Detection of decoupled surface and bulk states in epitaxial orthorhombic SrIrO3 thin films

Prescott E Evans,Takashi Komesu,Shiv Kumar,Kenya Shimada,Eike F Schwier,P A Dowben,Evgeny Y Tsymbal,Xia Hong,Le Zhang,Andrew J Yost,Ding-Fu Shao

doi:10.1063/1.5135941

Prescott E Evans, Takashi Komesu + Show 9 more

Open Access

https://doi.org/10.1063/1.5135941

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Abstract

We report the experimental evidence of evolving lattice distortion in high quality epitaxial orthorhombic SrIrO3(001) thin films fully strained on (001) SrTiO3 substrates. Angle-resolved X-ray photoemission spectroscopy studies show that the surface layer of 5 nm SrIrO3 films is Sr–O terminated, and subsequent layers recover the semimetallic state, with the band structure consistent with an orthorhombic SrIrO3(001) having the lattice constant of the substrate. While there is no band folding in the experimental band structure, additional super-periodicity is evident in low energy electron diffraction measurements, suggesting the emergence of a transition layer with crystal symmetry evolving from the SrTiO3 substrate to the SrIrO3(001) surface. Our study sheds light on the misfit relaxation mechanism in epitaxial SrIrO3 thin films in the orthorhombic phase, which is metastable in bulk.

Highlights

While the crystal structure of bulk SrIrO3 favors the monoclinic distortion of the hexagonal BaTiO3 structure6,13 rather than the perovskite phase, high-quality orthorhombic SrIrO3(001) films have been grown on SrTiO3(001),7,9,14 (LaAlO3)0.3(SrAl1/2Ta1/2O3)0.7,8,14 and GdScO312 substrates
Epitaxial SrIrO3 films in the ultrathin limit exhibit a range of interesting properties, including dimensionality crossover, metal–insulator transition, and enhanced spin relaxation time,9,11,15 and it is essential to understand how the electronic structure and lattice distortion evolve in these thin films
In a previous study,15 using the reciprocal space mapping and pole figure techniques, we have shown that the SrIrO3 films are fully strained and conform to the four-fold symmetry of SrTiO3, indicating that the films are in the orthorhombic phase

Summary

INTRODUCTION

The combination of strong spin–orbit interactions and electron correlation has driven the recent research interest in 5d irridates, including the correlated semimetal SrIrO3.7–12 Orthorhombic SrIrO3, as schematically shown in Figs. 1(a) and 1(b), has been theoretically predicted to host non-trivial topological phases. While the crystal structure of bulk SrIrO3 favors the monoclinic distortion of the hexagonal BaTiO3 structure rather than the perovskite phase, high-quality orthorhombic SrIrO3(001) films have been grown on SrTiO3(001), (LaAlO3)0.3(SrAl1/2Ta1/2O3)0.7,8,14 and GdScO312 substrates. While the crystal structure of bulk SrIrO3 favors the monoclinic distortion of the hexagonal BaTiO3 structure rather than the perovskite phase, high-quality orthorhombic SrIrO3(001) films have been grown on SrTiO3(001), (LaAlO3)0.3(SrAl1/2Ta1/2O3)0.7,8,14 and GdScO312 substrates. The goal of this research is to further examine the crystal symmetry in epitaxial orthorhombic SrIrO3 thin films. This is key to any effort to exploit the interfacial charge and control quantum confinement to engineer the electronic and magnetic states in SrIrO3. A key question is how the effect of the epitaxial strain is entangled with the quantum confinement in determining the electronic properties of these ultrathin films

EXPERIMENTAL AND THEORETICAL METHODS

SURFACE TERMINATION

SUPER-PERIODICITY

CONCLUSIONS