Riyi Zheng

Valley Hall phases (VHPs) in electronic and classical wave systems have been extensively explored and brought fruitful wave manipulations and associated applications. However, the concept of VHPs is so far restricted to two-dimensional (2D) systems. VHPs in 3D platforms remain an open question, which is not a straightforward extension of theirs 2D counterparts. In this work, we generalize the 2D VHPs into the 3D case in phononic crystals, which are derived from a nodal-line semimetal.

As a distinctive feature unique to non-Hermitian systems, non-Hermitian skin effect displays fruitful exotic phenomena in one or higher dimensions, especially when conventional topological phases are involved. Among them, hybrid skin-topological effect is theoretically proposed recently, which exhibits anomalous localization of topological boundary states at lower-dimensional boundaries accompanied by extended bulk states.

We report the realization of an acoustic quadratic Weyl semimetal, which hosts a pair of quadratic Weyl points with opposite charges ±2 in the two lowest bands. The quadratic Weyl points, protected by C4 and time-reversal symmetries, have quadratic dispersions along the kx and ky directions and linear dispersion along the kz direction. Nonzero charges of the quadrative Weyl points give rise to the Fermi-arc boundary states.