Calorimetric evidence for two phase transitions in Ba1−xKxFe2As2 with fermion pairing and quadrupling states

Ilya Shipulin, Nadia Stegani, Ilaria Maccari, Kunihiro Kihou, Chul-Ho Lee, Quanxin Hu, Yu Zheng, Fazhi Yang, Yongwei Li, Chi-Ming Yim, Ruben Hühne, Hans-Henning Klauss, Marina Putti, Federico Caglieris (), Egor Babaev () and Vadim Grinenko ()
Additional contact information
Ilya Shipulin: Leibniz-IFW Dresden
Nadia Stegani: University of Genoa
Ilaria Maccari: KTH Royal Institute of Technology
Kunihiro Kihou: National Institute of Advanced Industrial Science and Technology (AIST)
Chul-Ho Lee: National Institute of Advanced Industrial Science and Technology (AIST)
Quanxin Hu: Shanghai Jiao Tong University
Yu Zheng: Shanghai Jiao Tong University
Fazhi Yang: Shanghai Jiao Tong University
Yongwei Li: Shanghai Jiao Tong University
Chi-Ming Yim: Shanghai Jiao Tong University
Ruben Hühne: Leibniz-IFW Dresden
Hans-Henning Klauss: Technische Universität Dresden
Marina Putti: University of Genoa
Federico Caglieris: University of Genoa
Egor Babaev: KTH Royal Institute of Technology
Vadim Grinenko: Shanghai Jiao Tong University

Nature Communications, 2023, vol. 14, issue 1, 1-9

Abstract: Abstract Materials that break multiple symmetries allow the formation of four-fermion condensates above the superconducting critical temperature (Tc). Such states can be stabilized by phase fluctuations. Recently, a fermionic quadrupling condensate that breaks the Z2 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 Z2 time-reversal symmetry is broken ( $${T}_{{{{{{{{\rm{c}}}}}}}}}^{{{{{{{{\rm{Z2}}}}}}}}} \, > \, {T}_{{{{{{{{\rm{c}}}}}}}}}$$ T c Z2 > T c ). 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 Z2 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.

Date: 2023
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DOI: 10.1038/s41467-023-42459-0

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