A window to the atomic scale in superconductivity paves the way for new quantum materials

A window to the atomic scale in superconductivity paves the way for new quantum materials

Illustration of the Andreev reflection between a superconductor and an atomic sharp metal tip. Credit: Aalto University / Jose Lado.

In a study published in nano messagesFor the first time, researchers demonstrated a new technique for measuring quantum excitations in superconducting materials with atomic precision. Discovering these excitations is an important step toward understanding exotic superconductors, which could help us improve quantum computers and perhaps even pave the way toward room-temperature superconductors.

Superconductors are materials that do not contain electrical resistance At all, they usually require very low temperatures. They are used in a wide range of fields, from medical applications to a central role in quantum computers. Superconductivity results from specially bonded pairs of electrons known as Cooper pairs. So far, the occurrence of Cooper pairs has been measured indirectly and microscopically in large quantities, but a new technique developed by researchers at Aalto University and Oak Ridge National Laboratories in the United States can detect their occurrence with atomic precision.

The experiments were carried out by Wonhi Ko and Petro Maksimovic at Oak Ridge National Laboratory, with theoretical support from Professor Jose Lado of Aalto University. Electrons can “quantum tunnel” through energy barriers, jumping from one system to another through space in a way that cannot be explained by classical physics. For example, if an electron pairs with another electron at the point where a metal and a superconductor meet, it can form a Cooper pair that enters the superconductor while “recovering” another type of particle in the metal in a process known as Andreev reversal. The researchers looked for these Andreev reflections to discover Cooper pairs.

To do this, they measured electric current between an atomically sharp metal terminal and a superconductor, as well as how the current depends on the separation between the terminal and the superconductor. This enabled them to detect how much Andreev’s reflection goes back to the superconductor, while maintaining similar imaging resolution for individual atoms. The results of the experiment are fully consistent with Lado’s theoretical model.

This experimental discovery of Cooper pairs at the atomic level provides an entirely new way to understand quantum materials. For the first time, researchers can uniquely determine how the wave functions of the Cooper pairs are reconstructed at the atomic scale and how they interact with impurities at the atomic scale and other obstacles.

“This technology establishes an important new methodology for understanding the inner quantum structure of exotic types of superconductors known as unconventional superconductors, which may allow us to address a variety of open problems in quantum materials,” says Lado. Unconventional superconductors are a potential building block for quantum computers and could provide a platform for achieving room-temperature superconductivity. Cooper pairs feature unique internal structures in unconventional superconductors that have hitherto been difficult to understand.

This finding allows direct investigation of the state of Cooper pairs in unconventional superconductors, and the creation of an important new technology for a whole group of quantum materials. It represents a major step forward in our understanding of quantum materials and helps advance the work of developing quantum technologies.


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more information:
Wenhi Kuo et al., Non-contact Andreev reflection as a direct probe of superconductivity at the atomic scale, nano messages (2022). DOI: 10.1021 / acs.nanolett.2c00697

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