Speaker
Description
Although $\mu^+$SR is widely used as a tool for studying a microscopic internal magnetic field in condensed matters over 40 years, the counterpart technique, i.e., $\mu^-$SR is less common for such purpose mainly due to a low counting rate for reaching reliable statistics. However, the recent progress in the beam power and counting system overcame such problem. We therefore started a new $\mu^-$SR project to measure a nuclear magnetic field in hydrogen storage materials and battery materials since 2018 [1].
In order to expand the $\mu^-$SR work, we have attempted to measure the $\mu^-$SR spectra on superconducting MgB$_2$ in ISIS to join the time reversal symmetry breaking business. This is because the past $\mu^+$SR work on MgB$_2$ [2] reported the dynamic change in a nuclear magnetic field even below $T_c=39$ K due to muon diffusion, resulting in difficulty to know the variation of the nuclear magnetic field below $T_c$. From a $\mu^-$SR viewpoint, Mg almost lacks nuclear magnetic moments (since the natural abundance of $^{25}$Mg with $I=5/2$ is 10%), and as a result, the $\mu^-$s captured by Mg feel a nuclear magnetic field formed by surrounding B and could detect the change in it accompanied with the superconducting transition. Note that the natural abundance of $^{10}$B with $I=3$ is 19.9% and that of $^{11}$B with $I=3/2$ is 80.1%. Thus, the $\mu^-$ captured by B should exhibits a fast decay due to its own nuclear magnetic moment, and the corresponding asymmetry will disappear.
[1] J. Sugiyama et al., Phys. Rev. Lett. ${\bf121}$, 087202 (2018).
[2] Ch. Niedermayer et al., Phys. Rev. B ${\bf65}$, 094512 (2002).
