Potential for detecting a monopole-like topological magnetoelectric response using image potential states

Magnetic monopoles, hypothetical particles behaving as isolated magnetic charges, have long been predicted by theories beyond the standard model but remain experimentally elusive. Qi et al. proposed that magnetic monopoles could be realized in real space by applying an active electric field at a topological-insulator/vacuum interface. Here, we use field-emission scanning tunneling microscopy to investigate image potential states (IPSs) on higher-order topological insulator Bi(111) and observe anomalous IPS splitting. By varying substrate dielectric properties and film thickness, we find that the splitting correlates with the radial active electric field and the emergence of Bi(111) topological surface states. These observations are compatible with a phenomenological scenario in which the active radial electric field couples to the Bi (111) surface states and induces an effective monopole-like topological magnetoelectric response. Our results establish field-emission IPS spectroscopy as a sensitive platform for probing active-field-induced magnetoelectric responses at topological surfaces.