NMR-Experimente zur elektronischen Struktur der Si(111)(7x7)-Oberfläche

Despite of being one of the most intensively studied semiconductor surfaces the electronic structure of the Si(111)-(7x7) surface is still under discussion particularly in the vicinity of the Fermi energy. The density of states (DOS) at the Fermi level can be derived from temperature dependent measurements of the 8Li nuclear spin relaxation rate. The measurements were carried out at lithium coverages in the 10^(-3) ML range. In order to enhance the energy resolution of states close to E_F, which is proportional to k_B T, I modified the cooling of the surface to operate with liquid Helium instead of liquid Nitrogen. In order to be able to perform experiments down to 50K Sample temperature. A fit of a schematic model DOS to the data resulted in a band at the Fermi energy as narrow as 5~meV. This band is probably generated by many body effects among the surface electrons, which is a hint that the Si(111)-(7x7) surface stands at the verge of a Hubbard metal insulator transition which is driven by electron correlations. This is the first indication of this transition at the Si(111)-(7x7) surface. From the weak dependence of the relaxation rate from the external magnetic field together with its constancy in between 48 K and 400 K I estimated a lower limit of 0.5 eV for the activation energy of diffusion. Slightly shifted absorption lines were found in beta-NMR-resonance absorption experiments for positive and negative polarisation of the 8Li ensemble respectively. From this shift the sign of the electric field gradient (EFG) of Li adsorbed at Si(111)-(7x7) was deduced. The EFG was found to be positive,indicating that Li is adsorbed not on top but within the rather open (7x7) reconstruction. Due to the width of the lines of 12 Mhz the the calulation of the EFG from the lineshift underestimates the actual value, Vzz>2,44*10^15 V/cm^2.Despite of being one of the most intensively studied semiconductor surfaces the electronic structure of the Si(111)-(7x7) surface is still under discussion particularly in the vicinity of the Fermi energy. The density of states (DOS) at the Fermi level can be derived from temperature dependent measurements of the 8Li nuclear spin relaxation rate. The measurements were carried out at lithium coverages in the 10^(-3) ML range. In order to enhance the energy resolution of states close to E_F, which is proportional to k_B T, I modified the cooling of the surface to operate with liquid Helium instead of liquid Nitrogen. In order to be able to perform experiments down to 50K Sample temperature. A fit of a schematic model DOS to the data resulted in a band at the Fermi energy as narrow as 5~meV. This band is probably generated by many body effects among the surface electrons, which is a hint that the Si(111)-(7x7) surface stands at the verge of a Hubbard metal insulator transition which is driven by electron correlations. This is the first indication of this transition at the Si(111)-(7x7) surface. From the weak dependence of the relaxation rate from the external magnetic field together with its constancy in between 48 K and 400 K I estimated a lower limit of 0.5 eV for the activation energy of diffusion. Slightly shifted absorption lines were found in beta-NMR-resonance absorption experiments for positive and negative polarisation of the 8Li ensemble respectively. From this shift the sign of the electric field gradient (EFG) of Li adsorbed at Si(111)-(7x7) was deduced. The EFG was found to be positive,indicating that Li is adsorbed not on top but within the rather open (7x7) reconstruction. Due to the width of the lines of 12 Mhz the the calulation of the EFG from the lineshift underestimates the actual value, Vzz>2,44*10^15 V/cm^2.