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Higher-order topological insulators and semimetals, which generalize theconventional bulk-boundary correspondence, have attracted extensive researchinterest. Among them, higher-order Weyl semimetals feature two-fold linearcrossing points in three-dimensional (3D) momentum space, 2D Fermi-arc surfacestates, and 1D hinge states. Higher-order nodal-point semimetals possessingWeyl points or Dirac points have been implemented. However, higher-ordernodal-line or nodal-surface semimetals remain to be further explored inexperiments in spite of many previous theoretical efforts. In this work, werealize a second-order nodal-line semimetal in 3D phononic crystals. The bulknodal lines, 2D drumhead surface states guaranteed by Zak phases, and 1D flathinge states attributed to kz-dependent quadrupole moments, are observed insimulations and experiments. Our findings of nondispersive surface and hingestates may promote applications in acoustic sensing and energy harvesting.

Observation of Higher Order Nodal Line Semimetal in Phononic Crystals