A Breakthrough Experiment Unlocking the Mystery of Unconventional Superconductivity

A Breakthrough Experiment Unlocking the Mystery of Unconventional Superconductivity ...

The two spheres represent holes in an ordered magnetic array of spins (illustrated with compasses). Due to this magnetic environment, hole dopants bind in pairs.

For the first time, a group of scientists, led by physicist Eugene Demler of ETH Zurich, analyzed how magnetic correlations play a role in mediating hole pairing.

The charge carriers must pair up in order for electrical resistance to occur in certain materials. This is well known in traditional superconductors, but there are increasing numbers of materials that don't fit within this conventional theoretical framework.

Magnetic fluctuations, not phonons, are thought to be the cause of unexpected superconductors, and, surprisingly, magnetic interactions arise from the repellent Coulomb interaction between electrons. However, verifying these models in experiments is difficult.

Sarah Hirthe, Prof. Immanuel Bloch, and Dr. Timon Hilker from the Max Planck Institute of Quantum Optics in Garching (Germany), Prof. Fabian Grusdt from the Ludwig Maximilian University of Munich (Germany) and Prof. Eugene Demler from the ETH Zurich (Switzerland) have published papers that support central interpretations of these theories.

The team's synthetic crystal consists of atoms trapped in complex optical structures formed by intersecting laser beams, which are typically out of reach in real materials. Furthermore, individual atoms can be traced while also investigating their interactions with the other atoms, providing microscopic insight into the quantum many-body system at hand.

These abilities were leveraged to realize a magnetically mediated pairing that at first appeared to be unphysical, in that fermions repel each other, making it energetically undesirable. Still, electrons end up being paired, despite the repulsive interactions between them in cups, the first class of unconventional superconductors discovered in 1986.

The key is a technique introduced by Grusdt and Demler (then at Harvard) together with colleagues in 2018. Their research explored clever methods of modifying a fermions model with repellent interactions so that strong pairing emerges. They dubbed their approach the mixed-dimensional (mixD) t–J model, which dates back to the early 1990s when a few scientists — including Maurice Rice at ETH Zurich — developed so-called t–J ladder models to investigate magnetically mediated pairing

The remarkable experimental flexibility in generating synthetic crystals based on atomic quantum gases and light fields has enabled the first demonstration of such binding as had been predicted for mixD systems. The physicists were able to also directly compare the mixD situation with the standard scenario in which the repulsive interactions between holes prevent the formation of tightly bound pair states.

One of the most surprising findings of the Harvard numerical simulation, which is supported by Bohrdt's numerical simulations, is that the binding energy can be increased by one order of magnitude, since this energy scale determines the maximum temperature at which the system is still superconducting.

These are encouraging findings that open up a vast possibility for future research. On the one hand, the systems investigated so far are still relatively small in size, and larger systems should allow for more detailed investigations, providing in turn unique microscopic insight into the mechanisms underlying unconventional superconductivity.

Sarah Hirthe, Thomas Chalopin, Petar Bojovi, Annabelle Bohrdt, Fabian Grusdt, Immanuel Bloch, and Timon A. Hilker, 18 January 2023, Nature. DOI: 10.1038/s41586-022-05437-y

The European Research Council, the Max Planck Society, the Max Planck Harvard Research Center for Quantum Optics, the German Federal Ministry of Education and Research, Excellence Strategy Germany, Alexander von Humboldt-Stiftung, the European Research Council, the National Science Foundation, and the Air Force Office of Scientific Research have all funded the research.

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