Time-periodic corner states from Floquet higher-order topology

• Published in: Nature communications Vol. 13; no. 1; pp. 11 - 6
• Main Authors: Zhu, Weiwei, Xue, Haoran, Gong, Jiangbin, Chong, Yidong, Zhang, Baile
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 10.01.2022; Nature Publishing Group; Nature Portfolio
• Subjects: 639/766/119/2792/4128; 639/766/25/3927; Acoustic insulation; Acoustic measurement; Acoustic waveguides; Acoustics; Boundaries; Crystals; Humanities and Social Sciences; Lattice vibration; multidisciplinary; Oscillations; Science; Science (multidisciplinary); Time dependence; Topological insulators; Topology
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-021-27552-6

The recent discoveries of higher-order topological insulators (HOTIs) have shifted the paradigm of topological materials, previously limited to topological states at boundaries of materials, to include topological states at boundaries of boundaries, such as corners. So far, all HOTI realisations have been based on static systems described by time-invariant Hamiltonians, without considering the time-variant situation. There is growing interest in Floquet systems, in which time-periodic driving can induce unconventional phenomena such as Floquet topological phases and time crystals. Recent theories have attempted to combine Floquet engineering and HOTIs, but there has been no experimental realisation so far. Here we report on the experimental demonstration of a two-dimensional (2D) Floquet HOTI in a three-dimensional (3D) acoustic lattice, with modulation along a spatial axis serving as an effective time-dependent drive. Acoustic measurements reveal Floquet corner states with double the period of the underlying drive; these oscillations are robust, like time crystal modes, except that the robustness arises from topological protection. This shows that space-time dynamics can induce anomalous higher-order topological phases unique to Floquet systems. A combination of Floquet engineering and higher-order topological insulators has been lately only theoretically proposed. Here the authors experimentally realize an acoustic waveguides array to demonstrate a Floquet higherorder topological insulator extending its band topology from first order to higher order.