Symmetry constraints on the macroscopic response of AV3Sb5 kagome metals

In addition to superconductivity, the AV3Sb5 (A=K, Rb, Cs) family of kagome metals exhibits intriguing charge order phenomena, some of which have been proposed to involve not only changes in the magnitude of hopping parameters (bond order) but also changes in their phases (loop currents or orbital magnetism), resulting in time reversal symmetry breaking (TRSB). Determining the presence of spontaneous TRSB in these systems is crucial to understanding their phase diagram, albeit challenging given the absence of a macroscopic magnetization. To address this issue, we use group and representation theoretical approaches to classify the phases of AV3Sb5 in which both bond order and loop currents condense. We compare the form of various bulk response tensors in each allowed phase by exploiting the fact that the symmetry operations associated with each corresponding magnetic space group enforce constraints on observable bulk properties. We then suggest a set of optical and piezomagnetic response tensors that can be measured to uniquely determine the combined loop-current and bond-order patterns in these systems.