The effect of mechanical strain on the Dirac surface states in the (0001) surface and the cohesive energy of the topological insulator BiSe.

The band gap ( ) engineering and Dirac point tuning of the (0001) surface of 8 QLs (quintuple layers) thick BiSe slab are explored using the first-principles density functional theory calculations by varying the strain. The strain on the BiSe slab primarily varies the bandwidth, modifies the p - orbital population of Bi and moves the Dirac point of the (0001) surface of BiSe. The Dirac cone feature of the (0001) surface of BiSe is preserved for the entire range of the biaxial strain. However, around 5% tensile uniaxial strain and even lower value of volume conservation strain annihilate the Dirac cone, which causes the loss of topological (0001) surface states of BiSe. The biaxial strain provides ease in achieving the Dirac cone at the Fermi energy ( ) than the uniaxial and volume conservation strains. Interestingly, the transition from direct to indirect state of the (0001) surface of BiSe is observed in the volume conservation strain-dependent . The strain on BiSe, significantly modifies the conduction band of Se2 atoms near compared to Bi and Se1, and plays a vital role in the conduction of the (0001) surface of BiSe. The atomic cohesive energy of the BiSe slab is very close to that of (0001) oriented nanocrystals extracted from the Raman spectra. The strain-dependent cohesive energy indicates that at a higher value of strain, the uniaxial and volume conservation strain provides better stability than that of the biaxial strain (0001) oriented growth of the BiSe nanocrystals. Our study establishes the relationship between the strained lattice and electronic structures of BiSe, and more generally demonstrates the tuning of the Dirac point with the mechanical strain.