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In solid state physics, the electronic structure is often explained by examples of periodic systems. In reality, quasicrystals of metallic alloys with aperiodic atomic structure are well-known, whereas their valence electronic structure is still under debate [1]. Moreover, only very recently their unoccupied electronic structure has been studied [2]. In this contribution, we present angle-resolved photoelectron spectroscopy on the occupied and unoccupied electronic structure of a two-dimensional oxide quasicrystal (OQC), which is formed by the rewetting process of barium titanate on Pt(111) [3,4].
The occupied valence bands are investigated by the momentum microscope with He I excitation, and the photoelectron distribution over the whole hemisphere above the surface is mapped [5]. We decompose the energy-dependent photoelectron patterns according to symmetry considerations. As a results, clear dispersion of the oxygen 2p bands with a bandwidth of more than 0.5 eV at 5.2 eV below the Fermi level can be identified. Moreover, localized oxygen 2p states are observed at around 4 and 6 eV below the Fermi level, which are comparable to the O non-bonding and the Ti-O dpσ bands in bulk, respectively [6].
As preliminary studies on the unoccupied electronic states and their femtosecond dynamics, two-photon photoemission spectroscopy using a megahertz fiber laser system is performed. With a pump and probe photon energy of around 3.9 and 1.7 eV, the n=1 and 2 image potential states on OQC are identified at around 3.5 and 3.6 eV above the Fermi level.
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[5] C. Tusche, A. Krasyuk, and J. Kirschner, Ultramicroscopy 159, 520 (2015).
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