Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U10: Charge Density Waves and 1D Systems |
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Sponsoring Units: DCMP Chair: Weiqiang Yu, University of Maryland Room: Colorado Convention Center Korbel 1E |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U10.00001: A Kadanoff-Wilson renormalization group analysis of half-filled one-dimensional quantum electron-phonon models Hassan Bakrim, Claude Bourbonnais We study the zero temperature phase diagrams of the half-filled one-dimensional Su- Schrieffer-Heeger (SSH) and molecular crystal (CM) models using the Kadanoff-Wilson renormalization group approach. At the one-loop level, the full frequency dependence of the phonon induced electron-electron coupling constants is taken into account in the vertex corrections and the quantum interference between the Cooper and Peierls diffusion channels. This enters as a key ingredient for the description of the quantum to classical transition for the Peierls instability. Our results confirm that finite phonon frequency introduces quantum fluctuations that depress the Peierls gap $\Delta$ compared to the classical - mean field - limit $\Delta_0$. It is found that in the spinless fermion case, the Peierls gap vanishes at the threshold $\omega_D\sim \pi \Delta_0$, whereas for fermions with spins, the gap remains in the quantum spin- charge separated regime. We extend our study to the XY spin-Peierls chain and confirm the DMRG result about the existence of a power law relation between the critical spin-phonon coupling $\alpha_c$ and frequency at the quantum-classical boundary, namely $\alpha_c\sim \omega_D^{0.7}$. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U10.00002: Small Bipolarons in the anisotropic 2-dimensional Holstein-Hubbard model. Jun Zhou, Jerome Dorignac, David Campbell We will investigate the bipolaron states in the anisotropic two-dimensional Holstein-Hubbard model. The interplay between attractive electron-phonon coupling g and the repulsive electron-electron interaction v will generate many different ground states. The anisotropic electron hopping in two dimensions also plays a role in affecting the bipolaron state. The bipolaron could be located on a single site or be two polarons separated by several sites or a quadrisinglet state which is the superposition of 4 electronic singlets with a common central site. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U10.00003: Fermi surface pockets and Luttinger sum rule in low-dimensional systems Christophe Berthod, Thierry Giamarchi, Silke Biermann, Antoine Georges We investigate the Mott transition in a quasi-one dimensional system of weakly coupled interacting fermionic chains. Within a generalization of dynamic mean field theory, we study by quantum Monte Carlo the evolution of the electron self-energy with increasing inter-chain coupling. Our approach is able to capture the closing of the Mott gap at some critical coupling, and is thus ideally suited to examine the characteristics of the Mott transition in this and similar systems. We find that the transition proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. Although these pockets can be very small, we show that the Luttinger sum rule is obeyed throughout the phase diagram. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U10.00004: Does nesting really cause Charge Density Waves? Michelle Johannes, Igor Mazin The concept of a CDW induced by Fermi-surface nesting originated from the Peierls idea of electronic instabilities in purely 1D metals and is now often applied to charge ordering in real, low-dimensional materials. The idea is that if Fermi surface contours coincide when shifted along the observed CDW wave vector, then the CDW is considered to be nesting-derived. We argue that only a tiny fraction, if any, of the observed charge ordering phase transitions are true analogues of the Peierls instability, {\it i.e.} that (a) there is substantial nesting of the FS, as quantified by a peak in Im $\chi_0({\bf q})$ at the CDW wave vector; (b) this peak translates (as in the 1D case) into a peak in Re$ \chi_0({\bf q})$ at the same wave vector; (c) a divergence in the full electronic susceptibility causes the electronic subsystem to be unstable {\it without} ion shifts; and (d) {\it all} phonons are softened at the CDW vector. Using prototypical CDW materials NbSe$_2$, NbSe$_3$, and CeTe$_3$, we show that these conditions are hardly ever fulfilled, and that the CDW phases are actually structural phase transitions, driven by ionic rather than electronic instabilities. We also show mathematically that the original Peierls construction is so fragile as to be unlikely to apply to any real material. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U10.00005: The pseudogap phase in (TaSe$_4$)$_2$I Andras Vanyolos, Balazs Dora, Attila Virosztek We have developed the mean-field theory of coexisting charge-density wave (CDW) and unconventional charge-density wave (UCDW). The double phase transition manifests itself in the thermodynamic quantities and in the magnetic response, such as spin susceptibility and nuclear spin-lattice relaxation rate. Our theory qualitatively applies to the quasi-one dimensional CDW material (TaSe$_4$)$_2$I. This material exhibits peculiar properties: above the CDW transition temperature $T_c$, thermal fluctuations were found to die out rapidly, but robust pseudogap behavior is still detected. Namely, the experimental findings include: (i) sharp increase of the static spin susceptibility above $T_c$, (ii) smooth increase of the spin-lattice relaxation rate above $T_c$, (iii) as opposed to conventional CDW, no sharp feature in the spin-lattice relaxation rate below $T_c$. We have found that our coexisting CDW+UCDW model qualitatively describes these observed properties of (TaSe$_4$)$_2$I. Direct calculations for the magnetic response are shown to evidence the agreement. We also argue, that the fluctuations around $T_c$ are suppressed due to the presence of the ``hidden'' UCDW phase, which partially gaps the Fermi surface, and causes non-Fermi-liquid (pseudogap) behavior. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U10.00006: Charge density wave formation and physical properties of $R_{2}$Te$_{5}$ ($R$=Nd, Gd) compounds K.Y. Shin, N. Ru., M.F. Toney, I.R. Fisher $R_{2}$Te$_{5}$ ($R$=Nd, Gd) has a layered tetragonal structure based on alternating single and double square planar Te sheets separated by corrugated $R$Te layers, and has a quasi-two dimensional electronic structure. The material shares important physical properties with other two single and double Te layer variants, $R$Te$_{2}$ and $R$Te$_{3}$ (R=La~Yb), including a charge density wave (CDW) instability. Using a binary self-flux method, we have grown high-quality single crystals of Gd$_{2}$Te$_{5}$ and Nd$_{2}$Te$_{5}$ compounds and have characterized their structural, thermodynamic and transport properties. Our measurements reveal, for the first time, the charge density wave in this material. We will discuss the properties and origin of the CDW, and the relation to the better known $R$Te$_{2}$ and $R$Te$_{3}$ compounds. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U10.00007: Effect of Chemical Pressure on the Charge Density Wave Transition in Rare-earth Tritellurides $R$Te$_3$ N. Ru, G. Y. Margulis, K. Y. Shin, M. F. Toney, I. R. Fisher The charge density wave transition is investigated in the bi-layer family of rare earth tritelluride $R$Te$_3$ compounds ($R$ = Sm, Gd, Tb, Dy, Ho, Er, Tm) via high resolution x-ray diffraction and electrical resistivity. The transition temperature $T_{CDW}$ increases monotonically with increasing lattice parameter by an extraordinarily large amount, from 244(3) K for TmTe$_3$ to 416(3) K for SmTe$_3$. It is suggested that this behavior, and the observation of a secondary transition for the heaviest members of the series, is intimately linked to the effect of chemical pressure on the degree of bilayer splitting of the Fermi surface. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U10.00008: High-resolution STM imaging and spectroscopy of Cu$_{x}$TiSe$_{2}$ Dale Kitchen, Kenjiro K. Gomes, Abhay Pasupathy, Aakash Pushp, Pedram Roushan, Emilia Morosan, Robert J. Cava, Ali Yazdani The discovery of superconductivity in Cu-doped TiSe$_{2}$ has created a new opportunity to study the competition between charge density wave (CDW) formation and superconductivity in layered chalcogenides [1]. Using a cryogenic scanning tunneling microscope (STM), we have obtained atomic resolution images of \textit{in situ} cleaved Cu$_{x}$TiSe$_{2}$ and perform spatially resolved mapping of the electronic states of this compound. The STM images measured on samples at low Cu doping, reveal the atomic lattice, the CDW organization, and show local signatures consistent with individual Cu-dopants. Imaging and spectroscopy are used to identify how the Cu-dopants alter the local electronic structure of this material and destroy the CDW organization, which eventually gives way to the rise of superconductivity. [1] E. Morosan \textit{et al}., \textit{Nature Physics} \textbf{2}, 544 (2006). [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U10.00009: STM Studies of the CDW System TbTe$_3$ Alan Fang, Zhanybek Alpichshev, Nancy Ru, Ian Fisher, Aharon Kapitulnik We present STM data on the Charge Density Wave (CDW) in the Rare Earth Tri-Telluride TbTe$_3$. Topography scans as large as 250$\times$250 $\AA^2$ were taken with voltage bias as high as 0.8 Volt. Fourier analysis shows an incommensurate unidirectional modulation with wave-vector q $\approx$ 0.71 a*. The topographic scans at different bias voltages are used to highlight the difference in structure of the CDW and lattice period-doubling effects, either from the Te-Te dimerization, or from the Te-Tb layer directly below the surface. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U10.00010: APRES study of a complex charge density wave material Gd$_{2}$Te$_{5}$ Ruihua He, Kyungyun Shin, Hong Yao, Jude Laverock, Stephen Dugdale, Nancy Ru, Donghui Lu, Worawat Meevasana, Steve Kivelson, Ian Fisher, Zhi-Xun Shen By using angle-resolved photoemission spectroscopy based on synchrotron radiation as well as monochromatic He-I UV, we have investigated for the first time a complex charge density wave (CDW) material, Gd$_{2}$Te$_{5}$, of the rare earth (R) telluride family R$_{2}$Te$_{5}$, with a hybrid crystalline structure of its two well-studied relatives, RTe$_{2}$ and RTe$_{3}$. Based on a tight-binding model, combining with the LDA calculation, we analyze the experimental Fermi surface, energy band dispersions and their temperature dependence in detail, which provides valuable insights into its complex CDW phase revealed recently by the TEM and XRD measurements. The nature of different CDW wave vectors involved and the roles of different interlayer split bands in the CDW formation are discussed. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U10.00011: Electronic Structure and CDW Physics in LaTe$_{2}$ using ARPES Daniel Garcia, Gey-Hong Gweon, Shuyun Zhou, Jeff Graf, Chris Jozwiak, Myung-Hwa Jung, Yong Seung Kwon, Alessandra Lanzara We report a direct study of the band structure and charge density wave (CDW) formation in LaTe$_{2}$, by using high-resolution angle-resolved photoemission spectroscopy (ARPES). The nature of the CDW formation, the momentum dependence of the CDW gap and the role of dimensionality in the tellurides will be presented. Finally the transition from a stripe to a checkerboard phase and its layer dependence will be addressed. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U10.00012: Doping (x)- and pressure-dependence of the CDW state in Cu$_{x}$TiSe$_{2}$ using inelastic light scattering Harini Barath, Minjung Kim, S.L. Cooper, Emilia Morosan, R.J. Cava TiSe$_{2}$ has long been known to form a rather simple commensurate charge-density-wave (CDW) below T$_{CDW }\sim $ 200K. Interest in this material has grown recently, however, with the discovery that Cu intercalation between the TiSe2 layers [1] suppresses the CDW transition, and at intermediate compositions (x $\ge $ 0.04 in Cu$_{x}$TiSe$_{2})$, gives rise to a superconducting state. Consequently, Cu$_{x}$TiSe$_{2}$ is a particularly interesting system in which to investigate the competition between CDW and superconducting correlations. In this talk, we discuss our inelastic light scattering studies of the effects of chemical tuning on the CDW state in Cu$_{x}$TiSe$_{2}$, which we compare to pressure-dependent studies of the CDW state in TiSe$_{2}$. By monitoring both the CDW amplitude modes and phonons with chemical substitution and pressure, we are able to sensitively study the different routes to CDW melting in this interesting system. [1]. E. Morosan et al., \textit{Nature Physics }\textbf{2, }544 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U10.00013: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U10.00014: CDW dynamics in NbSe$_3$ probed by $^{93}$Nb NMR S. Suh, P. Monceau, W. G. Clark, R. E. Thorne, S. E. Brown Using $^{93}$Nb NMR spin echoes applied on a single crystal of NbSe$_3$, we have probed electric-field induced displacements of the CDW forming below T$_2$=144K. In our experiments, good S/N was achieved by aligning the chain axis of a single large crystal (cross-section O(500$\mu m^2$) with the coil symmetry axis. Evidence for CDW motion throughout the sample for $E>E_T$ was observed in motional narrowing experiments. For $E\le 0.9E_T$, we find a wide distribution of displacements less than one CDW wavelength, for both unipolar and bipolar electric field pulse excitations. At $E\sim 0.9E_T$, the mean displacement is approximately 6-7 degrees, and the width of the distribution is about twice the mean displacement. We discuss the results in the context of the Fukuyama-Lee phase Hamiltonian, and describe the constraints imposed by these experiments on the proposal that CDW depinning is an example of a dynamic critical phenomenon. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U10.00015: Transmission Electron Microscopy of Charge Density Wave Transitions of rare-earth transition-metal silicide R$_{5}$Ir$_{4}$Si$_{10}$ (R=Dy, Ho). C.H. Chen, C.M. Tseng, H.D. Yang The metallic ternary rare-earth transition-metal silicides of R$_{5}$Ir$_{4}$Si$_{10}$ where R=(Dy, Ho) exhibit charge density wave (CDW) transitions despite its seemingly three-dimensional crystal structure. In this talk we present the observation of the CDW phase transitions in this class of materials by electron diffraction and electron microscopy. These compounds exhibit incommensurate to commensurate phase transitions as temperature decreases. The modulation wave vector is found to be along the c-axis of the crystal with a modulation periodicity approximately four times of the unit cell. Real space imaging of CDW domains and/or domain walls using the dark-field technique in transmission electron microscopy will also be presented. [Preview Abstract] |
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