Condensed-matter physics has now opened a different route into this territory, one that does not require finding a fundamental particle at all. Topological magnetoelectric theory predicts that the surface electrons of a topological insulator respond to an electric field in a remarkable way: when an active electric field is applied near the boundary between a topological insulator and a topologically trivial material such as vacuum, the electrons generate a magnetic response that can be mathematically described as an image magnetic monopole. In effect, the surface behaves as though a magnetic monopole were lurking just beneath it, mirroring the applied electric field the way an ordinary image charge mirrors an electric charge. The prediction is elegant, but realizing it in the laboratory demands two difficult things at once: a way to create an electric field that is intense and localized on nanometre scales, and a way to detect the faint magnetic response it induces. A new experimental study suggests that both requirements can be met with a single, well-established instrument.

Scanning tunnelling microscopy, or STM, turns out to be a natural engine for this kind of physics. When a sufficiently large bias voltage is applied between the microscope’s metallic tip and the sample, the instrument enters the field-emission regime: electrons are torn from the tip apex and travel across the tunnelling junction toward the sample surface. This process produces an intense, highly localized active electric field confined to the tiny gap between tip and surface, exactly the kind of field that topological magnetoelectric theory calls for. Crucially, the field in the junction has more structure than a simple perpendicular pull. It comprises the tip-induced field together with a transient field generated by the moving, field-emitted electrons themselves, and the tip-induced field possesses a radial component, Er, that points outward from the junction axis. The strength of this radial component depends inversely on the dielectric constant of the substrate and on the tip-sample distance, a dependence that would soon prove decisive in interpreting the experiment.

The same field-emission conditions create a second phenomenon that serves as the experiment’s readout mechanism. An electron that has been emitted but has not yet joined the sample remains outside the surface, where it is attracted by the image charge it induces in the material beneath it. This attractive potential can briefly trap the electron just outside the surface, binding it into a ladder of Rydberg-like electronic states known as image potential states, or IPSs. These states hover nanometres above the surface, exquisitely sensitive to anything that perturbs the electrostatic landscape there, including the active electric field produced by the tip. On ordinary, topologically trivial surfaces, IPSs form a conventional, predictable series of spectroscopic peaks. That predictability is precisely what makes them powerful: any deviation from the expected pattern signals that something unusual is happening at the surface, and the details of the deviation encode what that something might be.

A research team led by Lan Chen at the Institute of Physics of the Chinese Academy of Sciences exploited this sensitivity to examine IPSs on Bi(111), the (111) surface of bismuth, which possesses higher-order topological character. Higher-order topological insulators, or HOTIs, are materials whose unusual topology manifests not in conducting edge channels on every boundary, as in conventional topological insulators, but in more subtle surface electronic structure that emerges under specific conditions. Using low-temperature STM, the researchers first established a baseline: on topologically trivial reference surfaces, the spectra displayed the conventional series of IPS peaks, exactly as textbook physics predicts. When they turned to Bi(111), however, the spectra revealed a strikingly unexpected splitting of the IPS features, an anomaly with no place in the standard description of image-potential states and an immediate hint that the higher-order topology of the surface was somehow involved.

To trace the origin of the splitting, the researchers systematically varied two experimental parameters: the thickness of the Bi(111) films and the dielectric properties of the substrate supporting them. The results were revealing. In the thinnest Bi(111) films, the anomalous splitting was entirely absent; it appeared only as the films grew thick enough for the characteristic Bi(111) surface electronic structure to develop. This thickness dependence tied the anomaly directly to the emergence of the topological surface states rather than to any bulk property of bismuth or to a generic artifact of the measurement. The surface electronic structure, in other words, was not a bystander but an active participant in producing the splitting, consistent with the idea that the effect involves the special response of topological surface electrons to the local field environment created by the STM junction.

The substrate experiments delivered the second, and arguably most telling, clue. When Bi(111) was supported on semiconducting Si(111), which screens electric fields only weakly, the splitting grew as the tunnelling current was raised and the STM tip moved closer to the surface, intensifying the field in the junction. When Bi(111) rested instead on metallic Sb(111), a far more effective screening substrate, the splitting remained nearly unchanged regardless of tip approach. This contrast indicates that the anomalous splitting is governed not by the perpendicular electric field in the junction, the component one might naively expect to dominate, but by the radial component of the active electric field, the component whose strength is controlled by substrate screening. A purely electrostatic explanation tied to the perpendicular field could not account for this behavior, pointing instead toward a coupling between the radial field and the surface’s electronic structure that carries magnetic-response character.

Building on these observations, the researchers proposed a phenomenological picture that connects all the pieces. In their interpretation, the radial active electric field couples to the Bi(111) surface electronic structure and induces an effective magnetic-response-like perturbation, one that is compatible with the monopole-like topological magnetoelectric response predicted by theory. Such a perturbation would lift the degeneracy of the orbital-like sublevels of the image potential states, splitting what should be a single peak into the multiple components observed in the spectra. The thickness dependence enters because the surface states that participate in this coupling only exist once the film is thick enough; the substrate dependence enters because screening controls the radial field strength. Within this framework, every experimental observation finds a natural place, and the anomalous splitting becomes a candidate spectroscopic fingerprint of a phenomenon that has otherwise been accessible only through theoretical formalism.

The broader significance of the work lies in the experimental method itself. Field-emission IPS spectroscopy, as demonstrated here, accomplishes in a single measurement what previously required separate capabilities: it simultaneously creates a highly localized active electric field through the STM tip and sensitively probes the surface’s response to that field through the trapped image-potential states. This dual role transforms the STM from a passive imaging tool into an active probe of magnetoelectric coupling at higher-order topological surfaces. For a field in which the central prediction, the image magnetic monopole, has remained a theoretical construct awaiting a practical detection strategy, the demonstration that a splitting of Rydberg-like surface states can serve as its signature represents a genuine methodological advance, opening a new experimental route for investigating active-field responses in topological materials.

Much work remains before the interpretation is settled. The study, published in National Science Review with DOI 10.1093/nsr/nwag548, reports an experimental observation together with a phenomenological framework, and the proposed connection between the splitting and a monopole-like magnetoelectric response is presented as a possible, physically consistent explanation rather than a definitive proof. Nevertheless, the findings give physicists something they have rarely had in this area: a reproducible, tunable spectroscopic anomaly whose dependence on film thickness, substrate screening and tip-sample distance can be systematically mapped, and whose theoretical meaning can be sharpened with each new measurement. If subsequent studies confirm the monopole-like interpretation, the humble image-potential states that have hovered above surfaces since the early days of surface physics will have become the medium through which one of Dirac’s most enduring ideas finally meets a laboratory analogue, not in the cosmos or a collider, but in the nanometre-scale gap beneath a scanning tunnelling microscope tip.

Subject of Research: Anomalous image-potential state splitting on higher-order topological insulator Bi(111) as a spectroscopic probe of monopole-like topological magnetoelectric response

Article Title: Anomalous splitting of image-potential states on HOTI Bi(111) provides a new spectroscopic window into possible monopole-like topological magnetoelectric responses

Article References: Anomalous splitting of image-potential states on HOTI Bi(111) provides a new spectroscopic window into possible monopole-like topological magnetoelectric responses. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: magnetic monopoles, topological insulators, bismuth, image-potential states, scanning tunnelling microscopy, magnetoelectric effect, field emission, higher-order topology, surface states, spectroscopy, condensed matter physics, National Science Review

The achievement rests on a technique that has become one of the most powerful control knobs in attosecond science: two-colour waveform synthesis. Instead of hitting the tunnel junction with a single laser beam, the researchers combined two coherent light fields of different colours, a fundamental frequency and its second harmonic. When these two fields overlap, their sum is no longer a simple sinusoidal oscillation. Within each optical cycle, the combined electric field becomes an asymmetric waveform, with a strong half-cycle in one direction and a weaker, differently shaped excursion in the other. For an electron contemplating a quantum jump across the tunnel barrier, that asymmetry matters enormously. The instantaneous field tilts the barrier, lowers it on one side, and determines the precise instant, within the optical cycle, when tunnelling is most probable. By adjusting the relative phase between the two colours, the team could shift that favoured instant back and forth, and with it, flip the direction of the resulting current pulse.

What makes the result remarkable is the combination of timescale and spatial precision. The junction in a scanning tunnelling microscope is a few nanometres or less across, and the tunnelling process itself is confined to a region far smaller than that. The researchers demonstrated sub-ångström topographic sensitivity, meaning the laser-driven current responded to height changes on the surface at the level of individual atoms, while achieving a lateral spatial resolution of two nanometres. In other words, the attosecond current they generated and steered remained a genuinely microscopic probe, not a diffuse photoresponse smeared across the sample. And they did it under ambient conditions, at room temperature and in air, rather than in the cryogenic vacuum that most ultrafast microscopy experiments demand.

A persistent obstacle in laser-driven STM has been heat. When laser light strikes the tip or the sample, some of its energy is inevitably absorbed, raising the temperature of the junction and producing thermal expansion of the tip, thermally assisted currents and other artefacts that can masquerade as genuine ultrafast dynamics. Earlier femtosecond and picosecond experiments in laser-assisted STM were often haunted by exactly this problem, and distinguishing a true sub-cycle electronic response from a slow thermal signal became a central methodological challenge for the field. The Technion team addressed this directly. Because their current control depends on the sub-cycle waveform of the two-colour field, flipping the carrier-envelope phase flips the current direction, whereas a thermal effect would be completely indifferent to the sign of the electric field. The observed phase-dependent, direction-reversing current is therefore an unambiguous signature of coherent, lightwave-driven electron transport, and the experiment was explicitly designed to be free of thermal artefacts.

To understand the mechanism, the researchers turned to theory at two levels of description. On the one-electron side, they employed a strong-field model of tunnelling, in the spirit of the Keldysh framework that has underpinned strong-field physics since 1965, treating the electron as a wavepacket launched through a field-tilted barrier. On the many-body side, collaborators performed time-dependent density functional theory calculations of the full metal-insulator-metal junction, capturing the collective response of the electrons in the electrodes. Projecting the experimental data onto both theoretical descriptions revealed a consistent physical picture: a three-step transport process operating in the non-adiabatic tunnelling regime. In this regime, the optical field changes so quickly that the electron cannot follow it adiabatically; instead, the field launches the electron into the barrier, the wavepacket propagates across the gap while the field continues to evolve, and the electron is finally collected on the far side, with the whole sequence compressed into a fraction of an optical cycle.

From their theoretical analysis, the team extracted a concrete number for the duration of the current burst: 860 attoseconds. That figure, less than a single femtosecond, quantifies how briefly the electrons stream through the junction during each pulse. It also places the experiment firmly in the lightwave electronics regime, where the oscillating carrier wave of light, rather than a sequence of electronic switches, dictates the timing of current flow. Lightwave electronics has been advancing rapidly in recent years, with petahertz-scale field control demonstrated in nanotips, nanoantennas and solid-state devices, but the STM junction offers something none of those platforms can: the ability to position the driven current with atomic-scale precision on a surface and, in principle, to image ultrafast charge dynamics as they unfold at specific sites, such as a single molecule or a defect.

The lineage of this work runs through several landmark experiments. Attosecond science itself was born from the recognition that strong laser fields can ionize atoms within a fraction of an optical cycle, and that the emitted electrons carry timing information about that process. In 2011, researchers at the Max Planck Institute of Quantum Optics showed attosecond control of electrons emitted from a nanoscale metal tip, extending strong-field physics to nanostructures. Later experiments traced attosecond electron emission from tungsten nanotips and demonstrated attosecond field emission from nanotips driven by terahertz-scale waveforms. In parallel, ultrafast STM developed along its own track, first with pulsed lasers that suffered from thermal complications, then with single-cycle terahertz pulses that enabled femtosecond imaging of molecular motion, and more recently with all-optical sub-cycle microscopy on atomic length scales. The new work synthesizes these threads, bringing waveform-level attosecond control, the hallmark of gas-phase strong-field physics, into the spatially resolved world of the STM.

The two-colour approach itself has a distinguished history. Phase-controlled ionization with two colours was explored in atoms in the 1990s, and coherent control of above-threshold photoemission from tungsten nanotips followed in 2016. What the Technion team added was the junction geometry and the diagnostic power of the STM platform. In a tunnel junction, the current direction is a measurable, meaningful quantity: electrons can flow from tip to sample or from sample to tip, and the net laser-induced current reflects the asymmetry of the driving waveform. By scanning the two-colour phase and recording how the current reverses, the researchers effectively performed attosecond streaking inside the junction, using the optical waveform itself as the clock that timestamps the tunnelling event.

The practical implications reach toward a long-sought goal: triggering and imaging ultrafast charge dynamics at specific locations on a surface. Many of the most important processes in chemistry, materials science and nanotechnology, from charge transfer in molecular junctions to carrier dynamics in photovoltaic materials, begin with electron motion on attosecond to femtosecond timescales localized to nanometre-scale regions. A microscope that can launch a directional attosecond current pulse at a chosen point and then read out the response could watch those processes unfold in real space and real time simultaneously. The two-nanometre lateral resolution and sub-ångström vertical sensitivity reported here, achieved under ambient conditions, suggest that such spatiotemporal microscopy need not be confined to specialized cryogenic laboratories.

There remain challenges to overcome before attosecond STM becomes a routine tool. The current bursts are generated by intense optical fields, and extending the technique to more delicate samples, to insulating substrates and to single-molecule junctions will require careful control of the near-field enhancement at the tip. The theoretical framework, spanning one-electron strong-field models and full many-body simulations, will need to keep pace as experiments move to more complex materials. But the conceptual barrier has now been crossed. Electrons tunnelling through the gap of a scanning tunnelling microscope have been caught in the act, timed to within a few hundred attoseconds, and steered at will by the waveform of light. The fastest event in electronics and the sharpest eye in microscopy are, at last, looking at the same thing at the same time.

Subject of Research: Attosecond control of laser-driven electron tunnelling currents in a scanning tunnelling microscope junction

Article Title: Attosecond current control and timing in a scanning tunnelling microscope

Article References: Davidovich, D., Ma, B., Goldner, A., Cohen, S., Chen, Z., Borisov, A. G., & Krüger, M. (2026). Attosecond current control and timing in a scanning tunnelling microscope. Nature Photonics. https://doi.org/10.1038/s41566-026-02021-y

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02021-y

Keywords: attosecond science, scanning tunnelling microscopy, lightwave electronics, two-colour laser pulses, quantum tunnelling, ultrafast microscopy, strong-field physics, TDDFT, nanoscale junctions, carrier-envelope phase, electron transport, Nature Photonics