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Homotopy, symmetry, and non-Hermitian band topology
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0002-1784-4619
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0001-7065-5828
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Number of Authors: 62024 (English)In: Reports on progress in physics (Print), ISSN 0034-4885, E-ISSN 1361-6633, Vol. 87, no 7, article id 078002Article in journal (Refereed) Published
Abstract [en]

Non-Hermitian matrices are ubiquitous in the description of nature ranging from classical dissipative systems, including optical, electrical, and mechanical metamaterials, to scattering of waves and open quantum many-body systems. Seminal line-gap and point-gap classifications of non-Hermitian systems using K-theory have deepened the understanding of many physical phenomena. However, ample systems remain beyond this description; reference points and lines do not in general distinguish whether multiple non-Hermitian bands exhibit intriguing exceptional points, spectral braids and crossings. To address this we consider two different notions: non-Hermitian band gaps and separation gaps that crucially encompass a broad class of multi-band scenarios, enabling the description of generic band structures with symmetries. With these concepts, we provide a unified and comprehensive classification of both gapped and nodal systems in the presence of physically relevant parity-time ( PT ) and pseudo-Hermitian symmetries using homotopy theory. This uncovers new stable topology stemming from both eigenvalues and wave functions, and remarkably also implies distinct fragile topological phases. In particular, we reveal different Abelian and non-Abelian phases in PT -symmetric systems, described by frame and braid topology. The corresponding invariants are robust to symmetry-preserving perturbations that do not induce (exceptional) degeneracy, and they also predict the deformation rules of nodal phases. We further demonstrate that spontaneous PT symmetry breaking is captured by Chern-Euler and Chern-Stiefel-Whitney descriptions, a fingerprint of unprecedented non-Hermitian topology previously overlooked. These results open the door for theoretical and experimental exploration of a rich variety of novel topological phenomena in a wide range of physical platforms.

Place, publisher, year, edition, pages
2024. Vol. 87, no 7, article id 078002
Keywords [en]
exceptional points, metamaterials, non-Hermitian systems, PT symmetry, topological bands
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:su:diva-238586DOI: 10.1088/1361-6633/ad4e64ISI: 001261089100001PubMedID: 38957897Scopus ID: 2-s2.0-85197683379OAI: oai:DiVA.org:su-238586DiVA, id: diva2:1931591
Available from: 2025-01-27 Created: 2025-01-27 Last updated: 2025-04-25Bibliographically approved
In thesis
1. Topology and non-Hermiticity
Open this publication in new window or tab >>Topology and non-Hermiticity
2025 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Topology is a branch of mathematics that studies properties that remain unchanged under continuous deformations. In physics, topology is used to describe phenomena that are robust against small perturbations. A well-known example is topological insulators—materials that act as insulators in their interior while conducting along their surface. These conducting states are protected by topological properties and persist even when the material is slightly modified. Over the past few decades, significant effort has been devoted to understanding and classifying different types of topological phases, which describe the various ways in which such robust properties can emerge in nature.

In recent years, interest has grown in dissipative systems, where energy losses play a central role. These systems are described using non-Hermitian Hamiltonians, which extend the conventional quantum mechanical framework.

This dissertation explores how non-Hermitian physics affects the topology and classification of topological phases. In particular, we investigate a type of topological charge known as exceptional points, which arise exclusively in non-Hermitian systems. These points are characterized by a topological charge that describes how energy bands intertwine around them. We focus specifically on how certain symmetries can stabilize exceptional points and shape their properties. Finally, we examine multifold exceptional points—a more intricate class of these singularities—and their topological characteristics.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2025. p. 73
Keywords
non-Hermitian, Topology, Exceptional points, Topological phases
National Category
Condensed Matter Physics
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-242514 (URN)978-91-8107-274-7 (ISBN)978-91-8107-275-4 (ISBN)
Public defence
2025-06-12, FB52, Roslagstullsbacken 21 and online via Zoom, public link is available at the department website, Stockholm, 14:00 (English)
Opponent
Supervisors
Available from: 2025-05-20 Created: 2025-04-25 Last updated: 2025-05-15Bibliographically approved

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König, J. Lukas K.Rødland, LukasStålhammar, MarcusJohansson Bergholtz, Emil

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