In the elusive realm of quantum materials, the discovery of new states of matter often challenges classical notions and reshapes our fundamental understanding of symmetry and topology. A recent breakthrough from Princeton University has unveiled a long-hidden chiral quantum state within a material previously believed to be achiral. This revelation not only deepens our grasp of quantum phenomena in topological systems but also signals a paradigm shift in how subtle symmetry breakings can manifest in complex materials.
At the heart of this discovery is the Kagome lattice structure found in the compound KV₃Sb₅. The Kagome lattice—a two-dimensional pattern composed of corner-sharing triangles—has historically been regarded as non-chiral, meaning it inherently lacks “handedness” or mirror asymmetry. Yet, by probing this lattice with an innovative approach, researchers detected a spontaneous emergence of chirality tied to an exotic charge density wave state, a modulated distribution of electrical charges that breaks translational symmetry in the electronic system.
The challenge in unearthing such chiral states lies in the subtlety of their electromagnetic signatures. Conventional tools have struggled to distinguish between left- and right-handed quantum states in bulk topological materials due to their intricate symmetry properties. Overcoming this obstacle, the Princeton team developed a sophisticated scanning photocurrent microscope (SPCM) capable of measuring nonlinear electromagnetic responses under circularly polarized light—a method particularly sensitive to broken inversion and mirror symmetries often muted in standard scanning tunneling microscopy.
This technique, while complementary to the atomic-scale imaging power of scanning tunneling microscopes (STM), uniquely captures optically induced photocurrent behavior at localized regions within the material. By illuminating KV₃Sb₅ with right- and left-circularly polarized light separately and measuring the resulting photocurrent disparities below its charge density wave transition temperature, the researchers directly observed a pronounced circular photogalvanic effect—a hallmark of emergent chirality in the system.
Remarkably, this emergent chirality arises spontaneously as the crystal is cooled to cryogenic temperatures near 4 Kelvin, signaling a phase transition whereby the material’s electronic structure reconfigures into a chiral charge-ordered state. This spontaneous symmetry breaking is a fundamental process whereby the initial symmetrical electronic configuration gives way to one that preferentially adopts a left- or right-handed orientation, fundamentally altering the material’s electromagnetic characteristics.
The discovery addresses a thorny debate in condensed matter physics regarding whether topological materials harbor intrinsic mechanisms to spontaneously break symmetry and develop chiral quantum states. Prior observations of similar phenomena appeared only in non-topological systems or at surfaces where symmetry constraints differ. Identifying such behavior in a bulk topological material firmly establishes chirality as an inherent feature of certain quantum phases, bridging a crucial gap between theory and experiment.
Despite this milestone, the underlying theoretical framework explaining why and how this chiral symmetry breaking occurs remains incomplete. As M. Zahid Hasan, the lead investigator, poignantly remarks, the definitive microscopic origin of this order and its relation to the topological nature of the material have yet to be fully elucidated. Nonetheless, this finding opens fertile grounds for further theoretical and experimental exploration into emergent many-body quantum states governed by intertwined symmetry and topology.
Beyond its deep scientific significance, the manifestation of chiral quantum states in topological materials carries profound implications for future technology. Chirality in electronic systems can generate anisotropic electromagnetic responses exploitable in next-generation optoelectronic and photovoltaic devices. The pronounced circular photogalvanic effect observed hints at potential applications where control over handedness could be harnessed to design novel quantum sensors or energy-harvesting systems with enhanced efficiencies.
The Kagome lattice’s role in this discovery underscores the importance of lattice geometry and electronic correlations in stabilizing unconventional quantum phases. Since the Kagome structure is characterized by inherent geometrical frustration and flat electronic bands, it serves as an ideal platform for quantum orderings that defy traditional symmetry classifications. This study highlights how even lattice motifs long thought to be achiral might harbor hidden avenues for symmetry lowering under precise conditions.
Notably, this research leverages decades of foundational work in topological physics, including insights gleaned from the celebrated quantum Hall effect and the theoretical development of topological insulators. Princeton physicists like Daniel Tsui and F. Duncan Haldane, Nobel laureates for their contributions in these areas, laid conceptual groundwork that enables the present exploration of intricate symmetry phenomena within topological matter.
The specialized synthesis and ultra-clean fabrication of quantum crystal devices were also essential for these experiments. Cooling the samples to near absolute zero minimized thermal fluctuations, allowing the fragile charge-ordered and chiral states to stabilize and be detected. Coupled with advanced instrumentation sensitive to nonlinear optical effects, these technical feats were critical in revealing the once-hidden chiral quantum state.
Future research is expected to broaden the application of scanning photocurrent microscopy and similar nonlinear electromagnetic probes to other candidate topological materials. Such efforts promise to uncover a rich landscape of emergent phases where topology and symmetry intertwine to produce unexpected quantum behaviors. The methodological innovation itself paves the way for resolving elusive many-body wavefunctions that evade conventional spectroscopic techniques.
In summary, the uncovering of a chiral charge order within the nominally achiral Kagome lattice material KV₃Sb₅ marks a significant advance in quantum materials science. This finding resolves a longstanding controversy by definitively showing that bulk topological materials can spontaneously break mirror and inversion symmetries to form chiral electronic states with novel electromagnetic properties. As such, it provides a new window into the complex dance of symmetry and topology in quantum phases, heralding exciting prospects for both fundamental physics and transformative quantum technologies.
Subject of Research:
Not applicable
Article Title:
Broken symmetries associated with a Kagome chiral charge order
News Publication Date:
22-Apr-2025
Web References:
https://doi.org/10.1038/s41467-025-58262-y
References:
Cheng, Z.-J., Hossain, M. S., Zhang, Q., et al. "Broken symmetries associated with a Kagome chiral charge order," Nature Communications, 22-Apr-2025. DOI: 10.1038/s41467-025-58262-y
Image Credits:
Shafayat Hossain and Zahid Hasan Lab
Keywords
Chirality, Quantum states
