Observation of $5f$ states in $\mathrm{U}∕\mathrm{W}\left(110\right)$ films by means of scanning tunneling spectroscopy

We have performed scanning tunneling spectroscopy (STS) investigations on clean metallic uranium epitaxially grown on the (110) surface of a tungsten single crystal at room temperature. This system is known to exhibit an hcp phase with the band structure close to the Fermi level dominated by $5f$ states. The STS spectra are found to be in good agreement with the results of relativistic band structure calculations based on the local density approximation. The calculated density of states of the Uranium film indicates that the features in the STS spectra are related to $5f$-states.

Figure 1

(a) and (b) Constant-current STM topographies ($U_{\mathrm{st}}=-250\mathrm{mV}$ and $I_{\mathrm{st}}=0.5\mathrm{nA}$, scan ranges $1\mu \mathrm{m}\times 1\mu \mathrm{m}$, and $198\mathrm{nm}\times 198\mathrm{nm}$) of a nominally $30\mathrm{ML}$ thick U film on W(110) annealed at $800\mathrm{K}$ for $5\min$. (a) Shows an overview of the closed uranium film with regular steps. (b) Shows a closed view of the uranium surface. The surface of the film is not uniform; it is possible to observe lines of dislocations (like the one marked with a white arrow) and lines (black arrow) caused by the steps of the W(110) substrate. The uranium steps have a height of $(2.7\pm 0.3)\mathrm{\AA }$. (c) Atomically resolved constant-current STM topography ($U_{\mathrm{st}}=-53\mathrm{mV}$ and $I_{\mathrm{st}}=2.45\mathrm{nA}$, scan range $5.48\mathrm{nm}\times 5.48\mathrm{nm}$). The image was taken on top of a 26 layer high uranium island of a nominally $8\mathrm{ML}$ thick U film on W(110) annealed at $800\mathrm{K}$ for $5\min$, and shows the hexagonal arrangement of the uranium atoms on the surface. (d) Differential conductivity (averaged over 300 spectra) on top of the surface of the clean uranium film (see Figs. 1a, 1b) with stabilization parameters $U_{\mathrm{st}}=-0.6\mathrm{V}$ and $I_{\mathrm{st}}=1.5\mathrm{nA}$.

Figure 2

Dependence of total energy on volume for bulk $\alpha \text{−}\mathrm{U}$ (at optimized axes ratios and internal parameters), bcc $\gamma \text{−}\mathrm{U}$, and hcp-U (at the optimized $c∕a$-ratio). The vertical line denotes the experimental volume of the $\alpha$-phase.

Figure 3

Calculated total DOS (black), $5f$-DOS (light gray), and $6d$-DOS (dark gray) for the surface layer in the hexagonal supercell (see text). $70\mathrm{meV}$ Gaussian broadening was applied.

Figure 4

Comparison between the local DOS (with $70\mathrm{meV}$ Gaussian broadening) at different distances from the surface of the uranium slab and the STS data taken on top of the $30\mathrm{ML}$ uranium film. For the sake of better comparability, the LDOS at different distances is scaled with a factor $\exp \left[2\kappa z\right]$.