New perspectives on quantum geometry, superconductivity and Bose-Einstein condensation

Superconductivity, superfluidity and Bose-Einstein condensation (BEC) are many-body phenomena where quantum statistics are crucial and the effect of interactions may be intriguing. Theoretical understanding of superconductivity and condensation in several real-world systems is still a challenge, and superconductivity at room temperature remains a grand goal. We have discovered that superconductivity has a connection to quantum geometry [1-3]. The superfluid weight in a multiband system has a previously unnoticed geometric component, proportional to the quantum metric and Berry curvature. Due to this, superconductivity is possible also in a flat band where individual electrons would not move. We have shown that the geometric contribution is essential in explaining the recent observation of superconductivity in bilayer graphene [4], and may eventually help realize superconductors at elevated temperatures. 

Bose-Einstein condensation has been realized for various particles or quasi-particles, such as atoms, molecules, photons, magnons and semiconductor exciton polaritons. We have recently experimentally realized a new type of condensate: a BEC of hybrids of surface plasmons and light in a nanoparticle array [5,6]. 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 [7,8], and eventually also studies of topological phenomena due to the easy tunability of the array geometry.  

[1] S. Peotta, P. Törmä, Nat. Commun. 6, 8944 (2015)
[2] A. Julku, S. Peotta, T.I. Vanhala, D.-H. Kim, P. Törmä, Phys. Rev. Lett. 117, 045303 (2016)
[3] P. Törmä, L. Liang, S. Peotta, Phys. Rev. B 98, 220511(R) (2018)
[4] A. Julku, T.J. Peltonen, L. Liang, T.T. Heikkilä, P. Törmä, arXiv:1906.06313 (2019)
[5] T.K. Hakala, A.J. Moilanen, A.I. Väkeväinen, R. Guo, J.-P. Martikainen, K.S. Daskalakis, H.T. Rekola, A. Julku, P. Törmä, Nat. Phys. 14, 739 (2018)
[6] A.I. Väkeväinen, A.J. Moilanen, M. Necada, T.K. Hakala, K.S. Daskalakis, P. Törmä, arXiv:1905:07609 (2019)
[7] T.K. Hakala, H.T. Rekola, A.I. Väkeväinen, J.-P. Martikainen, M. Necada, A.J. Moilanen, and P. Törmä, Nat. Commun. 8, 13687 (2017)
[8] R. Guo, M. Necada, T.K. Hakala, A.I. Väkeväinen, P. Törmä, Phys. Rev. Lett. 122, 013901 (2019)

Published: Fri 20 Sep 2019.