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

by Dr Ilaria Maccari (PSI)

Tuesday 31 Oct 2023, 14:00 → 15:00 Europe/Zurich

WHGA/121

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., Nature Communications 14, 6734 (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).