By using a combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction and first-principles calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite SrIrO3 can be lifted simply by applying epitaxial constraints. In particular, the Dirac gap opens without breaking the P bnm mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the n-glide operation, which can be selectively broken by different heteroepitaxial structures. This symmetry protection renders the nodal line a nonsymmorphic Dirac semimetallic state. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures.

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We report an experimental investigation of the two-dimensional J eff = 1/2 antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [(SrIrO3)1, (SrTiO3)m] (m = 1, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the m = 2 and m = 3 samples than the m = 1 sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive SrTiO3 layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the Neel temperature from 150 K for m = 1 to 40 K for m = 2 demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the SrTiO3 layer number m, for maintaining canting-induced weak ferromagnetism. The nearly unaltered transition temperature between the m = 2 and the m = 3 demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.

Fundamental control of magnetic coupling through heterostructure morphology is a prerequisite for rational engineering of magnetic ground states. We report the tuning of magnetic interactions in superlattices composed of single and bilayers of srIro 3 inter-spaced with srtio 3 in analogy to the Ruddlesden-popper series iridates. Magnetic scattering shows predominately c-axis antiferromagnetic orientation of the magnetic moments for the bilayer, as in sr 3 Ir 2 o 7. However, the magnetic excitation gap, measured by resonant inelastic x-ray scattering, is quite different between the two structures, evidencing a significant change in the stability of the competing magnetic phases. In contrast, the single layer iridate hosts a more bulk-like gap. We find these changes are driven by bending of the caxis Ir-o-Ir bond, which is much weaker in the single layer, and subsequent local environment changes, evidenced through x-ray diffraction and magnetic excitation modeling. Our findings demonstrate how large changes in the magnetic interactions can be tailored and probed in spin-orbit coupled heterostructures by engineering subtle structural modulations. Atomic scale layering of disparate materials to form an artificial heterostructure is a promising method for the intelligent design of emergent properties 1-3. Systems incorporating strong spin-orbit coupling are particularly intriguing as candidates for designer magnetic phases due to the coupling of the magnetic moments to structural distortions. Towards this goal, iridates, compounds composed of active Ir 5d orbitals in oxygen octahedra, have emerged as an important new class of correlated material 4-13. The combination of crystal field interactions and strong spin-orbit coupling, λ~0.5 eV, generates narrow bands often leading to Mott insulating antiferromagnetic ground states despite the modest values of the Coulomb repulsion U~1 eV 5,14,15. The resulting = J eff 1 2 pseudospin states are then intimately coupled with the Ir orbitals and thus are much more sensitive to structural changes than pure spin = S 1 2 moments. This is exemplified by the different magnetic ground states appearing in iridates composed of similar Ir-O octahedra when they are subtly distorted or interspaced with different atoms. For instance, Sr 2 IrO 4 , hosting isolated IrO 2 layers, forms an ab-plane canted antiferromagnetic state 6,16 , while Sr 3 Ir 2 O 7 , hosting isolated IrO 2 bi-layers, has robust c-axis collinear antiferromagnetic ordering stabilized by a 90 meV magnon gap 17,18. These competing magnetic ground states establish the iridates as ideal candidates for utilizing controlled structural perturbation to rationally engineer the magnetic behavior. Towards this goal, layered iridate heterostructures, or superlattices (SL), were recently realized via alternating layers of nSrIrO 3 and SrTiO 3 (nSIO/1STO) opening up routes to tailor magnetic phenomena within artificial Ruddlesden-Popper analogues 14,19. How magnetic couplings change within such heterostructures as compared to their bulk...

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