Fig. 4
IR conductivity of SS in magnetic fields. a A schematic of the SS band structure (dark blue) and that of the bulk (red) along two high-symmetry directions for one of the four Dirac cones inside the surface Brillouin zone. \(E_{{\rm{H1}}}^{{\rm{DP}}}\), \(E_{{\rm{H2}}}^{{\rm{DP}}}\), and \(E_{{\rm{L}} \pm }^{{\rm{DP}}}\) are energies of the Dirac points associated with the two Dirac cones located at the \(\overline {\rm{X}} \) points and the Dirac nodes along the \(\overline {\Gamma {\rm{X}}} \) line, respectively. E VHS1 and E VHS2 are energies of the two Van Hove singularities in the band structure. b A schematic of LLs of the SS in the same energy scale as that in a. While the LLs between E VHS1 and E VHS2 in the gray-shaded area have nontrivial dispersions32 (not shown here), the LLs near E F are well approximated by LLs of two independent Dirac cones at \(E_{{\rm{H}}1}^{{\rm{DP}}}\) and \(E_{{\rm{H}}2}^{{\rm{DP}}}\) as illustrated by the dashed lines in the shaded area. The CR of the SS associated with the Dirac cone at \(E_{{\rm{H}}2}^{{\rm{DP}}}\) is shown by the vertical arrows. c The real part of the model optical conductivity Re σ xx(ω) in several magnetic fields that are used to simulate the R(ω, B) spectra below 70 meV. The strong peak below 15 meV in Re σ xx (ω) arises from CR of the SS, which represents the spatially averaged 3D optical conductivity of the SS within the IR penetration depth. d The CR energy of SS \(\omega _{\rm{c}}^{{\rm{ss}}}\) as a function of magnetic field. The error bars represent the range of \(\omega _{\rm{c}}^{{\rm{ss}}}\) values that can be used in our model to produce excellent fit of the R(ω, B) data within experimental uncertainties as discussed in Supplementary Note 2. The shaded area indicates the range of \(\omega _{\rm{c}}^{{\rm{ss}}}\) values estimated from previous STM and ARPES experiments
