Giant anisotropic spin splitting in epitaxial graphene

Our recently reported spin and angular resolved photoemission (SARPES) results on an epitaxial graphene monolayer on SiC(0001) suggested the presence of a large Rashba-type spin splitting of \Delta k=(0.030+-0.005)1/A [1]. Although this value was orders of magnitude larger than predicted theoretically, it could be reconciled with the line width found in conventional spin-integrated high resolution angular resolved photoemission spectroscopy (ARPES) data. Here we present novel measurements for a hydrogen intercalated quasi free-standing graphene monolayer on SiC(0001) that reveal a spin polarization signal that - when interpreted in terms of the Rashba-Bychkov effect [2,3] - corresponds to a spin splitting of \Delta k=(0.024+-0.005)1/A. This splitting is significantly larger than the half width at half maximum of spin-integrated high resolution ARPES measurements which is a strong indication that the measured polarization signal does not originate from a Rashba-type spin splitting of the graphene pi-bands as we suggested in our previous report [1].