Electronic structure of a Co-decorated vicinal Cu(775) surface: High-resolution photoemission spectroscopy

We measure the electronic structure of low-coverage Co on a Cu(775) stepped substrate using high-resolution photoemission spectroscopy with the particular goal of relating the electronic dispersion to the coverage-dependent surface structure. In particular, we follow the evolution of the electronic dispersion of the $sp$-like Cu surface state and the position of the band minimum as a function of Co coverage. On the bare Cu(775) surface, we observe band folding of this state due to the stepped surface-superlattice array. In addition, we determine that the reference plane, as measured by the position of the band minimum of this state, changes dramatically after addition of just 0.03 ML Co. At 0.06 ML, we observe the formation of a second surface state at a binding energy of $0.68\mathrm{eV}$. This feature is attributed to a quantum-well state hybridized with the substrate.

Figure 1

(Color online) Sketch of well ordered Cu(775); the terrace width is $14.3\mathrm{\AA }$ and the sample cut is such that the step descends along the $\left[11\overline{2}\right]$ direction.

Figure 2

(Color online) (A) The measured bare Cu(775) surface-state dispersion using $25\mathrm{eV}$ photon energy. Note that the main band is centered on the surface Brillouin-zone boundary of the superlattice, designated by the dashed line (a). The parallel momentum corresponding to the terrace-normal direction is indicated by the dotted line (b). The parabolic continuous curve labeled (i) is a quadratic fit to the main band. The second continuous curve (ii) is a $0.44{\mathrm{\AA }}^{-1}$ $(2\pi ∕d)$-shifted replica of the fit. The curve indicated by (iii) is a weak- to nondispersive state, typical of many stepped surfaces. (B) An example of peak fitting the weak features to the right of the main band. After an appropriate background subtraction a sum of two Gaussian curves was used to fit the data. Three energy-distribution curves are shown at the parallel momenta indicated on the right. The downward- and upward-facing arrows indicate the nondispersive state and the umklapp band, respectively.

Figure 3

(Color online) Shift of the band minimum versus inverse terrace width. Data are collected from (i) this work and Refs. (ii) 1, (iii) 3, (iv) 6, (v) 8, (vi) 9, (vii) 10, and (viii) 11.

Figure 4

(Color online) Evolution of EDCs as a function of Co coverage at $k_{\parallel }=0.22{\mathrm{\AA }}^{-1}$. The inset to the figure shows features close to the Fermi level in greater detail for the same sequence in Co coverage as in the main figure. Experiments are performed using $25\mathrm{eV}$ incident photon energy.

Figure 5

(Color online) Angle-resolved photoemission from (a) clean Cu(775), and (b) 0.03 ML Co-covered surface at $21\mathrm{eV}$ incident photon energy. On clean Cu, the surface state is centered at the surface-superlattice Brillouin-zone (SSBZ) boundary, i.e., $k_{\parallel }=0.22{\mathrm{\AA }}^{-1}$. Upon cobalt deposition, the band shifts to a position close to terrace normal at $k_{\parallel }=0.31{\mathrm{\AA }}^{-1}$. Note that the parallel momentum corresponding to the terrace-normal direction [the dotted line in (b)] is a function of binding energy since the (111) terraces are oriented at a fixed angle (8.5°) with respect to the (775) surface.

Figure 6

(Color online) (a) Angle-resolved photoemission from 0.06 ML Co-covered Cu(775) taken at a photon energy of $25\mathrm{eV}$. The nondispersive feature labeled (iii) at $\sim 1.2\mathrm{eV}$ binding energy is due to emission from Co. In addition to the shifted Cu(775) surface state (blue squares), a new state with $0.68\mathrm{eV}$ binding energy appears at the SSBZ boundary (red dots). (b) EDCs taken at parallel momenta corresponding to (i) the SSBZ boundary and (ii) the terrace normal are shown along with Gaussian fits to the background subtracted EDCs. The shifted Cu(775) surface state is indicated by a downward-facing blue arrow and the new state by an upward-facing red arrow.

Figure 7

(Color online) Angle-resolved photoemission measurements along the steps ($y$ direction, see text), taken at a 1° interval from 0.04 ML Co-covered Cu (775). The top and bottom curves correspond to 19° and $-5°$, respectively. ARPES data at (a) $k_x\sim 0.34{\mathrm{\AA }}^{-1}$ (terrace normal) and (b) $k_x=0.22{\mathrm{\AA }}^{-1}$ (the SSBZ boundary) in the direction across the steps; (c) comparison of EDCs for the two cases at $k_y=0{\mathrm{\AA }}^{-1}$ along with Gaussian fits to the background subtracted EDCs.