Ferroelectric control of the electronic properties of ultra-thin metallic films
Final Report Abstract
Barium titanate BaTiO3 (BTO) is a highly interesting material because of its ferroelectric properties and its large number of phase transitions. Used as a single-crystalline substrate in layered systems with epitaxial thin films, these properties might influence indirectly the physical properties of the film, in particular the electronic structure which is sensitive to local electric fields and temperature-dependent crystallographic parameters. Different types of overlayer systems were studied in order to clarify the question whether BTO can serve as a model system to control the physical properties of the thin film by the ferroelectricity of the substrate. By use of angle resolved photoemission spectroscopy (ARPES) a) the surface electronic structure of clean BTO surfaces and b) metallic thin-film overlayers were investigated. Particularly epitaxial bismuth films gave promising results by revealing interesting features in the electronic band structure. Bismuth has a strong spinorbit coupling which leads e.g. to a large Rashba splitting in various metallic systems or to the characteristic "topological" surface states in famous TI systems as, e.g., Bi 2Te3. There is a close relation of ferroelectricity and d-p hybridization in BTO. We found that also the in-gap state in BTO is composed of Ti3d states that hybridize with O2p valence states as already stated for the related material SrTiO 3. The occurrence of the in-gap state as well as the quasi-particle state with mainly Ti3d character in BTO is closely related to the creation of oxygen vacancies at its surface. The order of occupation of either one of these states, however, depends on the position of high-symmetry points in k-space and on the amount of doping. Hence, states with certain orbital character are favored while reaching a specific level of doping. For the development of the hybridized in-gap state and the metallic quasi-particle state the creation of oxygen vacancies is an important requirement. These oxygen impurities might illustrate an opportunity to tune ferroelectric and metallic behavior in transition metal oxides at the same time assuming that the oxygen vacancies at the surface do not destroy ferroelectricity in the bulk. A combination of these two properties would represent a milestone for the realization of oxide hetero-structures. In this project it could be demonstrated that epitaxial Bi films can be grown with rather high quality on ferroelectric BTO(001) with layer thicknesses down to 4 ML. The films grow in bilayers along (0001) orientation and form two twinned domains, rotated relatively by 90°. By use of ARPES we have directly observed metallic electronic states with large spinsplittings being located inside the large band gap of the BTO substrate. We found an enhanced coupling between substrate and Bi films with decreasing layer thickness. This is manifested by the emergence of an characteristic electronic state at the Bi/BTO interface, and by rather subtle changes in the electronic structure of the film. The findings pave the way to exploit proximity effects between spin-polarized electronic states with strong spinorbit coupling and ferroelectricity. For example, ferroelectric field effects on the spinpolarized states in the Bi film, as predicted theoretically, could provide opportunities to manipulate spin-polarized states in Rashba/ferroelectric hybrid systems.
Publications
- "Barium Titanate: A promising candidate for the use of oxide-based electronics." DPG Spring Meeting Dresden 2014
P. Lutz, O. Copie, M. Mulazzi, H. Bentmann and F. Reinert
- Universal properties of the near-gap spectra of SmB6: dynamical mean-field calculations and photoemission experiments
C.-H. Min, K.-S. Chen, P. Lutz, H. Bentmann, B. Y. Kang, B. K. Cho, J. Werner, F. Goth, F. Assaad and F. Reinert
- "Surface states of n-doped BaTiO3 ." SFB Fall School/ FOR Workshop Würzburg 2016
P. Lutz, S. Moser, Y. J. Chang and F. Reinert
- "Ultrathin Bi-films on BaTiO3 (001)." DPG Spring Meeting Regensburg 2016
P. Lutz, T. Figgemeier, H. Bentmann and F. Reinert
- "Valence characterisation of the subsurface region in SmB 6." Philosophical Magazine, 2016, 1-15
P. Lutz, M. Thees, T. R. F. Peixoto, B. Y. Kang, B. K. Cho, C.-H. Min, F. Reinert
(See online at https://doi.org/10.1080/14786435.2016.1192724)
