State with spontaneously broken time-reversal symmetry above the superconducting phase transition

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 approximate to 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. A state that breaks time-reversal symmetry is observed in the normal phase above the superconducting critical temperature in a multiband superconductor. This could be explained by correlations between the Cooper pairs formed in different bands.

Automated summary

Experimental observations in hole-doped Ba1-xKxFe2As2 show the formation of fermionic bound states at lower temperatures, but a generation of self-induced magnetic fields in the resistive state is observed when the doping level is x approximate to 0.8 instead of the expected diamagnetic screening and zero resistance. This indicates the existence of a bosonic metal state breaking time-reversal symmetry, consistent with the theory of a state with fermionic quadrupling.