Electron quadruplets

The condensate of electron quadruplets is a proposed state of matter in which Cooper pairs are formed but 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 superconductivity first proposed by Berg, Fradkin and Kivelson.^[3] Another example is "quartic metal" phase ^[4][5] is related to but 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.^[6]

Related states can form in pair-density-wave systems.^[7] In systems with a greater number of broken symmetries, theoretical studies have demonstrated the existence of charge-6e and more complex orders.^[8][9] After the theoretical possibility was raised, observations consistent with electron quadrupling were published using hole-doped Ba_1-xK_xFe₂As₂ in 2021,^[6][10] with evidence of vestigial quadrupling reported in CsV₃Sb₅ soon after, in early 2022.^[11]

See also

• List of states of matter