Ph.D. Candidate: Tao Jia
Research Advisor: Zhi-Xun Shen
Date: Wednesday, February 19th, 2020
Location: McCullough Building, Room 335
Title: Substrate Effect for Correlated Chalcogenide Thin Films
The intersection between correlated electron systems and thin films provides exciting research opportunities. Quantum confinement effects, interfacial interactions and other substrate effects can be engineered to acquire desired physical properties in heterostructures. To create and characterize complex thin films, we apply Molecular Beam Epitaxy (MBE) and Angle-resolved Photoemission Spectroscopy (ARPES) as precise tools for film growth and electron property measurement.
Monolayer (1ML) FeSe films on SrTiO3 (STO) substrates demonstrate a large increase in electron doping and superconducting transition temperature. By comparing 1ML FeSe/TiO2 to 1ML FeSe/STO and other superconductors in iron chalcogenide family, we argue that doping and interfacial electron-phonon interaction are the two key factors to the enhanced superconductivity. Then we further study the origin of the doping effects in 1ML FeSe/titanate thin films using a versatile platform: 1ML FeSe/LaTiO3 (LTO) /STO. To our surprise, although the charge density of two-dimensional electron gases (2DEGs) on LTO/STO can be increased to a few times higher than FeSe carrier density, the doping of 1ML FeSe on top of LTO/STO heterostructure does not increase. This provides strong confinements to theories that aim to explain the doping of 1ML FeSe/titanates. Our research on 1ML FeSe with TiO2 and LTO/STO as substrates helps researchers understand the origin of increased superconductivity in this material system and provides insights on how to apply similar mechanisms to other materials.
We switch gears and discuss the substrate effects for TiSe2/TiO2 heterostructure. TiSe2, both as bulk single crystals and as thin films on graphene, is in an ordered charge density wave (CDW) state at low temperatures. We find that hexagonal TiSe2 can be grown epitaxially on tetragonal TiO2 substrates, and the resulting films show large electron doping and no CDW order. We further demonstrate that the doping and suppression of CDW order is due to Se vacancies, which are created by substrate induced change in the film growth condition. This indirect substrate effect of doping may be applied to other transition metal dichalcogenide (TMDC) films on transitional metal oxide (TMO) substrates as a powerful way to tune the electrical and optical properties of such heterostructures.