Valence band structure of the icosahedral Ag-In-Yb quasicrystal

The valence band structure of the icosahedral $\left(i\right)$ Ag-In-Yb quasicrystal, which is isostructural to the binary $i\text{-Cd-Yb}$ system, is investigated by ultraviolet photoemission spectroscopy (UPS). Experimental results are compared with electronic-structure calculations of a cubic approximant of the same phase. UPS spectra from the fivefold, threefold, and twofold $i\text{-Ag-In-Yb}$ surfaces reveal that the valence band near to the Fermi level is dominated by $\text{Yb}4f$-derived states, in agreement with calculations. The spectra also exhibit peaks which are surface core level shifted, caused by changes in the electronic structure in surface layers. Calculations yield a pseudogap in the density of states due to a hybridization of the $\text{Yb}5d$ band with the $\text{Ag}5p$ and $\text{In}5p$ bands. Both experimental and calculated band features are very similar to those of Cd-Yb. The modification of the band structure after surface treatment by sputtering and by oxidation is also studied. Additionally, the work function of $i\text{-Ag-In-Yb}$ measured from the width of UPS spectrum is found to be almost unaffected by surface orientation, but increases after sputtering or oxidation.

Figure 1

Valence band spectrum of the fivefold $i\text{-Ag-In-Yb}$ surface after annealing taken with He-I radiation. Inset shows a closeup near the Fermi level. Gray curves correspond to the results of a fit as described in the text. The Fermi level $E_F$ is marked by a dotted line.

Figure 2

(a) Total density of states of 1/1 cubic approximant of $i\text{-Ag-In-Yb}$ calculated with the tight-binding linear muffin-tin orbitals method. (b) Partial density of states for $\text{Yb}4d$ (black), $\text{Ag}/\text{In}5p$ (gray), and $\text{Ag}/\text{In}5s$ (light gray). Vertical dotted lines mark the Fermi level.

Figure 3

He-I valence band spectra of the fivefold $i\text{-Ag-In-Yb}$ surface and pure Ag, In, and Yb surfaces. Intensity of each spectrum is subtracted from background intensity above the Fermi level. Intensity of Ag spectrum is scaled by a factor of 0.3. The broad peak at around 6 eV in Yb spectrum is due to oxygen $2p$ states. Inset shows a closeup near the Fermi level.

Figure 4

He-I valence band spectra of the fivefold $i\text{-Ag-In-Yb}$ surface after sputtering, annealing, and oxidation. Valence band spectrum of pure Yb after oxidation is also given for comparison (dotted line, intensity is scaled by a factor of 0.7). The intensity of each spectrum is subtracted from the background intensity above the Fermi level. The $+$ symbol marks a shoulder developed due to oxidation. The inset shows a closeup near the Fermi level.

Figure 5

He-I valence band spectra of the fivefold, threefold, and twofold surfaces of $i\text{-Ag-In-Yb}$ after annealing. The intensity of each spectrum is subtracted from the background intensity above the Fermi level. The inset shows a closeup near the Fermi level. The arrow indicates a shoulder which develops due to surface contamination.

Figure 6

He-I valence band spectra of the fivefold $i\text{-Ag-In-Yb}$ surface after sputtering, annealing, and oxidation with a $-10\text{V}$ bias applied to the sample. The inset shows a closeup of the secondary cutoff. The shift of the secondary cutoff $\left(\Delta \varphi \right)$ to lower binding energy manifests an increase in work function, i.e., decrease in width of the spectrum, $W=h\nu -\varphi$.