Spin-Orbit Hybridization Points in the Face-Centered-Cubic Cobalt Band Structure

Linear magnetic dichroism is observed in spin-, time-, and energy-resolved two-photon photoemission from valence bands of epitaxial fcc cobalt on Cu(001). With image-potential states as spectator states we identify initial bulk and surface states with minority spin character as the source for dichroic intensities and apparent dichroic lifetimes. Excellent agreement with ab initio fully relativistic calculations of the cobalt fcc band structure allows us to precisely determine spin-orbit hybridization points close to the Fermi level. These spin hot spots enhance spin-flip scattering by several orders of magnitude and are therefore assumed to be crucial in ultrafast demagnetization.

Figure 1

(a) Schematic of the 2PPE experiment. (b) Dependence of the spin-averaged 2PPE intensity on the in-plane magnetization of the cobalt film (${\mathbf{M}}_{\uparrow }$ vs ${\mathbf{M}}_{\downarrow }$) for $p$-polarized pump pulses ($3hv$). The spectra are plotted on an initial energy scale. The asymmetry $A$ in the lower panel was extracted where intensity is resonantly enhanced by the IPS. Dashed lines mark the position of the maxima of $A$.

Figure 2

Time-resolved measurements of the dichroic decay of the $n=2$ and $n\ge 3$ IPS. Quantum beats occur among the coherently excited higher states [16].

Figure 3

(a) Spin- and energy-resolved 2PPE data. Red down triangles and green up triangles denote minority and majority electrons. The left (right) panel is recorded for magnetization up ${\mathbf{M}}_{\uparrow }$ (down ${\mathbf{M}}_{\downarrow }$). (b) Spin-polarization of the IPS for $s$- and $p$-polarized pump pulses. Dashed lines are a guide to the eye.

Figure 4

Spin-polarized fully relativistic band structure calculation of fcc cobalt. Off the hybridization points (square, circles) majority and minority spin bands are colored green and red. The minority-surface resonance experimentally found at $-0.45\mathrm{eV}$ is indicated by the dashed line labeled SR [29].