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 ...
Metal electrodes are a universal element of all electronic devices. Conducting SrRuO3 (SRO) epitaxial thin films have been extensively used as electrodes in complex-oxide heterostructures due to good lattice mismatches with perovskite substrates. However, when compared to SRO single crystals, SRO thin films have shown reduced conductivity and Curie temperatures (TC), which can lead to higher Joule heating and energy loss in the devices. Here, we report that highquality SRO thin films can be synthesized by controlling the plume dynamics and growth rate of pulsed laser epitaxy (PLE) with real-time optical spectroscopic monitoring. The SRO thin films grown under the kinetically controlled conditions, down to ca. 16 nm in thickness, exhibit both enhanced conductivity and TC as compared to bulk values, due to their improved stoichiometry and a strain-mediated increase of the bandwidth of Ru 4d electrons. This result provides a direction for enhancing the physical properties of PLE-grown thin films and paves a way for improved device applications. I (a.u.) (deg.) FWHM = 0.06
We present a pulsed laser deposition (PLD) system that can monitor growth by simultaneously using in situ optical spectroscopic ellipsometry (SE) and reflection highenergy electron diffraction (RHEED). The RHEED precisely monitors the number of thin-film layers and surface structure during the deposition and the SE measures the optical spectra of the samples simultaneously. The thin-film thickness information obtained from RHEED facilitates the SE modeling process, which allows extracting the in situ optical spectra, i.e. the dielectric functions, of thin-films during growth. The in situ dielectric functions contain indispensable information about the electronic structure of thin-films. We demonstrate the performance of this system by growing LaMnO 3+δ (LMO) thin-films on SrTiO 3 (001) substrates. By using in situ SE and RHEED simultaneously, we show that real-time thickness and dielectric functions of the LMO thin-films can be effectively extracted. The simultaneous monitoring of both optical SE and RHEED offers important clues to understand the growth mechanism of atomic-scale thin-films. a)
Dimensional tunability from two dimensions to one dimension is demonstrated for the first time using an artificial superlattice method in synthesizing 1D stripes from 2D layered materials. The 1D confinement of layered Sr IrO induces distinct 1D quantum-confined electronic states, as observed from optical spectroscopy and resonant inelastic X-ray scattering. This 1D superlattice approach is generalizable to a wide range of layered materials.
We have investigated the electronic and optical properties of (Sr1-xCax)2IrO4 ( = 0 -0.375) and (Sr1-yBay)2IrO4 ( = 0 -0.375) epitaxial thin-films, in which the bandwidth is systematically tuned via chemical substitutions of Sr ions by Ca and Ba. Transport measurements indicate that the thin-film series exhibits insulating behavior, similar to the eff = 1/2 spin-orbit Mott insulator Sr2IrO4. As the average A-site ionic radius increases from (Sr1-xCax)2IrO4 to (Sr1-yBay)2IrO4, optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated eff = 1/2 and eff = 3/2 bands. We suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the eff = 1/2 band. Our experimental results are consistent with this interpretation as implemented by a multi-orbital Hubbard model calculation: namely, incorporating a strong Fano-like coupling between the spin-orbit exciton and inter-site d-d transitions within the eff = 1/2 band.
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