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Shin, Hyungki

University of British Columbia

The perovskite structure oxides, ABO₃, have been the subject of considerable interest in the fundamental science and industrial fields due to their distinctive physical properties and the potential for novel applications resulting from a strongly correlated system with sensitivity to variation. Recent advanced techniques enable perovskite oxides to be grown in artificial structures, such as ultra-thin films and heterostructures/superlattices. Such artificial perovskite oxide structures can provide extra degrees of freedom. Among these techniques, molecular beam epitaxy (MBE) was used in this thesis to grow and study two titanate films, EuTiO₃ and LaTiO₃. EuTiO₃ and LaTiO₃ are perovskite structure oxides with insulating ground states and have an antiferromagnetic transition at 5.5 K and 160 K, respectively. However, they have distinctive electronic ground states; EuTiO₃ is a band insulator with a 1 eV energy gap, while LaTiO₃ is a Mott–Hubbard insulator with a 0.2 eV energy gap. Furthermore, LaTiO₃ has polarity along the (001) direction, while EuTiO₃ has charge-neutral layers. Thus, the similarities and differences between these two materials can produce unique properties and motivate scientific research in this thesis. This thesis studied the solid solutions and heterostructures of EuTiO₃ and LaTiO₃. The solid solution of EuTiO₃ and LaTiO₃, Eu₁₋ₓLaₓTiO₃ for 0 ≤ x ≤ 1, were grown, and their electronic and magnetic properties were studied. These structures show a diverse phase diagram from an insulator to a metal, changing from an antiferromagnetic to a ferromagnetic state, with increasing x. Furthermore, the heterostructure of the monolayer of FeSe and EuTiO₃ was studied. The high crystalline quality of monolayer FeSe, an iron-based high-temperature superconducting material, was grown on EuTiO₃ using a dual MBE system. This structure shows an antiferromagnetic state below 5.5 K and superconductivity with a transition temperature of 30 K. Lastly, the heterostructure of the two titanate insulators exhibits clear metallic behavior and an exotic magnetic state because of their polar/non-polar combinations and the competition of two antiferromagnetic states at the interface. Furthermore, the result suggests the possible closing and opening of the Mott–Hubbard gap due to the evolution of the electronic reconstruction at the interface.

Text

2023-02-27

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/83861

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/