13 September 2019 Nicolas Doiron-Leyraud

Giant thermal Hall effect reveals novel particles in cuprate high-temperature superconductors

Louis Taillefer and Gaël Grissonanche

Photo : Martin Blache - UdeS

Electrons in a magnetic field can’t travel in a straight line: their path gets heavily bent under the action of the Lorentz force. In a metal, this causes charges to accumulate at the sides of a sample, giving rise to the well-known Hall effect. A team led by Sherbrooke Professor and IQ Member Louis Taillefer now reports in Nature that cuprate high-temperature superconductors display a very puzzling thermal Hall effect – the thermal analog of the Hall effect. Not only do they observe an unusually large thermal Hall conductivity, they find it to be largest in specimen where electrons can’t actually move, which points to novel exotic excitations in cuprates.

A time will come when superconductors will revolutionize our daily life, in areas as diverse as energy transmission, medicine, and communications. For this to happen, the critical temperature at which superconductivity occurs must first be increased up to room temperature. As of today, copper-oxide materials known as “cuprates” are the most promising candidates for achieving that goal. However, these are materials where complexity and the weirdness of quantum mechanics seems to have created a perfect storm, creating a wealth of spectacular and anomalous properties that defy our understanding.

The most puzzling feature of cuprates is the “pseudogap phase”, a mysterious electronic phase of matter that coexists with superconductivity and is considered one of the great enigmas in physics today. Elucidating the nature of that phase is thought to be key to understanding how electrons behave in these materials, and how they give rise to an exceptionally strong superconductivity.

In experiments led by post-doc Gaël Grissonnanche, Taillefer and his team have now found something entirely new and unexpected about the pseudogap phase: it harbors exotic excitations that produce a negative thermal Hall effect. In conventional metals one would normally associate the thermal Hall effect with conducting electrons, but this scenario is safely excluded here because the measured signal is way too large to be coming from electrons. Moreover, the thermal Hall signal grows as the material – while still in the pseudogap phase – becomes increasingly insulating, reaching a maximal value when there are in fact no mobile electrons left at all to conduct electricity. As a result, the heat producing the thermal Hall signal is likely to be carried instead by spin excitations. But spin excitations do not normally respond to a magnetic field like electrons do, so to produce a thermal Hall signal they must be rather exotic. One possibility are so-called “spinon” excitations, chargeless spin excitations that emerge in so-called “spin-liquid” materials. Another scenario proposes that the pseudogap has some form of topological spin excitations. Identifying the excitations that underpin the huge thermal Hall signal in cuprates will trigger a new wave of research on cuprates, and the outcome will be key to understanding their pseudogap phase.

“One of the things that makes this discovery so exciting is the simple observation of a large thermal Hall signal in an insulator, which no one was expecting in cuprates. It may not be clear what its exact origin is, but it shakes up much of our current thinking on cuprates – as none of the existing conventional models can account for this thermal Hall effect”, says Grissonnanche, the lead author of the study.

The Gordon & Betty Moore Foundation contributed to funding this project, via a grant to Taillefer as part of its program on Emergent Phenomena in Quantum Systems.

This research benefitted from the international collaborative network of researchers within the Quantum Materials Program of CIFAR.

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