  We study the intervalley scattering in defected graphene by low-temperature
transport measurements. The scattering rate is strongly suppressed when defects
are charged. This finding highlights "screening" of the short-range part of a
potential by the long-range part. Experiments on calcium-adsorbed graphene
confirm the role of a long-range Coulomb potential. This effect is applicable
to other multivalley systems, provided that the charge state of a defect can be
electrically tuned. Our result provides a means to electrically control valley
relaxation and has important implications in valley dynamics in valleytronic
materials.
