Loop currents in two-leg ladder cuprates

New phases with broken discrete Ising symmetries are uncovered in quantum materials with strong electronic correlations. The two-leg ladder cuprate Sr14−xCaxCu24O41 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. Magnetism in the cuprates is crucial to understanding their superconducting properties and most studies focus on the antiferromagnetism related to Cu spins. Here, using polarised neutron diffraction the authors find evidence of a type of short range magnetic ordering in Ca-doped SrCuO spin-ladder compound whose origins may lie in magnetoelectric loop currents rather than Cu spins