Calorimetric evidence for two phase transitions in Calorimetric evidence for two phase transitions in Ba 1−x K x Fe 2 As 2 with fermion pairing and quadrupling states

• Published in: Nature communications Vol. 14; no. 1
• Main Authors: Shipulin, Ilya, Stegani, Nadia, Maccari, Ilaria, Kihou, Kunihiro, Lee, Chul Ho, Hu, Quanxin, Zheng, Yu, Yang, Fazhi, Li, Yongwei, Yim, Chi Ming, Hühne, Ruben, Klauss, Hans Henning, Putti, Marina, Caglieris, Federico, Babaev, Egor, Grinenko, Vadim
• Format: Journal Article
• Language: English
• Published: 2023
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-023-42459-0

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 Ba1−xKxFe2As2. 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 Ba1−xKxFe2As2 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.