Boundary-sensitive non-Hermiticity of Floquet Hamiltonian: spectral transition and scale-free localization

• URL: http://arxiv.org/abs/2603.22746v1
• Date: Tue, 24 Mar 2026 03:18:11 GMT
• Title: Boundary-sensitive non-Hermiticity of Floquet Hamiltonian: spectral transition and scale-free localization
• Abstract summary: We report a novel mechanism of boundary-sensitive PT symmetry breaking in one-dimensional Floquet systems.<n>We establish that a PT symmetry breaking transition occurs when the quasienergy bandwidth expands to cover the entire frequency Brillouin zone.
• Score: 7.1119324508676955
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: We report a novel mechanism of boundary-sensitive PT symmetry breaking in one-dimensional Floquet systems. By designing a time-periodic driving protocol, we realize a Floquet Hamiltonian that is Hermitian under periodic boundary conditions yet acquires non-Hermitian boundary terms under open boundary conditions due to the non-commutativity of driving Hamiltonians. We establish that a PT symmetry breaking transition occurs when the quasienergy bandwidth expands to cover the entire frequency Brillouin zone. This condition highlights a crucial difference from static non-Hermitian systems, where such transitions typically require band touching. Furthermore, we demonstrate that in the PT-broken phase, the eigenstates exhibit scale-free localization, a phenomenon arising from the specific system-size scaling of non-Hermitian terms. Finally, we provide a general framework for constructing multi-band models that exhibit this boundary-induced phase transition.

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