Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P44: Organic Conductors |
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Sponsoring Units: DCMP Chair: Michael Naughton, Boston College Room: Baltimore Convention Center 347 |
Wednesday, March 15, 2006 11:15AM - 11:27AM |
P44.00001: Interlayer magnetoresistance as a probe of the quantum coherence of electronic excitations in layered metals Malcolm Kennett, Ross McKenzie Angle-dependent magnetoresistance oscillations (AMRO) have been used as a powerful tool to map out Fermi surfaces in layered metals, such as organic metals, strontium ruthenate, and an over-doped cuprate. We derive a general formula for AMRO in systems with anisotropic interlayer hopping, anistropic in-plane scattering and an anisotropic 2$d$ Fermi surface. We discuss the ability of AMRO to discriminate between coherent transport when there is a 3$d$ Fermi surface and weakly incoherent transport, where there is hopping between 2$d$ Fermi surfaces that are only defined in each layer. We illustrate these ideas by comparison with experimental measurements of AMRO in thallium cuprate [1]. \\ $[$1$]$ N. E. Hussey {\it et al.}, Nature {\bf 425}, 814 (2003). [Preview Abstract] |
Wednesday, March 15, 2006 11:27AM - 11:39AM |
P44.00002: Interlayer Aharonov-Bohm interference in tilted magnetic fields in quasi-one-dimensional organic conductors Victor Yakovenko, Benjamin Cooper Different types of angular magnetoresistance oscillations in quasi-one-dimensional organic conductors, such as $\rm(TMTSF)_2X$, are explained in terms of Aharonov-Bohm interference in interlayer electron tunneling. A two-parameter pattern of oscillations for generic orientations of a magnetic field is visualized and related to the experimental data. \\ Reference: cond-mat/0509039 [Preview Abstract] |
Wednesday, March 15, 2006 11:39AM - 11:51AM |
P44.00003: Unified Theory of Magic Angles and Interference Commensurate Oscillations. Si Wu, Andrei Lebed, Heon-Ick Ha, Michael Naughton We suggest the unification theory of angular magnetoresistance oscillations in low-dimensional metals with open sheets of Fermi surfaces. It is based on an idea that effective space dimensionality of electron spectrum and electron wave functions is changed at some special directions of a magnetic field. These 1D -$>$ 2D dimensional crossovers are shown to be due to interference effects, which occur when electrons move in the extended Brillouin zone in a magnetic field. Our quantum mechanical approach allows to derive an equation which describes analytically both Magic Angles and Interference Commensurate oscillations in resistivity component, perpendicular to conducting layers, and reveals their common physical origin. We compare our results with experimental data obtained on (TMTSF)$_{2}$ClO$_{4}$ and (TMTSF)$_{2}$PF$_{6}$. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P44.00004: Lebed Magic Angles in (TMTSF)$_{2}$X $_{ }$Probed by Torque, Transport and NMR J. I. Oh, K. Kobayashi, P. M. Chaikin, J. Shinagawa, S. E. Brown, M. J. Naughton We've investigated the Lebed effect [1] in the quasi-1D molecular organic conductor (TMTSF)$_{2}$X for magnetic fields in the $b^{\prime }-c^{\ast }$-plane, via angle-dependent torque, magnetoresistance, and NMR relaxation rate. In torque versus field angle measurements for X = ClO$_{4}$ at 0.1 K, we observed distinct field induced spin density wave (FISDW) transitions but, to within our experimental accuracy of 3$\times $10$^{-11}$ Nm, we found no evidential anomalies at the Lebed magic angles. We compare this result with earlier reports of torque measurements in X = ClO$_{4}$ [2] and $^{77}$Se NMR relaxation rate measurements in X = PF$_{6}$ [3]. In fixed angles $T-$ (NMR) and $B-$sweeps (torque and magnetoresistance) in the vicinity of magic angles, no change in the FISDW position was observed. These measurements suggest that magic Lebed orientations have no effect on the metal-FISDW transition. [1] A. G. Lebed, JETP Lett. \textbf{43}, 174 (1986). [2] M. J. Naughton \textit{et al}., Phys. Rev. Lett. \textbf{67}, 3712 (1991). [3] W. Wu et al., Phys. Rev. Lett. 94, 097004 (2005).. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P44.00005: Thermoelectric power and Nernst effect studies in the metallic and field-induced spin density wave states in (TMTSF)$_{2}$ClO$_{4}$ Eun Sang Choi, James S. Brooks, Haeyong Kang, Younjung Jo, Woun Kang We have measured the angular dependence of thermoelectirc power (TEP) and Nernst effect of (TMTSF)$_{2}$ClO$_{4}$. At low temperatures and in the metallic state, Nernst effect shows giant resonant signals around the Lebed magic angles, while TEP is small without noticiable angular dependence. This behavior is very similar to what was observed in (TMTSF)$_{2}$PF$_{6}$ in the metallic state [Wu et al., Phys. Rev. Lett. \textbf{91} 56601(2003)]. By entering the field-induced spin density wave (FISDW) state, both TEP and Nernst signal show complicated behaviors reflecting the FISDW subphase transitions. Remarkably, the resonant Nernst effect still persists in the FISDW state and with even lager amplitude. By increasing the perpendicular field above $\sim $ 6.5 T, both TEP and Nernst effect becomes small again at all angles. Our Nernst effect results are inconsistent with some proposed models for the metallic state of (TMTSF)$_{2}$PF$_{6}$, which may suggest this phenomenon is beyond the Fermi liquid description. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P44.00006: Coexistence of spin density wave and triplet superconductivity in quasi-one-dimensional Bechgaard salts Wei Zhang, Carlos Sa de Melo The interplay between magnetic order and superconductivity is a very important problem in condensed matter physics. In the quasi-one-dimensional (quasi-1D) organic conductor (TMTSF)$_2$PF$_6$, an antiferromagnetic state charaterized by a spin density wave (SDW) order neighbors a triplet superconducting (TSC) state on the pressure-temperature phase diagram. Experiments [1,2] suggest a coexisting region of SDW and TSC orders in the vicinity of the phase boundary. We consider a tight-binding quasi-1D electron system, and construct the Ginzburg-Landau (GL) free energy with two order parameters. In the absence of a magnetic field, the rotational symmetry of this system is broken due to spin anisotropy and spin-orbit coupling. Thus, the GL free energy has a similar form as the ordinary $\phi_1$-$\phi_2$ model, except additional gradient terms. We calculate the GL coefficients microscopically and obtain a phase diagram in zero magnetic field. This phase diagram shows a coexistence region for SDW and TSC. \\ Reference:\\ $[1]$ T. Vuletic {\it et al.}, Eur. Phys. J. B {\bf 25}, 319 (2002).\\ $[2]$ I. J. Lee {\it et al.}, Phys. Rev. Lett. {\bf 94}, 197001 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P44.00007: Anomalous temperature dependence of the single-particle spectrum in the organic conductor TTF-TCNQ Nejat Bulut, Hiroaki Matsueda, Takami Tohyama, Sadamichi Maekawa The angle-resolved photoemission spectrum of the quasi-one-dimensional organic-conductor TTF-TCNQ exhibits an unusual temperature dependence in the sense that a transfer of spectral weight over an energy range of $\approx 1$\textit{eV} takes place as the temperature decreases below 260$K$. In order to investigate the origin of this behavior, we have performed Dynamical Density-Matrix-Renomalization-Group (DDMRG) calculations at zero temperature and Quantum Monte Carlo (QMC) calculations at finite temperatures for the single-particle spectral weight of the doped one-dimensional (1D) Hubbard model. We present DDMRG and QMC results for a range of the model parameters of the 1D Hubbard model and make comparisons with the photoemission data. In addition, we present zero-temperature DDMRG results on the doped 1D Hubbard-Holstein model in order to explore how the electron-phonon coupling influences the single-particle spectrum in 1D correlated conductors. [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P44.00008: Multiple spin sites in an organic conductor without magnetic ions. Takahisa Tokumoto, Yugo Oshima, David Graf, James Brooks, Johan Van Tol, Loius-Claude Brunel, George Papavassilliou The anisotropic low dimensional organic conductors are attractive because of the variety of ground states with unusual and exotic electronic properties. One of them is tau-[P-($S$,$S)$-DMEDT-TTF]$_{2}$(AuBr$_{2})_{1+y}$ [where y$\sim $0.75 and P-($S$,$S)$-DMEDT-TTF stands for pyrazino-($S$,$S)$-dimethyl-ethylenedithio-tetrathiafulvane], which has tetragonal crystal structure with unit cell dimensions \textbf{a}=\textbf{b}=7.3546 {\AA} and \textbf{c}=67.977 {\AA}$^{1}$. Even though there are no magnetic ions in the compound, transport measurements show magnetic ordering at low temperature and in magnetic fields. To investigate the origin of the magnetic behavior, we are conducting an ESR study. We do observe multiple resonances which indicate the existence of the multiple spins although the system does not contain magnetic ions. Moreover, the in-plane angular dependent ESR measurements reveal 4 fold symmetry. Both in-plane and out of plane ESR signal show evidence of antiferromagnetic behavior below 12 K. It is possible that the ion stoichiometric charge transfer (1+y) is the origin of the magnetic effects. Further analysis will be presented. [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P44.00009: Investigating the Charge Ordering Pattern in the Organic Spin Ladder (DT-TTF)$_2$Cu(mnt)$_2$ S. Brown, J. T. Haraldsen, J. Cao, J. L. Musfeldt, M. Mas-Torrent, C. Rovira, J. C. Dias, R. T. Henriques, M. Almeida Quantum spin ladders have attracted considerable interest as intermediaries between one-dimensional chains and two- dimensional square lattices. In order to elucidate the charge ordering pattern in a model organic spin ladder, we measured the temperature-dependent infrared spectra of the organic spin- ladder candidate (DT-TTF)$_2$Cu(mnt)$_2$. We interpret the results within the context of recent theoretical predictions of various charge ordering patterns in spin ladders [1]. Comparision with spectra of the isostructural Au analogue and neutral DT-TTF compound aid in this analysis. [1] R. T. Clay, S. Mazumdar, Phys. Rev. Lett. \textbf{94}, 207206 (2005) [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P44.00010: Magnetic field effects on the coexisting Bond-Charge-Density waves in the quasi-one-dimensional quarter-filled bands Sumit Mazumdar, R. Torsten Clay Magnetic field effects on spin-Peierls systems have been of interest for a long time. The theoretical phase diagram consists of three different regions containing the homogenous dimerized and undimerized phases, and a magnetic phase consisting of a soliton lattice or an incommensurate phase. We have investigated numerically spin excitations and magnetic field effects on the bond-charge-density wave (BCDW) that appears below the spin-Peierls transition in the quarter-filled band organic charge transfer solids (CTS), with the specific goal of determining whether the simplest phase diagram, obained within the spin model, applies also to the quarter-filled band where both charge and spin degrees of freedom exist. We also discuss recent experiments in quarter-filled band CTS within the context of our theory. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P44.00011: Temperature dependent competition between charge-ordering and spin-Peierls transition in (TMTTF)$_2$X: the role of quantum phonons R. T. Clay, R. P. Hardikar, S. Mazumdar The (TMTTF)$_2$X salts are quasi-one-dimensional materials with complex phase diagrams that feature a large number of ordered states including superconductivity. The ground states of these materials are often spin-Peierls (SP) states. However, at intermediate temperatures (100 K) a transition to a charge ordered state is also present, which may compete with the ground state SP ordering. We investigate numerically models for these materials that include three components: electronic interactions, bond-coupled phonons, and Holstein-type phonons coupled to the local charge density. These three components have different energy scales are hence expected to dominate at different temperatures. We explicitely include finite phonon frequency in our calculations using quantum Monte Carlo methods. We present charge, spin, and bond susceptibilities as a function of temperature and discuss recent experiments on these materials. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P44.00012: Discrete breather energy thresholds in Discrete Nonlinear Schrodinger (DNLS) systems Discrete breather energy thresholds in Discrete Nonlinear Schrodinger (DNLS) systems Jun Zhou, Jerome Dorignac, David Campbell The DNLS equation has been used successfully to model physical systems as varied as the Holstein polaron, the Davydov soliton, local modes of small molecules and, more recently, optical wave guide arrays and Bose-Einstein condensates trapped in optical lattices. In addition, the DNLS also governs the slow modulations of plane waves in Klein-Gordon systems (network of oscillators). In one dimension and for a cubic nonlinearity, the DNLS is known to support discrete breather solutions - time-periodic, spatially localized excitations - with arbitrary low energy (or norm). In contrast, for higher nonlinearities or in higher dimensions, an energy (norm) threshold is known to exist, below which discrete breathers cannot be found. Using two different approaches to treat the DNLS equation--namely, an exponential ansatz and the so-called ``single nonlinear impurity'' approximation-- we derive analytical expressions for these energy thresholds and compare them to the exact numerical solutions. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P44.00013: Bipolaron phase diagram of the 1D adiabatic Holstein-Hubbard model in the strong coupling limit Jerome Dorignac, Jun Zhou, David Campbell We derive the phase diagram for bipolarons in the one-dimensional adiabatic Holstein-Hubbard model in the strong coupling (small hopping) limit. We show the existence of a threshold value for the Coulomb interaction beyond which the ground state of the system consists of two free polarons (infinitely far apart from each other). This result, obtained by means of an exact perturbative expansion, cannot be reproduced by the usual exponential ansatz for the wave function. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P44.00014: First-Principles Study of Electronic Structure in $\alpha $-(BEDT-TTF)$_{2}$I$_{3}$ at Ambient Pressure and under Uniaxial Strains. Hiori Kino, Tsuyoshi Miyazaki We calculate the electronic structure of $\alpha $-(BEDT-TTF)$_{2}$I$_{3}$ at 8K and room temperature at ambient pressure and under the uniaxial strains along the a- and a-axes within the density functional theory. We discover \textit{anisotropic Dirac cone dispersion} near the chemical potential. We also extract the orthogonal tight binding parameters to analyze physical properties. An investigation of the electronic structure near the chemical potential clarify that effects of the uniaxial strain along the a-axis is different from that along the b-axis. The Dirac cone dispersion yields the linear density of states to give $T^{2}$ dependence of the carrier density upto about 100K. It may explain the experimental findings not only qualitatively but also quantitatively. [Preview Abstract] |
Wednesday, March 15, 2006 2:03PM - 2:15PM |
P44.00015: Power-law Current-Voltage Characteristics of Charge-Ordered Organic Crystals ${\theta}$-(BEDT-TTF)$_2$MZn(SCN)$_4$ (M=Cs, Rb) Yamaguchi Takahide, Takako Konoike, Kengo Enomoto, Mitsuka Nishimura, Taichi Terashima, Shinya Uji, Hiroshi M. Yamamoto We have measured the current-voltage characteristics of charge- ordered organic crystals ${\theta}$-(BEDT-TTF)$_2$MZn(SCN)$_4$ (M=Cs, Rb) in a low current range down to $10^{-13}$ A. The current-voltage characteristics follow the power law $I{\propto} V^a$ with a large exponent (e.g., $a=8.4$ at 0.3 K for M=Cs) over a wide range of currents. The power-law characteristics are attributed to electric field-induced unbinding of electron- hole pairs which are thermally excited in the background of the two-dimensional charge order. From analysis of crossover electric fields from ohmic to the power-law characteristics, we obtain strong evidence that the electron-electron Coulomb interaction is significantly long-ranged, i.e., the screening length is greater than 10 molecule sites. A novel magnetoresistance effect, possibly due to the Pauli exclusion principle, is also presented. [Preview Abstract] |
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