Observation of acoustic Floquet π modes in a time-varying lattice

Time modulation provides an important degree of freedom for classical wave steering in topological manners. Rather than using the spatial dimension to mimic time, here we design and implement an acoustic temporal lattice with time-varying couplings to demonstrate a unique type of strongly localized Floquet \ensuremath{\pi} modes that are gauge independent and can be robustly excited at the frequency in the nontrivial band gap. We observe the topological phase transitions by changing modulation frequency, consistent with the calculated quasienergy spectra. Our work paves the way for investigating intrinsic characteristics of various temporal systems and may inspire other intriguing discoveries in topological systems, such as higher-order temporal Floquet topological insulators.