Longitudinal-fieldμ+spin relaxation via quadrupolar level-crossing resonance in Cu at 20 K

We report the first detection of resonant longitudinal-field ${\mathrm{\mu }}^{+}$ spin relaxation via dynamic depolarization at a level-crossing resonance between the ${\mathrm{\mu }}^{+}$ Zeeman splitting and the induced quadrupolar coupling of the muon’s nearest-neighbor nuclei. The ${\mathrm{\mu }}^{+}$ relaxation rate as a function of external magnetic field B has a peak at the level-crossing resonance. In single-crystal Cu at 20 K with the [111] axis parallel to B the resonance occurs at a field of ${\mathrm{B}}_{\mathrm{res}=80.9\mathrm{}\pm 0.4}$ G with a full width at half maximum of 21.7±1.7 G. This approach should help clarify the controversial low-temperature motion of the ${\mathrm{\mu }}^{+}$ in Cu.