Periodic Reporting for period 4 - ODDSUPER (New mechanisms and materials for odd-frequency superconductivity)
Reporting period: 2022-08-01 to 2023-07-31
Odd-frequency superconductivity is a very unique superconducting state that is odd in time or, equivalently, frequency, which is opposite to the ordinary behavior of superconductivity. It has been realized to be the absolute key to understand the surprising physics of superconductor-ferromagnet (SF) structures and has also enabled the whole emerging field of superconducting spintronics. This project discovers and explores entirely new mechanisms and materials for odd-frequency superconductivity, to both generate a much deeper understanding of superconductivity and open for entirely new functionalities. Importantly, it generalizes and applies my initial discoveries of two new odd-frequency mechanisms, present in bulk multiband superconductors and in hybrid structures between topological insulators and conventional superconductors, respectively. In both cases odd-frequency superconductivity is generated without any need for ferromagnets or interfaces, completely different from the situation in SF structures. The result provides a significant expansion of the concept and importance of odd-frequency superconductivity to a very wide class of materials, ranging from multiband, bilayer, and nanoscale superconductors to topological superconductors. The project also establishes the connection between topology and odd-frequency pairing, which needs to be addressed in order to understand topological superconductors. Moreover, the project establishes several possible measurements for detecting odd-frequency superconductivity.
The project has made significant progress in discovering and exploring new mechanisms and materials for odd-frequency superconductivity. This has resulted so far in 23 peer-reviewed publications, including two review articles that have summarized the current status on odd-frequency superconductivity in multiband superconductors and in one-dimensional hybrid structures, respectively, both areas at the core of the ERC project. The original research works have centered around fundamental understanding on where and how odd-frequency pairing appears, explored entirely new materials and systems where we have discovered substantial odd-frequency pairing and established possible measurement tools for its detection. Overall, the project as generated successful results within all Tasks A-E listed in the original proposal. Below are a few examples of some of our more noteworthy results.
We have found pure odd-frequency pairing in Josephson junctions of topological Weyl nodal loop semimetals, which generates a huge Josephson effect, even orders of magnitude larger than in more traditional odd-frequency superconducting systems. These junctions we have quantified as optimal odd-frequency junctions.
We have found dominating odd-frequency pairing in the known superconductor Cu-doped Bi2Se3, a result that challenges the current understanding of this material. We have also discovered an unexpected diamagnetic Meissner effect from the odd-frequency pairing in this material, which means the odd-frequency pairing is remains stable in the presence of a magnetic field. We have extended this result to general two-band superconductors and demonstrated that for topological (normal-state) band structure odd-frequency pairing gives generically rise to a diamagnetic Meissner signal.
We have extended the classification of odd-frequency superconductivity to spin-3/2 systems, where orbital and spin degrees of freedom are strongly linked. We applied these results to known half-Heusler superconductors, establishing the likely presence of odd-frequency pairing.
We have extended the multiband mechanism for odd-frequency pairing to also include superconductors with pair density waves and applied the results to the underdoped cuprate superconductors, showing that odd-frequency is ubiquitous also in this family of superconductors.
We have found multiple distinctive experimental tools for detecting the otherwise elusive odd-frequency pairing. This includes finite Kerr effect in multiband superconductors such as Sr2RuO4 and UPt3 as well as quasiparticle interference as a direct experimental tool for bulk odd-frequency pairing, probing even the odd-frequency dependence directly.
We have explored the connection of odd-frequency pairing in multiple different topological Josephson junctions, including using topological insulators and semiconducting nanowires.
We have investigated the possibilities for odd-frequency superconducting pairing in driven systems, and their establishing the possibility of Floquet engineering of bulk odd-frequency pairs.
We have discovered how odd-frequency pairing can be the only superconducting pairing present at non-Hermitian exceptional points. The two last projects were not even envisaged in the grant proposal but naturally grew out of our results within the project and very nicely fits within the overall aim of the project.
All work has been disseminated in peer-reviewed open access journals. All work is also available on the preprint server arxiv.org. The work has been presented at a number of international conferences and workshops, where the PI has been an invited speaker.
We have found pure odd-frequency pairing in Josephson junctions of topological Weyl nodal loop semimetals, which generates a huge Josephson effect, even orders of magnitude larger than in more traditional odd-frequency superconducting systems. These junctions we have quantified as optimal odd-frequency junctions.
We have found dominating odd-frequency pairing in the known superconductor Cu-doped Bi2Se3, a result that challenges the current understanding of this material. We have also discovered an unexpected diamagnetic Meissner effect from the odd-frequency pairing in this material, which means the odd-frequency pairing is remains stable in the presence of a magnetic field. We have extended this result to general two-band superconductors and demonstrated that for topological (normal-state) band structure odd-frequency pairing gives generically rise to a diamagnetic Meissner signal.
We have extended the classification of odd-frequency superconductivity to spin-3/2 systems, where orbital and spin degrees of freedom are strongly linked. We applied these results to known half-Heusler superconductors, establishing the likely presence of odd-frequency pairing.
We have extended the multiband mechanism for odd-frequency pairing to also include superconductors with pair density waves and applied the results to the underdoped cuprate superconductors, showing that odd-frequency is ubiquitous also in this family of superconductors.
We have found multiple distinctive experimental tools for detecting the otherwise elusive odd-frequency pairing. This includes finite Kerr effect in multiband superconductors such as Sr2RuO4 and UPt3 as well as quasiparticle interference as a direct experimental tool for bulk odd-frequency pairing, probing even the odd-frequency dependence directly.
We have explored the connection of odd-frequency pairing in multiple different topological Josephson junctions, including using topological insulators and semiconducting nanowires.
We have investigated the possibilities for odd-frequency superconducting pairing in driven systems, and their establishing the possibility of Floquet engineering of bulk odd-frequency pairs.
We have discovered how odd-frequency pairing can be the only superconducting pairing present at non-Hermitian exceptional points. The two last projects were not even envisaged in the grant proposal but naturally grew out of our results within the project and very nicely fits within the overall aim of the project.
All work has been disseminated in peer-reviewed open access journals. All work is also available on the preprint server arxiv.org. The work has been presented at a number of international conferences and workshops, where the PI has been an invited speaker.
Several of the results listed above clearly classify as progress beyond the state-of-the-art and also not envisaged in the grant proposal, such as finding optimal odd-frequency junctions, Floquet engineering, non-Hermitian effects, dominating odd-frequency pairing in some existing multiband supercondcutors such as doped Bi2Se3, and importantly, discovering distinctive experimentally measurable signatures of odd-frequency pairing in quasiparticle interference, Kerr effect, and phase biased transport.