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State with spontaneously broken time-reversal symmetry above the superconducting phase transition
Grinenko, V.; Weston, D.; Caglieris, F.; Wuttke, C.; Hess, C.; Gottschall, T.; Maccari, I.; Gorbunov, D.; Zherlitsyn, S.; Wosnitza, J.; Rydh, A.; Kihou, K.; Lee, C.-H.; Sarkar, R.; Dengre, S.; Garaud, J.; Charnukha, A.; Hühne, R.; Nielsch, K.; Büchner, B.; Klauss, H.-H.; Babaev, E.
The most well-known example of an ordered quantum state—superconductivity—is caused by the formation and condensation of pairs of electrons. Fundamentally, what distinguishes a superconducting state from a normal state is a spontaneously broken symmetry corresponding to the long-range coherence of pairs of electrons, leading to zero resistivity and diamagnetism. Here we report a set of experimental observations in hole-doped Ba1−xKxFe2As2. Our specific-heat measurements indicate the formation of fermionic bound states when the temperature is lowered from the normal state. However, when the doping level is x ≈ 0.8, instead of the characteristic onset of diamagnetic screening and zero resistance expected below the superconducting phase transition, we observe the opposite effect: the generation of self-induced magnetic fields in the resistive state, measured by spontaneous Nernst effect and muon spin rotation experiments. This combined evidence indicates the existence of a bosonic metal state in which Cooper pairs of electrons lack coherence, but the system spontaneously breaks time-reversal symmetry. The observations are consistent with the theory of a state with fermionic quadrupling, in which long-range order exists not between Cooper pairs but only between pairs of pairs.
Nature Physics 17(2021)11, 1254-1259
Online First (2021) DOI: 10.1038/s41567-021-01350-9
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