Topological insulators and their aging

Insulators are materials having an energy gap between the highest occupied band (valence band) and the lowest unoccupied band (conduction band). Topological insulators are a special type of such materials, which possess gapless states with novel electromagnetic properties protected by time reversal symmetry at the surface of bulk insulators. Enormous research has been carried out on these materials as they are expected to bring immense technological advances and new possibilities in the fields of spintronics, quantum computation, dissipationless charge transfer etc. Since the surface states of topological insulators are time reversal symmetry protected, they are expected to be immune to weak disorder, chemical passivation of the surface or temperature change. However, significant discrepancy from such behavior has been found experimentally in various materials. We studied the detailed electronic structure and its aging of a topological insulator, Bi2Se3 employing high resolution photoemission spectroscopy. Both the band structure results and high resolution angle resolved photoemission data reveal significantly different surface electronic structure for different surface terminations. Furthermore, oxygen impurity on Se terminated surface exhibits an electron doping scenario, while oxygen on Bi terminated surface corresponds to a hole doping scenario. The intensity of the Dirac states reduces with aging indicating fragility of the topological order due to surface impurities. References: 1. D. Biswas, S. Thakur, K. Ali, G. Balakrishnan,, and K. Maiti, Sci. Rep. 5, 10260 (2015). 2. D. Biswas and K. Maiti, EPL 110, 17001 (2015). 3. D. Biswas, S. Thakur, G. Balakrishnan, and K. Maiti, Sci. Rep. 5, 17351 (2015).