Unconventional Topological Weyl Dipole Phonon

A pair of Weyl points (WPs) with opposite topological charges can exhibit an additional higher-order Z₂ topological charge, giving rise to the formation of a Z₂ Weyl dipole (WD). Owing to the nontrivial topological charge, Z₂ WDs should also appear in pairs, and the WPs within each Z₂ WD can not be annihilated when meeting together. As a novel topological state, the topological Weyl dipole (TWD) phase has garnered significant attention, yet its realization in crystalline materials remains a challenge. Here, through first-principles calculations and theoretical analysis, the existence of the nontrivial unconventional WD phase is demonstrated in the phonon spectra of the P6₃ type Y(OH)₃. Particularly, the nontrivial unconventional WD in this system is protected by a quantized quadrupole moment, and it is distinguished from conventional WD, as it comprises an unconventional charge-3 WP with charge of -3 and three conventional charge-1 WPs with charge of +1. Consequently, the nontrivial unconventional WD phase in Y(OH)₃ features unique 2D sextuple-helicoid Fermi-arc states on the top and bottom surfaces, protected by the topological charges, as well as 1D hinge states that connect the two nontrivial unconventional WDs along the side hinges, guaranteed by the quantized quadrupole moment.