  Band splittings, chiral spin polarization and topological surface states
generated by spin-orbit interactions at crystal surfaces are receiving a lot of
attention for their potential device applications as well as fascinating
physical properties. Most studies have focused on sp states near the Fermi
energy, which are relevant for transport and have long lifetimes. Far less
explored, though in principle stronger, are spin-orbit interaction effecs
within d states, including those deep below the Fermi energy. Here, we report a
joint photoemission/ab initio study of spin-orbit effects in the deep d orbital
surface states of a 24-layer Au film grown on Ag(111) and a 24-layer Ag film
grown on Au(111), singling out a conical intersection (Dirac cone) between two
surface states in a large surface-projected gap at the time-reversal symmetric
M points. Unlike the often isotropic dispersion at Gamma point Dirac cones, the
M point cones are strongly anisotropic. An effective k.p Hamiltonian is derived
to describe the anisotropic band splitting and spin polarization near the Dirac
cone.
