The study investigates the formation of a massive Dirac fermion state on the surface of a three-dimensional topological insulator, Bi$_2$Se$_3$, by introducing magnetic dopants to break the time-reversal symmetry (TRS). This process opens an energy gap at the Dirac point, and by carefully controlling the Fermi-energy (EF) to reside within this gap, an insulating massive Dirac fermion state is achieved. The researchers used angle-resolved photoemission spectroscopy (ARPES) to analyze the electronic structures of intrinsic, non-magnetically and magnetically doped Bi$_2$Se$_3$. They found that Fe-doped samples exhibit a gap at the Dirac point, while Mn-doped samples, which also introduce p-doping, allow EF to be tuned to reside within the surface Dirac gap. This realization of the insulating massive Dirac fermion state provides a platform for studying various topological phenomena, including the image magnetic monopole, half-quantum Hall effect, and topological contributions to the Faraday and Kerr effects. The ability to control the EF position through surface or bulk doping further enhances the material's potential for applications in condensed matter and particle physics.The study investigates the formation of a massive Dirac fermion state on the surface of a three-dimensional topological insulator, Bi$_2$Se$_3$, by introducing magnetic dopants to break the time-reversal symmetry (TRS). This process opens an energy gap at the Dirac point, and by carefully controlling the Fermi-energy (EF) to reside within this gap, an insulating massive Dirac fermion state is achieved. The researchers used angle-resolved photoemission spectroscopy (ARPES) to analyze the electronic structures of intrinsic, non-magnetically and magnetically doped Bi$_2$Se$_3$. They found that Fe-doped samples exhibit a gap at the Dirac point, while Mn-doped samples, which also introduce p-doping, allow EF to be tuned to reside within the surface Dirac gap. This realization of the insulating massive Dirac fermion state provides a platform for studying various topological phenomena, including the image magnetic monopole, half-quantum Hall effect, and topological contributions to the Faraday and Kerr effects. The ability to control the EF position through surface or bulk doping further enhances the material's potential for applications in condensed matter and particle physics.