The condensate of electron quadruplets is a proposed state of matter in which Cooper pairs do not exhibit long-range order, but electron quadruplets do. Such states emerge in systems with multiple broken symmetries due to the partial melting of the underlying low-temperature order, which destroys the condensates of Cooper pairs but preserves the condensates formed by pairs of preformed fermion pairs.[1][2] One example of the proposed electron quadruplet condensates is
charge-4e[3][4] Another example is "quartic metal" phase [5][6] is distinct from those superconductors explained by the standard BCS theory; rather than expelling magnetic field lines as in the Meissner effect, it generates them, a spontaneous Nernst effect that indicates the breaking of time-reversal symmetry.[7]
Related states can form in pair-density-wave systems.[8] In systems with a greater number of broken symmetries, theoretical studies have demonstrated the existence of charge-6e and more complex orders.[9][10]
After the theoretical possibility was raised, observations consistent with electron quadrupling were published using hole-doped Ba1-xKxFe2As2 in 2021,[7][11] with claims of charge-4e state reported in mesoscopic samples of CsV3Sb5 soon after, in early 2022.[12]
^Smiseth, J.; Smørgrav, E.; Babaev, E.; Sudbø, A. (6 June 2005). "Field- and temperature-induced topological phase transitions in the three-dimensional N -component London superconductor". Physical Review B. 71 (21): 214509. arXiv:cond-mat/0411761. Bibcode:2005PhRvB..71u4509S. doi:10.1103/PhysRevB.71.214509.{{cite journal}}: CS1 maint: article number as page number (link)
^Herland, Egil V.; Babaev, Egor; Sudbø, Asle (8 October 2010). "Phase transitions in a three dimensional U ( 1 ) × U ( 1 ) lattice London superconductor: Metallic superfluid and charge- 4 e superconducting states". Physical Review B. 82 (13): 134511. arXiv:1006.3311. Bibcode:2010PhRvB..82m4511H. doi:10.1103/PhysRevB.82.134511.{{cite journal}}: CS1 maint: article number as page number (link)
^Agterberg, D. F.; Geracie, M.; Tsunetsugu, H. (26 July 2011). "Conventional and charge-six superfluids from melting hexagonal Fulde-Ferrell-Larkin-Ovchinnikov phases in two dimensions". Physical Review B. 84 (1): 014513. arXiv:1106.1685. Bibcode:2011PhRvB..84a4513A. doi:10.1103/PhysRevB.84.014513.{{cite journal}}: CS1 maint: article number as page number (link)
