Effect of impurities on Tamm-like lanthanide-metal surface states

The effects of isolated residual-gas adsorbates on the local electronic structure of the Dy(0001) surface were spatially mapped by scanning tunneling microscopy and spectroscopy at $12\mathrm{K}$. Less than $15\mathrm{\AA }$ away from an adsorbate, a strong reduction of the intensity and a significant increase of the width of the majority component of the surface state due to impurity scattering were observed, with essentially no change of the minority component; this reflects a high lateral localization of the Tamm-like surface state. Furthermore, an adsorbate-induced state was found that behaves metastably.

Figure 1

(Color online) (a) STM topograph of a Dy(0001) surface covered with $\simeq 0.01\mathrm{ML}$ of hydrogen adsorbates (see text; tunneling parameters: $-200\mathrm{mV}$, $0.5\mathrm{nA}$). A zoom of the marked area is shown at (b) $-200\mathrm{mV}$ and (c) $100\mathrm{mV}$. (d) Bias dependence of the apparent height of the adsorbates. A contrast reversal occurs at the position of the MAC (see Fig. 2).

Figure 2

(Color online) Local tunneling spectra of a Dy(0001) surface $⩾15\mathrm{\AA }$ away from any adsorbate (top) and at various distances relative to an adsorbate (marked by arrows in the STM image). The intensity of the MAC (at $\simeq -100\mathrm{mV}$) is halved on an adsorbate, while the width is approximately doubled. In addition, an adsorbate-induced state appears at $U\simeq 100\mathrm{meV}$. The solid lines are results of the fit analysis.

Figure 3

(Color online) Top: Topographical cross section through a single adsorbate at $U=-300\mathrm{mV}$. Bottom: Lateral dependences of the width (${\mathrm{Dy}}_{\uparrow }$, red/gray squares) and the $dI∕dU$ intensity of the MAC (blue/dark gray circles), and comparison with the $dI∕dU$ intensity of the adsorbate-induced state (green/light gray triangles).

Figure 4

(Color online) Some of the STS spectra exhibit signal jumps at a bias voltage corresponding to the position of the adsorbate-induced state ($U\simeq 100\mathrm{mV}$; see arrow in red/gray spectrum), while others do not (blue/dark gray spectrum). Inset: Jumps can also occur “upwards” at $\simeq 100\mathrm{mV}$, as if the state suddenly appears at this bias (see green/light gray spectrum), and even several jumps can occur during data acquisition.