Unconventional Dirac-Weyl semimetal with ultralong surface arc in phononic crystals

The fascinating properties of unconventional Dirac and Weyl nodes in crystalline systems, carrying a topological charge of $\ifmmode\pm\else\textpm\fi{}2$, have captured the attention of researchers. Here, applying $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ analysis, we demonstrate that space group 92 facilitates the emergence of charge-2 Dirac point and charge-2 Weyl point. Notably, the constructed phononic crystal is allowed to be a concise configuration containing only a charge-2 Dirac point and a charge-2 Weyl point in pairs appearing simultaneously, that is, the realization of the unconventional Dirac-Weyl semimetal (UDWS). The projected dispersion bands reveal the unique double helical surface arcs of UDWS, which alternate from the projected charge-2 Dirac point to the projected charge-2 Weyl point, spanning the entire Brillouin zone with ultralong trajectories. The unique global surface arc characteristic demonstrates robust resilience against defects, underscoring its suitability as an optimal platform for investigating ultralong surface arcs.