Electron quadruplets

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 Ba_1-xK_xFe₂As₂ in 2021,^[7][11] with claims of charge-4e state reported in mesoscopic samples of CsV₃Sb₅ soon after, in early 2022.^[12]

See also

• List of states of matter