Manipulating The Energy Gap Of A Three Dimensional Topological Insulator In The Thickness Limit Region

Topological state is currently a hot topic in the field of condensed matter physics.Due to the strong spin-orbit coupling of the system and the protection by time-reversal symmetry,the bulk energy bands are inverted and a gapless surface state band structure is formed within the bulk gap,forming conduction channels at the edges/surfaces,while the bulk still exhibits insulating properties.The second generation of strong three-dimensional topological insulators Bi₂Se₃,Bi₂Te₃,and Sb₂Te₃ all have a single Dirac-cone shaped surface state.These materials,since their discovery,attracted great attention due to the simple surface state structures therein.Among them,the Dirac points of Bi₂Te₃ and Bi₂Se₃ are either deeply buried in the bulk valence band or close to the bulk valence band,which is not easily distinguishable.Sb₂Te₃with the Dirac point detached from the bulk states,appears to be an excellent system for us to study the topological surface state structure.However,the growth of high-quality Sb₂Te₃ films is relatively difficult.In addition,there are relatively few experimental studies of the structures of ultra-thin films.We used molecular beam epitaxy to grow high quality ultra-thin Sb₂Te₃ films and characterized their energy gaps below the thickness limit using scanning tunneling microscopy.The experimental results obtained in the paper are as follows1.We found that high quality thin-layer Sb₂Te₃ films can be grown on graphene substrates by a two-step method(substrate temperature of 180°C+220-230°C,respectively)through growth-parameter tuning.We obtained films with wide terraces necessary for our subsequent Landau quantization studies.2.We derived the energy gap of ultra-thin Sb₂Te₃ films,which follows an exponential decay with the film thickness by scanning tunneling spectroscopy(STS).Meanwhile,based on the previous study,we further increase the magnetic field and find that the 4 QL Sb₂Te₃ film has a double-peak structure in STS with a large separation at the original zero-level peak position,which somehow verifies that 4QL is not the lowest phase transition point and there is still an energy gap of about 20 me V.Two possible oscillatory phase transition patterns of 1-4QL Sb₂Te₃ films between NIs and QSHs are derived by simple curve fitting according to the theoretical results.3.Sb₂Te₃ films composed of more domains with different lattice orientations and of different domain densities in each quintuple layer are grown on STO substrates to obtain twisted structure between quintuple layers.On this basis,the effect of the interlayer twist-angle on the energy gap is analyzed by combining the Moiréstripe period and gap size in STS.We found that the large anti-parallel stacking between the second and third layers leads to an increase of the energy gap.