Calorimetric evidence for two phase transitions in Ba<sub>1-x</sub>K<sub>x</sub>Fe<sub>2</sub>As<sub>2</sub> with fermion pairing and quadrupling states.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 37872158.
- Also identified by DOI 10.1038/s41467-023-42459-0 and PMC identifier 10593811.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
Materials that break multiple symmetries allow the formation of four-fermion condensates above the superconducting critical temperature (T<sub>c</sub>). Such states can be stabilized by phase fluctuations. Recently, a fermionic quadrupling condensate that breaks the Z<sub>2</sub> time-reversal symmetry was reported in Ba<sub>1-x</sub>K<sub>x</sub>Fe<sub>2</sub>As<sub>2</sub>. A phase transition to the new state of matter should be accompanied by a specific heat anomaly at the critical temperature where Z<sub>2</sub> time-reversal symmetry is broken ([Formula: see text]). Here, we report on detecting two anomalies in the specific heat of Ba<sub>1-x</sub>K<sub>x</sub>Fe<sub>2</sub>As<sub>2</sub> 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<sub>2</sub> 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.