Fig. 4: Measurements of the Berry phase.
a The theoretical energy spectrum of the Hamiltonian \(H^{\prime} ({{{{{{{\boldsymbol{k}}}}}}}})\) in Eq. (8) over the Brillouin zone, showing the existence of four Dirac points between the two lowest energy bands located at k = (kx, ky) ≈ (±0.26π, ± 0.4π). (b) The experimentally measured Berry phases for the lowest energy eigenstate \(\left|{u}_{1}({{{{{{{\boldsymbol{k}}}}}}}})\right\rangle\) along four closed paths composed of discrete momenta (blue lines and points) in the Brillouin zone enclosing the corresponding Dirac points (red crosses). For each Dirac point, the measured Berry phase is γ = π.
