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Non-Hermiticity may significantly influence the topology of a photonic/electronic lattice, giving rise to point gaps, which have attracted much attention in recent years. While the influences of the point-gap topology on the bulk-boundary correspondence for the line gap have been widely studied, the topological lattice that simultaneously processes the line-gap and point-gap topological transitions has not been reported. Here, a strategy to simultaneously manipulate the line-gap and point-gap topologies in non-Hermitian lattices is proposed. By introducing the asymmetric intercell coupling, the line-gap topological transition process is demonstrated. By further considering the nonreciprocal coupling between the nearest neighboring unit cells, the point-gap and line-gap topological transitions can be simultaneously realized. Finally, the influence of the next nearest coupling on non-Hermitian line-gap and point-gap topologies is also discussed. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version This work was sup-ported by Singapore Ministry of Education (No. MOE2018-T2-2-189(S)),A*Star AME IRG Grant (No. A20E5c0095), Programmatic Funds (No.A18A7b0058), and National Research Foundation Singapore CompetitiveResearch Program (Nos. NRF-CRP22-2019-0006 and NRF-CRP23-2019-0007).

Topological phases of matter have remained an active area of research in the last few decades. Periodic driving is known to be a powerful tool for enriching such exotic phases, which leads to various phenomena with no static analogs. One such phenomenon is the emergence of the elusive $pi/2$ modes, i.e., a type of topological boundary state pinned at a quarter of the driving frequency. The latter may lead to the formation of Floquet parafermions in the presence of interaction, which is known to support more computational power than Majorana particles. In this work, we experimentally verify the signature of $\pi/2$ modes in an acoustic waveguide array, which is designed to simulate a square-root periodically driven Su-Schrieffer-Heeger model. This is accomplished by confirming the $4T$-periodicity ($T$ being the driving period) profile of an initial-boundary excitation, which we also show theoretically to be the smoking gun evidence of $\pi/2$ modes. Our findings are expected to motivate further studies of $\pi/2$ modes in quantum systems for potential technological applications., Comment: 6 pages, 3 figure. Comments are welcome

Symmetry plays a key role in modern physics, as manifested in the revolutionary topological classification of matter in the past decade. So far, we seem to have a complete theory of topological phases from internal symmetries as well as crystallographic symmetry groups. However, an intrinsic element, i.e., the gauge symmetry in physical systems, has been overlooked in the current framework. Here, we show that the algebraic structure of crystal symmetries can be projectively enriched due to the gauge symmetry, which subsequently gives rise to new topological physics never witnessed under ordinary symmetries. We demonstrate the idea by theoretical analysis, numerical simulation, and experimental realization of a topological acoustic lattice with projective translation symmetries under a $Z_2$ gauge field, which exhibits unique features of rich topologies, including a single Dirac point, M\"{o}bius topological insulator and graphene-like semimetal phases on a rectangular lattice. Our work reveals the impact when gauge and crystal symmetries meet together with topology, and opens the door to a vast unexplored land of topological states by projective symmetries., Comment: 7 pages, 4 figures