Kang Yang

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Geometric phases and Berry curvatures are features of physical states following adiabatic evolutions. They lead to rich phenomena in transports and topological phases in metals. Many these interesting properties can also arise in photons transmitting in periodic media. There, dissipation provides another controlling method and gives rise to effective non-Hermitian evolutions. In this seminar, I will demonstrate a new form of topological transition in parity-time symmetric systems, where upon increasing loss and gain, the eigenstates can spontaneously break the symmetry. Their non-Abelian Berry curvatures quantise qualitatively differently across the symmetry breaking. I will show that the sponntaneous PT-breaking obeys a universal topological duality principle: the topology of the eigenstates must remain conserved, but it manifests as Euler numbers in the symmetry-preserving regime while as Chern numbers in the symmetry-breaking regime. This contrast paradigm to conventional topological transitions originates from the coexistence of two different gauge structures that goes beyond Cartan's symmetric spaces. The results demonstrate that loss and gain can engineer non-Abelian gauge structures and provide new ideas to prepare topological bands.

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FU Berlin
0.03
Contact: Sebastian Diehl