Ultrasound evidence for multicomponent superconducting order parameter in Ba1−xKxFe2As2 with electron quadrupling phase

Experiments have pointed to the formation of the electron quadrupling condensate in Ba 1− x K x Fe 2 As 2 at x ~ 0.8. The state spontaneously breaks time-reversal symmetry and is sandwiched between two critical points, separating it from the broken time-reversal symmetry (BTRS) superconducting state at T_cU(1) T c U ( 1 ) and a normal-metal state at T_c^Z2 T c Z2 . We report a theory of the acoustic effects spectroscopy of systems with an electron quadrupling phase based on ultrasound-velocity measurements. We show that the experimental results are consistent with BTRS superconductivity at x ~ 0.8, fulfilling the necessary condition for the formation of electron quadrupling in Ba 1− x K x Fe 2 As 2 . We provide the theoretical basis and the experimental strategy to study the order parameter symmetry of emerging quadrupling condensates in superconductors.