Multi Weyl Points and the Sign Change of Their Topological Charges in   Woodpile Photonic Crystals
  We show that Weyl points with topological charges 1 and 2 can be found in very simple chiral woodpile photonic crystals, which can be fabricated using current techniques down to the nano-scale. The sign of the topological charges can be tuned by changing the material parameters of the crystal, keeping the structure and the symmetry unchanged. The underlying physics can be understood using a tight binding model, which shows that the sign of the charge depends on the hopping range. Gapless surface states and their back-scattering immune properties are also demonstrated in these systems. 
Ming-Li Chang, Meng Xiao, Wen-Jie Chen, C. T. Chan
physics.optics cond-mat.mes-hall