Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B49: Focus Session: Electron Correlations and Spin Orbit: Iridates |
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Sponsoring Units: DMP Chair: James Rondinelli, Drexel Room: Mile High Ballroom 1C |
Monday, March 3, 2014 11:15AM - 11:51AM |
B49.00001: Exploring the correlated phase behavior and electronic properties of parent and doped spin-orbit Mott phases Invited Speaker: Stephen Wilson An unusual manifestation of Mott physics dependent on strong spin-orbit interactions has recently been identified in a growing number of classes of 5d transition metal oxides built from Ir$^{4+}$ ions. Instead of the naively expected increased itinerancy of these iridates due to the larger orbital extent of their 5d valence electrons, the interplay between the amplified relativistic spin-orbit interaction (intrinsic to large Z iridium cations) and their residual on-site Coulomb interaction $U$, conspires to stabilize a novel class of spin-orbit assisted Mott insulators with a proposed J$_{eff}=$1/2 ground state wavefunction. The identification of this novel spin-orbit Mott state has been the focus of recent interest due to its potential of hosting a variety of new phases driven by correlated electron phenomena (such as high temperature superconductivity or enhanced ferroic behavior) in a strongly spin-orbit coupled setting. Currently, however, there remains very little understanding of how spin-orbit Mott phases respond to carrier doping and, more specifically, how relevant $U$ remains for the charge carriers of a spin-orbit Mott phase once the bandwidth is increased. Here I will present our group's recent experimental work exploring carrier doping and the resulting electronic phase behavior in one such spin-orbit driven Mott material, Sr$_{3}$Ir$_{2}$O$_{7}$, with the ultimate goal of determining the relevance of $U$ and electron correlation effects within the doped system's ground state. Our results reveal the stabilization of an electronically phase separated ground state in B-site doped Sr$_{3}$Ir$_{2}$O$_{7}$, suggestive of an extended regime of localization of in-plane doped carriers within the spin-orbit Mott phase. This results in a percolative metal-to-insulator transition with a novel, global, antiferromagnetic order. The electronic response of B-site doping in Sr$_{3}$Ir$_{2}$O$_{7\, }$will then be compared with recent results exploring A-site doping if time permits. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B49.00002: Controlling spin-orbital and electronic structure in iridate thin films and heterostructures Jian Liu, Jiun-Haw Chu, Claudy Rayan Serrao, Di Yi, Xavi Marti, Ramamoorthy Ramesh Epitaxial thin films and heterostructures have been showed to be a versatile platform for tuning the Mott physics and inducing novel phase in complex oxides with 3$d$ transition metal elements. The strong spin-orbit coupling in 5$d$ transition metal oxides adds a new dimension to this area and attracts strong interests since many theoretical proposals have been put forward for unconventional electronic, magnetic and topological phases. While the combination of correlation and spin-orbit coupling holds huge potential for appealing quantum states and functionalities, the fundamental challenge of realizing experimental manipulation on the spin-orbitals of the $d$-electrons and modulation on the resulting electronic structure has yet to be addressed. Here we report our study on gaining possible control on these spin-orbitals by epitaxial layering. We take perovskite SrIrO$_{\mathrm{3}}$ as a model system of 5$d$ complex oxides and investigate its response to heteroepitaxial strain. Results from x-ray spectroscopy, optical spectroscopy and transport measurements demonstrate that epitaxial constrain offers a unique pathway to tuning the spin-orbit coupling, its interaction with the ligand field, and the macroscopic electronic properties. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B49.00003: Magnetoresistance of Sr$_2$IrO$_4$ Epitaxial Thin Films O.B. Korneta, J. Nichols, J. Terzic, L.E. De Long, G. Cao, S.S.A. Seo Recent studies on Sr$_2$IrO$_4$ single crystals and thin films have revealed an intriguing insulating ground state, even though there are continued debates whether to classify this material as a Mott or Slater insulator. We have recently synthesized epitaxial Sr$_2$IrO$_4$ thin films on various substrates, which allow for the deposition of films under either tensile or compressive strain. The measurements of temperature-dependent magnetoresistance (MR), $\Delta R/R = [R(H)-R(0)]/R(0)$ on these samples reveal a negative linear MR near the room temperature, which is well above the antiferromagnetic ordering temperature ($T_{N} \approx 240$~K). However, as the temperature decreases, the {MR} becomes larger with a positive parabolic response. This behavior is very robust showing no noticeable dependence on magnetic field direction, strain, or film thickness and is remarkably different from the {MR} observed on Sr$_2$IrO$_4$ single crystals. This intriguing effect can be potentially explained by the presence of multiple conducting channels within the sample. We will discuss this model as well as other possible mechanisms for this unique phenomenon. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B49.00004: Tuning Electronic Structure of Epitaxial Sr$_{2}$IrO$_{4}$ Thin Films via Strain S.S.A. Seo, J. Nichols, J. Terzic, E.G. Bittle, O.B. Korneta, L.E. De Long, J.W. Brill, G. Cao Recent research on Sr$_{2}$IrO$_{4}$ has shown that the energy scale associated with spin-orbit coupling is comparable to the crystal-field energy and the on-site Coulomb interaction. The strong competition between these fundamental interactions creates the potential for the emergence of novel electronic states. To understand the physics of Sr$_{2}$IrO$_{4}$ and to find a way of tuning its multiple competing interactions, we have investigated the transport, magnetic, and optical properties of $c$-axis oriented Sr$_{2}$IrO$_{4}$ epitaxial thin films grown on various oxide substrates. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angles are expected to result in increased (decreased) electronic bandwidths. However, the films under various strains have little change in their transport properties. In optical spectroscopic measurements, we have observed that 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 affected by in-plane lattice-strain and interlayer-spacing. Our observations strongly suggest that not only the electronic bandwidth, but also the magnitude of the electronic correlation energy can be manipulated by lattice strain, which provides an important insight into the physics driven by the coexistence of strong spin-orbit coupling and electronic correlation. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B49.00005: Anisotropic Electronic Properties of $a$-Axis Oriented Sr$_{2}$IrO$_{4}$ Epitaxial Thin Films J. Nichols, O.B. Korneta, J. Terzic, L.E. De Long, J.W. Brill, G. Cao, S.S.A. Seo We have successfully synthesized $a$-axis oriented Sr$_{2}$IrO$_{4}$ epitaxial thin films on LaSrGaO$_{4}$ (100) substrates by pulsed laser deposition. The tetragonal structure of the substrate allows for the film to grow with compressive strain along both in-plane directions ($b$- and $c$-axes). This results in the $c$-axis of the film being in-plane. We will present the anisotropic structural, electronic, and optical properties of these $a$-axis oriented thin films along both the $b$- and $c$-axes. X-ray diffraction confirms these films are of high quality and are fully strained along the $c$-axis while the $b$-axis undergoes strain relaxation. The $c$-axis resistivity is approximately one order of magnitude larger than that of the \textit{ab}-plane. Optical absorption spectra with E$\bot c$ polarization show both Ir 5$d$ intersite transitions and charge-transfer transitions (O 2$p$ to Ir 5$d)$, while E//$c$ spectra show only the latter. The structural anisotropy created by biaxial strain in $a$-axis-oriented thin-films also changes the electronic structure and gap energy. These $a$-axis-oriented, epitaxial thin-films provide a powerful tool to investigate the highly anisotropic electronic properties of Sr$_{2}$IrO$_{4}$. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B49.00006: First Principles Prediction of Topological Phases in Thin Films of Pyrochlore Iridates Xiang Hu, Zhicheng Zhong, Mehdi Kargarian, Andreas R\"uegg, Penghao Xiao, Chandrima Mitra, Gregory A. Fiete Using density functional theory and Hartree-Fock theory, we predict topological phases in thin pyrochlore iridate films grown along the [111] direction. Including the full orbital structure of the the relevant d-orbitals and the strong but finite-spin orbit coupling strength, we find a two-dimensional time-reversal invariant topological insulator with a gap of up to ~.15eV is possible in a bilayer geometry, and a zero magnetic field quantum anomalous Hall state is possible in a trilayer geometry with a gap of up to ~0.1eV. Our results show that while the bulk pyrochlore iridates experimentally explored so far may not be promising for insulating topological phases, the thin film geometries are. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B49.00007: Topological insulating phases in oxide multilayers using perovskites and rutiles Victor Pardo, Jose L. Lado, Daniel Baldomir Ab initio calculations combined with tight-binding modelling have been performed\footnote{Phys. Rev. B 88, 155119 (2013).} in 5d-electron-based perovskite multilayers in the large spin-orbit coupling limit. The topological properties of the systems (SrTiO$_3$)$_7$/(SrIrO$_3$)$_2$ and isoelectronic (KTaO$_3$)$_7$/(KPtO$_3$)$_2$ grown along the (111) direction have been analyzed as a function of on-site Coulomb repulsion $U$, parity asymmetry and uniaxial strain. The former is found to be a topological semimetal and the latter is a topological insulator describable as the high-U limit of the other one. This high-U phase can be driven to a trivial insulating phase by a perpendicular external electric field. In the talk, we will describe how to proceed in a similar way with rutile-based multilayered structures, where a 4d/5d electron dioxide with rutile structure, sandwiched by a band insulator like TiO$_2$ or SnO$_2$ can lead to topologically non-trivial properties if band filling and strain are tuned. We discuss also the possibility of obtaining similar topological states using isoelectronic fluorides. The electronic structure and properties of free-standing thin films will be also briefly discussed. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B49.00008: Chiral orbital angular momentum perspective on surface electronic states of SrTiO3 and KTaO3 Kyeong Tae Kang, Panjin Kim, Jung Hoon Han Tight-binding models suitable for the recently observed surface electronic bands of $\rm{SrTiO_3}$ and $\rm{KTaO_3}$ are analyzed with a view to bringing out the relevance of chiral orbital angular momentum (OAM) structure in the $t_{2g}$-derived bands. With the inversion symmetry breaking at the surface, orbital chiralities of the three bands (neglecting spin splitting) are $m = +1,0,-1$. Further inclusion of spin-orbit interaction induces linear Rashba splitting on the chiral OAM bands, but not in the non-chiral, $m=0$ band structure. Our predictions can be easily verified by circular dichroism ARPES experiment. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B49.00009: Electric Field Tuning of the Rashba splitting in $d$-electron systems Shanavas Veedu, Sashi Satpathy It has been found that the Rashba spin splitting is proportional to the electric field so that one can manipulate the electron spin through electric fields leading to potential applications in spintronics devices. Theoretical models based on phenomenological and symmetry arguments have been successful in reproducing the effects, but a complete understanding for $d$ orbital systems is still lacking. Using tight-binding Hamiltonian approaches, we show that the effect can be understood by treating the electric field as a perturbation that leads to mixing of $p$ and $f$ states with $d$ orbitals which can be shown to result in effective Hamiltonians of the Rashba type. We also propose a recipe for deriving the Hamiltonian terms using Gaunt coefficients for general lattice and orbital configurations. We have tested our predictions with density functional theory based calculations for various 3$d$ and 5$d$ systems. In the case of the perovskite oxide surface of KTaO$_3$, we find that Rashba effect originates from the first few layers near the surface and can be altered by moving the 2DEG in and out of the surface using applied fields and the model agrees well with the calculations. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B49.00010: Interaction-induced quantum anomalous Hall phase in bilayers of 3d transition-metal oxide Yilin Wang, Zhong Fang, Xi Dai In the present paper, we have studied the electronic structure of 3d transition-metal oxide LaCoO$_3$ thin film grown on the [111] surface of SrTiO$_3$. By using first-principles calculation under local density approximation implemented with Gutzwiller variational method (LDA+G), we have studied the bilayer systems of LaCoO$_3$ thin films grown along the [111] direction on SrTiO$_3$. The LDA results show that two nearly flat bands locate at the top and bottom of eg bands of Co atoms, and the Fermi level crosses the lower one, which is almost half-filled. After including both the spin-orbit coupling and the rotational invariant Coulomb interaction in the LDA+G method, we found that the Coulomb interaction will enhance the effective spin-orbit coupling, and a ferromagnetic insulator phase with a gap as large as 0.15 eV will be stabilized. Further calculations indicate that such a ferromagnetic insulator phase will have non zero Chern number one leading to quantum anomalous Hall effect. Increasing Hund's rule coupling in this system will generate a low spin to high spin transition and destroy the quantum anomalous Hall phase. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B49.00011: Total Energy Calculations using DFT+DMFT: Application to the Pressure-composition Phase Diagram of Rare-earth Element Nickelates Invited Speaker: Hyowon Park Ab-initio total energy calculations have been implemented within the fully self-consistent density functional theory plus dynamical mean field theory (DFT+DMFT) method, using a Wannier orbital basis. The method is used to calculate the structural and metal-insulator transition phase diagrams of the rare-earth element nickelate $R$NiO$_3$ perovskites as a function of rare-earth ion, pressure and temperature. This phase diagram is of interest because the insulating phase arises from a remarkable site-selective Mott state, in which unusual electronic physics is strongly coupled to a breathing-mode Ni-O bond disproportionation. Conventional DFT fails to stabilize the breathing distortion and thus does not reproduce the insulating phase. DFT+U overpredicts order, in particular finding that $La$NiO$_3$ is disproportionated, in disagreement with experiment. In contrast to these theories, the DFT+DMFT method can quantitatively reproduce the metal-insulator and structural phase diagram of all $R$NiO$_3$ perovskites in the plane of pressure and rare-earth elements. The calculated temperature dependence of the energetics of the phase transformation indicates that the thermal transition is driven by phonon entropy effects. This present method can be generally applied to nano-structured or artificially structured strongly correlated materials including heterostructures and thin films, whose electronic phases are strongly coupled to their lattice degrees of freedom. [Preview Abstract] |
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