Investigation of potential fluctuating intra-unit cell magnetic order in cuprates by muon spin relaxation

TL;DR

This study uses muon spin relaxation to investigate intra-unit cell magnetic order in cuprate superconductors, finding no static order and placing limits on fluctuation rates, thus challenging some theoretical models.

Contribution

It provides the first muSR measurements at very low temperatures to test for static IUC magnetic order in cuprates, constraining fluctuation dynamics.

Findings

No static IUC magnetic order detected at 25 mK.

Fluctuation rates of magnetic order are faster than muSR detection limits.

Results challenge the existence of static magnetic order predicted by some theories.

Abstract

We report low temperature muon spin relaxation (muSR) measurements of the high-transition-temperature (Tc) cuprate superconductors Bi{2+x}Sr{2-x}CaCu2O{8+\delta} and YBa2Cu3O6.57, aimed at detecting the mysterious intra-unit cell (IUC) magnetic order that has been observed by spin polarized neutron scattering in the pseudogap phase of four different cuprate families. A lack of confirmation by local magnetic probe methods has raised the possibility that the magnetic order fluctuates slowly enough to appear static on the time scale of neutron scattering, but too fast to affect $\mu$SR or nuclear magnetic resonance (NMR) signals. The IUC magnetic order has been linked to a theoretical model for the cuprates, which predicts a long-range ordered phase of electron-current loop order that terminates at a quantum crictical point (QCP). Our study suggests that lowering the temperature to T ~ 25 mK and moving far below the purported QCP does not cause enough of a slowing down of fluctuations for the IUC magnetic order to become detectable on the time scale of muSR. Our measurements place narrow limits on the fluctuation rate of this unidentified magnetic order.