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, H.‐H. Klauß, M. Putti, Federico Caglieris, Egor Babaev, Vadim Grinenko
Abstract
Abstract 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−x K x Fe 2 As 2 . 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 ( $${T}_{{{{{{{{\rm{c}}}}}}}}}^{{{{{{{{\rm{Z2}}}}}}}}} \, > \, {T}_{{{{{{{{\rm{c}}}}}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Z2</mml:mi> </mml:mrow> </mml:msubsup> <mml:mspace/> <mml:mo>></mml:mo> <mml:mspace/> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> </mml:math> ). Here, we report on detecting two anomalies in the specific heat of Ba 1−x K x Fe 2 As 2 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.