Many-body effects and topology in magnets and superconductors
Doctoral thesis
Permanent lenke
https://hdl.handle.net/11250/3146878Utgivelsesdato
2024Metadata
Vis full innførselSamlinger
- Institutt for fysikk [2790]
Sammendrag
Solid materials where quantum mechanical effects dominate the behavior can contribute to faster and more energy-efficient computers. The road to technological application starts with fundamental research. This thesis concerns my theoretical investigations of a few select systems where quantum effects are important. My research is in the field of quantum condensed matter theory and has been published in ten papers with a focus on magnetism, topology, and superconductivity.
The first paper studies the two-dimensional interface between a magnetic insulator and a topological insulator. A topological insulator has conducting surface states with spin-momentum locking. We explore how the surface states are affected by the coupling to spin fluctuations in the magnetic insulator using Green's function techniques. We also explore the relevance to experimental measurements.
The next three papers deal with skyrmions. Skyrmions are special states in magnetic insulators where the spins rotate in all directions away from a center. This fascinating spin texture carries a real-space topology which leads to a great stability. The stability of skyrmions has inspired a wide range of potential applications. With a mind on both fundamental physics and storage density in magnetic memory technology, we look at small skyrmions. Quantum skyrmions are so small that the quantum nature of the spins is much more relevant than in traditional treatments of larger skyrmions. We explore how stable such small skyrmions are, and how to quantify their existence when the usual topological invariant is no longer well-defined. Furthermore, we find a topological magnon band structure with topological phase transitions when tuning the easy-axis anisotropy. This means that chiral edge states, with applications in spintronics, can be turned on and off. Finally, we couple the skyrmions to a normal metal across an interface. Spin fluctuations in the skyrmions mediate an effective electron-electron interaction between electrons in the normal metal. This interaction is exotic enough to yield topological superconductivity. Topological superconductors have excitations at edges or defects which can be used in topological quantum computers. In a follow-up paper, we find topological superconductivity when the magnet instead hosts a helical state, which is a topologically trivial magnetic state with topologically trivial magnons. A consideration of symmetries therein, gives us a greater understanding of the mechanism giving magnon-mediated topological superconductivity.
Two papers deal with magnon-mediated superconductivity at the interface between antiferromagnets and a normal metal. Also here, we investigate the possibility of topological superconductivity. These papers employ weak- and strong-coupling approaches to superconductivity. We also employ a strong-coupling theory of superconductivity in three further papers. In one, we consider the largest magnetic field permissible in a flat-band superconductor. Superconductors which can handle large magnetic fields are important in the field of superconducting spintronics, where interfaces between magnets and superconductors demonstrate exciting behavior. In the second, we look at time-reversal symmetry breaking in a multiband superconductor. Lastly, one paper deals with magnon-mediated superconductivity in an altermagnet, where the electron bands have a momentum-dependent spin splitting.
Består av
Paper 1: Kristian Mæland, Håkon I. Røst, Justin W. Wells, and Asle Sudbø, Electron-magnon coupling and quasiparticle lifetimes on the surface of a topological insulator, Phys. Rev. B 104, 125125 (2021), ©2021 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.104.125125Paper 2: Kristian Mæland and Asle Sudbø, Quantum fluctuations in the order parameter of quantum skyrmion crystals, Phys. Rev. B 105, 224416 (2022), ©2022 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.105.224416
Paper 3: Kristian Mæland and Asle Sudbø, Quantum topological phase transitions in skyrmion crystals, Phys. Rev. Res. 4, L032025 (2022), CC 4.0, DOI https://doi.org/10.1103/PhysRevB.105.224416
Paper 4: Kristian Mæland and Asle Sudbø, Topological Superconductivity Mediated by Skyrmionic Magnons, Phys. Rev. Lett. 130, 156002 (2023), © 2023 American Physical Society, DOI https://doi.org/10.1103/PhysRevLett.130.156002
Paper 5: Chi Sun, Kristian Mæland, and Asle Sudbø, Stability of superconducting gap symmetries arising from antiferromagnetic magnons, Phys. Rev. B 108, 054520 (2023), ©2023 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.108.054520
Paper 6: Kristian Mæland and Asle Sudbø, Exceeding the Chandrasekhar-Clogston limit in flat-band superconductors: A multiband strong-coupling approach, Phys. Rev. B 108, 214511 (2023), ©2023 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.108.214511
Paper 7: Niels Henrik Aase, Kristian Mæland, and Asle Sudbø, Multiband strong-coupling superconductors with spontaneously broken time- reversal symmetry, Phys. Rev. B 108, 214508 (2023), ©2023 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.108.214508
Paper 8: Kristian Mæland, Sara Abnar, Jacob Benestad, and Asle Sudbø, Topological superconductivity mediated by magnons of helical magnetic states, Phys. Rev. B 108, 224515 (2023), ©2023 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.108.224515
Paper 9: Kristian Mæland, Bjørnulf Brekke, and Asle Sudbø, Many-body effects on superconductivity mediated by double-magnon processes in altermagnets, Phys. Rev. B 109, 134515 (2024), ©2024 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.109.134515
Paper 10: Chi Sun, Kristian Mæland, Even Thingstad, and Asle Sudbø, Strong-coupling approach to temperature dependence of competing orders of superconductivity: Possible time-reversal symmetry breaking and nontrivial topology, Phys. Rev. B 109, 174520 (2024), ©2024 American Physical Society, DOI https://doi.org/10.1103/PhysRevB.109.174520