Loop currents in quantum matter

TL;DR

This paper discusses the evidence and implications of intra-unit-cell loop current order in quantum materials, especially high-temperature superconducting cuprates, highlighting its potential broader relevance.

Contribution

It reviews experimental evidence for intra-unit-cell loop currents and explores their significance in understanding quantum states of matter.

Findings

Polarized neutron diffraction supports intra-unit-cell magnetism.

Loop current order breaks parity and time-reversal symmetries.

Possible existence in other quantum materials beyond cuprates.

Abstract

In many quantum materials, strong electron correlations lead to the emergence of new states of matter. In particular, the study in the last decades of the complex phase diagram of high temperature superconducting cuprates highlighted intra-unit-cell electronic instabilities breaking discrete Ising-like symmetries, while preserving the lattice translation invariance. Polarized neutron diffraction experiments have provided compelling evidences supporting a new form of intra-unit-cell magnetism, emerging concomitantly with the so-called pseudogap state of these materials. This observation is currently interpreted as the magnetic hallmark of an intra-unit-cell loop current order, breaking both parity and time-reversal symmetries. More generally, this magneto-electric state is likely to exist in a wider class of quantum materials beyond superconducting cuprates. For instance, it has been already observed in hole-doped Mott insulating iridates or in the spin liquid state of hole-doped 2-leg ladder cuprates.