Low temperature raman study of the electron coherence length near graphene edges

We developed a novel optical defocusing method for studying spatial coherence of photoexcited electrons and holes near edges of graphene. Our method is applied to measure the localization l(D) of the disorder-induced Raman D band (～1350 cm(-1)) with a resolution of a few nanometers. Raman scattering experiments performed in a helium bath cryostat reveal that as temperature is decreased from 300 to 1.55 K, the length l(D) increases. We found that the localization of the D band varies as 1/T(1/2), giving strong evidence that l(D) scales with the coherence length of photoexcited electrons near graphene edges.