Electron momentum-space densities of Li metal: A high-resolution Compton-scattering study

Directional Compton profiles (CP's) of Li metal were measured for 11 directions of the momentum transfer q with 0.14 a.u. (a.u.=atomic units: h=e=m=1) momentum-space resolution using synchrotron radiation from the DORIS (Doppel-Ring-Speicheranlage) storage ring monochromatized to 31 keV. Both the total valence-electron CP's and the directional differences of the CP's exhibit considerable deviations from the most recent density-functional calculations, performed by Sakurai et al. [Phys. Rev. Lea. 74, 2252 (1995)] within the limits of the local density approximation. These discrepancies are attributed to self-energy effects connected with the excitation of so-called plasmaron modes. The three-dimensional (3D) valence-electron momentum density, rho(p), as well as the 3D occupation number density N(k), were reconstructed using the Fourier-Bessel method. The reconstructed rho(p) exhibits clear evidence of higher momentum components due to 110 umklapp processes. The reconstructed N(k) enables a direct experimental access to the Fermi-surface anisotropy of Li, which was found to be 3.6+/-1.1%. The reconstructed N(k) for k parallel to[001] was fitted to a model with the renormalization factor z as the only free parameter, which was found to be z=0.1+0.1.