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Qiangqiang Gu and Séamus Davis's breakthrough identifies unprecedented state of topological quantum matter

Graphic of Nb tip interacting with UTe2 surface

DOI: 10.1038/s41586-023-05919-7

Cornell scientists have revealed a new phase of matter in candidate topological superconductors that could have significant consequences for condensed matter physics and for the field of quantum computing and spintronics.

Researchers at the Macroscopic Quantum Matter Group at Cornell University have discovered and visualized a crystalline yet superconducting state in a new and unusual superconductor Uranium Ditelluride (UTe2) using one of the world’s most powerful millikelvin Scanned Josephson Tunnelling Microscopes. This “spin-triplet electron-pair crystal” is a previously unknown state of topological quantum matter.

The findings are published in the paper, “Detection of a Pair Density Wave State in UTe2,” published 29 June in Nature. Cooper-pair density waves are a form of electronic quantum matter in which pairs of electrons, instead of forming a conventional “superconductive” fluid where all are in the same freely moving state, freeze into a superconductive electron-pair crystal called a pair density wave state (PDW).

Lead author Dr. Qiangqiang Gu, a postdoctoral researcher working in Prof. Séamus Davis’s lab at Cornell University, co-led with Joe Carroll of University College Cork and Dr. Shuqiu Wang of Oxford University, explains the essence of the paper.

Frontiers of condensed matter physics

Superconductors are topological when the pairing potential exhibits odd-parity, leading to each electron pair adopting a spin-triplet state, with both electron spins oriented in the same direction. Topological superconductors are the target of intense research by physicists because they can, in theory, form the materials platform for ultra-stable quantum computers. However, even after a decade of intense investigation into topological superconductivity, no bulk materials, (except superfluid 3He which was also discovered at Cornell), have been definitively recognized as spin-triplet odd-parity superconductors. Recently, the exotic new material Uranium Ditelluride (UTe2) has emerged as a highly promising candidate for this classification. However, its superconductive order parameter remains elusive. In 2021, theoretical physicists began to propose that UTe2 is actually in topological pair density wave state. No such form of quantum matter had ever been detected.

Pioneering technology leads to exciting discovery

In simple terms, a Pair Density Wave (PDW) is like a stationary dance of the paired electrons found in in a superconductor, but here the pairs form periodic crystalline patterns in space. Our team at Cornell discovered the first PDW ever observed in 2016 using the superconductive-tip Scanned Josephson Tunnelling Microscope that we invented for that purpose. Since then we have pioneered SJTM studies at millikelvin temperatures and with microvolt energy resolution. For the UTe2 project we have directly visualized the spatial modulations of the superconducting pairing potential at atomic scale and found them to modulate exactly as predicted in a PDW state as the density of electron pairs modulates periodically in space. What we detected is a new quantum matter state - a topological pair density wave composed of spin-triplet Cooper pairs.

When asked about the fundamental implications of this work Dr. Gu explained

“The discovery of the first PDW in spin-triplet superconductors is exciting. Uranium-based heavy fermion superconducting compounds are a new and exotic class of materials that provide a promising platform for realisation of topological superconductivity. Collaboration at Cornell, Oxford and University College Cork is an ideal opportunity to make use of the complementary research skills and experimental instruments at both institutions and exposes us to the research cultures in the US and Europe. An extensive year-long study led to the exciting news that pair density waves are present in spin-triplet superconductors. Our scientific discovery also points out the ubiquitous nature of this intriguing quantum state in s-wave, d-wave and p-wave superconductors, and it sheds light on new avenues for identifying such states in a broad spectrum of materials.”