Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H41: CDW, SDW and Charge Order in Organic Conductors |
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Sponsoring Units: DMP DCMP Chair: Claude Bourbonnais, Universite de Sherbrooke Room: 413 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H41.00001: X-ray Diffraction measurements of the CDW phase in doped TiSe$_{2}$ and NbSe$_{2}$ J.P. Castellan, C.D. Malliakas, M. Lavarone, S. Rosenkranz, R. Osborn, F. Weber Layered transition metal chalcogenides display a phase diagram similar to that of high Tc superconducting materials. In high Tc materials the superconductivity coincides with the suppression of magnetic order. In the case of TiSe$_{2}$ and NbSe$_{2}$ the onset of superconductivity involves a charge density wave (CDW) rather than magnetic order. The phase diagram of doped TiSe$_{2}$ and NbSe$_{2}$ has so far been interpreted as a competition of the CDW order and superconductivity order parameters. Another interpretation is that the softening of the excitons responsible for the CDW order enables the superconductivity. We have measured diffuse scattering associated with the fluctuations in the order parameter of the CDW on either side of the quantum phase transition using X-ray scattering. We will present the behavior of the critical exponents as the quantum phase transition is approached and discuss whether there is a coupling of the CDW and superconducting order parameters. Work supported by US DOE BES-DMS DE-AC02-06CH11357 [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H41.00002: Interplane and Intraplane Coupling in Charge-Density-Wave Phases of $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $ Yizhi Ge, Amy Liu At low temperatures, the layered transition metal dichalcogenides $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $ adopt similar charge-density wave structures corresponding to a $\sqrt {13} \times \sqrt {13} $ reconstruction in which the Ta atoms cluster within the triangular layers. Yet $\mbox{TaS}_2 $ also undergoes a metal-insulator transition, while $\mbox{TaSe}_2 $does not. Here we present a density-functional-theory study of the electronic structure of $\mbox{1T-TaS}_2 $ and $1\mbox{T-TaSe}_2 $. The half filled Ta d band at the Fermi level is found to differ significantly in the two materials: in $\mbox{TaSe}_2 $ the band is three dimensional, while in $\mbox{TaS}_2 $ it is highly one dimensional. These results are analyzed using maximally localized Wannier functions. The effects of stacking sequence and the spin-orbit interaction will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H41.00003: A time-resolved observation of collective phenomena in the CDW compound TbTe3 F. Schmitt, P. S. Kirchmann, U. Bovensiepen, R. G. Moore, L. Rettig, M. Krenz, J.-H. Chu, N. Ru, L. Perfetti, D.-H. Lu, M. Wolf, I. Fisher, Z.-X. Shen Strong correlations and collective phenomena in solids are a fascinating and challenging area of physics. Understanding how the interactions between the constituents give rise to collective phenomena like phase transitions and collective modes will greatly enhance our understanding in solid state physics. One such system, the Rare Earth Tritelluride RTe$_3$, is an excellent model system for a systematic study of charge-density wave (CDW) physics. We have performed time-resolved angle-resolved photoelectron spectroscopy (trARPES), a novel form of pump-probe spectroscopy, on TbTe3 and we will present our newest results. We observe two collective modes and what we term the time-dependent melting of the CDW, which we discussed earlier\footnote{F. Schmitt et al., Science \textbf{321}, 1649-1652 (2008)}. Here, we will focus primarily on the detailed properties of the two collective modes. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H41.00004: Voltage Induced Torsional Strain in Tantalum Trisulfide in its CDW State J. Nichols, Jun Zhou, L. Ladino, J.W. Brill In 2007, Pokrovskii et al [Phys. Rev. Lett. 98, 206404 (2007)] reported that crystals of tantalum trisulfide spontaneously twist when a voltage near the CDW depinning threshold is applied. The direction of twist reverses when the voltage is reversed, so that the twist angle describes a characteristic hysteresis loop as a function of applied voltage or current. We have studied this effect by placing the sample inside a resonant RF cavity so that the twisting sample modulates the resonant frequency of the cavity. A magnetic wire is attached to the sample so that magnetic torque can also be applied to the sample. We have reproduced the ``Pokrovskii effect'' and also observed the complement: application of torque induces an additional voltage when the sample is biased above threshold. We have also found that the onset of the torsional strain occurs at a voltage slightly below that at which the shear modulus of the sample softens. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H41.00005: Evidence for coupling between charge-density-wave and phonons in two-dimensional rare-earth tri-tellurides M. Lavagnini, M. Baldini, A. Sacchetti, D. Di Castro, B. Delley, R. Monnier, J.H. Chu, N. Ru, I.R. Fisher, P. Postorino, L. Degiorgi We report on a Raman scattering investigation of the charge-density-wave (CDW), quasi two-dimensional rare-earth tri-tellurides $R$Te$_3$ ($R$= La, Ce, Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe$_3$ and CeTe$_3$ under externally applied pressure. The observed phonon peaks can be ascribed to the Raman active modes for both the undistorted as well as the distorted lattice in the CDW state by means of a first principles calculation. The latter also predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition. The integrated intensity of the two most prominent modes scales as a characteristic power of the CDW-gap amplitude upon compressing the lattice, which provides clear evidence for the tight coupling between the CDW condensate and the vibrational modes. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H41.00006: Pressure dependence of the single particle excitation in the charge-density-wave CeTe$_3$ system L. Degiorgi, M. Lavagnini, A. Sacchetti, C. Marini, M. Valentini, R. Sopracase, A. Perucchi, P. Postorino, S. Lupi, J.H. Chu, I.R. Fisher We present data on the pressure dependence at 300 K of the optical reflectivity of CeTe$_3$, which undergoes a charge-density-wave (CDW) phase transition well above room temperature. The collected data cover an unprecedented broad spectral range from the infrared up to the ultraviolet, which allows a robust determination of the gap as well as of the fraction of the Fermi surface affected by the formation of the CDW condensate. Upon compressing the lattice there is a progressive closing of the gap inducing a transfer of spectral weight from the gap feature into the Drude component. At frequencies above the CDW gap we also identify a power-law behavior, consistent with findings along the $R$Te$_3$ series (i.e., chemical pressure) and suggestive of a Tomonaga-Luttinger liquid scenario at high energy scales. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H41.00007: Far infrared magnetospectroscopy of quasi-1D and 2-D density wave compounds. A.F. Isakovic, G.L. Carr We report a far infrared reflectance study of quasi-1D blue bronze (K$_{0.3}$MoO$_{3})$ and magnetically doped quasi-2D Mn$_{0.01}$NbSe$_{2}$ over the temperature range from 10K up to 180K and fields up to 10T. For blue bronze, several features in the magnetoreflectance spectra, including amplitudon and phonon-like modes, change substantially (up to 50{\%}) when a magnetic field is applied. For 100K$<$T$<$180K (below the CDW T$_{C}$ of 183K), the spectra change nearly monotonically with T. Changes at lower T involve primarily the amplitudon mode (CDW order parameter), consistent with creep transport of the CDW. The B and T dependent spectra of Mn doped NbSe$_{2}$ reveal the presence of an isosbestic point near 5 meV photon energy and having a shape suggesting a local transfer of oscillator strength. The magnetoreflectance changes abruptly around a temperature of 16K for fields below 6T. Though a Drude-Lorentz fit can be applied to the spectra, a full understanding of charge transport in these materials will require a more detailed model. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H41.00008: Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)$_{2}$PF$_{6}$ salts Mario Poirier, Alexandre Langlois, Claude Bourbonnais, Pascale Foury-Leylekian, Alec Moradpour, Jean-Paul Pouget Using a microwave cavity perturbation technique at 16.5 GHz, we report a temperature and magnetic field study of the complex dielectric function along the stacking $a$-axis for a (TMTTF)$_{2}$PF$_{6}$ single crystal and its deuterated analog (d$_{12})$. For both salts, the charge ordering transition (CO) is characterized by a decrease of the dielectric constant $\varepsilon _{a}$ centered at T$_{CO}$ (65 K and 85 K); concomitantly, the dielectric losses go through a maximum near T$_{CO}$ and decreases rapidly below. The spin-Peierls transition (SP) is rather signalled by a rapid increase of $\varepsilon _{a}$ below T$_{SP}$ (16.5 K and 13 K) accompanied by a small peak in the losses. For the deuterated salt, we have observed important relaxation effects below 40 K that complicate the analysis of the dielectric function in the SP ground state. The temperature dependence of the SP anomalies was analysed in magnetic field values up to 18 Tesla. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H41.00009: NMR spin echo measurements of sliding spin density wave repinning in (TMTSF)$_{2}$PF$_{6}$ W.G. Clark, M.E. Hanson, Ernest W.H. Wong We report the repinning rate of sliding spin density waves (SDW's) in (TMTSF)$_{2}$PF$_{6}$ obtained from proton spin echo measurements. This method provides a local measurement of the square of the magnitude of the SDW velocity averaged over the sample as a function of time after the electric field responsible for the SDW sliding is reduced to below its depinning value. It shows a slow decay of the SDW current which we attribute to the corresponding adjustment of the SDW phase to the pinning centers and relaxation of the strain of the sliding SDW. Above the temperature $T\sim $4~K, a thermally activated behavior for the characteristic decay time is observed whose activation energy of 22$\pm $3 K is close to the single electron excitation gap. This indicates that above 4 K, the repining rate is dominated by thermally excited electrons. Below 4 K, a much weaker $T$-dependence is observed suggesting that tunneling may become the dominant repinning mechanism at lower $T$. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H41.00010: Low-temperature states in quasi-one-dimensional charge transfer salt (TMTTF)$_{2}$SbF$_{6}$ studied by $^{13}$C NMR under hydrostatic high pressures Fumitatsu Iwase, Koichi Sugiura, Kou Furukawa, Toshikazu Nakamura We report $^{13}$C NMR study of a quasi-one-dimensional charge transfer salt, (TMTTF)$_{2}$SbF$_{6}$, under the application of the hydrostatic pressure. The antiferromagnetic phase transition at ambient pressure was confirmed by the line splitting and the divergent increase of the spin-lattice relaxation rate 1/$T_{1}$. Under $\sim $5 kbar, 1/$T_{1}$ deceases abruptly at low temperatures without the enhancement, indicating that the ground state is a spin-gapped phase. However, the decrease of the Knight shift expected for spin-Peierls phase transition has not been observed. We observed additional lines, which split symmetrically, at low temperatures under the pressure of $\sim $17 kbar. The possible reentrant antiferromagnetic phases are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H41.00011: The Theory of the Field-Induced Charge-Density-Wave Phases. Andrei Lebed We study a problem about the appearance of the Field-Induced Charge-Density-Wave (FICDW) phases in a quasi-one-dimensional conductor in high magnetic fields. The calculated ratios for critical magnetic fields of the different FICDW phase transitions are in very good agreement with the existing experimental data in alpha-(ET)2KHg(SCN)4 materials. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H41.00012: Realistic Parameters for the Description of Organic Metals Andreas Dolfen, Erik Koch, Volker Blum, Laura Cano-Cort\'es, Jaime Merino In molecular crystals correlation effects are often significant. For a non-perturbative description of the full Coulomb interaction we have therefore to resort to a model description in terms of generalized Hubbard models. The derivation of parameters for such models is crucial for realistic simulations. While hopping parameters are easily derived from density-functional theory (DFT) the Coulomb parameters pose a significant problem due to screening processes. We decompose their contributions into intra- and inter-molecular parts. The intra-molecularly screened Coulomb parameters are treated within DFT whereas the inter-molecular corrections are evaluated using classical electrostatics with DFT-derived polarizabilities and the distributed-dipole approach in combination with a Ewald summation. Even for simple lattices of polarizable point dipoles we find intriguing screening phenomena. As realistic applications we discuss the one- and two-dimensional organic metals TTF-TCNQ and $\Theta$-(BEDT-TTF)$_2$I$_3$. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H41.00013: Spinodal decomposition of the charge ordering in the $\theta $-based organic salt Majed Abdel-Jawad, Ichiro Terasaki, Hatsumi Mori, Takehiko Mori We have measured with X-ray diffraction, resistivity and thermopower the spinodal decomposition of the long range charge ordering in the organic compounds $\theta $-(BEDT-TTF)$_{2}$RbZn(SCN)$_{4}$ and $\theta $-(BEDT-TTF)$_{2}$RbCo(SCN)$_{4}$ . Details of this spinodal decomposition reveals that the charge ordering growth in these compounds follows at first a two dimensional diffusion controlled mode followed abruptly by a change to a three dimensional diffusion controlled growth. Thermopower reveals that the long ranged charge ordered state in these compounds is characteristic of a system with strong on-site repulsion with narrow bandwidths. In contrasts to this, the thermopower of the short ranged charge ordering is anomalous not only in its value but also in its temperature dependence. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H41.00014: Possible verification of tilted and anisotropic Dirac cone in quasi-two dimensional organic compound $\alpha$-(BEDT-TTF)$_2$ I$_3$ using inter-layer magnetoresistance Takao Morinari, Takahiro Himura, Takami Tohyama It is proposed that a tilted and anisotropic Dirac cone is verified using the interlayer magnetoresistance in the layered Dirac fermion system, which is realized in $\alpha$-(BEDT-TTF)$_2$I$_3$, quasi-two-dimensional organic compound. The formula for the inter-layer magnetoresistance is derived using the analytic Landau level wave functions and assuming local tunneling of electrons. Reflecting the deformation of the Landau level wave function due to tilt and anisotropy of the Dirac cone, the inter-layer magnetoresistance depends on the direction of the magnetic field in the plane. We discuss how to determine the parameters of the tilt and anisotropy experimentally using the theoretical formula. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H41.00015: Interlayer Magnetoresistance of the Layered Organic Conductor (DMET)$_{2}$I$_{3}$ P. Dhakal, J.I. Oh, H. Yoshino, K. Kikchui, M.J. Naughton The interlayer magnetoresistance $\rho _{zz}(\theta $,$\phi )$ of the layered organic conductor (DMET)$_{2}$I$_{3}$ is reported for all possible orientations in a 9T magnetic field at 100 mK, and compared with existing theoretical models. We have also calculated $\rho _{zz}(\theta $,$\phi )$ for such Q1D conductors using semiclassical calculations employing, for the first time$^{1}$, the true triclinic crystal structure. These calculations are in qualitative agreement with our data, especially in the $y-z$ plane, where all previous models fail to reproduce the experimentally observed$^{2}$ Lebed magic angle effect$^{3}$. These results will be useful to understand the relationship between the different kinds of angular effects in this and other families of layered organic conductors. This work was supported by NSF Grant No. DMR 0605339. [1]. H. Yoshino, \textit{et al.}, (unpublished). [2]. S. Uji, \textit{et al.}, Proceeding of Physical Phenomena at High Magnetic Fields III (World Scientific, London, 1998), p 227. [3]. A. G. Lebed, JETP Lett. \textbf{43}, 174 (1986); T. Osada, \textit{et al.}, Phys. Rev. Lett. \textbf{66}, 1525 (1991); M. J. Naughton, \textit{et al.}, Phys. Rev. Lett. \textbf{67}, 3712 (1991). [Preview Abstract] |
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