Crystal-quasicrystal interfaces on Al-Pd-Mn

Two decades after their discovery, quasicrystals continue to fascinate our imagination. Even though there remain many outstanding questions about the basic structure, our focus has lately been shifting beyond the basic structural aspects, towards the search for interesting physical properties and technological applications. In this regard, undoubtedly, thin film growth on quasicrystal surfaces promises to be a very fertile ground. Despite their true long-range orientational order, quasicrystals possess forbidden fivefold or tenfold point-group symmetries and lack translational periodic order. Therefore, the interface between a quasicrystal and a crystal is of immense scientific interest, because it can metastabilize previously unknown structures with unique properties and potentially disclose structural mysteries of quasicrystalline surfaces. In our studies, we used various electron probes and observed that binary Al alloys exist as commensurate single crystals on the fivefold- and threefold-symmetry surfaces of the Al-Pd-Mn alloy. Thus, epitaxial growth conditions, like lattice and chemical matching observed in crystals are apparently satisfied on a local scale. We have also grown Al films at different temperatures on quasicrystal surfaces, and found that above RT Al readily diffuses into the bulk quasicrystal leading to a huge increase of its Debye temperature, implying that the quasicrystalline structure contains vacancies, which are filled with the additional Al. For thin films grown at RT, epitaxy locks the Al atoms to a strained quasicrystalline lattice, whereas for thicker films, excess strain energy cannot be supported and the structure relaxes to the bulk stable face-centered cubic phase by breaking into multi-twinned domains. The crystal-quasicrystal transformations also present an opportunity to investigate the electronic structure in a comparative way. We highlight a relevant result on the filling of the Mn 3d band in Al-Pd-Mn upon quasicrystal formation that gives rise to the pseudo gap.