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Chirality—the property of an object that is distinct from its mirror image—has long captivated scientists across biology, chemistry, and physics. The phenomenon is sometimes called "handedness," because it refers to an object possessing a distinct left- or right-handed form. It is a universal quality that is found across various scales of nature, from molecules and amino acids to the famed double-helix of DNA and the spiraling patterns of snail shells.

Now, researchers at Princeton University have uncovered a hidden chiral quantum state in a material previously thought to be non-chiral. The finding sheds light on an intense debate within the physics community and expands our understanding of what is possible in the quantum realm.

In a study recently in Nature Communications, the team led by M. Zahid Hasan, the Eugene Higgins Professor of Â鶹ÒùÔºics at Princeton University, used a newly developed scanning photocurrent microscope (SPCM) to uncover the elusive broken symmetries underlying a charge density wave in KV₃Sbâ‚…, a Kagome lattice topological material.

Their findings help settle a long-debated controversy over whether such materials can spontaneously break symmetry to form chiral quantum states—a discovery that could pave the way for novel quantum technologies.

Researchers have seen a similar phenomenon in non-topological systems, but this is the first time that such chiral symmetry has been broken in a bulk topological quantum material.

"This is somewhat like pointing the James Webb telescope at the quantum world and discovering something new," said Hasan. "We're finally able to resolve subtle quantum effects that had remained hidden in a topological quantum material."

The Kagome lattice is a two-dimensional geometrical pattern composed of corner-sharing triangles. It is named after a traditional woven bamboo basket pattern that is a common design in Japan and has long been a central platform for exploring exotic quantum phases.

For a long time, it was considered inherently achiral, meaning that it lacks handedness. Yet, in 2021, Hasan's group used a high-resolution scanning tunneling microscope (STM) and discovered that, under certain conditions, KV₃Sb₅ spontaneously forms an unusual charge density wave—a periodic modulation of electronic density.

This discovery, which resulted in a paper in Nature, raised tantalizing questions about whether chirality in the form of a charge order could emerge atop a non-chiral Kagome lattice. The paper is among the three most cited papers in the field because of the issues it has raised.

A spontaneous charge order in physics is a type of phase transition (like water turning to ice) that occurs when electric charges form non-random patterns. In essence, an ordered state is created from an initially disordered state through a process known as spontaneous symmetry breaking.

However, detecting the specific symmetries broken during this transition have proved notoriously difficult in certain classes of topological materials. Subtle differences between left- and right-handed quantum states in such quantum materials have long eluded conventional measurement techniques.

To tackle this, graduate student Zi-Jia Cheng and postdoctoral researcher Shafayat Hossain, two co-lead authors of the paper, engineered a scanning photocurrent microscope capable of detecting this topological material's nonlinear electromagnetic response under circularly polarized light.

This microscope is different from a scanning tunneling microscope, which has typically been used in these types of experiments. The SPCM, though not as high resolution as the STM, is used when the goal is to characterize optically active materials and study their photocurrent behavior at a local scale. A combination of STM and SPCM then provides the complete imaging of the many-body quantum wavefunction.

"In this set up, we shine and focus coherent light on the sample placed in a specially designed quantum device and as the light interacts with the sample it generates a photocurrent that we can measure," said Hasan.

Together with former postdoctoral fellow Qi Zhang, the researchers fabricated ultra-clean quantum crystal devices and cooled them down to a frigid 4 degrees Kelvin for the measurement.

At high temperatures, the photocurrent showed no preference between right- and left-handed circular light. But as the material was cooled past its charge density wave transition, a remarkable shift occurred: the photocurrent became handed, a definitive signature of chirality known as the circular photogalvanic effect.

The researchers achieved this by first shining right-circularly coherent polarized (right-handed) light on the lattice and then measured its current. Then they shined left-handed light and measured its current. They were able to see a very clear difference between the two.

"Our measurements directly pinpoint broken inversion and mirror symmetries and shed light on the topological nature of this quantum material that exhibits charge order," said Cheng. "This conclusively establishes the intrinsic chiral nature of the charge-ordered state in a topological material for the first time."

Despite this, an explanation for this phenomenon remains elusive. "We confirmed the phenomenon, but we don't yet have a rigorous theory as to why it occurs," added Hasan. "We still don't fully understand it."

However, the implications stretch beyond basic science. According to Hasan, chiral quantum states could one day power new optoelectronic and photovoltaic technologies. "It's surprising that an emergent chiral state can generate such a pronounced response that was never reported before," he said. "This work also shows that second-order electromagnetic measurements are a powerful tool for detecting subtle symmetry breakings in topological materials."

Symmetry breaking is important because it explains the emergence of ordered states in nature and understanding how the process works is a fundamental goal of scientific investigation. Symmetric theories in physics are frameworks in which the laws governing the universe remain constant under specific conditions. These theories are essential for understanding the universe and, indeed, are fundamental to the advancement of scientific inquiry.

However, much of the real world is, in fact, asymmetrical in nature. Therefore, understanding how and under what conditions symmetries are broken is crucial to understanding many concepts in physics, such as phase transitions, magnetism and superconductivity, and topological behaviors, to name a few.

As for the future? Hasan is optimistic: "This is just the beginning. With these sensitive tools, who knows what hidden worlds of topological quantum matter we'll uncover next."