Calorimetric evidence for two phase transitions in Ba_1−xK_xFe₂As₂ with fermion pairing and quadrupling states

Materials that break multiple symmetries allow the formation of four-fermion condensates above the superconducting critical temperature (T _c). Such states can be stabilized by phase fluctuations. Recently, a fermionic quadrupling condensate that breaks the Z ₂ time-reversal symmetry was reported in Ba_1−xK_xFe₂As₂. A phase transition to the new state of matter should be accompanied by a specific heat anomaly at the critical temperature where Z ₂ time-reversal symmetry is broken (TcZ2>Tc). Here, we report on detecting two anomalies in the specific heat of Ba_1−xK_xFe₂As₂ at zero magnetic field. The anomaly at the higher temperature is accompanied by the appearance of a spontaneous Nernst effect, indicating the breakdown of Z ₂ symmetry. The second anomaly at the lower temperature coincides with the transition to a zero-resistance state, indicating the onset of superconductivity. Our data provide the first example of the appearance of a specific heat anomaly above the superconducting phase transition associated with the broken time-reversal symmetry due to the formation of the novel fermion order.