Quasiperiodic layers of free-electron metals studied using electron diffraction

Using electron diffraction, we show that free-electron metals, such as sodium and potassium, form a highly regular quasiperiodic monolayer on the fivefold surface of icosahedral Al-Pd-Mn and that the quasiperiodicity propagates up to the second layer in sodium. Our photoelectron spectroscopy results show that the quasicrystalline alkali-metal adlayer does not exhibit a pseudogap near the Fermi level thought to be characteristic for the electronic structure of quasicrystalline materials. Calculations based on density functional theory provide a model structure for the quasicrystalline alkali-metal monolayer and confirm the absence of a pseudogap.

Figure 1

(a) LEED pattern of a clean $i\text{-Al-Pd-Mn}$ surface. Diffraction spots are numbered to facilitate their identification in the intensity profiles [shown in (d)] along the dashed line going through different spots. (b) and (c) show LEED patterns measured at 130 K for different Na and K coverages on $i\text{-Al-Pd-Mn}$, respectively. Images are shown in inverted gray scale where black indicates the highest brightness. All the LEED patterns have been recorded at 78 eV electron-beam energy. (d) Intensity profile as a function Na coverage along the dashed line is shown in (a). (e) The atomic structure of an adsorbed K monolayer on the Al-Pd-Mn substrate where Al: small open circles, Pd: small gray circles, Mn: small black circles, and K: large gray circles. The quasiperiodic ordering is represented by the DHBS tiling (cf. text).

Figure 2

LEED patterns of 1.2 ML Na grown on $i\text{-Al-Pd-Mn}$ surface at several incident electron-beam energies. Images are shown in inverted gray scale where black indicates the highest brightness. All the LEED patterns have been recorded at 130 K.

Figure 3

Photoemission spectra in the near $E_F$ region (open circles) of (a) 1.3 ML K coverage on $i\text{-Al-Pd-Mn}$ and (b) clean $i\text{-Al-Pd-Mn}$ surface at 130 K (open circles). The fitted curve (thick solid line) and Fermi function (dashed line) are also shown. Residual of fitting is shown at the top of each spectra. (c) Closeup of near Fermi edge region as a function of K coverage. Spectra are normalized to the same height at 0.1 eV. (d) The difference spectra are obtained by subtracting the near $E_F$ region spectrum of 1.3 ML K from those of other K coverages shown in (c). (e) The calculated DOS of Na and K monolayer adsorbed on $i\text{-Al-Pd-Mn}$.