Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L47: Metal-Insulator and Other Electronic Phase Transitions: Experiment II |
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Sponsoring Units: DCMP Chair: Rajeswari Kolagani, Towson University Room: Mile High Ballroom 4F |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L47.00001: Hysteretic melting transition of a soliton lattice in IrTe$_2$ Weida Wu, Pin-Jui Hsu, Tobias Mauerer, Matthias Vogt, Matthias Bode, J.J. Yang, Yoon Seok Oh, S-W. Cheong We report on the observation of the hysteretic transition of a commensurate charge modulation in IrTe$_2$ from transport and scanning tunneling microscopy (STM) studies. Below the transition ($T_{\rm C} \approx 275$ K on cooling) a $q = 1/5$ charge modulation was observed, which is consistent with previous studies [1,2]. Additional modulations [$q_n = (3n+2)^{-1}$] appear below a second transition at $T_{\rm S}\approx 180$ K on cooling. The coexistence of various modulations persist up to $T_{\rm C}$ on warming. The atomic structures of charge modulations and the temperature dependent STM studies suggest that 1/5 modulation is a periodic soliton lattice which partially melts below $T_{\rm S}$ on cooling. Our results provide compelling evidence that the ground state of IrTe$_2$ is a commensurate 1/6 charge modulation, which originates from periodic dimerization of Te atoms visualized by atomically resolved STM images [3]. [1] Yang, {\it et al.}, Phys. Rev. Lett. 108, 116402 (2012). [2] Oh, {\it et al.}, Phys. Rev. Lett. 110, 127209 (2013). [3] Hsu, {\it et al.}, arXiv:1311.3015, (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L47.00002: Electronic signatures of dimerization in IrTe$_{2}$ Jixia Dai, Weida Wu, Yoon Seok Oh, S.-W. Cheong, J.J. Yang Recently, the mysterious phase transition around T$_{c} \quad \approx $ 260 K in IrTe$_{2}$ has been intensively studied. A structural supermodulation with q$=$1/5 was identified below T$_{c}$. A variety of microscopic mechanisms have been proposed to account for this transition, including charge-density wave due to Fermi surface nesting, Te p-orbital driven structure instability, anionic depolymerization, ionic dimerization, and so on. However, there has not been an unified picture on the nature of this transition. To address this issue, we have performed low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) experiments on IrTe$_{2}$ and IrTe$_{2-x}$Se$_{x}$. Our STM data clearly shows a strong bias dependence in both topography and local density of states (STS) maps. High resolution spectroscopic data further confirms the stripe-like electronic states modulation, which provides insight to the ionic dimerization revealed by X-ray diffraction. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L47.00003: A series of modulated states in the dimerized IrTe$_2$ V. Kiryukhin, L.G. Pascut, S.-W. Cheong, M.J. Gutmann, J.J. Yang IrTe$_2$ is a layered compound exhibiting stripes of dimerized Ir ions. With decreasing temperature, a series of structures with different periodicities is observed. These structures are distinguished by the arrangement of the dimerized stripes, as found using x-ray diffraction. The structural transitions appear to be driven by the Ir dimerization and Ir-Ir bonding in this compound. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L47.00004: Dimerization-Induced Cross-Layer Quasi-Two-Dimensionality in Metallic Iridate IrTe$_{2}$ Gheorghe-Lucian Pascut, Kristjan Haule, Matthias J. Gutmann, Sarah A. Barnett, Alessandro Bombardi, Sergey Artyukhin, Turan Birol, David Vanderbilt, Junjie Yang, Sang-Wook Cheong, Valery Kiryukhin IrTe$_{2}$ a layered chalcogenide metal composed of stacked layers of IrTe$_{6}$ octahedra, has recently received a lot of interest due to: (1) the structural phase transition ($T_{S}=$280 K) to a modulated structure characterized by the wave vector \textbf{\textit{q}}$_{\mathbf{0}}=$(1/5, 0, 1/5) and (2) the superconducting properties of the Pd/Pt-doped/intercalated samples. Using the crystal structure obtained from single crystal X-ray diffraction and first principle calculations we show that the mechanism for the structural phase transition is driven by the Ir dimerization and bonding, contrary to mechanisms proposed before based on: orbital-driven Peierls instability, crystal field splitting the Te $p$ orbitals and depolymerization of the inter-layer Te bonds. In this talk I will describe the Ir/Te dimers and the electronic structure calculations which reveal an intriguing quasi-two-dimensional electronic state, with planes of reduced density of states cutting diagonally through the Ir and Te layers. These planes are formed by the dimers exhibiting a signature of covalent bonding character development. The role of the electronic correlations and spin orbit coupling will be also discussed. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L47.00005: Metal-insulator transition in SrTi$_{\mathrm{1-x}}$V$_{\mathrm{x}}$O$_{3}$ thin films Man Gu, Stuart Wolf, Jiwei Lu Epitaxial SrTi$_{\mathrm{1-x}}$V$_{\mathrm{x}}$O$_{3}$ (0 $\le $ x $\le $ 1) thin films with thicknesses of $\sim$ 16 nm were grown on (001)-oriented LSAT substrates using the pulsed electron-beam deposition technique. The transport study revealed a temperature driven metal-insulator transition (MIT) at 95 K for the film with x $=$ 0.67. The films with higher vanadium concentration (x \textgreater 0.67) were metallic, and the electrical resistivity followed the T$^{2}$ law corresponding to a Fermi liquid system. In the insulating region of x \textless 0.67, the temperature dependence of electrical resistivity for the x $=$ 0.5 and 0.33 films can be scaled with Mott's variable range hopping model. The possible mechanism behind the observed MIT might be associated the interplay between electron-electron interactions and disorder-induced localization. The Ti$^{4+}$ ion substitution introduces Anderson-localized states as well as lattice distortions that result in a reduction in the effective 3d bandwidth W. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L47.00006: Epitaxial growth of Ruddlesden-Popper La$_{n+1}$Ni$_{n}$O$_{3n+1}$ series using reactive molecular-beam epitaxy June Hyuk Lee, I-Cheng Tung, Jarrett Moyer, Guangfu Luo, Seo Hyoung Chang, Dane Morgan, Hawoong Hong, Peter Schiffer, Dillon Fong, John Freeland We report the growth of single crystalline La$_{n+1}$Ni$_{n}$O$_{3n+1}$ epitaxial thin films using reactive molecular-beam epitaxy. Ruddlesden-Popper La$_{n+1}$Ni$_{n}$O$_{3n+1}$ compounds, consisting of LaO$^{+}$ and NiO$_{2}$$^{-}$ layers, have been considered a potential candidate for solid-oxide fuel cell cathodes and thermoelectrics. However, the growth of higher order La$_{n+1}$Ni$_{n}$O$_{3n+1}$ single crystals has not been possible so far. We utilize synchrotron x-ray diffraction at the Advanced Photon Source during layer?by?layer deposition together with density functional theory calculations to understand how LaO$^{+}$ and NiO$_{2}$$^{-}$ oxide layers re-arrange dynamically during growth. Using this layer re-arrangement, epitaxial La$_{2}$NiO$_{4}$, La$_{3}$Ni$_{2}$O$_{7}$, and La$_{4}$Ni$_{3}$O$_{10}$ films on (001)-oriented SrTiO$_{3}$ have been synthesized with the proper nickel valance state and structure. Here we will discuss the connection between structure and electrical transport properties. Work at the APS, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L47.00007: Understanding Metal-Insulator transitions in ultra-thin films of LaNiO$_{3}$ Jayakanth Ravichandran, Philip D.C. King, Darrell G. Schlom, Kyle M. Shen, Philip Kim LaNiO$_{3}$ (LNO) is a bulk paramagnetic metal and a member of the family of RENiO$_{3}$ Nickelates (RE = Rare Earth Metals), which is on the verge of the metal-insulator transition. Ultra-thin films of LNO has been studied extensively in the past and due to its sensitivity to disorder, the true nature of the metal-insulator transition in these films have been hard to decipher. We grow high quality ultra-thin films of LNO using reactive molecular beam epitaxy (MBE) and use a combination of ionic liquid gating and magneto-transport measurements to understand the nature and tunability of metal-insulator transition as a function of thickness for LNO. The underlying mechanisms for the transition are discussed in the framework of standard transport models. These results are discussed in the light of other Mott insulators such as Sr$_{2}$IrO$_{4}$, where we have performed similar measurements around the insulating state. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L47.00008: Photoinduced insulator-to-metal dynamics in strained NdNiO3 ultrathin films Richard D. Averitt, Elsa Abreu, Jingdi Zhang, Derek Meyers, Kun Geng, Jak Chakhalian Epitaxial rare-earth nickelate thin films are ideally suited for the study and control of electronic correlations associated with the insulator-to-metal phase transition since strain can be precisely controlled. We investigate picosecond conductivity dynamics of strained NdNiO3 utilizing time-domain terahertz spectroscopy. Starting from the insulating phase, photoexcitation results in a conductivity increase towards the metallic state. The dynamics consist of a fast ~1ps increase accompanied by a slower increase occurring over tens of picoseconds. These results will be compared to those obtained on the vanadates where the slow rise time is ascribed to nucleation and growth of the metallic phase. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L47.00009: Renormalization of the nickelate phase diagram in strained thin films Ankit Disa, Divine Kumah, Joseph Ngai, Fred Walker, Charles Ahn, Eliot Specht, Dario Arena As a result of strong electron-lattice coupling, the bulk electronic phase diagrams of correlated oxides can be modified in epitaxial thin films using reduced dimensionality and substrate-induced strain. Taking advantage of the unique features of these materials, such as correlation-driven magnetic and metal-insulator transitions, requires a systematic understanding of how the thin film phase diagram differs from the bulk. Here, we explore the phase diagram of thin films of rare-earth nickelates, $R$NiO$_3$, which in the bulk exhibit a systematic dependence of the transition temperature, $T_{MI}$, with $R$. Studying solid solutions of NdNiO$_3$ and LaNiO$_3$ (Nd$_y$La$_{1-y}$NiO$_3$ with $0 \leq y\leq 1$) under compressive epitaxial strain, we observe a consistent renormalization of $T_{MI}$ to lower temperatures. By examining the physical and electronic structure of the films using synchrotron x-ray diffraction and absorption spectroscopy, we determine that the renormalization is due to an enhanced Ni-O overlap as a result of coherent compressive strain. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L47.00010: Magnetotransport of NdNiO$_{3}$ thin films Adam Hauser, Evgeny Mikheev, Nelson Moreno, Tyler Cain, Jinwoo Hwang, Jack Zhang, Susanne Stemmer The Hall coefficient of epitaxial NdNiO$_{3}$ films is evaluated in a wide range of temperatures, from the metallic into the insulating phase. It is shown that for temperatures for which metallic and insulating regions co-exist, the Hall coefficient must be corrected for the time-dependence in the longitudinal resistance, which is due to a slow evolution of metallic and insulating domains. The positive Hall and negative Seebeck coefficients, respectively, in the metallic phase are characteristic for two bands participating in the transport. We report on magnetoresistance measurements at low temperature and interpret them in terms of the specific magnetic ordering in these films, as a function of epitaxial film strain. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L47.00011: Tunneling spectroscopy in Mott insulators, LaNiO$_{3}$ and NdNiO$_{3}$: Pseudo gaps and real gaps S. James Allen, Adam Hauser, Evgeny Mikheev, James Kally, Alex Kozhanov, Daniel Ouellette, Susanne Stemmer To explore the low lying excitations of prototypical charge transfer Mott insulators, we fabricated 4-terminal tunnel junctions and measured the temperature dependent tunnel conductance, along with the sheet resistivity, for LaNiO$_{3}$ a metal, and NdNiO$_{3}$ which underwent a metal insulator transition at 125 K. Films were deposited by rf magnetron sputtering on LaAlO$_{3}$ substrates. At low temperatures the tunneling conductance in LaNiO$_{3}$ develops a pronounced pseudogap 20 meV wide. NdNiO$_{3}$ exhibits a pseudo gap above the transition temperature. Just below the transition, the tunnel conductance at 0 bias is strongly suppressed but enhanced at larger bias voltages, signaling a redistribution of the quasi-particle density states as the system enters the insulating phase. At the lowest temperatures the tunnel conductance is suppressed by 4-5 orders of magnitude and a well developed gap appears - 25 meV wide. Comparisons are made with extant optical conductivity as well as recent theories based on Fermi surface instabilities.- SungBin Lee, Ru Chen, and Leon Balents, ``Landau Theory of Charge and Spin Ordering in the Nickelates,'' Phys. Rev. Lett. \textbf{106}, 016405 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L47.00012: Electronic properties of nickelates across the metal-insulator transition: a high-resolution ARPES study R.S. Dhaka, Z. Ristic, N.C. Plumb, W. Kong, M. Medarde, M. Shi, L. Patthey, M. Radovic, J. Mesot The metal-insulator transition (MIT) in rare earth nickelate thin films has started to be a focus of the research in condensed matter physics [1]. Transport, magnetization and neutron scattering studies have shown a temperature driven MIT and magnetism in strongly correlated RNiO3 bulk samples [2] as well as in thin films [1]. In these systems it is believed that strong electron-electron correlations play an important role in MIT phenomena [3]. However, no direct information of the momentum resolved electronic structure across the MIT has been provided so far. Here, by combining in-situ PLD and high-resolution angle-resolved photoemission study, we report the band structure and Fermi surface (FS) of NdNiO3 thin films across the temperature driven MIT. In the metallic phase, we prove the existence of electron and hole FS pockets at the center and corner of the Brillouin zone (BZ), respectively. These FS pockets show strongly three-dimensional nature along the c-axis. Upon cooling across the MIT, we observe transfer of the spectral weight from near the Fermi level to higher binding energy in the entire BZ. Our results demonstrate the loss of coherent quasiparticle spectral weight associated with strong electron correlations involved in the MIT. [1] J. Liu et al., Nature Comm 4, 2714 (2013); R. Scherwitzl, et al., Advanced Materials 22, 5517 (2010). [2] M. Medarde, JPCM 9, 1679 (1997). [3] M. Imada, et al., Rev. Mod. Phys. 70, 1039 (1998). [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L47.00013: Specific Heat of Single Crystalline Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ Carlos Sanchez, Victor Aguilar, Oscar Bernal, Guo-meng Zhao Substantial studies of magnetization and specific heat on Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ have demonstrated the existence of a charge ordering (CO), ferromagnetic (FM), antiferromagnetic (AFM) transitions. In this work, the specific heat of two single crystalline Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ samples, one containing $^{16}$O and the other highly concentrated $^{18}$O, was measured as a function of temperature, from 3K to 300K, at both zero and 50 kOe applied field. Measurements were done using a Quantum Design Physical Property Measurement System (PPMS) with Specific Heat option. The FM transition was found to depend on the isotope mass, which seems to agree with previous works. The CO transitions was observed as a sharp peak at the CO temperature (T$_{co}$), which seems to depend strongly on field, oxygen isotope mass, and thermal cycling history. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L47.00014: Nematicity in charge and orbital ordered structure: a new description of phase transitions in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ Jing Tao, W.G. Yin, Y. Zhu, K. Sun Doped manganites have a well-known unidirectional superlattice modulation at low temperatures, although the origin of the modulation is still under debate. The phase transition of the modulation in this compound has been characterized by the superlattice reflections and the transition temperature was determined when the modulation becomes long-range. Here we report a new description of the phase transition in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ from the aspect of symmetry by measuring anisotropy based on transmission electron microscopy results. Instead of one phase transition, we found that the electronic structures undergo smectic, nematic and isotropic behaviors upon warming. Comparing to previous characterizations of the phase transition in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$, this symmetry measurement enables a better unification between electronic structure and other properties such as the crystal lattice variation. Moreover, we directly observed the creation of dislocation pairs in the smectic phase, which is consistent with the dislocation-proliferation mechanism predicted by the nematicity theory in correlated systems. The defect observations also suggest the charge and orbital ordering nature of the modulation in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L47.00015: Charge disproportionation without charge transfer in the rare-earth nickelates as a possible mechanism for the metal-insulator transition Steven Johnston, Anamitra Mukherjee, Ilya Elfimov, Mona Berciu, George Sawatzky We study a model for the metal-insulator (MI) transition in the rare-earth nickelates RNiO$_3$, based upon a negative charge transfer energy and coupling to a rock-salt like lattice distortion of the NiO$_6$ octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with ground state of predominantly $d^8 \, ligand$ character, where $ligand$ denotes a ligand hole. For sufficiently large distortions ($\delta d_{\rm Ni-O} \sim 0.05 - 0.10$ {\AA}), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with $(d^8 \, ligand^2)_{S=0}(d^8)_{S=1}$ character, where $S$ is the total spin. Thus the MI transition may be viewed as being driven by an internal volume ``collapse'' where the NiO$_6$ octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge, similar to that reported in a prior DMFT study. [Preview Abstract] |
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