Microscopic Insights into Magnetic Warping and Time-Reversal Symmetry Breaking in Topological Surface States of Rare-Earth-Doped Bi2Te3

symmetry (TRS) at the topological surface state (TSS) enables the opening of a Dirac gap, which is essential for realizing quantum anomalous Hall physics. This work investigates the impact of submonolayer deposition of magnetic rare-earth adatoms on the prototypical topological insulator Bi2Te3, characterized by a strongly warped Fermi surface. Scanning tunneling microscopy (STM), core-level photoemission spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), and quasiparticle interference (QPI) mapping are combined to reveal direct evidence of local interactions between erbium (Er) atoms and the substrate, leading to significant modifications of the TSS. Erbium deposition induces a warping transition of the Fermi surface from a snowflake to a star-of-David–like geometry, along with a Dirac point gap opening and spectral splitting near the Γ point. QPI maps confirm the reconstructed surface band topology through modified scattering patterns consistent with TRS breaking. These results identify a microscopic mechanism for magnetic interaction at the surface of a topological insulator and establish magnetic rare-earth doping as an effective strategy to tailor topological electronic states with atomic-scale control.