Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A22: Organic Conductors & Other Correlated Electron Systems |
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Sponsoring Units: DMP Chair: Claude Bourbonnais, Universite de Sherbrooke Room: 324 |
Monday, March 18, 2013 8:00AM - 8:12AM |
A22.00001: Optical evidence of competitive nature between charge-order and dimer-Mott insulators Ryuji Okazaki, Yukio Yasui, Ichiro Terasaki, Yuka Ikemoto, Taro Moriwaki, Takahisa Shikama, Hatsumi Mori, Kazuyuki Takahashi, Hideki Nakaya, Takahiko Sasaki A family of two-dimensional (2D) quarter-filled organic materials exhibits various intriguing electronic and magnetic states. These salts are essentially metallic due to the partially-filled band, however, several materials show the correlated insulating states such as charge-order and dimer-Mott insulators owing to strong correlation effects coupled with their unique internal degrees of freedom. In this talk, we show a competitive nature between charge-order and dimer-Mott insulating phases in the 2D quarter-filled organic salt $\beta$-(meso-DMBEDT-TTF)$_2$PF$_6$ through the optical conductivity measurements. This material has been known to exhibit charge ordering below $T_c$ = 70 K. We find optical evidence of a dimer-Mott insulating phase above $T_c$, indicating that the transition in this material is a transition from dimer-Mott to charge-order insulator. Below $T_c$, the optical peak feature of dimer-Mott insulator is significantly suppressed by the formation of charge order, implying a competition of these two insulators in this system. Furthermore our infrared imaging spectroscopy reveals a spatially inhomogeneous electronic state far below $T_c$, which is attributed to the competition between charge-order and dimer-Mott insulators. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A22.00002: Superconductivity and polar charge fluctuation in low dimensional organic salts Sumio Ishihara, Akihiko Sekine, Joji Nasu Organic conductors are one of the families in which exotic superconductivities have been examined intensively. Recently, dielectric anomaly is reported in one of the $\kappa $-type BEDT-TTF salts, $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$Cu$_{\mathrm{2}}$(CN)$_{\mathrm{3}}$. A dielectric anomaly is observed in the temperature dependence of the dielectric constant around 30K. These experimental results trigger reinvestigations of the electronic structure in the dimer-Mott insulating systems, and a mechanism of the superconductivity. Superconductivity and polar charge fluctuation are studied in an organic conductor where the dimer-molecule degree of freedom exists. The two-types of the extended Hubbard models, where the intra- and inter-dimer Coulomb interactions are taken into account, are analyzed by the random-phase approximation and the fluctuation-exchange approximation. The superconductivity appears in a vicinity of the charge-density wave (CDW) phase where the electronic distributions are polarized inside dimers. The extended s-wave type paring is favored and is cooperative with the d$_{\mathrm{xy}}$-type paring due to the spin fluctuation. This superconductivity is compared with that realized near the CDW phase where the charge is not polarized inside dimers. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A22.00003: Collective charge excitation in low dimensional organic salts Makoto Naka, Sumio Ishihara Electronic ferroelectricity is known as phenomena where electric polarization is attributed to the charge order without inversion symmetry. This is seen in some transition metal oxides, e.g. LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$, and charge transfer salts. Quasi 2-dimesional organic salt kappa-(ET)$_{\mathrm{2}}$Cu$_{\mathrm{2}}$(CN)$_{\mathrm{3\thinspace }}$is one of the electronic ferroelectricities. Two ET molecules construct a dimer and are arranged on a triangular lattice. Recently, it is reported that a dielectric anomaly is experimentally observed around 30K. An origin of this dielectric anomaly is thought to be an ?electronic? dipole generated by a localized hole in one side of the ET molecules in dimers. Motivated by the experimental results, we study charge dynamics in dimer-Mott insulating system with internal charge degree of freedom in a dimer. We adopt the three kinds of models, extended Hubbard model, V-t model and its effective pseudo-spin model. We analyze these models by utilizing the exact diagonalization method and spin wave approximation, and focus on the collective charge excitation. In the ground state, paraelectric dimer-Mott phase and ferroelectric charge ordered phase compete with each other. We find the low-energy intra-dimer charge excitations which show a strong light polarization dependence. The collective excitation mode which is observable by light being parallel to the electric polarization shows a softening and a remarkable frequency dispersion around the phase boundary. This collective charge excitation of the ?electronic? dipole explains the recently observed peak structure in optical conductivity for the THz region. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A22.00004: Microscopic investigation of Fabre charge transfer salts as function of temperature and pressure Helene Feldner, Anthony Jacko, Eva Rose, Martin Dressel, Roser Valenti, Harald O. Jeschke The Fabre charge transfer salts are quasi-1D materials with a rich temperature and pressure phase diagram. We use literature as well as newly obtained crystal structures to sample many temperatures and pressures (both chemical and physical). We find that general trends in their electronic properties can be connected to their phase diagram. Finally, we analyze the importance of correlations in these systems using an extended Hubbard model, parametrized using DFT Wannier orbital overlaps. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A22.00005: LDA+DMFT investigation of the organic charge transfer salt $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl Johannes Ferber, Kateryna Foyevtsova, Harald O. Jeschke, Roser Valenti We combine density functional theory with dynamical mean field theory for the study of organic molecular crystals using a new scheme to construct molecular Wannier functions. We calculate spectral and optical properties for the strongly correlated material $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. The new method allows us to analyze the contributions of intradimer and interdimer contributions to the optical conductivity on the same footing. We find in agreement with experiment that strong correlations lead to a Hubbard peak in the optical conductivity. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A22.00006: Charge Induced Spin Polarization in Thiophene Oligomers Avadh Saxena, Dong Hou, Junjie Qiu, Shijie Xie Charge induced spin polarization in organic small molecules is a key factor for spin transport and magnetic effects in related organic devices. We study the spin polarization in charged thiophene oligomer molecules by calculating the magnetic moment with density functional theory (DFT). We find that the emergence and variation of the net magnetic moment is related to both the amount of charge injected and the polymerization of the oligomer, i.e. the number of monomer units. Combined with model analysis, we conclude that the strong electron-electron (e-e) interaction and electron-lattice (e-l) interaction in organic materials are responsible for charge induced spin polarization in organic oligomers. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A22.00007: Inter-chain transport in the quasi-one-dimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$ Joshua Cohn, Benjamin D. White, Carlos A.M. dos Santos, John J. Neumeier We report measurements of electrical resistivity ($\rho$) and thermoelectric power ($S$) transverse to the conducting chains (crystallographic \textit{c} axis) on single crystals of the quasi-one-dimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$. While $\rho_c(T)$ exhibits metallic behavior at $T\leq T_{max}\sim 270$~K, it decreases with increasing $T$ above this temperature similar to the behavior of $\rho$ transverse to the conducting planes in a variety of two-dimensional metals.\footnote{See, e.g., D. B. Gutman and D. L. Maslov, Phys. Rev. Lett. {\bf 99}, 196602 (2007).} We discuss the corresponding thermopower, $S_c$, which is relatively $T$-independent and a modest 30 $\mu$V/K at low $T$, increases sharply with increasing $T$ near $T_{max}$, and exceeds 200 $\mu$V/K at $T>400$~K. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A22.00008: Nanoscale interplay of inhomogeneity and electron interactions in the quasi one-dimensional purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ Jung Hoon Liu, Anjan Soumyanarayanan, Michael Yee, Yang He, Martha Greenblatt, Nigel Hussey, Jennifer Hoffman The marked deviation from Fermi liquid behavior for the quasi one-dimensional (1D) purple bronze, Li$_{0.9}$Mo$_6$O$_{17}$ (LPB), has been observed by both bulk transport and surface sensitive spectroscopic probes, and has generated much theoretical interest. Here we report on spectroscopic scanning tunneling microscopy (STM) studies of 1D `chains' on the surface of LPB in the presence of a magnetic field. While we can consistently identify high-energy features in the tunneling density of states corresponding to the bulk band structure, we find that the Coulomb suppression of tunneling around the Fermi energy is inhomogeneous on the nanometer length scale. We discuss the inhomogeneity in the context of the 1D `chains', and its implications on other measurements. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A22.00009: Direct observation of electronic nematicity in charge and orbital ordered La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ J. Tao, K. Sun, J.M. Zuo, Y. Zhu Nematic and smectic states have been demonstrated to be very important in understanding high-T$_{\mathrm{c}}$ superconductivity. Here we report similar observations of electronic nematicity in doped manganites. Both the electron diffraction results and HRTEM images obtained from single crystal domain of La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ clearly show a C4 to C2 symmetry broken in charge ordered (CO) and orbital ordered superstructures at intermediate temperature range. The electronic nematicity persists in the crystal until long-range CO forms as a stripe phase at lower temperatures upon cooling. During warming process, we observed topological defects in the charge ordering superstructures, indicating that the melting of the CO superstructure is defect mediated. Theoretical simulations will also be provided for better interpretation of the phenomenon. Research at Brookhaven National Laboratory was sponsored by the US Department of Energy (DOE)/Basic Energy Sciences, Materials Sciences and Engineering Division under Contract DE-AC02-98CH10886. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A22.00010: Checkerboard to Stripe Charge Ordering Transition in TbBaFe$_2$O$_5$ Daniel Pratt, Sung Chang, Wei Tian, Alexey Taskin, Yoichi Ando, Jerel Zarestky, Andreas Kreyssig, Alan Goldman, Robert McQueeney A combined neutron and x-ray diffraction study of TbBaFe$_2$O$_5$ reveals a rare checkerboard to charge ordering transition. TbBaFe$_2$O$_5$ is a mixed valent compound where Fe$^{2+}$/Fe$^{3+}$ ions are known to arrange into a stripe charge-ordered state below $T_{V} =$ 291 K, that consists of alternating Fe$^{2+}$/Fe$^{3+}$ stripes in the basal plane running along the \textbf{\textit{b}} direction. Our measurements reveal that the stripe charge-ordering is preceded by a checkerboard charge-ordered phase between $T_{V}$\textit{ \textless\ T} \textit{\textless\ T}* $=$ 308 K. The checkerboard ordering is stabilized by inter-site coulomb interactions which give way to a stripe state stabilized by orbital ordering. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A22.00011: Novel electronic transition in layered IrTe$_{2}$ Yoon Seok Oh, J.J. Yang, Y. Horibe, S.-W. Cheong Layered chalcogenides such as 1T-TaS$_{2}$, 1T-TiSe$_{2}$, Bi$_{2}$Se$_{3}$, and MoS$_{2}$ exhibit rich low-dimensional physical properties such as superconductivity, topological insulator, charge density waves (CDW), and field-effect-transistor with high mobility. IrTe$_{2}$ forms in the layered CdI$_{2}$ structure, and exhibits diamagnetism and superlattice modulations below $\sim$260 K. In addition, superconductivity appears when the $\sim$260 K transition is fully suppressed by, for example, chemical doping. The origin of the $\sim$260 K transition in IrTe$_{2}$ has been controversial. It was claimed to be a structural transition, which suppresses electronic conduction. It was also reported that Fermi surface instability drives the transition - $i.e.$ it is charge density wave-type. In this talk, we present our comprehensive studies on electron diffraction and transport experiments under chemical/hydrostatic pressure to unveil the origin of the novel electronic transition in IrTe$_{2}$. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A22.00012: Superstructure and its domain structure in layered IrTe$_2$ at low temperatures Y. Horibe, F.T. Huang, J.J. Yang, Y.S. Oh, Y.J. Choi, A. Hogan, S.-W. Cheong IrTe$_2$, forming in layered CdI$_2$ structure, exhibits a unique phase transition accompanied by the appearance of diamagnetism and a sharp increase of electrical resistivity. This transition has been discussed to be due to a charge-density-wave formation related to the Fermi surface nesting [1]. In this talk, we will report the three dimensional superstructure below the transition temperature, obtained from electron diffraction patterns using low-temperature transmission electron microscopy. The superstructure is characterized by the presence of the superlattice reflections with the modulation wave vector q$=$ 1/5 [101]. Its domain structure and chemical doping effects on the modulation wave vector will also be discussed.\\[4pt] [1] J. J. Yang et al., PRL 108, 116402 (2012). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A22.00013: Resonant Ultrasound Studies of Mo$_{3}$Sb$_{7}$ Lindsay VanBebber, Jiaqiang Yan, David Mandrus, Brian C. Sales, Veerle Keppens The elastic behavior of a series of Mo$_{3-x}$M$_{x}$Sb$_{7-y}$X$_{y}$ (M= Cr, Ru, X= Te) single crystals is examined with resonant ultrasound spectroscopy (RUS) as a function of temperature (300 K - 5 K). The elastic response of the parent compound Mo$_{3}$Sb$_{7}$ reveals a transition at around 53K, evidenced by a dramatic softening in the shear modulus c=(c$_{11}$-c$_{12}$)/2. This softening is associated with a cubic-to-tetragonal structural transition as well as a spin gap formation. The transition temperature is lowered by a few degrees upon doping with Cr. Doping with Ru and Te is known to suppress the structural transition, but the softening in the shear modulus suggests that a structural instability remains present in these compounds. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A22.00014: Electronic thermoelectric power factor and metal-insulator transition in FeSb2 Cedomir Petrovic, Qing Jie, Rongwei Hu, Emil Bozin, Anna Llobet, Igor Zaliznyak, Qiang Li We show that synthesis-induced metal-insulator transition (MIT) for electronic transport along the orthorhombic c axis of FeSb2 single crystals has greatly enhanced electrical conductivity while keeping the thermopower at a relatively high level. By this means, the thermoelectric power factor is enhanced to a new do a new record high S2$\sigma \sim $8000 $\mu $WK$-$2 cm$-$1 at 28 K. We find that the large thermopower in FeSb2 can be rationalized within the correlated electron model with two bands having large quasiparaticle disparity, whereas MIT is induced by subtle structural differences. The results in this work testify that correlated electrons can produce extreme power factor values. [Preview Abstract] |
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