| 1. |
- Rødland, Lukas, 1993-, et al.
(författare)
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Disorder and non-Hermiticity in Kitaev spin liquids with a Majorana Fermi surface
- 2024
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 109:15
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Tidskriftsartikel (refereegranskat)abstract
- We study the effect of disorder on ?2 quantum spin liquids with a Majorana Fermi line (respectively a surface in three dimensions), and we show that depending on the symmetries that are preserved on average, qualitatively different scenarios will occur. In particular, we identify the relevant non-Hermitian symmetries for which disorder will effectively split the Fermi line into two exceptional lines, with Re(?)=0 states filling the area in between. We demonstrate the different scenarios using both toy models as well as large-scale numerical simulations.
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| 2. |
- Sayyad, Sharareh, et al.
(författare)
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Symmetry-protected exceptional and nodal points in non-Hermitian systems
- 2023
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Ingår i: SciPost Physics. - 2542-4653. ; 15:5
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Tidskriftsartikel (refereegranskat)abstract
- One of the unique features of non-Hermitian (NH) systems is the appearance of NH degeneracies known as exceptional points (EPs). The extensively studied defective EPs occur when the Hamiltonian becomes non-diagonalizable. Aside from this degeneracy, we show that NH systems may host two further types of non-defective degeneracies, namely, non-defective EPs and ordinary (Hermitian) nodal points. The non-defective EPs manifest themselves by i) the diagonalizability of the NH Hamiltonian at these points and ii) the non-diagonalizability of the Hamiltonian along certain intersections of these points, resulting in instabilities in the Jordan decomposition when approaching the points from certain directions. We demonstrate that certain discrete symmetries, namely parity-time, parity-particle-hole, and pseudo-Hermitian symmetry, guarantee the occurrence of both defective and non-defective EPs. We extend this list of symmetries by including the NH time-reversal symmetry in two-band systems. Two-band and four-band models exemplify our findings. Through an example, we further reveal that ordinary nodal points may coexist with defective EPs in NH models when the above symmetries are relaxed.
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| 3. |
- Stålhammar, Marcus, 1994-, et al.
(författare)
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PT symmetry-protected exceptional cones and analogue Hawking radiation
- 2023
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 25:4
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Tidskriftsartikel (refereegranskat)abstract
- Non-Hermitian Hamiltonians, which effectively describe dissipative systems, and analogue gravity models, which simulate properties of gravitational objects, comprise seemingly different areas of current research. Here, we investigate the interplay between the two by relating parity-time-symmetric dissipative Weyl-type Hamiltonians to analogue Schwarzschild black holes emitting Hawking radiation. We show that the exceptional points of these Hamiltonians form tilted cones mimicking the behavior of the light cone of a radially infalling observer approaching a black hole horizon. We further investigate the presence of tunneling processes, reminiscent of those happening in black holes, in a concrete example model. We interpret the non-trivial result as the purely thermal contribution to analogue Hawking radiation in a Schwarzschild black hole. Assuming that our particular Hamiltonian models a photonic crystal, we discuss the concrete nature of the analogue Hawking radiation in this particular setup.
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| 4. |
- Yang, Kang, et al.
(författare)
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Homotopy, Symmetry, and Non-Hermitian Band Topology
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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 K-theory classifications of non-Hermitian systems based on line and point gaps have deepened the understanding of many physical phenomena. However, ample systems remain beyond this description; reference points and lines are in general unable to distinguish whether multiple non-Hermitian bands exhibit band crossings and braids. To remedy this we consider the complementary notions of non-Hermitian band gaps and separation gaps that crucially include a broad class of multi-band scenarios, enabling the description of generic band structures with symmetries. With these concepts, we provide a unified and systematic 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 fragile phases and, remarkably, also implies new stable phenomena stemming from the topology of both eigenvalues and eigenvectors. 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 close band gaps, and they also predict the deformation rules of nodal phases. We further demonstrate that spontaneous PT symmetry breaking is captured by a Chern-Euler description, a fingerprint of unprecedented non-Hermitian topology. 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.
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