Epitaxy-distorted spin-orbit Mott insulator in Sr${}_{2}$IrO${}_{4}$ thin films

Epitaxy-distorted spin-orbit Mott insulator in Sr $_{2}$ IrO $_{4}$ thin films

C. Rayan Serrao, Jian Liu, J. T. Heron, G. Singh-Bhalla, A. Yadav, S. J. Suresha, R. J. Paull, D. Yi, J.-H. Chu, M. Trassin, A. Vishwanath, E. Arenholz, C. Frontera, J. Železný, T. Jungwirth, X. Marti, and R. Ramesh

Phys. Rev. B 87, 085121 – Published 15 February 2013

Abstract

High-quality epitaxial thin films of $J_{eff}$ $=$ 1/2 Mott insulator Sr $_{2}$ IrO $_{4}$ with increasing in-plane tensile strain have been grown on top of SrTiO $_{3}$ (001) substrates. Increasing the in-plane tensile strain up to ∼0.3 $%$ was observed to drop the $c$ / $a$ tetragonality by 1.2 $%$ . X-ray absorption spectroscopy detected a strong reduction of the linear dichroism upon increasing in-plane tensile strain towards a reduced anisotropy in the local electronic structure. While the most relaxed thin film shows a consistent dependence with previously reported single crystal bulk measurements, electrical transport reveals a charge gap reduction from 200 meV down to 50 meV for the thinnest and most epitaxy-distorted film. We argue that the reduced tetragonality plays a major role in the change of the electronic structure, which is reflected in the change of the transport properties. Our work opens the possibility for exploiting epitaxial strain as a tool for both structural and functional manipulation of spin-orbit Mott systems.

Received 20 August 2012

DOI:https://doi.org/10.1103/PhysRevB.87.085121

Authors & Affiliations

C. Rayan Serrao¹, Jian Liu^2,*, J. T. Heron¹, G. Singh-Bhalla^1,3, A. Yadav¹, S. J. Suresha⁴, R. J. Paull¹, D. Yi¹, J.-H. Chu¹, M. Trassin¹, A. Vishwanath², E. Arenholz⁵, C. Frontera⁶, J. Železný⁷, T. Jungwirth^7,8, X. Marti^1,7,9,†, and R. Ramesh^1,2,3,4

¹Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
²Department of Physics, University of California, Berkeley, California 94720, USA
³Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁴National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁵Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁶Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, E-08193 Bellaterra, Spain
⁷Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 162 53 Praha 6, Czech Republic
⁸School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom
⁹Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Praha 2, Czech Republic

^*jian.liu@berkeley.edu
^†xaviermarti@berkeley.edu

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Vol. 87, Iss. 8 — 15 February 2013

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Images

Figure 1
(a) Scanning tunnel electron microscopy image of a 10-nm-thick epitaxial Sr $_{2}$ IrO $_{4}$ (001) thin film grown on top of a SrTiO $_{3}$ (001) substrate. The image evidences the 2D layered structure with ABA-ABA stacking of the iridium (red) and strontium (blue) oxide planes. Inset: Sketch of the Sr $_{2}$ IrO $_{4}$ layered structure (Ir, Sr, and O are depicted by big, medium, and small spheres, respectively). Bottom inset: θ/2θ scan showing only expected peaks for a $c$ -oriented Sr $_{2}$ IrO $_{4}$ (60-nm-thick film). (b) Atomic force microscopy images showing terraces and steps with less than 1 monolayer root mean square roughness in a 10-nm-thick samples. (c) X-ray diffraction azimuthal φ scans indicating [110]SrTiO $_{3} (001) ∥ [100]$ Sr $_{2}$ IrO $_{4}$ (001) epitaxial relationship. Rocking curves around the Sr $_{2}$ IrO $_{4}$ (0 0 12) and SrTiO $_{3}$ (002) reflections for films with thickness (d) 5 nm and (e) 10 nm, respectively. (f) 60-nm-thick sample two-dimensional projection of the reciprocal space of Sr $_{2}$ IrO $_{4}$ (001)/SrTiO $_{3}$ (001) thin films. The abscissa Q $_{ip}$ represents the total in-plane component of the momentum transfer [Q $_{ip}$ $=$ ( $Q_{x}$ $^{2}$ $+$ $Q_{y}$ $^{2}$ ) $^{1 / 2}$ ], while the ordinate is the vertical component Q $_{z}$ . Inset shows the calculated versus observed structure factors refinement for the 60- and 10-nm-thick samples. Sketch illustrates the obtained Ir-octahedra distortion.Reuse & Permissions
Figure 2
(a) X-ray diffraction θ/2θ scans for Sr $_{2}$ IrO $_{4}$ (001)/SrTiO $_{3}$ (001) films with increasing thickness (dynamical theory diffraction simulation is shown as the red line). (b)–(d) Reciprocal space maps around the Sr $_{2}$ IrO $_{4}$ (2 0 24) reflection. Data indicate that the in-plane parameter monotonically contracts upon a reduction of the film thickness with a concomitant expansion of the out-of-plane parameter. Dashed red lines indicate the vertical and horizontal coordinates for bulk Sr $_{2}$ IrO $_{4}$ ; the solid green line signals the in-plane parameter of the SrTiO $_{3}$ substrate.Reuse & Permissions
Figure 3
(Top) Linear polarization-dependent x-ray absorption spectra at oxygen K-edge for samples with 5, 10, and 60 nm (as indicated). (Bottom) Difference spectra corresponding to the three thicknesses. While the large linear dichroism (shown in the bottom panel) of the relaxed sample reflects the expected two-dimensional behavior, it gradually fades out due to the increasing in-plane tensile strain and the concomitant collapse of the out-of-plane parameter of the unit cell.Reuse & Permissions
Figure 4
In-plane resistance measurements on the set of Sr $_{2}$ IrO $_{4}$ thin films. The resistivity normalized to the room temperature value is presented in the inset. Analyses revealed a temperature-dependent effective band gap (Δ) tunable with the unit cell distortion. The most relaxed sample mimics the reported (asterisks, adapted from Ref. 20) bulk behavior, while the largest strain-induced distortion severely modifies both the slope and abscissa of the temperature dependence. Panels (b)–(d) plot the gap versus structural parameters. Panel (e) shows the theoretical increase of the Ir-O-Ir bonding angle with epitaxial strain.Reuse & Permissions

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