Surface States of Cobalt Nanoislands on Cu(111)

The electronic structure of thin Co nanoislands on Cu(111) has been investigated below and above the Fermi level ($E_F$) by scanning tunneling spectroscopy at low temperature. Two surface related electronic states are found: a strong localized peak 0.31 eV below $E_F$ and a mainly unoccupied dispersive state, giving rise to quantum interference patterns of standing electron waves on the Co surface. Ab initio calculations reveal that the electronic states are spin polarized, originating from $d_{3z^2-r^2}$-minority and $sp$-majority bands, respectively.

Figure 1

(a) Constant-current image and (b) $dI/dV$ map of 2 ML high Co nanoislands on Cu(111). The scan range is $32\mathrm{n}\mathrm{m}\times 41\mathrm{n}\mathrm{m}$ and the tunneling parameters are $V=0.5\mathrm{V}$, $I=1.8\mathrm{n}\mathrm{A}$. In the gray scale, brighter areas correspond to higher topography and larger LDOS, respectively. (c) Fourier spectrum of standing electron wave pattern on a Co island.

Figure 2

Dispersion relation of the electronic state determined from standing electron waves on Co islands. The inset shows the coverage dependence of the dispersion. The solid lines represent parabolic fits. The fitting parameters $E_0$ and $m^{*}$ are given in Table .

Figure 3

Representation in 3D of the topography of a Co island on Cu(111), where the gray scale coding is given by the $dI/dV$ signal at $V=0.5\mathrm{V}$, $I=1.1\mathrm{n}\mathrm{A}$. The second, third, and fourth ML of Co as well as the Cu(111) surface are visible, with standing electron waves on all layers. The image size is $28\mathrm{n}\mathrm{m}\times 28\mathrm{n}\mathrm{m}$.

Figure 4

STS spectra on 2 ML high Co nanoislands and on the bare Cu(111) surface taken with the same tip.

Figure 5

Calculated LDOS $2.1\AA$ above a 2 ML Co film on Cu(111) [16]. The inset shows the charge density distribution of the minority $d_{3z^2-r^2}$ surface state at the $\overline{\Gamma }$ point in a (110) plane through the Co film.