Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G44: Charge-Density Waves |
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Sponsoring Units: DCMP Chair: Robert Thorne, Cornell University Room: Baltimore Convention Center 347 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G44.00001: Dynamics of the Electro-Optic Response of Blue Bronze Luis Ladino, Miraj Uddin, Mario Freamat, Joseph Brill We have measured the frequency, voltage, and position dependence of the electro-transmittance and electro-reflectance of the charge-density-wave (CDW) conductor blue bronze at T $\sim $ 80 K when square-wave voltages are applied to the sample. The electro-optic response, assumed to be proportional to the local strain of the CDW, is characterized in terms of a relaxation time and a surprisingly long ($>$ 0.1 ms) ``inertial'' delay for CDW repolarization. At a given position in the sample, the relaxation time increases with decreasing voltage, as expected. Both time constants increase away from the current contacts, indicating that the inertia is not due to barriers at the contacts. For one sample, the electro-optic response is also observed to decay for times $\sim $ 10 ms. This research was supported by NSF grants {\#} DMR-0100572 and DMR-0400938. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G44.00002: Temperature Dependence of the Electro-Optic Relaxation Time of Blue Bronze Joseph Brill, Luis Ladino, Mario Freamat, Miraj Uddin, Damir Dominko We have measured the temperature dependence of the electro-transmittance of the quasi-one dimensional charge-density-wave (CDW) conductor blue bronze at temperatures between 55 K and 125 K when square-wave voltages are applied to the sample. For voltages well-above the threshold for nonlinear current, the characteristic electro-optic relaxation time, at a given position in the sample, depends primarily on the CDW current, whereas the magnitude of the relative change in transmittance decreases with decreasing temperature even for constant CDW current. At lower voltages, the frequency dependence of the response broadens, suggesting non-uniform and temperature dependent barriers to CDW repolarization. This research was supported by NSF grant {\#} DMR-0400938. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G44.00003: Structure and Charge Density Waves of K0.3MoO3 (blue bronze) (20-1) Surfaces by Variable Temperature STM Maxim Nikiforov, Abdel Isakovic, Robert Thorne, Dawn Bonnell Scanning Tunneling Microscopy has been used extensively in the study of charge density waves in hexagonal compounds such as NbSe2, $\alpha $-TaSe2, $\alpha $-TaS2, NbSe3 etc. A limited number of studies have been done of the surfaces of K0.3MoO3 (blue bronze), a monoclinic compound. To date, charge density waves on blue bronze have not been imaged by STM. In this work we demonstrate unit cell spatial resolution on the blue bronze (20-1) surface at room temperature and at 110K. The contrast in images obtained at room temperature is attributed to the surface atomic structure. The role of the tip atom in the contrast formation is demonstrated by comparison of three different images of the (20-1) surface of K0.3MoO3. At low temperatures charge density waves are superimposed on the unit cell resolution pattern. To the best of our knowledge this is the first observation of charge density waves on blue bronze by STM. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G44.00004: The Phase Diagram of Driven Density Waves Robert Thorne Despite nearly 30 years of study, the physics of the velocity-driving force relation of density wave (DW) conductors has remained controversial. Recent experiments indicate that at low temperatures collective DW motion begins at a threshold field E$_{T}$, but occurs via a new mechanism that we call coherent collective creep. A first-order transition from creep to high velocity sliding occurs when the DW reaches a critical velocity. As the temperature is increased, the transition vanishes at a critical point, and collective sliding begins immediately at E$_{T}$. This phase diagram is inconsistent with all existing models of driven elastic media in the presence of disorder, which assume a single length and energy scale associated with pinning. As emphasized by Larkin and Brazovskii, there are, in fact, two length and energy scales associated with local and collective pinning, respectively. This interpretation has broad consequences for our understanding of driven disordered systems. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G44.00005: Spatial and temporal correlations in CDW dynamics Zachary M. Stum, Abdel F. Isakovic, Robert E. Thorne Noise phenomena associated with the collective mode provide a powerful probe of charge-density-wave (CDW) dynamics. We have developed a setup to allow measurements of spatial and temporal correlations in the noise on length scales of hundreds of microns at frequencies of up to 1 GHz. Details of the electric field, temperature and sample quality dependence of these correlations will be discussed. Our goal is to observe long-sought evidence for critical dynamics as the depinning transition is approached from above. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G44.00006: End current injection contacts for transport studies in CDW materials Abdel F. Isakovic, Katarina Cicak, Robert E. Thorne Current contacts to whisker- or rod-like crystals of quasi-one-dimensional conductors are generally applied to their sides. Effects associated with the resulting anisotropic current injection have long complicated interpretation of transport measurements. We have developed a method to microfabricate end current contacts. This method allows direct injection along the one-dimensional axis, greatly reduces spreading and contact resistances, and produces more uniform current densities. Using these contacts, we have been examining the physics of phase slip and single-particle to collective current conversion in the CDW conductors NbSe$_{3}$ and TaS$_{3}$, and in particular how the excess voltage required for phase slip varies with current and temperature. End contacts also modify the magnitude and temperature dependence of the CDW's depinning threshold. Our analysis shows that the activation voltage for the nucleation of topological dislocations increases with decreasing temperature, and varies from typically 20 mV at 110 K to 80 mV at 80 K. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G44.00007: Electronic Structure of LaTe$_{2}$ Daniel Garcia, Gey-Hong Gweon, Shuyun Zhou, Jeff Graf, Chris Jozwiak, M.H. Jung, Y.S. Kwon, Alessandra Lanzara Current work on the charge density wave system LaTe$_{2}$ indicates the existence of near Fermi energy gapping due to \textbf{q}$_{CDW}$=\textbf{a}*/2 and \textbf{b}*/2. We find this band gapping to be weaker than originally thought, particularly for the near $\Gamma $ point contour. In addition, we find evidence of gap anisotropy which maintains the expected four fold symmetry of the crystal. Finally, we observe evidence of non-negligible kz dependence, originally considered minor. This appears to alter the CDW nesting as well as the gap anisotropy and has considerable importance to future studies of Rare-Earth Dichalcogenides. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G44.00008: Effect of Pressure on the Competition between Charge Density Wave and Superconductivity in ZrTe$_{3}$ Single Crystal Y. Uwatoko, M. Abliz, M. Hedo, R. Yomo, K. Yamamya Competition and coexistence of CDW and superconductivity are studied from ancient times by NbSe$_{2}$, NbSe$_{3}$, etc. The rivalry of CDW and superconductivity was changed with pressure and the method of investigating the situation has been used. We report on an intricate competition between charge density wave (CDW) formation and superconductivity under pressure up to 11 GPa in the low-dimensional conductor ZrTe$_{3}$. The electrical resistivity measurements up to 11 GPa in the temperature 2.4K to 300K were performed using a standard 4-probe method. Hydrostatic pressure was produced by using a cubic anvil pressure cell in the Teflon cell filled with a fluid pressure transmitting medium of mixture of Fluorinert. As pressure increases, the CDW transition temperature T$_{CDW}$ initially increases, then begins to decrease at 2 GPa and abruptly disappears near 5 GPa. On the other hand, while the superconducting transition temperature T$_{C}$ falls to below 1.2 K at $\sim $0.5 GPa and is not observed at up to 5 GPa above 2.5 K, a superconducting transition emerges beginning at $\sim $5 GPa and T$_{C}$ increases steeply up to 11 GPa. This is an observation of pressure-induced reentrant superconductivity. The results are discussed in terms of the change in the reduced area of the Fermi surface due to CDW formation. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G44.00009: Incommensurate lattice modulation in CDW compound TbTe$_{3}$ N. Ru, A. Fang, A. Kapitulnik, I. R. Fisher, M. F. Toney The layered materials $R$Te$_{3}$ ($R=$ rare earth) are simple charge-density wave (CDW) compounds, for which large regions of the original quasi-2D Fermi surface are nested by a single incommensurate wavevector. We use high resolution x-ray diffraction and scanning tunneling microscopy (STM) to study how the lattice responds to the incommensurate ordering. The superlattice of TbTe$_3$ reveals a unidirectional lattice modulation characterized by $q_{CDW}$ = 0.296 c*. Higher harmonics are several orders of magnitude lower in intensity, implying an almost sinusoidal lattice modulation. The CDW is well correlated within and between planes, with correlation lengths in excess of 1000 $\AA$. STM measurements reveal the CDW gap and real-space lattice modulation. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G44.00010: Stripes in quasi 2D rare-earth tellurides Hong Yao, John Robertson, Eun-Ah Kim, Steven Kivelson Even though the rare-earth tellurides are tetragonal materials with quasi-2D band structure, they have a hidden 1D character. The consequent, near-perfect Fermi surface nesting results in the formation of a charge density wave (CDW) state. Interestingly, the CDW is unidirectional (?striped?), spontaneously breaking not only translational symmetry, but the discrete rotational symmetry, as well. We show that there are two possible ordered phases consistent with the band structure: A bidirectional ?checkerboard? state would occur if the CDW transition temperature is sufficiently low, whereas the observed striped state is favored when the transition temperature is larger. We comment, as well, on the implications of this finding for the issue of stripes vs. checkerboards in more strongly correlated systems, such as the cuprates. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G44.00011: The dimensionality of charge density waves in the presence of quenched disorder Adrian Del Maestro, Bernd Rosenow, Subir Sachdev We present the effects of quenched disorder on a model of charge density wave (CDW) ordering on the square lattice. Our model may be applicable to the cuprate superconductors, where a random electrostatic potential exists in the CuO planes as a result of the presence of charged dopants. We argue that the presence of a random potential can affect the uni-directionality of the CDW order. Coupling to a uni-directional CDW, the random potential can lead to the formation of domains with 90 degree relative orientation, thus tending to restore the rotational symmetry of the underlying lattice. For a checkerboard CDW on the other hand, disorder generates spatial anisotropies on short length scales and thus some degree of uni-directionality. Using both numerical and analytical techniques, we quantify these disorder effects. Contact will be made with different experimental approaches on various materials that seem to observe both stripe and checkerboard charge ordering. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G44.00012: Inhomogeneous and glassy electronic phases driven by competing orders I. Vekhter, Z. Nussinov, A. V. Balatsky Emergence of inhomogeneous and glassy states in interacting systems has been a focus of much attention recently. It has been well established that such states may arise a) in the presence of disorder; b) in pure systems in the presence of interactions at competing length scales. Here we investigate the emergence of inhomogeneous states as a result of competing orders. We use a Ginzburg-Landau theory and find that, even if the theory is local, negative amplitude-gradient coupling leads to states of inhomogeneous coexistence of order parameters. Proliferation of low lying modes in such systems triggers slow dynamics and low critical temperatures. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G44.00013: Dynamic Compressibility and aging in Wigner crystals and quantum glasses Thierry Giamarchi, Leticia Cugliandolo, Pierre Le Doussal We study the non-equilibrium linear response of quantum elastic systems pinned by quenched disorder with Schwinger-Keldysh real-time techniques complemented by a mean-field variational approach. We find (i) a quasi-equilibrium regime in which the analytic continuation from the imaginary-time replica results holds provided the marginality condition is enforced; (ii) an aging regime. The conductivity and compressibility are computed. The latter is found to cross over from its dynamic to static value on a scale set by the waiting time after a quench, an effect which can be probed in experiments in e.g. Wigner glasses. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G44.00014: Wigner crystallization in Na$_3$Cu$_2$O$_4$ and Na$_8$Cu$_5$O$_{10}$ chain compounds. P. Horsch, M. Sofin, M. Mayr, M. Jansen We report the synthesis of novel doped edge-sharing chain compounds Na$_3$Cu$_2$O$_4$ and Na$_8$Cu$_5$O$_{10}$, which form insulating states with commensurate charge order [1]. We identify these systems as one-dimensional Wigner lattices, where the charge order is determined by the long-range Coulomb interaction and the number of holes in the d-shell of Cu. Our interpretation is supported by X-ray structure data as well as by an analysis of magnetic susceptibility and specific heat data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems allow for an unambiguous distinction between the classical Wigner lattice and the $4k_F$ charge-density wave of quantum mechanical origin. Finally, we briefly discuss the domain-wall type charge excitations and the theoretical expectation for the optical conductivity of 1D Wigner lattices [2]. [1] P. Horsch, M. Sofin, M. Mayr, and M. Jansen, Phys. Rev. Lett. 94, 076403 (2005). [2] M. Mayr and P. Horsch, (unpublished). [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G44.00015: Fermi surface nesting and the origin of the charge density wave in NbSe2 Christopher Howells, Michelle Johannes, Igor Mazin The origin of a charge density wave (CDW) transition in NbSe$_2$ and related transition metal dichalcogenides has been studied and hypothesized about for the past four decades. The idea that CDW formation is the result of Fermi surface nesting was put forward as early as 1978 and continues to be suggested in current papers. Using highly accurate density functional calculations, we make a detailed study of the band structure and Fermi surfaces of equilibrium and pressurized NbSe$_2$. We calculate the real part of the non-interacting susceptibility, $\Re\chi_0(\mathbf{q})$, which is the relevant quantity for a CDW instability and the imaginary part,$\Im\chi_0(\mathbf{q})$, which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in $\Re\chi_0(\mathbf{q} )$ near the CDW wave vector, but that no such peaks are visible in $ \Im\chi_0(\mathbf{q})$, definitively eliminating FS nesting as a factor in CDW formation. We discuss the effects of pressure and provide calculated de Haas van Alphen frequencies and effective masses that can be compared to experiment. [Preview Abstract] |
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