Physicists report the discovery of new evidence for a mysterious quantum state in an unusual class of materials, in findings that could illuminate a long-running debate over how these systems transition into superconductivity.
In physics, symmetry breaking refers to processes in which a symmetric state collapses from its initial state of disorder into a more ordered one, albeit now possessing less symmetry.
The new research findings reveal evidence that time-reversal symmetry, a fundamental kind of symmetry in nature, appears to be broken at temperatures significantly higher than previously recognized.
The research was detailed in a recent study published in Nature Physics.
Mysterious Kagome Metals
In their study, an international team of researchers led by a team at the Korea Advanced Institute of Science and Technology focused on what are known as kagome metals—a variety of ferromagnetic quantum materials possessing unique qualities.
For their study, the researchers used a specific kagome metal called Cesium Vanadium Antimonide (CsV₃Sb₅), a novel material first discovered in 2021 by researchers at UC Santa Barbara’s NSF Quantum Foundry.
The researchers found that during experiments with the material, which is recognized as a candidate superconductor, CsV₃Sb₅ showed indications that time-reversal symmetry was broken at temperatures significantly higher than previously recognized.
This is significant because it is suggestive of an elusive phenomenon physicists call loop-current order, which describes a state in which the circulation of electrons throughout a material occurs in persistent microscopic loops.
A Signal Emerges
Using an advanced photoemission technique, the team was able to uncover subtle differences in the way electrons respond to circularly polarized light, which is light that is produced using an electric field vector that rotates in a circle as the light wave propagates, equaling one complete rotation for each wavelength.
Based on the team’s measurements, a distinctive signal emerged, which they say is consistent with a time-reversal symmetry-breaking state, meaning that their observations provide what may be the best experimental evidence to-date for an exotic quantum state of matter known as loop-current order, wherein electrons spontaneously assume microscopic wave-like formations.
Loop-current order in kagome metals has been an ongoing area of study for physicists, and the team’s new findings offer what may be the best evidence yet uncovered.
Exotic States and High-Temperature Superconductivity
One reason the team’s findings are so significant is that their results offer some clarity for the otherwise complex sequence of phase transitions that are known to occur within CsV₃Sb₅.
Based on their research, the team now believes loop-current order manifests as the temperature begins to decrease, which is then followed by a periodic rearrangement of electrons known as a charge density wave. Finally, superconductivity then follows, accompanied by the absence of electrical resistance altogether.
Fundamentally, the team’s work could offer new insights into the behavior and characteristics of strongly correlated quantum materials, which could hold promise for future studies involving the mechanisms that produce exotic states of matter.
“These insights deepen our understanding of the phase landscape in kagome metals and highlight connections with a correlated system exhibiting analogous transition hierarchy,” the team reports.
In the future, such studies may also help scientists finally attain the long-sought goal of high-temperature superconductivity, whereby materials capable of conducting electricity with zero resistance, and at temperatures above 30 K (-243°C), helping to reduce the current expenses associated with cooling systems required for superconducting, and ultimately helping to bring these high-power engineering technologies into commercial viability.
The team’s recent study, “Evidence of time-reversal symmetry breaking above the charge density wave order in a kagome metal,” was published in Nature Physics.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.