Anomaly in YBa2Cu4O8 charge distribution below Tc: a zero-field muSR   study
  Zero-field $\mu$SR measurements in $^{63}$Cu isotope enriched and natural YBa$_2$Cu$_4$O$_8$ powders are presented. The temperature dependence of the $\mu^+$ relaxation rate is characterized by a sizeable enhancement below T$_c$. The comparison of the asymmetry decay in the two samples reveals that the $\mu^+$ relaxation is driven by nuclear dipole interaction from 300 K down to 4.2 K. It is argued that the increase in the relaxation below T$_c$ originates from a change in $\mu^+$ site, possibly due to a modification in the charge distribution within CuO chains. 
P. Carretta, A. Keren, J.S. Lord, I. Zucca, S.M.Kazakov and J.   Karpinski