25 Sep 2025 Thu 4 pm

I would talk about my recent works on non-hermitian topological systems, where exceptional degeneracies appear. In one of our recent works[1], we report the circularly polarized light (CPL) driven exceptional physics in a multi-Weyl semimetals. We show that the driving is not only a key ingredient to control the position of the exceptional contours (ECs); light also has the ability to generate new ECs [1]. Interestingly, we also have demonstrated the topological charge distribution and Lifshitz transition, which are controllable by the driving field in such generated ECs. Apart from the CPL, one might also be interested in bi-circularly (BCL) polarized light driven topological phases. The ”BCL” refers to two superimposed circularly polarised lights with a frequency ratio of η merging and having a phase difference α. The main idea of using the BCL light is to have increased tunability on different topological phases. Importantly, BCL consists of two circularly polarized lights having different frequencies and has a crucial role in the spatial inversion symmetry and rotational symmetry of the system. The BCL shows a rose like pattern and has a rotational symmetry of (n1 + n2)/gcd(n1, n2) fold, where n1 and n2 are the two frequencies of the circularly polarized lights that produce BCL. The Fermi surface topology of a triple non-hermitian (NH) Weyl-semimetal (WSM) driven by BCL light is presented in a recent work [2]. BCL light, however, modifies the symmetry features of NH triple Weyl and causes an unusual new kind of band swapping. We observe swapping between the imaginary bands (with or without exceptional degenaracies), which causes unique Fermi surfaces in the form of double rings and self links. Interestingly these unique Fermi surfaces can be tuned by the combined action of non-hermicity and bi-polarized light parameters. The novelty of our work lies in the fact that the topology of a non-hermitian triple Weyl semimetal is tuned by the application of bi-circular light. The discussed topological phases in this work disappears for a periodic circularly/linearly polarized light, even in the presence of non-hermiticity. The tuning of various topological non-hermitian phases by the phase angles of the bi-circular light is something new and stands out from the line of work in non-hermitian topology. The Berry curvature analysis shows the evidence of such nodal structures, which appear as a result of unusual band swapping only between the imaginary bands (with or without exceptional degeneracies).

1. 1.Light-driven Lifshitz transitions in non-Hermitian multi-Weyl semimetals, Debashree Chowdhury, Ayan Banerjee, and Awadhesh Narayan Phys. Rev. A 103, L051101, (2021).
   

2. 2.Phase transition from Weyl to self-linked semimetal using bi-circular laser, Debashree Chowdhury, Accepted in Journal of Physics: Condensed matter, (2025).

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