Quartic metal: Spontaneous breaking of time-reversal symmetry due to four-fermion correlations in Ba$_{1-x}$K$_x$Fe$_2$As$_2$

TL;DR

This paper reports the discovery of a novel bosonic metal state in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ characterized by spontaneous breaking of time-reversal symmetry due to four-fermion correlations, distinct from conventional superconductivity.

Contribution

It introduces the experimental observation of a bosonic metal state with quartic fermion correlations and spontaneous time-reversal symmetry breaking above the superconducting transition.

Findings

Detection of fermionic bound states forming at higher temperatures

Observation of spontaneous magnetic fields indicating broken time-reversal symmetry

Absence of superconducting diamagnetism in the studied state

Abstract

Discoveries of ordered quantum states of matter are of great fundamental interest, and often lead to unique applications. The most well known example -- superconductivity -- is caused by the formation and condensation of pairs of electrons. A key property of superconductors is diamagnetism: magnetic fields are screened by dissipationless currents. Fundamentally, what distinguishes superconducting states from normal states is a spontaneously broken symmetry corresponding to long-range coherence of fermion pairs. Here we report a set of experimental observations in hole doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ which are not consistent with conventional superconducting behavior. Our specific-heat measurements indicate the formation of fermionic bound states when the temperature is lowered from the normal state. However, for $x \sim 0.8$, instead of the standard for superconductors, zero resistance and diamagnetic screening, for a range of temperatures, we observe the opposite effect: the generation of self-induced magnetic fields measured by spontaneous Nernst effect and muon spin rotation experiments. The finite resistance and the lack of any detectable diamagnetic screening in this state exclude the spontaneously broken symmetry associated with superconducting two-fermion correlations. Instead, combined evidence from transport and thermodynamic measurements indicates that the formation of fermionic bound states leads to spontaneous breaking of time-reversal symmetry above the superconducting transition temperature. These results demonstrate the existence of a broken-time-reversal-symmetry bosonic metal state. In the framework of a multiband theory, such a state is characterized by quartic correlations: the long-range order exists only for {\it pairs} of fermion pairs.