Magnetic monopoles are hypothetical particles that behave as isolated north or south magnetic poles. Unlike electric charges, which can exist independently as positive or negative charges, ordinary magnets always contain both north and south poles. Even when a magnet is repeatedly divided, an isolated magnetic pole cannot be obtained. In 1931, Paul Dirac showed that the existence of magnetic monopoles could provide a fundamental explanation for the quantization of electric charge. Since then, magnetic monopoles have become an important concept in particle physics, quantum field theory and grand unified theories. However, elementary magnetic monopoles have never been directly observed.
Condensed-matter physics offers another route to exploring monopole-like phenomena. Topological magnetoelectric theory predicts that when an active electric field is introduced near the interface between a topological insulator and a topologically trivial material such as vacuum, the surface electrons can generate a magnetic response that may be described as an “image magnetic monopole”. The challenge is how to create such a highly localized electric field experimentally and, more importantly, how to detect the resulting response.
A local electric field from STM
Scanning tunnelling microscopy, or STM, provides a natural way to create a strong and highly localized electric field. When a sufficiently high bias is applied between the metallic tip and the sample, the STM enters the field-emission regime. Electrons are emitted from the tip and travel across the junction towards the sample surface, producing an intense local active electric field.
At the same time, an emitted electron outside the sample is attracted by the image charge it induces inside the material. This attractive potential can trap the electron outside the surface and form a series of Rydberg-like electronic states known as image potential states, or IPSs, which provide a particularly sensitive spectroscopic probe of how a surface responds to an active electric field.
An anomalous splitting on Bi(111)
A research team led by Lan Chen at the Institute of Physics, Chinese Academy of Sciences, used low-temperature STM to investigate IPSs on Bi(111), a material with higher-order topological character. On topologically trivial reference surfaces, the researchers observed the conventional series of IPS peaks. On Bi(111), however, the spectra showed a strikingly unexpected splitting feature.
The researchers then changed both the thickness of the Bi(111) films and the dielectric properties of the supporting substrate. The anomalous splitting was absent in the thinnest Bi(111) films and appeared as the characteristic Bi(111) surface electronic structure developed with increasing thickness.
The substrate produced another important contrast. On Bi(111) supported by semiconducting Si(111), the splitting increased as the tunnelling current was raised and the STM tip approached the surface. On Bi(111) supported by more strongly screening metallic Sb(111), the splitting remained nearly unchanged. This difference indicates that the anomalous splitting is closely related to the radial component of the active electric field rather than simply to the perpendicular electric field in the STM junction.
A possible monopole-like topological magnetoelectric response
Based on these observations, the researchers proposed a phenomenological picture in which the radial active electric field couples to the Bi(111) surface electronic structure and induces an effective magnetic-response-like perturbation compatible with a monopole-like topological magnetoelectric response. Such a perturbation could lift the degeneracy of orbital-like IPS sublevels and produce the observed anomalous splitting. The study shows that field-emission IPS spectroscopy can simultaneously create a highly localized active electric field and sensitively probe the resulting surface response. It provides a new experimental route for investigating active-field responses and magnetoelectric coupling at higher-order topological surfaces.
National Science Review
Experimental study