Observation of gauge field induced non-Hermitian helical skin effects

Synthetic gauge fields and non-Hermitian skin effects are pivotal to topological phases and non-Hermitian physics, each recently attracting great interest across diverse research fields. Realizing skin effects typically require non-symmetrical couplings or on-site gain and loss. Here, we theoretically and experimentally report that, under gauge fields, symmetry dissipative couplings can give rise to a nonreciprocal skin effect, hosting a pseudospin degree of freedom and featuring helical transport, dubbed as the \helical pseudospin skin effect". Before introducing the gauge fields, this model exhibits localized pseudospin edge modes and extended bulk modes, without skin effects. As the gauge field strength is applied from 0 to π, we observe the emergence of a non-Hermitian topological phase transition between the hybrid-order and second-order helical pseudospin skin effects. Our findings not only highlight gauge field enriched non-Hermitian topology, but also bring pseudospin-dependent accumulation into skin effects.