Probe-dependent Dirac-point gap in the gadolinium-doped thallium-based topological insulator TlBi0.9Gd0.1Se2

A tunable gap in the topological surface state is of great interest for novel spintronic devices and applications in quantum computing. Here, we study the surface electronic structure and magnetic properties of the Gd-doped topological insulator $\mathrm{Tl}{\mathrm{Bi}}_{0.9}{\mathrm{Gd}}_{0.1}{\mathrm{Se}}_{2}$. Utilizing superconducting quantum interference device magnetometry, we show paramagnetic behavior down to 2 K. Combining spin- and angle-resolved photoemission spectroscopy with different polarizations of light, we demonstrate that the topological surface state is characterized by the Dirac cone with a helical spin structure and confirm its localization within the bulk band gap. By using different light sources in photoemission spectroscopy, various Dirac-point gap values were observed: 50 meV for $h\ensuremath{\nu}=18\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ and 20 meV for $h\ensuremath{\nu}=6.3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. Here, we discuss the gap observation by the angle-resolved photoemission spectroscopy method as a consequence of the scattering processes. Simulating the corresponding spectral function, we demonstrate that the asymmetric energy-distribution curve of the surface state leads to an overestimation of the corresponding gap value. We speculate that 20 meV in our case is a trustworthy value and attribute this gap to be originated by scattering both on magnetic and charge impurities provided by Gd atoms and surface defects. Given the complexity and importance of scattering processes in the topological surface state together with our observations of distinctive photoemission asymmetry, we believe our results are important for research of the massive Dirac fermions in novel quantum materials.