Fig. 2.

Low-magnetic-field Landau level transitions in ZrTe₅. (A) Absorption features T_n (1 $\le$ n $\le$ 7) in T(B)/T(B₀) spectra. The spectra are displaced from one another by 0.1 for clarity. (B, C) Squares of T_n and T₁* energies plotted as a function of magnetic field (B $\le$ 4 T). T_n correspond to the interband LL transitions: ${\text{LL}}_{-\left|N\right|}\to {\text{LL}}_{+\left|N-1\right|}$ (or ${\text{LL}}_{-\left|N-1\right|}\to {\text{LL}}_{+\left|N\right|}$). The nonzero $E_{{\text{T}}_1}^2\left(B\right)$ at zero field reveals a bandgap ∆. C, Inset shows the energy difference between T₁ and T₁*, which equals to the bandgap. (D) Absorption features T₁* in T(B)/T(B₀) spectra. T₁*, which is located at energies lower than T₁, arises from the intraband LL transition ${\text{LL}}_{+0}\to {\text{LL}}_{+1}$ (or ${\text{LL}}_{-1}\to {\text{LL}}_{-0}$). (E) Schematic of the Landau level spectrum without a Zeeman splitting and the allowed optical transitions, shown in a $E^2$−$B$ plot.