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On the interpretation of the fast relaxing signal in μSR experiments.

Authors: R. C. Vilão, R. B. L. Vieira, H. V. Alberto, J. M. Gil, A. Weidinger

Ref.: 14th International Conference on Muon Spin Rotation, Relaxation and Resonance (μSR2017) (2017)

Abstract: In a number of µSR experiments, a fast relaxing signal at the Larmor frequency of the muon has been observed (for example [1-4]). The signal is usually assigned to either µ+ or to Mu-; the relaxation is attributed to the conversion to paramagnetic muonium. However, neither of the two assignments is really satisfactory (a review of the “sapphire puzzle” is presented by Cox in ref. [5]). We suggest that the fast relaxing signal correspond actually to a paramagnetic state. The hyperfine interaction is not seen since it is small and fluctuates, averaging to almost zero. The underlying picture of our model is the following: The incoming muonium stops first at an interstitial site in the unrelaxed lattice. The formation of the final muon configuration requires a lattice rearrangement which takes some time, in particular if an energy barrier hinders the conversion. This transition situation between the initial and final muon configuration is, in our opinion, observed in the fast relaxing signal. Concrete indications of the paramagnetic character of the fast signal are: i) an upward frequency shift in low magnetic fields and ii) the decoupling possibility in longitudinal fields. Both phenomena were observed in recent zirconia experiments [4]. [1] S.R. Kreitzman, R. Kiefl, D.R. Noakes, J.H. Brewer, E.J. Ansaldo, Hyperfine Interactions 31 (1986) 521,Thermally activated muonium formation in Al2O3 and BaF2 [2] V. Storchak, J. H. Brewer, G.D. Morris, Phys. Rev. B 56 (1997) 55, Electric-field-induced muonium formation in sapphire [3] I. Fan et al. Physica B: Condensed Matter 404, 849 (2009) Diamagnetic states in germanium studied by the photo-excited TF-μ SR measurements [4] R B L Vieira et al. Phys. Rev. B 94 (2016) 115207, Isolated hydrogen configurations in zirconia as seen by muon spin spectroscopy and ab-initio calculations [5] S. F. J. Cox, J. Phys.: Condens. Matter 18 (2006) 1079 Oxide muonics: II. Modelling the electrical activity of hydrogen in wide-gap and high-permittivity dielectrics