Terahertz radiation, occupying the electromagnetic spectrum between infrared and microwave frequencies, has long captivated researchers due to its potential in applications ranging from high-resolution imaging to wireless communications. However, controlling and guiding terahertz waves with precision has remained a formidable challenge, often hindered by material constraints and diffraction limits. The emergence of plasmon polaritons—quasiparticles arising from the coupling of electromagnetic waves with collective electron oscillations at material interfaces—offers a tantalizing path towards overcoming these obstacles by confining and manipulating electromagnetic energy at scales below the diffraction limit.

In this context, topological insulators have emerged as a fertile ground for achieving exotic electromagnetic phenomena. These materials, characterized by insulating bulk states and conductive surface states protected by topological order, present unique avenues for plasmonic excitations. The study, conducted by Viti, Schiattarella, Sichert, and colleagues, expertly exploits these surface states to realize terahertz plasmon polaritons with an adjustable dispersion relationship—a critical parameter dictating how these quasiparticles propagate and interact.

The research centers on engineered metaelements constructed from topological insulator materials. By carefully designing the geometric and electrostatic parameters of these metaelements, the team achieved a tunable dispersion profile, allowing precise control over the phase velocity and confinement of terahertz plasmon polaritons. This level of tunability is significant because it enables the tailoring of plasmonic responses for specific application requirements, ranging from sensing and modulation to on-chip photonic circuitry.

Central to their methodology was the integration of advanced nanofabrication techniques with sophisticated terahertz spectroscopy measurements. The researchers employed near-field terahertz microscopy to visualize the propagation of plasmon polaritons across the topological insulator surface with nanoscale spatial resolution. These spatially resolved measurements not only confirmed the existence of tunable plasmonic modes but also allowed direct access to their dispersion characteristics, providing a firm experimental grounding to the theoretical models proposed.

The interplay between topological protection and plasmonic behavior represents a novel frontier harnessed by the team. The inherent robustness of surface states in topological insulators against scattering and defects imparts remarkable stability to the plasmon polaritons, ensuring low-loss propagation even in imperfect material conditions. This resilience is a pivotal advantage when designing practical devices that require stable, high-quality plasmonic signals.

Importantly, the tunability introduced in these metaelements is achieved “by design,” meaning that the dispersion properties can be predetermined through precise structural engineering rather than by post-fabrication adjustments or external stimuli alone. This represents a paradigm shift in plasmonics, where static material properties typically dictate electromagnetic responses. The work signals a move towards programmable photonic materials that can be optimized at the design phase for bespoke terahertz functionalities.

The potential applications of this research stretch across various high-impact domains. In telecommunications, for example, tunable terahertz plasmon polaritons could enable ultra-fast, miniaturized modulators and filters that enhance signal processing capabilities. Similarly, in spectroscopic sensing, these devices could achieve heightened sensitivity and selectivity by exploiting tailored dispersion to maximize light-matter interactions with target analytes.

Moreover, the findings complement and advance ongoing efforts to integrate topological photonic structures with metamaterials—artificial composites engineered to exhibit properties not found in nature. By combining the topological nature of surface states with the versatility of metamaterial design, the study opens avenues for producing reconfigurable, multifunctional optical platforms operating at terahertz frequencies.

The study also shines a light on the rich physics governing plasmon polaritons in nontrivial topological landscapes. The observed dispersion tuning can be theoretically understood through modifications in the electronic band structure and electromagnetic boundary conditions imposed by the engineered metaelements. These insights enrich the conceptual framework of plasmonics, suggesting new physics to explore in other correlated electron systems and two-dimensional materials.

As research in terahertz science accelerates, this work underscores the importance of marrying topological effects with plasmonics to surmount lingering technological challenges. The use of topological insulator metaelements with built-in tunability paves the way toward scalable, practical terahertz components that maintain performance while reducing complexity and energy consumption.

Looking ahead, the authors suggest exploring dynamic tuning mechanisms, such as electrical gating or optical pumping, to complement the design-based tunability and introduce real-time control over plasmon polariton dispersion. Such developments would significantly broaden the functional repertoire of terahertz plasmonic devices, enabling adaptive systems capable of responding to environmental changes or user-defined signals.

Additionally, expanding this platform to hybrid systems combining topological insulators with other two-dimensional materials, like graphene, could yield synergistic benefits by leveraging their complementary electronic and optical properties. This could lead to multi-band operation and enhanced nonlinear effects critical for advanced photonic applications.

In conclusion, this pioneering study by Viti and colleagues represents a remarkable stride in nanophotonics and topological materials science. By tracing and tuning terahertz plasmon polaritons through custom-designed topological insulator metaelements, they demonstrate profound control over electromagnetic waves at nanoscales. This fusion of theory, materials science, and cutting-edge experimental techniques heralds a new era in terahertz technology, promising transformative impacts across scientific research and industry.

The meticulous integration of topological concepts with plasmonics evidenced here not only expands the fundamental understanding of light-matter interaction but also catalyzes the ongoing evolution of next-generation photonic devices. As efforts continue to harness these phenomena, the vision of compact, efficient, and tunable terahertz platforms for communication, sensing, and quantum technologies moves steadily into reality.

Such advancements epitomize the power of interdisciplinary research, where physics, materials engineering, and optical science converge to unlock unprecedented technological capabilities. The tunable dispersions engineered within these metaelements stand as a testament to human ingenuity in manipulating the quantum and classical realms of light.

This work is set to inspire a new wave of experimental and theoretical inquiry aimed at exploring and expanding the boundaries of topological plasmonics. The implications for future research are vast, including the exploration of dissipative and nonlinear effects, the impact of external field perturbations, and the integration of such systems into complex optoelectronic architectures.

Ultimately, this research not only enriches the scientific landscape but also lays a solid foundation for real-world innovations that will shape communications, sensing, and computation technologies in the coming decades, reinforcing the pivotal role of terahertz science in the technological frontier.

Subject of Research: Terahertz plasmon polaritons with tunable dispersion in topological insulator metaelements

Article Title: Tracing terahertz plasmon polaritons with a tunable-by-design dispersion in topological insulator metaelements

Article References:
Viti, L., Schiattarella, C., Sichert, L. et al. Tracing terahertz plasmon polaritons with a tunable-by-design dispersion in topological insulator metaelements. Light Sci Appl 14, 288 (2025). https://doi.org/10.1038/s41377-025-01884-0

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01884-0

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

In a remarkable milestone for condensed matter physics, a research team led by He Qinglin at the Center for Quantum Materials Science, School of Physics, Peking University, has successfully observed non-reciprocal Coulomb drag in Chern insulators for the first time. This groundbreaking discovery, published recently in Nature Communications, ushers in a new era for exploring electron-electron interactions within magnetic topological systems and deepens our understanding of quantum states governed by topological principles. Their work pushes the boundary of quantum transport phenomena in materials that have captivated physicists for their exotic electronic behaviors.

Coulomb drag is an inherently fascinating phenomenon where the movement of charged particles, or current, in one conductor can induce a voltage in a nearby but electrically isolated conductor. This interaction arises purely through long-range Coulomb forces — the electrostatic repulsion or attraction between charged particles — without any direct electrical contact. Previous studies have characterized Coulomb drag extensively in conventional two-dimensional electron systems, but exploring this effect in topological materials marked by non-trivial band structures has remained an elusive challenge until now.

Chern insulators represent a unique class of magnetic topological materials distinguished by their capacity to exhibit the quantum anomalous Hall effect (QAH). Unlike the classic quantum Hall effect, which necessitates external magnetic fields, Chern insulators display quantized Hall conductance due to intrinsic magnetization combined with robust chiral edge states that allow dissipationless transport along their boundaries. These edge modes are resilient to disorder and scattering, making Chern insulators prime candidates for applications in spintronics and quantum information.

The significance of this research lies not only in the pioneering observation of a non-reciprocal Coulomb drag effect but also in its implications for the control and detection of quantum states in advanced materials. Non-reciprocal phenomena, where the physical response depends on the direction of applied stimuli, are of increasing interest because they can enable new electronic functionalities, such as rectification and isolation, fundamental to quantum circuits and devices. By demonstrating such asymmetry in Coulomb drag, the research reveals intricate coupling mechanisms between quantum edge states mediated by Coulomb interactions.

To execute these experiments, the team employed molecular beam epitaxy (MBE) to grow ultrathin films of vanadium-doped (Bi,Sb)₂Te₃, a prototypical topological insulator system chemically engineered to promote a high-temperature quantum anomalous Hall effect. Utilizing a dual Hall-bar device architecture separated by a nanoscale vacuum gap ensured that coupling between layers occurred exclusively through Coulomb forces, eliminating unwanted tunneling currents that could mask the pure electrostatic interaction signals. This meticulous device design allowed precise probing of Coulomb drag dynamics under stringent experimental conditions.

Measurements were conducted at ultra-low temperatures reaching as low as 20 millikelvin and under perpendicularly applied magnetic fields to investigate the detailed interplay of magnetization and quantum transport phenomena. The researchers recorded both longitudinal (along current direction) and transverse (perpendicular to current flow) drag voltages, supplementing these with current-voltage (I-V) characterizations to differentiate between shot noise and mesoscopic fluctuation regimes. Temperature-dependent scaling analysis further confirmed the mesoscopic origins of the observed behaviors.

One of the most striking findings was the fixed polarity of longitudinal drag signals regardless of the current direction or magnetic field polarity. This rectification-like property indicates an inherent directionality in Coulomb drag, breaking conventional expectations of reciprocal behavior in electronic transport. Conversely, the transverse drag exhibited a clear dependence on the magnetization’s orientation, pinpointing the role of chiral edge state couplings between the layers as the dominant conduit for non-reciprocal interactions.

Delving into the underlying mechanisms, the study identified mesoscopic fluctuations as the primary factor influencing Coulomb drag at ultra-low temperatures, with a characteristic quadratic temperature dependence (T²). As bias currents increased, shot noise—quantum noise intrinsic to discrete charge carriers—became the prevailing driver, introducing nonlinearities in the drag voltages that correspond to changes in quantum transport regimes. This duality underscores the rich complexity of electron correlations in topological insulator systems and opens avenues for tuning device responses by controlling temperature and bias conditions.

Beyond fundamental physics, these insights have profound implications for the rapidly advancing field of topological quantum computing. The non-contact detection technique introduced here provides a sensitive probe for quantum states, particularly those relevant to qubit operations based on Majorana fermions and other exotic quasiparticles. The ability to monitor quantum coherence and state transitions without perturbing fragile quantum information is a critical milestone toward scalable and robust quantum technologies.

Moreover, the asymmetric Coulomb drag effect uncovered in Chern insulators could inspire innovative device architectures that leverage magnetization dynamics to enable low-power, chiral electronic components. Devices exploiting such directional coupling could revolutionize spintronic circuits, offering new pathways to integrate magnetic control with topological robustness for improved performance and energy efficiency.

This breakthrough underscores the power of combining cutting-edge materials science with precision quantum transport measurements to unlock unforeseen physical phenomena. By charting previously unexplored territory in non-reciprocal Coulomb drag, He Qinglin’s group has expanded our comprehension of topology-driven quantum interactions and set the stage for future explorations that may transform quantum electronics and computation.

The publication of this work in Nature Communications attests to its significance within the physics community and its potential impact across multiple domains including condensed matter physics, quantum materials engineering, and information science. As researchers worldwide build on these findings, this report will stand as a seminal contribution highlighting the interplay of topology, magnetism, and Coulomb interactions in quantum materials.

Pioneering experimental techniques, such as the dual Hall bar nanoscale gap device employed by the team, illustrate the meticulous engineering necessary to study subtle quantum effects. This approach could be adapted to investigate other topological phases or explore dynamic control of quantum states via external stimuli. The synergy between intrinsic material properties and novel measurement strategies signals a vibrant future for the field.

In sum, the first observation of non-reciprocal Coulomb drag in magnetic Chern insulators marks a milestone that bridges fundamental quantum physics and emerging quantum technology. This achievement expands the horizon for identifying and harnessing new quantum phenomena where topology, symmetry breaking, and electron correlations converge, paving the way for breakthroughs in understanding and utilizing complex quantum systems.

Subject of Research: Observation and analysis of non-reciprocal Coulomb drag phenomena in magnetic Chern insulators exhibiting quantum anomalous Hall effects.

Article Title: Non-reciprocal Coulomb drag between Chern insulators

News Publication Date: April 24, 2025

Web References:

• Nature Communications article DOI: 10.1038/s41467-025-58401-5
• Full article (PDF)

Keywords

Topology, Quantum states, Quantum anomalous Hall effect, Chern insulators, Coulomb drag, Quantum materials, Mesoscopic fluctuations, Shot noise, Non-reciprocal transport, Majorana qubits, Molecular Beam Epitaxy, Quantum computing

The research team, led by Prof. Catherine Dubourdieu from the Helmholtz-Zentrum Berlin and the Free University of Berlin, has published significant findings in the prestigious journal Nature Communications. The study explores a novel class of nanoislands formed on silicon substrates, examining their capability for electrical control. The collaboration includes esteemed institutions like the CEMES-CNRS in France and the Jozef Stefan Institute in Slovenia, reflecting the international nature of this cutting-edge research.

To begin their investigation, the scientists successfully fabricated BaTiO3 nanostructures that take the form of tiny trapezoidal islands. These nanoislands, which measure between 30-60 nanometers in width, exhibit stable polarization domains that are key for their functionality. This innovative work emphasizes the impact of the initial silicon wafer passivation step, which is crucial for inducing the formation of these nanoislands. The meticulous adjustment of the fabrication parameters demonstrates the significant relationship between material design and functional outcomes.

One of the highlights of this research is the reversible switching of the polarization domains within these nanoislands via an applied electric field. The researchers employed advanced techniques, including vertical and lateral piezoresponse force microscopy (PFM), to study the resulting domain patterns. The data derived from PFM measurements, combined with phase field modeling, unveiled a downward convergent polarization, a finding that correlates well with the observations made using scanning transmission electron microscopy (STEM).

A particularly noteworthy aspect of this study is the identification of a swirling component in the polarization field surrounding the nanoislands. This chiral feature imparts the nanoisoands with unique topological properties, characterized by textures resembling liquid vortices flowing into a constricting funnel. The ability to switch the polarity of these domains through the application of an electric field opens new avenues for addressing stability challenges within the field of chiral topological materials.

The ability to manipulate these chiral textures experimentally signals a substantial advancement in the understanding of ferroelectric materials at the nanoscale. Prof. Dubourdieu’s statement underscores this achievement: "In this work, we have shown that chiral topological textures can be stabilized by shaping nanostructures in an appropriate way." This concept of structural design leading to functional properties is vital for the future development of novel electronic devices that leverage the unique characteristics of these materials.

The implications of this research extend far beyond mere academic curiosity. The stabilization and manipulation of chiral textures could revolutionize data storage technologies by enabling ultra-high-density storage solutions, which could vastly improve the capabilities of current memory devices. Additionally, the integration of these materials into field-effect transistors could result in exceptionally energy-efficient devices, thus contributing to the overarching goal of sustainable technology development.

As the demand for more efficient and compact electronic devices continues to escalate, this research offers a glimpse into the future of nanoelectronics. The findings provide a foundation for further investigation into how external electric or optical stimuli can be utilized to direct and stabilize topological textures, making them easier to incorporate into existing systems. Continued interdisciplinary collaboration will be essential to advance this research field further.

Moreover, the work contributes to a broader understanding of the physical phenomena associated with polar materials, where the interplay of symmetry and topology grants rise to innovative applications in various sectors. With applications ranging from computing to renewable energy technologies, the potential benefits of this research are monumental.

In conclusion, the exploration of switchable topological polar states in BaTiO3 nanoislands represents a significant stride in nanotechnology. This foundational research demonstrates how manipulating nanoscale structures can lead to extraordinary electronic properties, setting the stage for future advancements in efficient and powerful electronic components. The continued evolution of this field could one day lead to technologies that see vast improvements in energy consumption and computational power.

Subject of Research: Switchable topological polar states in epitaxial BaTiO3 nanoislands on silicon
Article Title: Switchable topological polar states in epitaxial BaTiO3 nanoislands on silicon
News Publication Date: 20-Nov-2024
Web References: Link to the article
References: [Reference details can be added if required]
Image Credits: Laura Canil / HZB
Keywords: Ferroelectrics, Nanoelectronics, Chiral textures, Barium Titanate, Nanoscale technology, Electric fields, Data storage technologies.