Strong Coulomb repulsion and spin–orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Initial attempts to investigate systems, where both of these fundamental interactions are comparably strong, such as 3d and 5d complex oxide superlattices, have revealed properties that only slightly differ from the bulk ones of the constituent materials. Here we observe that the interfacial coupling between the 3d antiferromagnetic insulator SrMnO3 and the 5d paramagnetic metal SrIrO3 is enormously strong, yielding an anomalous Hall response as the result of charge transfer driven interfacial ferromagnetism. These findings show that low dimensional spin–orbit entangled 3d–5d interfaces provide an avenue to uncover technologically relevant physical phenomena unattainable in bulk materials.

We have synthesized epitaxial Sr 2 IrO 4 thin-films on various substrates and studied their electronic structures as a function of lattice-strain. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angle is expected to result in increased (decreased) electronic bandwidth. However, we have observed that the two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strain, indicating that the electronic-correlation energy is also affected by in-plane lattice-strain. The effective tuning of electronic structures under lattice-modification provides an important insight into the physics driven by the coexisting strong spin-orbit coupling and electronic correlation. PACS: 71.70.Ej, 72.80.Sk, 81.15 In this letter, we report on the growth and optical properties of Sr 2 IrO 4 (SIO-214) thin films. The in-plane lattice mismatches between SIO-214 and various oxide substrates can exert both tensile (+) and compressive (-) strains to films, as shown in Fig. 1(a). We find that the electronic structure of SIO-214 films are effectively altered by lattice strain, and we observe 3 shifted optical transitions (absorptions) between the J eff = 1/2 lower Hubbard band (LHB) and the J eff = 1/2 upper Hubbard band (UHB), and between the J eff = 3/2 band and the J eff = 1/2 UHB band. Our observations strongly suggest that not only the electronic bandwidth, but also the magnitude of the effective electronic correlation energy (U eff ), can be manipulated by lattice strain. Our results demonstrate that epitaxial SIO-214 thin films can be used as a model system to study the physics of coexisting strong electron correlation and strong spin-orbit coupling under lattice modification.We have used a custom-built, pulsed laser deposition system equipped with in-situ Table I. The epitaxial growth conditions are found to be the following: an oxygen partial pressure (P O2 ) of 10 mTorr, a substrate temperature of 700 °C, and a laser (KrF excimer, λ = 248 nm) fluence of 1.2 J/cm 2 . Figure 2 shows θ-2θ X-ray diffraction scans of the samples discussed herein. Well-defined 00l-peaks are present due to the films' 00l-orientation along the perpendicular to the substrates. The full widths at half maximum in rocking-curve scans of the 00l peaks are all less than 0.05°, which confirms the high crystallinity of the films. Note that the thin films' 0012-peaks are shifted to low angles as the substrate lattice parameters decrease (from GSO to LAO). This behavior is consistent with the schematic diagrams in Fig. 1(b), since elongated (contracted) out-of-plane lattice parameters are expected as compressive (tensile) in-plane strain is exerted on thin films. 4Figure 3(a) shows X-ray reciprocal space maps, which reveal important information about both the in-plane and the out-of-plane lattice parameters of the SIO-214 thin films near the 332-reflection (103-reflection) of orthorhombic (pseudo-cubic) substrates. The 1118-peaks from the thin films are clearly observed, and are...

We have investigated the electronic properties of epitaxial orthorhombic SrIrO 3 thin-films under compressive strain. The metastable, orthorhombic SrIrO 3 thin-films are synthesized on various substrates using an epi-stabilization technique. We have observed that as in-plane lattice compression is increased, the dc-resistivity (ρ) of the thin films increases by a few orders of magnitude, and the dρ/dT changes from positive to negative values. However, optical absorption spectra show Drude-like, metallic responses without an optical gap opening for all compressively-strained thin films. Transport measurements under magnetic fields show negative magneto-resistance at low temperature for compressively-strained thin-films. Our results suggest that weak localization is responsible for the strain-induced metal-insulator transition for the orthorhombic SrIrO 3 thin-films. I. INTRODUCTIONThe 5d transition metal oxides have drawn much interest for their exotic phases arising from the interplay between strong spin-orbit interaction and electronic correlation. [1][2][3] These materials, which include the iridates, were originally predicted to be weakly correlated, paramagnetic metals due to the extended nature of the 5d orbitals and a partially-filled valence band. However, a few interesting phases have been theoretically proposed for systems exhibiting both strong spin-orbit coupling and strong electron correlation. The iridates are prime candidates for realizing these unusual states, such as topological insulators, unconventional superconductivity, quantum spin-Hall effect, and Weyl semimetals to name a few. [4][5][6][7] One system of recent interest is the Ruddlesden-Popper (R-P) series iridates, Sr n+1 Ir n O 3n+1 , whose electronic structure is tunable from a three-dimensional correlated metal, SrIrO 3 (n = ∞), to a two dimensional J eff = 1/2 Mott insulator, Sr 2 IrO 4 (n = 1). 1The insulating state emerges when an octahedral crystal field splits the degenerate 5d levels into e g and t 2g bands; the partially filled t 2g bands (L eff = 1) are further split into the J eff = 3/2 and J eff = 1/2 bands by the strong spin-orbit coupling inherent in iridium ions. The Mott gap opens in the J eff = 1/2 band if the on-site Coulomb interaction becomes energetically comparable to or larger than the bandwidth. In SrIrO 3 , there are six nearest neighbor Ir atoms, while in Sr 2 IrO 4 there are only four. This reduction of coordination number of Ir 5d orbitals reduces the bandwidth and opens a gap in the partially filled J eff = 1/2 band in Sr 2 IrO 4 . Hence, the metal-insulator transition in the R-P series iridates is driven by a dimensionally controlled decrease in bandwidth.In this paper, we have investigated whether a metal-insulator transition can also occur in epitaxial SrIrO 3 thin films via a strain-induced reduction in bandwidth. While the Ir-O bond length is rigid and difficult to change, the Ir-O-Ir bond angle can be readily affected by lattice strain. 8,9 As schematically illustrated in Fig. 1 We have grown epitaxial ...

We report x-ray resonant magnetic scattering and resonant inelastic x-ray scattering studies of epitaxially strained Sr2IrO4 thin films. The films were grown on SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates, under slight tensile and compressive strains, respectively. Although the films develop a magnetic structure reminiscent of bulk Sr2IrO4, the magnetic correlations are extremely anisotropic, with in-plane correlation lengths significantly longer than the out-of-plane correlation lengths. In addition, the compressive (tensile) strain serves to suppress (enhance) the magnetic ordering temperature TN, while raising (lowering) the energy of the zone-boundary magnon. Quantum chemical calculations show that the tuning of magnetic energy scales can be understood in terms of strain-induced changes in bond lengths.

Charge transfer in superlattices consisting of SrIrO3 and SrMnO3 is investigated using density functional theory. Despite the nearly identical work function and non-polar interfaces between SrIrO3 and SrMnO3, rather large charge transfer was experimentally reported at the interface between them. Here, we report a microscopic model that captures the mechanism behind this phenomenon, providing a qualitative understanding of the experimental observation. This leads to unique strain dependence of such charge transfer in iridate-manganite superlattices. The predicted behavior is consistently verified by experiment with soft x-ray and optical spectroscopy. Our work thus demonstrates a new route to control electronic states in non-polar oxide heterostructures.Electron density is one of the most important parameters controlling electronic phases in strongly correlated electron systems. As a milestone in condensed matter physics, high critical temperature superconductivity was discovered in Cu-based oxides by doping carriers into Mott insulating states [1]. This triggered an improvement in crystal synthesis techniques, leading to the discovery of a number of novel spin, charge and orbital states in complex oxide materials [2]. Thin film growth techniques have also improved dramatically [3,4]. In Ref.[4], Ohtomo et al. demonstrated atomically sharp interfaces between two insulating titanates with a metallic behavior. Such metallic interfaces led to the concept of electronic reconstruction originally proposed for K-doped C 60 systems [5,6]. One of the important aspects of the electronic reconstruction is that the valence state of constituent ions in such heterostructures can significantly differ from the corresponding valence state in bulk systems as a result of the electron transfer within the heterostructures. Such electron transfer can be manipulated by the polar discontinuity [7] or by the difference in the work functions [8]. The polar discontinuity was previously discussed in the context of III-V semiconductor heterostructures [9]. In this case, the discontinuity often leads to the atomic reconstruction because it is significantly more challenging to change the valence state than for transition-metal elements.Thus, hetero-structuring is expected to become a fascinating route to explore novel electronic states in complex * Copyright notice: This manuscript has been authored by UTBattelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan) † okapon@ornl.gov systems ...