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Scientists have long sought to understand the quantum metric (QM)—a fundamental quantity that measures how rapidly neighboring electronic states in a solid change across momentum space. Predicted to drive exotic nonlinear and topological responses, QM has remained experimentally unexplored because this hidden wavefunction property is thought to be fragile under ambient conditions.

“Previous observations hinted at QM effects, but only under conditions that required van der Waals magnets, liquid-helium cooling, and multi-tesla fields,” says Jiahao Han, a member of an AIMR research team.

In a 2024 article, Han et al. showed that the QM can not only be detected but also tuned under ambient conditions¹. They achieved this by engineering a heterostructure of the chiral antiferromagnet Mn₃Sn capped with Pt, where interfacial spin textures offer a means to control the underlying QM.

“The novelty of this work was that by capping Mn₃Sn with a thin Pt layer, we engineered interfacial spin textures that break both inversion and time-reversal symmetries,” explains Han. “This symmetry breaking is what allows the QM contributions to stand out clearly in electrical measurements.”

By tracking the nonlinear Hall response in their heterostructure, the team showed that QM manifests as a time-reversal-odd second-order Hall effect. Remarkably, this signal remained robust across a wide temperature range—from cryogenic levels up to well above room temperature—and could be reversibly switched with small external magnetic fields, confirming the ability to flexibly control the QM.

“The significance of this result is that it establishes a practical route to harnessing quantum geometry in real materials,” says Han. “Being able to control QM under ambient conditions opens new opportunities for topological electronics, from low-power memory and logic elements to sensors and rectifiers that exploit the geometry of electronic states rather than just their charge or spin.”

A future direction will use this approach to explore unconventional superconductivity in flat-band and topological systems, as well as other facets of quantum geometry for transport phenomena and device applications.

A personal insight from Dr. Jiahao Han

What idea or moment in this project gave you the deepest sense of accomplishment, and did anything about the results surprise you?

For me, the greatest accomplishment was conceiving the idea of using the Mn₃Sn interface to engineer a spin texture that simultaneously breaks inversion and time-reversal symmetries. This approach was crucial to making the quantum metric visible in a second-order Hall effect, and it drew on insights from materials science, spintronics, and geometric physics in a way that felt very innovative. What impressed me most was how effectively the experiment realized this conception. It wasn’t so much surprising as deeply satisfying to see our thorough understanding of chiral antiferromagnets translate so directly into successful experimental outcomes.

(Author: Patrick Han)

This research highlight has been approved by the authors of the original article and all information and data contained within has been provided by said authors.