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Beyond the Cooper pair paradigm: fermion quadrupling condensates above the superconducting transition

Ilaria Maccari

Stockholm University

Superconductivity arises as a consequence of the formation and condensation of electron pairs. However, multi-component systems may exhibit a different kind of order associated with the condensation of electron quadruplets both below and above the superconducting critical temperature. In this talk, I will present the first experimental observation, in the iron arsenide Ba1-xKxFe2As2, of a non-superconducting fermion-quadrupling condensate that spontaneously breaks time-reversal symmetry [1,2].  From a theoretical standpoint, this is a beyond mean-field state whose onset is driven by the proliferation of topological phase excitations. Studying the interplay between the different topological defects at play is a highly nontrivial problem that can be addressed via Monte Carlo simulations.  In the second part of my talk, I will discuss the emergence of this phase within a London model for s+is superconductors as a function of different intercomponent couplings [3]. Finally, I will briefly discuss the case of magic-angle twisted bilayer graphene, whose low-energy effective model has revealed a beyond-mean field phase diagram with a substantial fermion-quadrupling state for all the values of the parameters considered [4].

[1] V. Grinenko et al., Nature Physics 17, 1254–1259 (2021). [2] I. Shipulin et al., arXiv preprint arXiv: 2212.13515 (2023). [3] I. Maccari, E. Babaev, Phys. Rev. B 105 (21), 214520 (2022). [4] I. Maccari, J. Carlström, E. Babaev, Phys. Rev. B 107 (6), 064501 (2023).