Surface electronic structure of Y(0001): A consistent picture

The photoelectron spectrum of the (0001) surface of single-crystalline yttrium bulk samples is rich in structure from the surface state near the Fermi energy to a series of spectral features between 2 and $10\mathrm{eV}$ binding energies. Many of them appear on other rare-earth surfaces, e.g., Gd, Ho, Pr, and Er, as well. However, it is an unsolved puzzle that none of the observed features, except for the surface state at the Fermi energy, can be found in data from clean rare-earth thin films. While some of the additional features in the results from single-crystal surfaces were already attributed to impurities such as chlorine, oxygen, and hydrogen, the peak at $-9.6\mathrm{eV}$ remains a source of speculation for more than $20\text{years}$ now. In this Rapid Communication, we provide a consistent picture of the surface electronic structure of Y(0001), which explains all differences between the data obtained from single-crystal surfaces and thin films. We show that the peak at $-9.6\mathrm{eV}$, which was previously assigned to a well-ordered clean surface, is in fact due to the formation of ordered carbon compounds originating from impurities in the bulk material.

Figure 1

Photoemission spectra of a yttrium single crystal with (0001) surface at $300\mathrm{K}$ (open dots) and at $1100\mathrm{K}$ (filled dots). The gray line represents photoemission data obtained from a thin yttrium film grown on W(110) (taken from Ref. 5).

Figure 2

Photoemission spectra of a Y(0001) single-crystal surface taken at room temperature (black dots) during the annealing process from $300\text{to}900\mathrm{K}$ (gray lines), at $900\mathrm{K}$ (gray dots), and at $1100\mathrm{K}$ (open dots).

Figure 3

Photoemission spectra of adsorption experiments with $\mathrm{C}{\mathrm{O}}_2$ performed on the Y(0001) single crystal at two sample temperatures, 300 and $900\mathrm{K}$. The filled gray dots indicate the spectra as obtained from the nominally clean sample. The open dots, triangles, and squares show spectra for increasing $\mathrm{C}{\mathrm{O}}_2$ exposure. The filled black dots represent an exposure of $20\mathrm{L}$ $\mathrm{C}{\mathrm{O}}_2$ $(1\mathrm{L}=1.33\times {10}^{-4}\mathrm{Pa}\mathrm{s})$.

Figure 4

Photoemission spectra of a Y(0001) single-crystal surface taken from Ref. 10 in comparison with our results from $20\mathrm{L}$ $\mathrm{C}{\mathrm{O}}_2$ on Y(0001) at a temperature of $900\mathrm{K}$ (black dots) and after cooling down to $300\mathrm{K}$ (open dots).