Using muons to probe magnetic phases in quantum disordered systems through muon-induced distortions

In muon-spin spectroscopy (µ⁺SR) experiments, concerns are often raised about the influence of the implanted muon on its local environment. In this talk, we will present examples where the distortions induced by the muon provide sensitivity to the magnetic state of the system. Transverse-field µ⁺SR spectra for the strong-rung molecular spin ladder system (Hpip)₂CuBr₄ exhibit characteristic behaviour in each of the regions of the phase diagram [1]. Analysis of the muon stopping sites in this compound, obtained from density functional theory calculations, suggests that the magnetic field shift at the muon site in the quantum disordered phase results from a local muon-induced antiferromagnetic state. We propose that a similar mechanism enables the detection of field-induced transitions in the quantum spin liquids (QSLs) κ-(BEDT-TTF)₂Cu₂(CN)₃ and κ-(BEDT-TTF)₂Ag₂(CN)₃. In these cases, the field distributions seen by the muon correspond to those expected from adjacent ordered phases in the 2D triangular lattice QSL phase diagram.

[1] T. Lancaster et al. New J. Phys. 20, 103002 (2018).