Spin-dependent lifetime and exchange splitting of surface states on Ni(1 1 1)

Abstract

We report on a spin-resolved two-photon photoemission study of the Ni(1 1 1) surface states. Nickel thin films were grown by molecular beam epitaxy on a W(1 1 0) substrate. The first image-potential state is used as a sensor to map the spin polarization of the occupied surface states. This allows us to identify the majority spin component of the Shockley surface state as well as a majority and minority d-derived surface resonance. The n = 1 image-potential state is found to be exchange split by 14 ± 3 meV. In spite of the fact that the band structure at the Fermi level exhibits a strongly discerned density of states in both spin channels, we observe low spin asymmetries in the decay and dephasing rates of the photoexcited electrons. Varying the sample preparation reveals that the Shockley surface state contributes about 40% to the spin-dependent decay rate.

Formation of image-potential states at the graphene/metal interface

“…For Ru(0001), we have fitted these parameters to the experimental binding energies E 1 = 0.66 eV and E 2 = 0.19 eV reported by Gahl et al [38]. For Ni(111) we have used data reported by Andres et al [36]. Due to the lack of experimental data on the image-potential states of clean Ir(111), we have estimated the binding energies by using the Rydberg formula E n = 0.85 eV/(n + a) 2 where a is the quantum defect.…”

“…Binding energies and lifetimes of the image-potential states on g/Ir(111) have been measured by 2PPE [11] and can be directly compared with the results of our model calculations. No experimental data on g/Ni(111) are available, but 2PPE results on the binding energies for the clean Ni(111) surface [36] make it possible to properly adjust the parameter of the metal potential and to predict the binding energies for the graphene-covered surface. The ferromagnetic coupling of Ni results in a spin-split surface-projected band structure which is reflected by an exchange-splitting of the image-potential states.…”

“…The ferromagnetic coupling of Ni results in a spin-split surface-projected band structure which is reflected by an exchange-splitting of the image-potential states. The observed splitting of 14 ± 3 meV is, however, small compared to the linewidth and can only be observed with spin-resolved detection [36]. We therefore neglect the exchange-splitting here and use a spin-integrated surface-projected band gap extracted from [41] for our model calculations.…”

Surface states characterization in the strongly interacting graphene/Ni(111) system

“…Nevertheless, also the electronic structure at the G point on clean Ni(111), being dominated by spin polarized surface states, is relevant. A recent study [16] has definitively clarified the electronic structure at the G point of Ni(111). A Shockley surface state, derived from the sp band, with a majority spin component presides over the electronic structure at the Fermi level, while its minority component is unoccupied.…”

“…Moreover, an unoccupied image potential state (IPS) is present at 0.8eV below the vacuum level, with an exchange splitting of about 14meV, in agreement with the theoretical estimation based on the bulk penetration of the IPS wave function. While a complete and detailed study of the surface states on clean Ni(111) has been addressed [16,17] an equivalent investigation on graphene/Ni(111) is lacking.…”