Gap opening mechanism at the Dirac point in the electronic spectrum of GD-doped topological insulator

The electronic structure of magnetically doped topological insulator Bi$_{1.09}$Gd$_{0.06}$Sb$_{0.85}$Te$_{3}$ is studied in the vicinity of the Dirac point at various temperatures (above and below the Néel temperature, 1–35 K) and synchrotron radiation polarizations using angle-resolved photoelectron spectroscopy. It is shown that the energy gap exists in photoemission spectra at the Dirac point, which remains open above the long-range magnetic ordering temperature $T_N$. Measurements of magnetic properties by the superconducting magnetometry method show antiferromagnetic ordering with the paramagnetic transition temperature of 8.3 K. The studies of the temperature dependence of the Dirac cone state intensity by photoelectron spectroscopy confirm the existence of the magnetic transition and show the possibility of its indication directly from photoemission spectra. A more detailed analysis of the splitting between states of upper and lower Dirac cones (i.e., the energy gap) at the Dirac point in the photoelectron spectra shows the dependence of the gap at the Dirac point on the synchrotron radiation polarization type (28–30 meV for $p$-polarization and 22–25 meV for circularly polarized radiation of opposite chirality). The gap opening mechanism at the Dirac point above $T_N$ due to “coupling” of Dirac fermions with opposite momenta and spin orientations due to their interaction with the spin texture formed immediately during photoemission in the region of the photoemission hole at the magnetic impurity atom (Gd). It is shown that the gap at the Dirac point, measured above $T_N$, is dynamic and is formed immediately during photoemission. In this case, the gap nature remains magnetic (even in the absence of long-range magnetic ordering) and is caused by properties of magnetic topological insulator, which does control the gap invariability when passing through $T_N$. The dynamic nature of the generated gap is confirmed by its dependence on synchrotron radiation polarization.