Surface states in a 3D topological insulator: The role of hexagonal   warping and curvature
  We explore a combined effect of hexagonal warping and of finite effective mass on both the tunneling density of electronic states (TDOS) and structure of Landau levels (LLs) of 3D topological insulators. We find the increasing warping to transform the square-root van Hove singularity into a logarithmic one. For moderate warping an additional logarithmic singularity and a jump in the TDOS appear. This phenomenon is experimentally verified by direct measurements of the local TDOS in Bi$_2$Te$_3$. By combining the perturbation theory and the WKB approximation we calculate the LLs in the presence of hexagonal warping. We predict that due to the degeneracy removal the evolution of LLs in the magnetic field is drastically modified. 
E.V. Repin, V.S. Stolyarov, T. Cren, C. Brun, S.I. Bozhko, L.V.   Yashina, D. Roditchev, and I.S. Burmistrov
cond-mat.mes-hall