The problem with copper is fundamental to the physics of charge transport at small scales. In a bulk metal, electrons travel long distances before scattering off imperfections, giving copper its famously low resistivity of about 1.7 micro-ohm centimetres. But when a wire’s dimensions approach the mean free path of its electrons, two effects conspire to raise resistance dramatically. Electrons begin to scatter off the surfaces and grain boundaries of the wire, and the effective cross-section available for conduction shrinks. In state-of-the-art copper interconnects, which in the most advanced chips have critical dimensions below 20 nanometres, this size effect causes resistivity to soar, producing signal delays, wasted power and degraded reliability. As the semiconductor industry pushes toward ever-denser circuitry, the wires that tie transistors together have become a limiting factor in computing performance.

The Hong Kong Polytechnic University team, together with collaborators at South China Normal University, The Hong Kong University of Science and Technology, The University of Hong Kong, the Taiwan Semiconductor Research Institute and National Taiwan Normal University, turned to an unconventional class of conductors: topological semimetals. Materials such as CoSi host exotic electronic states in which electrons are protected from backscattering by the topology of their band structure. In particular, CoSi possesses long-lived Fermi-arc surface states, chiral electronic pathways that live on the surfaces of the crystal and are remarkably resistant to the scattering that plagues ordinary metals. The researchers reasoned that if these surface states are highly conductive, then making the material thinner, which increases the surface-to-volume ratio, should improve rather than degrade its overall conductivity.

To test this idea, the team grew high-quality single crystals of CoSi and fabricated nanoflakes with thicknesses spanning from one micrometre down to roughly 20 nanometres. The results were unambiguous and, by the standards of conventional metals, almost paradoxical. As the CoSi thickness decreased from 1 micrometre to about 20 nanometres, the resistivity dropped from 7.0 to 0.72 micro-ohm centimetres, a nearly tenfold improvement. The highly conductive surface path, dominated by the topological Fermi-arc states, progressively takes over as the bulk contribution diminishes with thickness. At room temperature, the resistivity of 20-nanometre-thick CoSi is one-tenth that of copper at the same thickness, a margin that could transform the design of the wiring layers in future chips.

Conductivity alone, however, is not enough to qualify a material as an interconnect candidate. Chip wiring must survive brutal operating conditions. Current densities in modern interconnects can exceed millions of amperes per square centimetre, and the resulting momentum transfer from electrons to metal atoms drives electromigration, the gradual transport of atoms that eventually opens voids and breaks the wire. Copper is particularly vulnerable at small dimensions, where grain boundaries and surfaces provide fast diffusion pathways for atoms. The CoSi semimetal, by contrast, is held together by exceptionally strong bonding. The researchers calculated a cohesive energy of 5.4 electronvolts and an atom migration barrier of 3.7 electronvolts, values far higher than those of copper. Experimentally, CoSi nanoflakes maintained reliable conduction at current densities of up to 10^8 amperes per square centimetre and at temperatures up to 450 degrees Celsius, performance that positions the material among the most robust thin-film conductors ever characterized.

The team also verified that CoSi can handle the high-frequency demands of modern communications circuitry. Radiofrequency measurements showed that CoSi interconnects operate cleanly at frequencies up to 40 gigahertz, a regime relevant to wireless transceivers, high-speed data links and advanced processors. Low loss and stable impedance at these frequencies are essential for any material hoping to replace copper in the back-end-of-line metallization of a chip, and the measurements suggest that CoSi’s smooth single-crystalline surfaces and low resistivity translate directly into excellent high-frequency behaviour.

Perhaps most convincingly, the researchers moved beyond isolated test structures and integrated a CoSi interconnect into a functioning silicon circuit. They connected a 16-nanometre-node silicon ring oscillator, a standard benchmark circuit used to evaluate process technology, using CoSi wiring. The oscillator operated at the same frequency as an identical circuit wired with conventional metal interconnects, demonstrating that the exotic semimetal can be married to mainstream silicon manufacturing without degrading circuit performance. This on-chip demonstration is a critical milestone, because many promising nanomaterials have faltered at exactly this step, proving difficult to integrate with the complementary metal-oxide-semiconductor processes that underpin the global electronics industry.

The findings arrive at a moment of intense searching within the semiconductor community. As copper interconnects approach their physical limits, researchers have explored a wide range of alternatives, including graphene, carbon nanotubes and other topological semimetals such as NbAs, NbP and MoP. Recent studies have shown surface-dominated transport in Weyl semimetal nanowires and ultrahigh conductivity in NbAs nanobelts, but questions of manufacturability, reliability and integration have kept any successor to copper out of production. CoSi stands out because it combines several advantages at once: a resistivity that improves with scaling, extraordinary electromigration resistance, thermal stability well beyond typical chip operating temperatures, proven high-frequency performance and demonstrated compatibility with a commercial silicon technology node.

There are, of course, hurdles between a laboratory demonstration and a production line. The CoSi nanoflakes in this study were grown and characterized as single crystals, and future work will need to establish scalable deposition methods, patterning techniques and via integration compatible with high-volume manufacturing. The contact resistance between CoSi and other chip materials, the chemical stability of the semimetal during processing, and the cost of adopting a new metallization scheme all remain open engineering questions. Still, the fundamental physics reported here inverts the central dilemma of interconnect scaling. Instead of fighting a material that gets worse as it gets smaller, chip designers could embrace one that gets better, turning the relentless miniaturization that once threatened copper wiring into an advantage for topological semimetals.

If CoSi and its relatives can clear the remaining manufacturing barriers, the implications extend beyond faster smartphones and data centres. Interconnect resistance is a growing share of the energy budget of modern computing, and taming it would reduce power consumption across everything from cloud servers to edge devices. The work also signals a broader convergence between topological quantum materials and mainstream electronics, a field long dominated by theoretical promise rather than practical devices. With a material that conducts better at 20 nanometres than at a micrometre, survives currents that vaporize copper and runs at 40 gigahertz inside a working silicon chip, the researchers have offered the semiconductor industry a glimpse of what may come after copper, and a reason to believe that the end of transistor scaling is not, after all, the end of computing progress.

Subject of Research: Single-crystalline cobalt silicide (CoSi) semimetal as a highly conductive and reliable nanoscale interconnect material to replace copper in advanced chips

Article Title: Single-crystalline CoSi semimetals with high conductivity and reliability

Article References: Chen, J., Yan, J., Fan, L., Zheng, T., Che, X., Zhu, C., Lu, W., Deng, M., Ng, Y. H., Wang, Z., Wan, Y., Jiang, X., Zhu, Y., Yang, Z., Chen, K. J., Liang, B.-W., Li, K.-S., Lan, Y.-W., Li, L.-J., & Chai, Y. (2026). Single-crystalline CoSi semimetals with high conductivity and reliability. Nature Materials. https://doi.org/10.1038/s41563-026-02740-1

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02740-1

Keywords: CoSi, semimetal, interconnects, copper replacement, topological materials, resistivity scaling, electromigration, nanoelectronics, Fermi arcs, ring oscillator, Nature Materials, chip wiring

The study centers around a meticulously designed atomic trimer array, a one-dimensional lattice composed of interlinked triplets of atomic sites. Such arrays belong to the broader family of topological insulators, materials known for their ability to carry robust edge modes protected against disorder and defects. However, the introduction of nonlinear interactions in these systems remains an experimental and theoretical challenge. The team’s approach leverages atomic interactions to break conventional linear regimes, effectively creating an interactive playground where new quantum edge states arise out of complex particle interplay. This breakthrough now bridges a critical gap between theory and experiment in nonlinear topological photonics.

At the core of the experiment is the realization that interactions within atomic trimers do not merely add complexity but give rise to fundamentally new edge-state behaviors that deviate from classical expectations. Unlike traditional edge modes that propagate linearly and maintain fixed energy dispersions, these nonlinear edge states exhibit dynamic and tunable properties influenced by particle density and on-site interactions. This discovery not only enriches the taxonomy of edge phenomena in topological materials but also opens pathways to harness nonlinearity for practical application in devices that require robust, switchable quantum states immune to environmental noise.

Methodologically, the researchers employed state-of-the-art ultracold atom trapping and optical lattice technologies, enabling them to assemble atomic trimers with exquisite precision. By tuning inter-atomic interactions via Feshbach resonances and controlling lattice parameters, they created an environment where the nonlinear effects become dominant at the edges of the chain. The signature of nonlinear edge modes emerged from detailed spectroscopy measurements, where the researchers observed shifts and intensity modulations of localized edge states as a function of interaction strength—clear evidence of underlying nonlinear dynamics rooted in the many-body quantum regime.

The theoretical framework supporting these experiments draws inspiration from topological band theory extended into the nonlinear realm. Traditionally, topological states are understood through linear Hamiltonians with fixed symmetries. However, once interactions complicate these systems, the Hamiltonian becomes nonlinear and non-Hermitian, challenging the established paradigms. The current work successfully extends theoretical models by incorporating interaction terms that capture the essence of nonlinear coupling within each trimer unit and between neighboring units. The resulting predictions accurately forecasted the emergence of edge state bifurcations and novel localization phenomena, subsequently validated by experimental data.

One of the most striking aspects of this study is the interplay between topology and nonlinearity, which forms a synergistic relationship that stabilizes edge states beyond the protective capabilities of symmetry alone. In linear systems, topological robustness is guaranteed by the topological invariants such as the Zak phase or Chern number. However, adding nonlinear interactions introduces new modes of stabilization, including self-trapping and interaction-induced topological transitions. The atomic trimer array acts as a minimal model capturing these complex effects, serving as a testbed for future research into intricate many-body quantum phases unachievable in bulk materials or classical systems.

From an application standpoint, nonlinear edge states in atomic trimer arrays promise revolutionary advances in quantum devices. The inherent robustness against external perturbations, coupled with the tunability via interaction strength, suggests that these systems could form the basis of next-generation quantum switches, sensors, and transducers. Moreover, the nonlinear character enables a form of state-dependent response, a feature crucial for developing adaptive quantum circuits where output states can be controlled dynamically by input excitations. This has vast implications for quantum computing architectures relying on topological protection to maintain coherence amidst environmental decoherence.

Further, the insights gained from this research will spur developments in photonics, where analogous topological and nonlinear principles can be engineered using coupled waveguides or resonator arrays. The atomic trimer model’s conceptual clarity provides a versatile blueprint to design photonic circuits capable of harnessing nonlinear edge modes for on-chip optical processing. Integrating such systems with existing silicon photonics infrastructure could accelerate the deployment of more sophisticated optical communication networks that benefit from topologically protected data channels with in-built nonlinear functionality for enhanced control and switching speeds.

The experimental techniques elaborated in this work also set a new standard for precision control in strongly correlated systems. By manipulating ultracold atoms trapped in configurable optical lattices, the researchers overcome the limitations imposed by material defects or fixed solid-state interactions. This atomic platform allows for real-time tuning of interaction parameters and lattice geometry, offering unparalleled versatility. As a result, complex phenomena such as interaction-induced topological phase transitions, many-body localization at edges, and nonlinear self-focusing of quantum states become accessible for systematic investigation, opening a new chapter in quantum simulation research.

Moreover, the nonlinear edge states detected in the atomic trimer array highlight the subtle physics that emerges when quantum systems are driven beyond weak-coupling approximations. The discovered phenomena challenge existing classification schemas by demonstrating that topological labels must be reconsidered when interactions dominate. This finding motivates a broader re-examination of topological phases in non-equilibrium and strongly correlated regimes, where traditional homotopy-based invariants may fail to capture the richness of the quantum landscape. Thus, the study not only advances immediate experimental capabilities but also provokes a fresh theoretical discourse in condensed matter physics.

For the scientific community, this research is a testament to the fruitful convergence of atomic physics, topology, and nonlinear dynamics. It exemplifies how a multidisciplinary approach can unravel complex emergent behavior previously obscured by conceptual or experimental limitations. The collaboration behind this breakthrough underscores the importance of combining refined experimental innovations with deep theoretical insight, pushing the frontier of how we understand and manipulate quantum matter at its most fundamental level.

Additionally, the research team’s findings carry fundamental implications for quantum transport phenomena and edge state lifetimes in interacting topological materials. By tuning interactions, the researchers observed modified transport signatures directly linked to edge-localized nonlinear modes, suggesting novel pathways to engineer controllable dissipation mechanisms in quantum channels. This insight paves the way for designing devices that exploit edge state lifetimes dependent on interaction regimes, a critical prerequisite for reliable quantum information transfer across extended networks.

Looking forward, the observation of nonlinear edge states compels new lines of inquiry into multi-dimensional topological systems incorporating more complex unit cells and richer interaction topologies. Extending the atomic trimer array concept to higher dimensions or incorporating long-range interactions could reveal entirely new classes of emergent topological excitations, with equally striking nonlinear characteristics. Such explorations would significantly deepen the current understanding of quantum matter far beyond the prototypical models studied to date, potentially revolutionizing the design principles of future quantum materials.

The significance of this discovery also resonates in the broader context of quantum technological development. As efforts intensify to build scalable quantum platforms, the ability to exploit and manipulate robust localized states at system boundaries will be paramount. The demonstration of nonlinear edge states controlled by atomic interactions signifies a major step toward integrating topological protection with active control mechanisms in quantum hardware, facilitating the development of devices that are both resilient and reprogrammable.

In sum, the work by Du and colleagues marks a milestone in the study of nonlinear topological physics by experimentally verifying nonlinear edge states in an interacting atomic trimer array. Their innovative use of ultracold atoms, coupled with advanced theoretical models, exposes a rich landscape of quantum phenomena arising from the synergy of topology and interactions. This discovery not only challenges existing paradigms but opens a promising frontier for engineering quantum matter with unprecedented functionalities designed at the nanoscale.

The future prospects stemming from this research inspire optimism that nonlinear topological edge states will become foundational elements in the next generation of quantum information systems, photonic devices, and beyond. As such, the scientific community eagerly anticipates how these new principles will be harnessed to forge transformative technologies that tap into the quantum world’s complex yet elegantly structured nature.

Subject of Research: Nonlinear edge states in interacting atomic trimer arrays and their implications for topological photonics and quantum materials.

Article Title: Observation of nonlinear edge states in an interacting atomic trimer array.

Article References:

Du, H., Zhao, H., Li, Y. et al. Observation of nonlinear edge states in an interacting atomic trimer array.
Light Sci Appl 14, 296 (2025). https://doi.org/10.1038/s41377-025-01997-6

DOI: https://doi.org/10.1038/s41377-025-01997-6