The charge dynamics of hydrogen-like centers formed by the implantation of energetic (4 MeV) muons in semi-insulating GaAs have been studied by muon spin resonance in electric fields. The results point to the significant role of deep hole traps in th
e compensation mechanism of GaAs. Electric-field-enhanced neutralization of deep electron and hole traps by muon-track-induced hot carriers results to an increase of the non-equilibrium carrier life-times. As a consequence, the muonium ($mu^+ + e^-$) center at the tetrahedral As site can capture the tracks holes and therefore behaves like a donor.
We report the first direct observation of the oxygen-isotope ($^{16}$O/$^{18}$O) effect on the in-plane penetration depth $lambda_{ab}$ in a nearly optimally doped YBa$_2$Cu$_3$O$_{7-delta}$ film using the novel low-energy muon-spin rotation techniqu
e. Spin polarized low energy muons are implanted in the film at a known depth $z$ beneath the surface and precess in the local magnetic field $B(z)$. This feature allows us to measure directly the profile $B(z)$ of the magnetic field inside the superconducting film in the Meissner state and to make a model independent determination of $lambda_{ab}$. A substantial isotope shift $Deltalambda_{ab}/lambda_{ab}=2.8(7)$% at 4 K is observed, implying that the in-plane effective supercarrier mass $m_{ab}^ast$ is oxygen-isotope dependent with $Delta m_{ab}^ast/m_{ab}^ast = 5.5(1.4)%$.
