$d$- and $sp$-like surface states on fcc Co(001) with distinct sensitivity to surface roughness

The sensitivity of electronic surface states to modifications of the surface topography has been investigated in regard to the orbital character of the respective states. The surface of face-centered-cubic Co(001) shows two characteristic electronic states: an occupied $d$-like minority surface state around the center of the surface Brillouin zone $\overline{\Gamma }$ and an unoccupied $sp$-like surface state with an exchange splitting of 0.56 eV around the zone boundary $\overline{\text{X}}$. These states were studied experimentally on a 15-monolayer Co film on Cu(001) by spin-resolved direct and inverse photoemission. In addition, the Co(001) surface was theoretically described by calculations of the Bloch spectral function and of the direct and inverse photoemission intensities within the one-step-model approach. Different surface topographies from atomically smooth to very rough, as proved by scanning tunneling microscopy, were achieved by choosing varied growth and annealing temperatures for the preparation of the Co films. Both types of surface states are highly sensitive to the film roughness, yet in a distinctly different way. The less localized $sp$-like wave function causes a much higher sensitivity of the unoccupied surface state to lateral inhomogeneities than the more localized $d$-type wave function of the occupied surface state.

Figure 1

Spin-resolved PE measurements for 15-ML Co grown on Cu(001) for different growth and annealing temperatures (right-hand panel). The corresponding surface roughness is apparent in the STM measurements on the left-hand side. The PE measurements were obtained for normal electron emission at a sample temperature of $T=145\text{K}$. The spectra were normalized to equal background intensity at $E-E_{\mathrm{F}}=-4.1\text{eV}$. The gray bar marks the position of the minority surface state $S{S}_d^{\downarrow }$. $B{S}_d^{\downarrow }$ and $B{S}_d^{\uparrow }$ are transitions from 3$d$ bulk states with minority and majority-spin character, respectively. In addition, theoretical PE spectra for a Co(001) surface from a one-step model calculation are shown in the left upper corner of the figure.

Figure 2

Spin-resolved PE measurements for two differently prepared 15-ML Co films on Cu(001) [$\wp$($T_{\mathrm{G}}=115\text{K}$, $T_{\mathrm{A}}=555\text{K}$), $\wp$($T_{\mathrm{G}}=295\text{K}$, no annealing)] before and after exposure to oxygen. For each preparation, the spectra for the clean surface, for the surface exposed to 0.1 L of oxygen, and the difference spectrum for minority spin are shown. The difference spectra reveal the energy $E-E_{\mathrm{F}}$ of $S{S}_d^{\downarrow }$ and its spectral width.

Figure 3

Color-coded contour plot for the calculated Bloch spectral function (left-hand side) and for the theoretical PE intensity obtained within the one-step model (right-hand side). High spectral intensity is indicated by dark colors. The dispersion of surface-related features, the minority surface state (labeled $S{S}_d^{\downarrow }$) and an additional surface-resonance-like state, was obtained by using the determinant criterion and is marked by circles and squares, respectively.

Figure 4

Spin-resolved IPE spectra for 15 MLs of Co on Cu(001) measured at room temperature for $\theta ={45}^{\circ }$ along $\overline{\Gamma }\overline{\mathrm{X}}$. The Co film was prepared at $T_{\mathrm{G}}=115\text{K}$ followed by an annealing to $T_{\mathrm{A}}=555\text{K}$. The experimental data are shown in comparison with theoretical spectra for fcc Co(001). The inset shows spin-integrated IPE spectra for different film thicknesses: 5, 15, and 30 MLs. The surface state $S{S}_{sp}$ (indicated by a gray bar) appears at the same energy for all thicknesses.

Figure 5

Spin-integrated IPE measurements of 15-ML Co grown on Cu(001) for different surface topographies (right-hand side) in relation to corresponding STM measurements (left-hand side). For the preparations $\wp$($T_{\mathrm{G}}=115\text{K}$, $T_{\mathrm{A}}=555\text{K}$) and $\wp$($T_{\mathrm{G}}=115\text{K}$, $T_{\mathrm{A}}=295\text{K}$), the sample temperature was $T=295\text{K}$ during the measurement. For the preparations $\wp$($T_{\mathrm{G}}=295\text{K}$, no annealing) and $\wp$($T_{\mathrm{G}}=115\text{K}$, no annealing) the sample temperature was $T=145\text{K}$ during the measurement. The spectra were normalized to equal background intensity.

Figure 6

Color-coded contour plot for the calculated Bloch spectral function of the unoccupied electronic structure of Co(001) (left-hand side). Calculations within the one-step model of IPE for $\hslash \omega =9.9\text{eV}$ (right-hand side). High spectral intensity is indicated by dark colors. The dispersion behavior of the $sp$-like surface resonance $S{R}_{sp}^{\downarrow \uparrow }$ is indicated by green up and red down triangles for the majority and minority-spin channels, respectively.