RF-μSR in electric fields studies of GaP

We report on combined RF-μSR and electric fields (EF) μSR measurements in semi-insulating GaP. At low temperature the capture of excess carriers by a neutral muonium (Mu=μ++e-) center is suggested to be a dominant process in muonium charge dynamics. The charged muonium fraction (Mu- or Mu+) formed on the microsecond time-scale is increased with lowering the temperature. Contrary to the usually observed scenario, at T<40K an EF prevents formation of the final charged state.