Electron movement and structures described in quantum physics allow researchers to better understand how and why materials like superconductors behave as they do. Rice University researchers Jianwei Huang and Ming Yi have developed a new capability, magnetoARPES, building on angle-resolved photoemission spectroscopy (ARPES) that allows researchers to study quantum behaviors they have been unable to resolve using ARPES alone.
MagnetoARPES adds a tunable magnetic field, external to the sample, to ARPES. This allows researchers to probe the full electronic response to a magnetic field, giving insights into why certain collective behaviors of electrons develop. Magnetic fields have, historically, been excluded from ARPES experiments, but over the course of a few years of experimentation and simulations, Yi's team found a viable way to incorporate this capability into the ARPES sample environment.
"This project started as a small exploratory exercise," said Yi, associate professor of physics and astronomy and corresponding author on the paper. "Then a series of simulations and tests gave increasingly promising results until we discovered that a small tunable magnetic field, generated by a coil, could allow momentum-resolved electronic spectral information to be largely retained."
To put magnetoARPES to the test, the team used a kagome superconductor — a superconductor with unusual electronic behavior that had been described in other experiments. By probing the electronic spectral information under a viable magnetic field, the team was able to detect collective behavior of the electrons that suggested a certain symmetry in the material was broken. This behavior is consistent with the theoretically predicted loop current orders, where the electrons on the crystal lattice circle around in opposite directions. By introducing an external, tunable magnetic field, domains with opposite electron motion could be aligned, allowing their collective behavior to be detected.
"Using magneto-ARPES allowed us to confirm that kagome's electrons work together to make the quantum state break time-reversal symmetry," explained Huang, a former Rice postdoctoral researcher now at Sun Yat-Sen University and first author on the paper. "The data showed this breaking was connected with another electron state called a charge density wave, allowing insight into how charge density waves may help form superconductivity."
The existence of time-reversal symmetry breaking in kagome had been proposed before, but this study offers the first experimental evidence confirming such unusual behavior directly in momentum space. Just like newborns learn about the world around them by banging their toys or chewing on them, physicists learn about enigmatic phases of matter by subjecting the material to different external stimuli and observing how the material responds, therefore gaining insights to emergent phenomena in quantum materials and how to control them for applications. The technique of magnetoARPES adds a new dimension along which we can learn about the electronic response to the magnetic field — one of the most useful tuning knobs that has been missing for the ARPES probe.
"Showing that useful information can be gained when performing ARPES in a field is an exciting starting point," Yi said. "We look forward to the discoveries to come from this capability as the collective creativity and momentum of an incredible research community continues to enhance and improve this technique as some independent efforts are already underway."
This work was supported by the Gordon and Betty Moore Foundation's EPiQS Initiative (BMF9470, 1520), the Robert A. Welch Foundation (C-2175, C-1839, C-1509), and the U.S. Department of Energy, Basic Energy Sciences (DF-SC0021421), the Air Force Office of Scientific Research (FA2386-21-1-4060, FA9550-21-1-0423, FA9550-24-1-0048), the David Lucile Packard Foundation, the Department of Energy, Basic Energy Sciences (DE-FG02-99ER45747, DE-SC0026179), the Israel Science Foundation (2974/23) and National Science Foundation (DMR 2011839) and the project Quantum materials for applications in sustainable technologies (QM4ST).
