+++ONLINE COLLOQUIUM+++ Quantum geometry and topology in superconductors and superfluids (Prof. Dr. Päivi Törmä )
- Datum: 22.06.2021
- Uhrzeit: 14:30
- Vortragende(r): Prof. Dr. Päivi Törmä
- Aalto University, Helsinki, Finland
- Ort: +++ONLINE KOLLOQUIUM+++
We have discovered that superconductivity (superfluidity) has a connection to quantum geometry . Namely, the superfluid weight in a multiband system has a previously unnoticed component which we call the geometric contribution. It is proportional to the quantum metric of the band. Quantum metric is connected to the Berry curvature, and this allows to relate superconductivity with the topological properties of the band. Using this theory, we have shown that superconductivity is possible also in a flat band where individual electrons would not move. We and other groups have shown  that these results are essential in explaining the observation of superconductivity in bilayer graphene and may eventually help realize superconductors at elevated temperatures. Recently, we have shown that quantum geometry can enhance also light-matter coupling , and quantum correlations of a weakly interacting Bose-Einstein condensate .
Bose-Einstein condensation has been realized for various particles or quasi-particles, such as atoms, molecules, photons, magnons and semiconductor exciton polaritons. We have experimentally realized a new type of condensate: a BEC of hybrids of surface plasmons and light in a nanoparticle array . The condensate forms at room temperature and shows ultrafast dynamics. We utilized a special measurement technique, based on formation of the condensate under propagation of the plasmonic excitations, to monitor the sub-picosecond thermalization dynamics of the system. This new platform is ideal for studies of differences and connections between BEC and lasing , and eventually also studies of topological phenomena due to the easy tunability of the array geometry and the system symmetries. Recently, we have observed formation of polarization textures and domain walls, and obtained the BEC phase for the first time using a phase retrieval algorithm .
 S. Peotta, P. Törmä, Nature Commun. 6, 8944 (2015); A. Julku, S. Peotta, T.I. Vanhala, D.-H. Kim, P. Törmä, Phys. Rev. Lett. 117, 045303 (2016); P. Törmä, L. Liang, S. Peotta, Phys. Rev. B 98, 220511(R) (2018)
 A. Julku, T.J. Peltonen, L. Liang, T.T. Heikkilä, P. Törmä, Phys. Rev. B 101, 060505(R) (2020); X. Hu, T. Hyart, D.I. Pikulin, E. Rossi, Phys. Rev. Lett. 123, 237002 (2019); F. Xie, Z. Song, B. Lian, B.A. Bernevig, Phys. Rev. Lett. 124, 167002 (2020); for a news article see L. Classen, Physics 13, 23 (2020) https://physics.aps.org/articles/v13/23
 G. Topp, C.J. Eckhardt, D.M. Kennes, M.A. Sentef, P. Törmä, arXiv:2103.04967 (2021)
 A. Julku, G.M. Bruun, P. Törmä, arXiv:2104.14257 (2021)
 T.K. Hakala, A.J. Moilanen, A.I. Väkeväinen, R. Guo, J.-P. Martikainen, K.S. Daskalakis, H.T. Rekola, A. Julku, P. Törmä, Nature Phys. 14, 739 (2018); A.I. Väkeväinen, A.J. Moilanen, M. Necada, T.K. Hakala, P. Törmä, Nature Commun. 11, 3139 (2020)
 T.K. Hakala, H.T. Rekola, A.I. Väkeväinen, J.-P. Martikainen, M. Necada, A.J. Moilanen, and P. Törmä, Nature Commun. 8, 13687 (2017); R. Guo, M. Necada, T.K. Hakala, A.I. Väkeväinen, P. Törmä, Phys. Rev. Lett. 122, 013901 (2019)
 J.M. Taskinen, P. Kliuiev, A.J. Moilanen, P. Törmä, to appear in Nano Letters, arXiv:2012.06456 (2021)