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 Ba_1−xK_xFe₂As₂. 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.

有序量子态最著名的例子——超导性——是由电子对的形成和凝聚引起的。从根本上说，超导状态与正常状态的区别在于自发破缺的对称性，对应于电子对的长程相干性，导致零电阻率和抗磁性。在这里，我们报告了在空穴掺杂的 Ba _{1− x} K _x Fe ₂ As ₂中的一组实验观察结果。我们的比热测量表明当温度从正常状态降低时，费米子束缚态的形成。然而，当掺​​杂水平为x ≈ 0.8，而不是在超导相变之下预期的抗磁屏蔽的特征开始和零电阻，我们观察到相反的效果：在电阻状态下产生自感应磁场，通过自发能斯特效应和 μ 子自旋旋转实验测量. 这一综合证据表明存在一种玻色子金属态，其中库珀电子对缺乏相干性，但系统自发地破坏了时间反转对称性。这些观察结果与具有费米子四倍的状态理论一致，其中长程有序不存在于库珀对之间，而仅存在于成对之间。