Spinful topological phases in acoustic crystals with projective PT symmetry

TL;DR

This paper demonstrates experimentally that acoustic crystals without intrinsic spin can exhibit spinful topological phases through projective symmetry representations, expanding the classification of topological matter.

Contribution

It provides the first experimental realization of spinful topological phases in spinless acoustic systems using projective PT symmetry, overcoming fundamental spin class constraints.

Findings

Realized a DIII-class topological phase with a 2Z winding number

Observed Kramers degenerate bands and boundary modes

Showed spinless systems can host spinful topological phases

Abstract

For the classification of topological phases of matter, an important consideration is whether a system is spinless or spinful, as these two classes have distinct symmetry algebra that gives rise to fundamentally different topological phases. However, only recently has it been realized theoretically that in the presence of gauge symmetry, the algebraic structure of symmetries can be projectively represented, which possibly enables the switch between spinless and spinful topological phases. Here, we report the first experimental demonstration of this idea by realizing spinful topological phases in "spinless" acoustic crystals with projective space-time inversion symmetry. In particular, we realize a DIII-class one-dimensional topologically gapped phase characterized by a 2Z winding number, which features Kramers degenerate bands and Kramers pair of topological boundary modes. Our work thus overcomes a fundamental constraint on topological phases by spin classes.