Mattia Iannetti

Doping tin surface epitaxially grown on silicon, Sn/Si(111), with boron atoms yields the appearance of a superconducting (SC) phase below T c ∼ 4 − 9 K . Even though the pairing mechanism remains unknown, experimental evidence of chiral d − wave superconductivity has been recently reported, thus ruling out a phonon-mediated pairing. Here, we study theoretically the SC phase of the doped Sn/Si(111) within a t − J model investigating different symmetries of the order parameter. We analyze the photoexcitation of the system by intense terahertz pulses and show that the polarization dependence of the induced current can distinguish between different symmetries of the SC gap, thus providing an experimental means to investigate the spectroscopic features of the Sn/Si(111), and possibly to many other systems, across the SC transition.

Spin-momentum-locked edge states of quantum spin Hall insulators provide a compelling platform for spintronic applications, owing to their intrinsic protection against backscattering from non-magnetic disorder. This protection emerges from time-reversal symmetry, which pairs Kramers partners of helical edge modes with opposite spin and momentum, thereby strictly forbidding elastic single-particle backscattering within the pair. Yet, contrary to the idealized notion of linear edge bands, the non-monotonic dispersions of realistic materials can host multiple Kramers pairs, reintroducing backscattering channels between them without violating time-reversal symmetry. Here, we investigate inter-Kramers pair backscattering in the non-linear edge bands of the quantum spin Hall insulator indenene, highlighting a critical aspect of edge state stability. Using quasiparticle interference in scanning tunneling spectroscopy – a direct probe of backscattering – we observe intra-band coupling between different Kramers pairs, while energy regions with only a single Kramers pair remain strictly protected. Supported by theoretical analysis, our findings provide an unprecedented experimental demonstration of edge state backscattering fully consistent with their underlying topological protection. This insight has profound implications for numerous quantum spin Hall insulator candidates, emphasizing that the mere presence of gap-traversing edge modes does not inherently guarantee their protection against backscattering.