers were able to uniquely determine how the wave functions of Cooper pairs are reconstructed at the atomic scale and how they interact with atomic-scale impurities and other obstructions.

This technique establishes a key new way to understand the internal quantum structure of an exotic type of material called an unconventional superconductor, potentially allowing us to address a wide range of open problems in quantum materials. Unconventional superconductors are a potential fundamental building block for quantum computers and could provide a platform to achieve superconductivity at room temperature. Cooper pairs have a unique internal structure in unconventional superconductors, and until now, understanding these structures has been very difficult.

"This discovery allows researchers to directly probe the state of Cooper pairs in unconventional superconductors, establishing a key new technology for an entire family of quantum materials. It represents a major step forward in the scientific community's understanding of quantum materials and will help advance work on developing quantum technologies."