Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F47: Metal-Insulator and Other Electronic Phase Transitions: Experiment I |
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Sponsoring Units: DCMP Chair: David Hsieh, California Institute of Technology Room: Mile High Ballroom 4F |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F47.00001: Local modification of spin orbit coupling in Sr2IrO4 Kyle McElroy, Jixia Dai, Eduardo Calleja, Gang Cao Sr$_{2}$IrO$_{4}$ has a novel Mott insulating ground state that is a result of strong spin orbit coupling (SOC) splitting the t$_{\mathrm{2g}}$ states leaving a small bandwidth Jeff$=$1/2 valence band that can then be localized by the small 5d Coulomb repulsion. In order to investigate the effects that the strong SOC has on the novel ground state we have doped them with Rh$^{4+}$ atoms, which lower the SOC, which substitute for the the Ir$^{4+}$ ions. In bulk it has been shown that with only a small Rh concentration changes the insulating state to a metallic one and the low temperature magnetic state weakens. We have found several interesting results in these doped materials and will discuss them and what they tell us about the ground state of Sr$_{2}$IrO$_{4}$. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F47.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F47.00003: A time- and wavelength-resolved optical pump-probe reflectivity study of the Metal-to-Insulator Transition in Sr$_{2}$IrO$_{4}$ Tejas Deshpande, Darius Torchinsky, Liuyan Zhao, Xiaoyue Ni, Tongfei Qi, Gang Cao, David Hsieh The iridates have been predicted to exhibit many exotic quantum phases due to a unique interplay of strong electron-electron correlations and spin-orbit coupling. The perovskite iridate Sr$_{2}$IrO$_{4}$ in particular has recently attracted a lot of attention owing to the possibility of high-temperature superconductivity upon doping and an unconventional phase transition between a metallic and spin-orbital entangled Mott Insulator ground state. Optical pump-probe reflectivity experiments using 1.5 eV pump and 1.5 eV probe light have demonstrated that the thermally induced metal-to-insulator transition in Sr$_{2}$IrO$_{4}$ exhibits both Mott- and Slater-type behavior [Phys. Rev. B 86, 035128 (2012)]. We extend these studies by performing optical pump-probe reflectivity experiments on Sr$_{2}$IrO$_{4}$ single crystals over a wide range of probe wavelengths in order to investigate the low energy electronic relaxation dynamics near the insulating gap. We will discuss the implications of our results on the nature of the metal-to-insulator transition. This work is supported by Army Research Office Grant Nos. W911NF-13-0059 and (ARO-DURIP) W911NF-13-1-0293. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F47.00004: Strongly-correlated 2D electron state by surface deposition in Sr$_2$IrO$_4$ Ilya Belopolski Strong electron correlations in crystals often play a more important role in lower dimensions. Here we use in situ potassium surface deposition to create a 2D strongly-correlated electronic state at the surface of Sr$_2$IrO$_4$. This compound is unusual because bandwidth, Coulomb repulsion and spin-orbit coupling are at comparable energy scales. In related compounds, it has been predicted that these competing interactions can give rise to exotic states such as a Weyl semimetal or an oxide topological insulator. Here, we use angle-resolved photoemission spectroscopy (ARPES) to study how the electron states in Sr$_2$IrO$_4$ change as a result of potassium deposition. We observe the formation of new electron states, which may be interpreted as a Rashba band splitting near the sample surface. Such a spin-textured surface electron state is unusual in a strongly-correlated compound. This result may allow us to realize novel strongly-correlated electron states by taking advantage of large spin-orbit coupling. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F47.00005: STM studies on the lightly doped Mott Insulator Sr$_{2-x}$Eu$_{x}$IrO$_{4}$ Carlos J. Arguello, Ethan P. Rosenthal, Qingbiao Zhao, Bum Joon Kim, Abhay N. Pasupathy Sr$_{2}$IrO$_{4}$ is a 5d$^{5}$ transition metal oxide that displays a novel J$_{eff}=$1/2 Mott insulator behavior. This has been attributed to a large spin-orbit coupling combined with narrow Hubbard bands. Several interesting effects of chemical doping on this system have been proposed, being of special interest the possibility of an insulating to metal transition and of induced superconductivity for large doping concentrations. However, a systematic study of this type requires an understanding of the effect of chemical dopants at the atomic scale. In this talk, we will present Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) measurements on lightly doped Sr$_{2-x}$Eu$_{x}$IrO$_{4}$. By obtaining atomic resolution images we have estimated the Eu doping concentration to be close to 0.5{\%}. This dilute doping allows us to isolate the effect of an individual doping atom or impurity on the electronic properties of the system. Furthermore, high spatial resolution STS measurements enable us to study the effective spatial range of the effect of a dopant on the local density of states. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F47.00006: Hole doping induced metal-insulator transition in Sr$_{1-x}$K$_x$IrO$_4$ Qing'an Li, Qingbiao Zhao, B.J. Kim, J.F. Mitchell We report a metal-insulator transition against temperature in hole doped Sr2IrO4. The temperature dependence of \textit{ab}-plane resistivity of the doped Sr2IrO4 shows a peak at 6.5K. The magnetization against temperature shows a magnetic transition temperature about 200 K that is significantly reduced compared with its pristine material (240K). Hall effect measurements confirm that the conduction carrier is hole. A small magnetoresistance $\sim$ 3.5{\%} with significant anisotropy with respect to magnetic field orientation is observed, indicating the importance of spin-orbital coupling on conduction mechanism of the materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F47.00007: Raman studies of electronic excitations in Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$ Jhih-An Yang, Dmitry Reznik, Tongfei Qi, Gang Cao A novel Mott insulator Sr$_{2}$IrO$_{4}$ driven by strong spin-orbital interaction (SOI) has recently attracted a lot of attention. Small onsite Coulomb repulsion can open a gap in the SOC-induced J=1/2 states due to the narrow band. Interesting electronic phases in Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$ were discovered by substituting Ir$^{4+}$ with Rh$^{4+}$, which can effectively tune the strength of spin-orbit interaction. We report results of a Raman scattering investigation of Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$ from x=0 to x=0.7. The evolution of the phonon spectrum with Rh doping and temperature as well as the resonant profiles of phonons will be discussed. In addition, we also observed broad luminescence-like signal whose origin is not well understood. Latest results on high energy electronic excitations and luminescence will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F47.00008: Metal-insulator transition in doped iridates Sr$_3$Ir$_2$O$_7$ Wenwen Zhou, Yoshinori Okada, Chetan Dhital, Tom Hogan, Ilija Zeljkovic, Daniel Walkup, Hsin Lin, Tay-Rong Chang, Arun Bansil, Ziqiang Wang, Stephen Wilson, Vidya Madhavan Bilayer perovskite iridate Sr$_3$Ir$_2$O$_7$ (Ir327) is a spin-orbit Mott insulator with a small charge gap, and as such provides an ideal playground for exploring carrier-induced metal-insulator transition (MIT). In particular, site-dependent introduction of carriers is proposed to lead to vastly different effects on this transition. To probe this, we use scanning tunneling spectroscopy (STS) to spatially map out the local density of states of Ir327 doped via two distinct routes: Ru-doped Ir327 (Sr$_3$(Ir$_{1-x}$Ru$_x$)$_2$O$_7$) with in-plane, and La-doped Ir327 ((Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$) with out-of-plane carriers. We find that in-plane Ru doping leads to MIT at x $\sim$ 35 $\%$, while out-of-plane La doping shows homogeneous metallic phase, even with dilute La concentration of a few percent, indicating a different MIT mechanism arising from the site-dependent doping process. Our STS data, combined with transport and neutron scattering results, offer potential routes to obtaining metallic states as well as novel phases from the parent insulating states in iridates. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F47.00009: Surface Structure and Property Coupling of Sr$_{3}$(Ru$_{\mathrm{1-x}}$Mn$_{\mathrm{x}})_{2}$O$_{7}$ Chen Chen, Corentin Durand, An-Ping Li, Jiandi Zhang, Rongying Jin, Ward Plummer The double-layered Ruthenate Sr$_{3}$Ru$_{2}$O$_{7}$ exhibits interesting behavior under the influence of pressure, while partial substitution of Mn for Ru generates a dramatic response in the physical properties. Even more striking is the structure-property relationship observed at the surface. Combining LEED $I-V, $with STM/STS and high-resolution electron energy loss spectroscopy (HREELS), we document a very unique surface phase diagram. The octahedra are tilted at the surface (not in the bulk) for low Mn doping, and the surface stabilizes and enhances the octahedra rotation, present in the bulk for Mn doping less that $\sim$ 20{\%}. The structure-property relationship at the surface is consistent with calculations of a tilt/rotation phase diagram (PRB 64, 020509 (2001)). Tilt distortion at the surface favors an insulating AFM ordered phase and when tilt is removed by doping of Mn the surface becomes conducting. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F47.00010: Metal-insulator transition in epitaxial perovskite SrIrO$_3$ thin films via strain J.H. Gruenewald, J. Terzic, J. Nichols, G. Cao, S.S.A. Seo Iridates have drawn considerable interest due to their exotic phases arising from the interplay of the strong spin-orbit interaction and the electronic correlation. Here we will discuss our experimental investigations of the electronic properties of epitaxially strained SrIrO$_3$ thin-films. The orthorhombic perovskite crystal phase of SrIrO$_3$ is synthesized as a thin film ($\sim$ 20 nm) on various substrates of (LaAlO$_3$)$_{0.3}$-(Sr$_2$AlTaO$_6$)$_{0.7}$, SrTiO$_3$, GdScO$_3$, and MgO using pulsed laser deposition. We have observed that when the in-plane lattice parameters are tuned from tensile to compressive strain, the electronic behavior of the strained SrIrO$_3$ thin-films changes from metallic to insulating. All samples have sheet resistance below 13 k$\Omega$/$\Box$, and the insulating samples were fit using the Mott variable-range-hopping equation at low temperatures ($<$ 15 K), which is believed to be the conducing mechanism of Anderson localization at finite temperature. The strain-dependent metal-insulator transition in epitaxial perovskite SrIrO$_3$ thin-films offers an important insight into the electronic structure of these strongly correlated, spin-orbit-coupled materials. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F47.00011: Electronic, magnetic, and structural properties of ferromagnetic insulator K$_{2}$Cr$_{8}$O$_{16}$ Sooran Kim, Kyoo Kim, B.I. Min The hollandite-type material, K$_{2}$Cr$_{8}$O$_{16}$, exhibits a couple of phase transitions upon cooling: the first magnetic transition at T = 180K and the second metal-insulator transition at T = 95K. Namely, for 95K $<$ T $<$ 180K, K$_{2}$Cr$_{8}$O$_{16}$ is a ferromagnetic metal, while, for T $<$ 95K, it is a ferromagnetic insulator. Moreover, the metal-insulator transition is accompanied by the structural transition from tetragonal to monoclinic structure. In order to explore the underlying mechanisms of these phase transitions, we have investigated systematically electronic, magnetic, and structural properties of K$_{2}$Cr$_{8}$O$_{16}$ based on the first principles DFT (density-functional theory) band structure calculations taking into account the on-site Coulomb correlation interaction ${\it U}$. The role of the electron-electron correlation on the magnetic, metal-insulator, and the structural transitions will be discussed by comparing the band structures of DFT and DFT+${\it U}$. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F47.00012: Impact of Interface Roughness on the Metallic Transport of Strongly Correlated 2D Holes in GaAs Quantum Wells Nicholas Goble, John Watson, Michael Manfra, Xuan Gao Understanding the non-monotonic behavior in the temperature dependent resistance, $R(T)$, of strongly correlated two-dimensional (2D) carriers in clean semiconductors has been a central issue in the studies of 2D metallic states and metal-insulator transitions. We have studied the transport of high mobility 2D holes in 20nm wide GaAs quantum wells with varying interface roughness by changing the Al fraction $x$ in the Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As barrier. Prior to this work, no comprehensive study of the non-monotonic resistance peak against controlled barrier characteristics has been conducted. We show that the shape of the electronic contribution to $R(T)$ is qualitatively unchanged throughout all of our measurements, regardless of the percentage of Al in the barrier. It is observed that increasing $x$ or short range interface roughness suppresses both the strength and characteristic temperature scale of the 2D metallicity, pointing to the distinct role of short range versus long range disorder in the 2D metallic transport in this 2D hole system with interaction parameter $r_{s}$ $\sim$ 20. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F47.00013: Nonactivated transport driven by Coulomb interactions with tunable shapes Jian Huang, Loren Pfeiffer, Ken West In high quality updoped GaAs field-effect-transistors, the two-dimensional (2D) charge carrier concentrations can be tuned to very low values similar to the density of electrons on helium surfaces. An important interaction effect, screening of the Coulomb interaction by the gate, rises as a result of the large charge spacing comparable to the distance between the channel and the gate. Based on the temperature -dependence of the resistivity results from measuring four different GaAs heterojunction-insulated-gate field-effect-transistor (HIGFET) samples, the power law characteristics are found for 2D hole densities $\leq 2\times10^{9}$ cm$^{-2}$ with an exponent universally depending on a single dimensionless parameter [1], the ratio between the mean carrier separation and the distance to the metallic gate that screens the Coulomb interaction. Thus, the interaction-driven electronic properties are not only sensitive to the relative strength, but also the shape of the interaction potential. \newline [1] Jian Huang, L. N. Pfeiffer, K. W. West, to appear on Phys. Rev. Lett. (2013) [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F47.00014: Exploring Fe$_{1-y}$Co$_{y}$Si near the insulator-to-metal transition Yan Wu, Brad Fulfer, Julia Chan, David Young, John DiTusa FeSi is a nonmagnetic narrow gap insulator with interesting temperature-dependent magnetic and optical properties. Charge carriers, either holes or electrons, accompanied by a more localized magnetic moments, can be introduced by doping FeSi with Mn or Co. It has been reported that Mn doping of FeSi near the insulator-to-metal transition (IMT) exhibits an intriguing field sensitive non-Fermi-Liquid behavior due to an undercompensated Kondo effect where the spin-1/2 carriers underscreen the $S=$1 impurity moments. To compare with the case of Mn substitution (hole doping), we investigate the effect of Co substitution (Fe$_{1-y}$Co$_{y}$Si, 0$\le y \le $ 0.1)(electron doping)results. Our magnetic property measurements indicate an interesting evolution of the impurity magnetic moments with $y$. Our transport studies indicate a temperature and field dependence that does not conform to the standard disordered Fermi-liquid form for small $y$. Standard semiconducting behavior is restored either by applying a magnetic field or increasing $y$. A more detailed analysis is underway to compare with disordered Fermi liquid theory as well as to the previously reported behavior of Fe$_{1-x}$Mn$_{x}$Si. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F47.00015: Strain-induced metal-insulator transitions in $d^1$ perovskites within DFT+DMFT Krzysztof Dymkowski, Claude Ederer We present results of combined density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations, assessing the effect of epitaxial strain on the electronic properties of the Mott insulator LaTiO$_3$ and the correlated metal SrVO$_3$. In particular, we take into account the effect of strain on the collective tilts and rotations of the oxygen octahedra in the orthorhombically distorted $Pbnm$ perovskite structure of LaTiO$_3$. We find that LaTiO$_3$ undergoes an insulator-to-metal transition under a compressive strain of about $-2$\,\%, consistent with recent experimental observations [1]. We show that this transition is driven mainly by strain-induced changes in the crystal-field splitting between the Ti $t_{2g}$ orbitals, which in turn are related to changes in the octahedral tilt distortion. We compare this with the case of SrVO$_3$, without octahedral tilts, where we find a metal-to-insulator transition under tensile epitaxial strain. Similar to LaTiO$_3$, this metal-insulator transition is linked to the strain-induced change in the crystal-field splitting within the $t_{2g}$ orbitals. [1] Wong et al., Phys. Rev. B 81, 161101 (2010) [Preview Abstract] |
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