Spin-Polarized Surface State in EuO(100)

High-quality films of the ferromagnetic semiconductor EuO are grown on epitaxial graphene on Ir(111) and investigated in situ with scanning tunneling microscopy and spectroscopy. Electron scattering at defects leads to standing-wave patterns, manifesting the existence of a surface state in EuO. The surface state is analyzed at different temperatures and energies. We observe a pronounced energy shift of the surface state when cooling down below the Curie temperature $T_{\mathit{C}}$, which indicates a spin polarization of this state at low temperatures. The experimental results are in agreement with corresponding density functional theory calculations.

Figure 1

(a) STM topography image of a closed EuO(100) film containing point defects, stabilization parameters: $U=1.25\mathrm{V}$, $I=0.77\mathrm{nA}$, scan size $10.3\mathrm{nm}\times 8.8\mathrm{nm}$, imaged at 5.3 K. (b) Simultaneously recorded differential conductivity $dI/dU$ map (contrast is enhanced by adding the spatial derivative to the signal). Circular standing-wave patterns of electrons scattered at point defects are visible. Inset: Fourier transform (width $36.6{\mathrm{nm}}^{-1}$) of a large-scale Laplace-filtered $dI/dU$ map containing the area shown in (b).

Figure 2

(a) STM topography and (b) same area differential conductivity $dI/dU$ map at 5.3 K measured on 3.3 nm EuO(100). Scattering is observed at step edges. Stabilization parameters: $U=1.50\mathrm{V}$, $I=0.073\mathrm{nA}$, scan width 16.5 nm. (c) STM topography and (d) same area $dI/dU$ map of the same sample, but different area at 81.0 K. Scattering is observed at line defects. Stabilization parameters: $U=1.50\mathrm{V}$, $I=0.033\mathrm{nA}$, scan width 16.5 nm. (e) Line profiles of $dI/dU$ as indicated by the red (gray) and black line in (b) and (d).

Figure 3

Dispersion relation: ($E-E_{\mathit{F}}$) vs wave vector $k$. Red (gray): Sample temperature 5.3 K. Black: Sample temperature 81.0 K. Full dots: Real-space method. Open triangles: Fourier transform method (see text). Lines and gray area: Results from density functional calculations (downshifted by 0.41 eV; see text). Lines: surface state, red (gray): 0 K, solid: spin up, dashed: spin down. Black: Spin averaged state $T>T_{\mathit{C}}$. Gray area: bulk band at 0 K, projected on the parallel momentum $k$. Note that the bulk band at $T>T_{\mathit{C}}$ is not at the same position.

Figure 4

(a) STM topography of EuO(100). Tunneling parameters: $U=1.51\mathrm{V}$, $I=0.15\mathrm{nA}$. (b) Atomically resolved STM topography image of the same sample area as in (a), imaged with $U=-0.68\mathrm{V}$, $I=0.10\mathrm{nA}$, scan width 2.5 nm. The black box in (a) and (b) marks the same sample area. (c) Height profile along the horizontal line in (a). (d) Statistics of the maximum apparent depth of depressions measured under identical tunneling conditions as in (a). (e) The apparent barrier height $\mathrm{\Phi }$ of the tunneling junction is determined by local $I(z)$) spectra measured along the red (gray) line in the inset, where $z$ is the vertical tip-sample distance. Two defect sites are marked by arrows. Spectra parameter: Stabilization at $U_{s\mathrm{tab}}=0.96\mathrm{V}$ and $I_{s\mathrm{tab}}=0.51\mathrm{nA}$, retraction to $U=0.70\mathrm{V}$. Inset: STM topography of EuO(100), scan size $18.3\mathrm{nm}\times 14.4\mathrm{nm}$, $U=0.95\mathrm{V}$, $I=0.51\mathrm{nA}$, imaged at 5.3 K.