The concept of exotic phases of matter has captivated physicists due to its dual significance. From a fundamental perspective, these phases inhabit a largely uncharted territory of the quantum realm, where electrons and their interactions give rise to emergent behaviors defying classical descriptions. Practically, such states promise robust platforms for technologies like quantum computing, where protection against decoherence—the loss of quantum information—is paramount. The discovery in CeRu₄Sn₆ elegantly illustrates this interplay, revealing that a topological semimetallic state can flourish precisely at the brink of a continuous quantum phase transition known as a quantum critical point (QCP).

Topological materials are characterized by electronic properties safeguarded by symmetries that render them resilient to minor imperfections or perturbations. This robustness arises because the global quantum state is protected by topological invariants, abstract mathematical quantities that remain unchanged under continuous deformations. These invariants convey stability much like the twist of a Möbius strip or the number of holes in an object, making such materials promising building blocks for devices that must operate reliably in noisy or fluctuating environments.

The study led by physicist Julio Larrea Jiménez and his international collaborators sheds light on how symmetries linked to nontrivial topologies—especially those connected to chirality, or handedness—can produce quantum states vastly different from those predicted by classic quantum mechanics based on Schrödinger’s equation. Although the Schrödinger framework underpins much of quantum physics, the team demonstrated that when unusual symmetries are at play, especially under extreme conditions, new horizons of electronic organization reveal themselves beyond conventional Bloch states and quasiparticle descriptions.

In metallic systems, electrons interact and organize according to well-understood paradigms involving quasiparticles—collective excitations that behave like independent particles with modified properties such as effective mass. In heavy fermion compounds like CeRu₄Sn₆, these quasiparticles are significantly heavier because of entanglement between conduction electrons and localized magnetic moments, a phenomenon known as the Kondo effect. Under normal conditions, this effect leads to a complex fluid of heavy electrons exhibiting conventional metallic behavior. Yet, experiments have now shown that close to the quantum critical point—the regime where the Kondo fluid dissolves amid intense quantum fluctuations—quasiparticles break down. In their place, an emergent topological semimetal state arises, defying traditional theory.

To decipher the underlying mechanism, the researchers constructed a theoretical model focusing on the Kondo breakdown limit where the heavy fermion fluid disintegrates. Remarkably, this model predicts the birth of topologically protected crossings in electronic bands, known as Weyl points, even in the absence of well-defined quasiparticles. Quantum fluctuations, which dominate near the QCP, are not destructive here; instead, they actively give rise to new topological features in the electronic landscape, stabilizing the Weyl-Kondo semimetal phase.

CeRu₄Sn₆’s response under extreme experimental conditions—high pressure, magnetic fields, and temperatures near absolute zero—manifests these phenomena vividly. Ordinarily, the Kondo entanglement binds the conduction electrons and cerium 4f electrons in a heavy fermion composite. As these conditions shift the system toward the quantum critical point, this intricate entanglement unravels, allowing quantum fluctuations to dictate the system’s behavior. Here, researchers observed the spontaneous Hall effect—a transverse voltage developing without any external magnetic field—which is a hallmark signature of Weyl semimetals and their associated chiral topological states.

The discovery is profoundly significant because it combines interactions, topology, and symmetry in a single unified framework. Previously, such phenomena were mostly theoretical conjectures. Now, experimental evidence firmly establishes that topological semimetallic states can emerge through quantum criticality, redefining the nature of phase transitions. This challenges the classical notion of an order parameter and well-defined excitations, since at the QCP, electronic bands become chaotic, and low-energy excitations supplant the traditional order parameters.

Topological phases, once considered a specialized niche, have grown into a central pillar of modern condensed matter physics. The 2016 Nobel Prize in Physics, awarded for discoveries related to topological phase transitions and states of matter, underscores their profound impact. Materials like topological insulators and Weyl semimetals usher in a new vocabulary for understanding quantum phenomena that resist perturbations, providing profound insights into electronic transport, magnetism, and superconductivity.

Technologically, the exploration of quantum matter under extreme conditions—ultra-low temperatures, enormous magnetic fields, and high pressures—creates access to states of matter unattainable under ordinary circumstances. Two-dimensional materials further extend this frontier, enabling new kinds of quantum order. This expanding landscape reveals that quantum matter possesses far more organizational potential than previously imagined, making the search for and understanding of exotic phases both a scientific challenge and a pathway to transformative technologies.

Larrea and his colleagues’ work exemplifies this frontier by demonstrating that quantum criticality, rather than merely suppressing order, can foster novel emergent phenomena. The realization of a Weyl-Kondo semimetal born from quantum critical fluctuations may eventually compel a reexamination of quantum materials and their applications, especially where strong correlations and topological protection intertwine. Such materials could lead the way to quantum devices that operate with unprecedented coherence and stability, crucial for the next generation of quantum computing.

Moreover, this experiment bridges a long-standing gap between theory and practice. The theoretical models envisaged Weyl-Kondo semimetals with topological nodes emerging at the Kondo breakdown, but until now, empirical validation remained elusive. The ability to experimentally pinpoint and manipulate these states opens fresh routes for tailoring quantum phases through external parameters, potentially tuning materials for specific functionalities by controlling pressure, temperature, and magnetic field.

In essence, the discovery that quantum criticality can seed a new topological state in CeRu₄Sn₆ invites a paradigm shift. It suggests that the chaotic, fluctuation-dominated regimes long considered inhospitable to order may instead offer fertile ground for novel phases. This insight propels the fields of condensed matter and quantum information science closer to understanding and harnessing the full potential of quantum matter.

Looking forward, the fusion of heavy fermion physics, topology, and quantum criticality promises rich physics to explore. The quest to understand how strong electronic correlations and topological invariants combine will undoubtedly inspire new experiments and theoretical frameworks. As researchers continue to probe matter under ever more extreme conditions, the emerging tapestry of exotic quantum phases will likely deepen our grasp of the quantum universe and accelerate the development of robust quantum technologies.

Subject of Research: Physics / Condensed Matter Physics / Quantum Phase Transitions / Topological States
Article Title: Emergent topological semimetal from quantum criticality
News Publication Date: 14-Jan-2026
Web References: https://doi.org/10.1038/s41567-025-03135-w
Image Credits: Julio Larrea Jiménez

Quantitatively, the emergence of this coherent state manifests through its unique sensitivity to confinement geometry and the underlying crystal symmetry. The kagome lattice structure, known for its inherent geometric frustration and topological complexity, appears to host a state whose coherence length scales extend across micron-sized domains. This macroscopic coherence contrasts markedly with ordinary metals and highlights the delicate interplay of spatial confinement, lattice symmetry, and many-body interactions in driving emergent electronic phases.

At the core of this phenomenon is the phase stiffness of the many-body coherent state, which lends it remarkable transport properties bearing some resemblance to the superconducting phase. However, critical differences remain: the state is still dissipative, disallowing the zero-resistance hallmark of superconductivity, yet it exhibits pronounced magnetoresistance oscillations reminiscent of Josephson plasma modes observed in layered cuprate superconductors. These oscillations underscore a collective, phase-coherent dynamic that is unprecedented outside the realm of superconductors, indicating a new paradigm of itinerant electronic states.

The nature of the microscopic constituents that condense into this coherent many-body state is a topic of ongoing investigation. Present hypotheses suggest that orbital loop currents, excitonic bound states, or coupled charge and spin fluctuations could serve as the fundamental building blocks of this coherence. Such candidates reflect the complex intertwining of charge, spin, and orbital degrees of freedom in kagome metals, and demand sophisticated experimental probes to unravel their precise contributions.

Scanning tunneling microscopy has provided pivotal insights by visualizing an emergent unidirectional coherent state forming within the charge density wave gap at T′. This observation offers a microscopic window into the evolution of coherence at the nanoscale and suggests that the coherent state is not confined to local fluctuations but represents a bulk electronic reconstruction. Whether this coherence represents the intrinsic bulk order or a precursor fluctuation heralding long-range order remains a compelling open question.

The implications for superconductivity in CsV3Sb5 are profound. This coherent non-superconducting phase lays the electronic groundwork from which superconducting order emerges at lower temperatures. Understanding how these pre-formed coherent states influence or compete with superconductivity could unlock new insights into pairing mechanisms and the complex phase diagram of kagome metals.

Beyond the intrinsic scientific value, the discovery of coherent but dissipative charge transport in the normal state points to novel device possibilities. Quantum interference devices operating at elevated temperatures without reliance on fragile superconducting order may become feasible. Geometry and frustration-driven mechanisms could be harnessed to engineer coherence in quantum materials, potentially revolutionizing high-temperature quantum electronics.

The kagome structure’s intrinsic frustration plays a central role, as it modifies electron correlations and stabilizes unconventional many-body states. This new coherent phase challenges prevailing theoretical frameworks and motivates the development of microscopic models incorporating the interplay of symmetry, topology, and many-body coherence.

In terms of experimental design, the study highlights the importance of micron-scale confinement and global lattice symmetry, pointing toward sophisticated device architectures as powerful probes. Controlled tuning of geometry may enable manipulation of coherence properties and open pathways toward tailored quantum materials design.

Moreover, this work situates kagome metals as a fertile platform for exploring exotic quantum phases beyond traditional superconductivity and charge density waves. The many-body interference effects documented here not only deepen fundamental understanding but expand the landscape of known states of quantum matter.

Future directions will likely include local and microscopic probes capable of resolving the internal structure of this phase, clarifying the precise nature of the incoherent constituents that underlie coherence onset. Advanced spectroscopic and transport measurements, combined with theoretical modeling, will be essential in fully characterizing this emergent state.

Ultimately, the discovery of coherent many-body interference in kagome crystals opens a novel paradigm in the physics of correlated electrons, embodying a new form of coherence distinct from but related to superconductivity. This breakthrough not only enriches fundamental knowledge but could catalyze transformative quantum technologies operating in regimes previously inaccessible due to the limitations of superconducting materials.

Subject of Research:
Kagome metal CsV3Sb5; Many-body coherence; Quantum interference; Charge transport; Correlated quantum matter.

Article Title:
Many-body interference in kagome crystals.

Article References:
Guo, C., Wang, K., Zhang, L. et al. Many-body interference in kagome crystals. Nature (2025). https://doi.org/10.1038/s41586-025-09659-8

Image Credits: AI Generated