Loop-current order through the kagome looking glass

TL;DR

This paper explores the theoretical framework and phenomenology of loop-current order in kagome materials, highlighting its unique magnetic properties, microscopic models, and challenges in experimental detection.

Contribution

It provides a comprehensive overview of the group-theoretical properties, microscopic models, and phenomenology of loop-current states in kagome systems, including comparisons with other magnetic orders.

Findings

Loop-current states break time-reversal symmetry via non-trivial hopping phases.

Comparison of loop-current order with spin-density waves and charge-density waves.

Discussion of experimental challenges in detecting loop-current order.

Abstract

In loop-current states, interacting electronic degrees of freedom collectively establish interatomic currents, in a rare example of magnetism in which spin degrees of freedom do not play the primary role. The main impact of such states on the electronic spectrum is not via the standard Zeeman term, but via the kinetic energy, in which hopping parameters develop non-trivial phases that break time-reversal symmetry. The recent proposal of loop-current states in kagome superconductors has stimulated renewed interest in this exotic type of magnetism. In this perspective, we use kagome materials as a scaffolding to frame the basic phenomenology of loop-current states. We provide an overview of the group-theoretical properties of loop currents, as well as of relevant microscopic models and ab initio methods. Particular emphasis is given to the comparison with spin-density waves in the presence of spin-orbit coupling, as well as to the anharmonic coupling with charge-density waves, which is present in systems with threefold rotational symmetry. We also provide a brief overview of the current status of loop-current order in kagome metals and discuss open challenges including their experimental detection and interplay with other orders.