Band-to-band coupling can partly explain the difference in the band shapes of Bi2Te3 and Bi2Se3. Although both systems exhibit inverted bands at the Γ-point (Figures and), the Γ + 4 valence band edge state of Bi2Te3 is warped, whereas the Γ + 4 band is parabolic in Bi2Se3. This can partly be attributed to the coupling between the Γ + 4 valence band edge state and secondary Γ - 5 ⊕ Γ - 6 band, where the coupling is stronger in Bi2Te3 due to the smaller energy separation. Note that the Γ + 4 band in Bi2Te3 is warped due to synergistic effects from both the degree to which bands are inverted (where the bands are more inverted in Bi2Te3 than in Bi2Se3) and band-to-band coupling.