Loop Currents in Two-leg Ladder Cuprates

TL;DR

This paper reports the discovery of loop current-like magnetism in two-leg ladder cuprates, revealing local symmetry breaking in a spin liquid system and suggesting a link to long-range magnetic order at higher doping levels.

Contribution

It provides experimental evidence of loop current magnetism in a one-dimensional spin liquid, a phenomenon previously only theoretically predicted.

Findings

Short-range magnetism observed in Sr14-xCaxCu24O41.

Magnetism breaks time-reversal and parity symmetries.

Loop current-like order may induce long-range magnetic order.

Abstract

New phases with broken discrete Ising symmetries are uncovered in quantum materials with strong electronic correlations. The two-leg ladder cuprate \textbf{$Sr_{14-x}Ca_{x}Cu_{24}O_{41}$} hosts a very rich phase diagram where, upon hole doping, the system exhibits a spin liquid state ending to an intriguing ordered magnetic state at larger $Ca$ content. Using polarized neutron diffraction, we report here the existence of short range magnetism in this material for two $Ca$ contents, whose origin cannot be ascribed to Cu spins. This magnetism develops exclusively within the two-leg ladders with a diffraction pattern at forbidden Bragg scattering which is the hallmark of loop current-like magnetism breaking both time-reversal and parity symmetries. Our discovery shows local discrete symmetry breaking in a one dimensional spin liquid system as theoretically predicted. It further suggests that a loop current-like phase could trigger the long range magnetic order reported at larger doping in two-leg ladder cuprates.