Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A22: Charge Density Wave Materials |
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Sponsoring Units: DCMP Chair: Dan Dessau, University of Colorado at Boulder Room: D163 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A22.00001: New High Energy Scales in Quasi-One-Dimensional K$_{0.3}$MoO$_3$ Revealed by High Resolution Angle-Resolved Photoemission Spectroscopy Daixiang Mu, Wentao Zhang, Lin Zhao, Haiyun Liu, Xiaowen Jia, Shanyu Liu, Guodong Liu, Xiaoli Dong, Jun Zhang, X.J. Zhou, Xiaoyang Wang, Qinjun Peng, Zhimin Wang, Shenjin Zhang, Feng Yang, Chuangtian Chen, Zuyan Xu High resolution angle-resolved photoemission (ARPES) measurements have been carried out on K$_{0.3}$MoO$_3$, a proto-typical quasi-one-dimensional material that exhibits Peierls transition at 180 K. Two high energy scales around 100meV and 300meV are revealed in the dispersion measured using super-high resolution vacuum ultra-violet (VUV) laser-based ARPES measurements. These new high energy features emerge in the charge-density-wave state. The origin of these new energy scales will be discussed. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A22.00002: Variable Temperature Scanning Tunneling Microscope study on CDW material 2H-TaSe$_2$ Jixia Dai, Yue Cao, Eduardo Calleja, Daniel Dessau, Helmuth Berger, Kyle McElroy As a layered quasi-2D material, 2H-TaSe$_2$ has a very rich phase diagram including a second order phase transition at 122K, a first order phase transition at 90K, and a superconductivity transition at 133mK. With our UHV Scanning Tunneling Microscope, we have performed temperature-controlled STM work to study the incommensurate and commensurate CDW phases of 2H-TaSe$_2$, from 77K to 110K. We will present temperature and tunneling bias voltage dependant topograph data, together with IV and dI/dV spectra in order to help understanding the nature of these two different CDW phases and the gapping mechanism of this material. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A22.00003: The Role of Electron-Phonon Coupling in the CDW phase transitions in TaSe$_{2}$ Yue Cao, Zhe Sun, Qiang Wang, Jixia Dai, Kyle McElroy, Michael Hermele, Helmuth Berger, Daniel Dessau In this talk, we will report our research progress of the classical charge density wave material 2H-TaSe$_{2}$. The formation of the CDW can be driven by the electronic instability or by the interplay between electrons and phonons, which is an essential ingredient of CDW. In this talk, we will provide a novel analyzing technique that can help distinguish the two scenarios. We will discuss the three possible nesting schemes in this talk and compare its electronic instability. We employ a novel band dissected technique to analyze the characteristic correlation functions for the CDW phase. By comparing the electronic instability to the actual band folding in the incommensurate CDW phase, we can tell the role of electronic structure / electron phonon coupling in this material. This discussion will help improve our understanding of the CDW and of the nesting picture in general. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A22.00004: Extended phonon collapse in the charge density wave compound NbSe$_2$ S. Rosenkranz, F. Weber, J.P. Castellan, R. Osborn, A. Said, R. Hott, R. Heid, D. Reznik The soft-phonons in the charge density wave (CDW) compound NbSe$_2$ were investigated using high-resolution inelastic X-ray scattering. As the CDW transition at T$_C$=33K is approached from high temperature, we observe a breakdown of the dispersion with the phonons becoming overdapmed over an extended region around the CDW wavevector. This is in contrast to the cusp in the phonon dispersion expected from the commonly invoked electronic nesting scenario of the CDW transition. Instead, our results, combined with $ab\ initio$ calculations, show that the wavevector of the CDW order is dictated by the momentum dependence of the intrinsic electron-phonon coupling. The strong influence of electron-phonon matrix-elements could also be of importance to other systems, where CDW-like correlations have been attributed to unusual physical properties. \\ \\ Work supported by US DOE BES-DMS DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A22.00005: Pressure induced enhancement of CDW fluctuations in 1T-TiSe2 Y.I. Joe, X.M. Chen, J. Geck, M. von Zimmermann, S. Yuan, S.L. Cooper, Peter Abbamonte 1T-TiSe2 is atypical quasi 2-dimensional CDW material showing a 2x2x2 superlattice modulation at low temperature. It's is known that high pressure suppresses the CDW order and interestingly induces superconductivity. While it is not agreed what drives the CDW transition, it has been proposed that the CDW fluctuations are closely related to the emergence of superconductivity . Here we present a detailed high pressure X-ray diffraction study of the CDW order in -TiSe2. We can directly probe not only the order parameter but also the fluctuations of the CDW order. At low pressure we observe no sizable deviation from mean field predictions. For pressures above around 2.5GPa, however, we observe changes in the CDW line shape, indicating enhanced CDW fluctuations. Those results are consistent with previous high pressure transport measurements and suggest a relationship between the superconductivity and the charge density wave in this system. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A22.00006: Fluctuations of CDW order at a quantum phase transition Yejun Feng, J. van Wezel, S. Haravifard, G. Srajer, J. Mitchell, Jiyang Wang, T.F. Rosenbaum, R. Jaramillo, Z.-A. Xu, Y. Liu 2H-NbSe2 is the archetypical two-dimensional charge-density-wave system. Using x-ray diffraction in a diamond anvil cell, we track the evolution of the CDW order towards the buried quantum critical point inside the superconducting phase. We observe a pressure-dependent nesting vector as well as fluctuation broadening, and compare these results to the behavior of the three-dimensional spin-density-wave system, Chromium, at its quantum critical point. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A22.00007: The effect of quantum fluctuations on charge ordered NbSe$_2$ Jasper van Wezel, Yejun Feng, Jiyang Wang, R. Jaramillo, T.F. Rosenbaum Among materials displaying charge density wave order, NbSe$_2$ stands out because its ordering vector does not correspond to any obvious nesting properties of its Fermi surface or band structure. The well known Peierls mechanism is thus less effective in singling out an ordering vector for NbSe$_2$, and the transition is driven instead by an increase of the susceptibility over a wide range of wave numbers. As the CDW transition is suppressed towards zero temperature, such a broad susceptibility gives rise to quantum fluctuations with an equally broad span in wavelengths. Here, we examine the role of these quantum fluctuations as the critical point is approached. We compare our theoretical findings to recent measurements of the ordering wave vector of NbSe$_2$ under pressure and show that its properties can be understood as arising from the combined effect of the presence of quantum fluctuations and the coupling of the CDW order parameter to the lattice. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A22.00008: Field-Effect Modulation of Charge Density Wave Conduction Ethan Geil, Robert Thorne We have constructed field-effect devices, analogous to MOSFETs, with crystals of the charge-density wave (CDW) conductor NbSe3 as the channel. Applying a gate voltage across an oxide insulator modulates the carrier density in the NbSe3 and also applies a transverse electric field. Surprisingly, relatively small ($\sim$ 0.1\%) changes in carrier density (as measured by the single particle conductivity) produce large ($\sim$ 40\%) decreases in the threshold field for collective conduction. We discuss this result in terms of collective screening of the applied field and modulation of the CDW order parameter. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A22.00009: Torque dependence of the voltage-induced torsional strain in tantalum trisulfide associated with charge-density-wave depinning J. Nichols, H. Zhang, J.W. Brill Crystals of orthorhombic tantalum trisulfide slowly twist (by $\sim $ 1/4 degree) when voltages near the charge-density-wave depinning threshold are applied. We have studied how this hysteretic voltage-induced torsional strain (VITS) is affected by additional torques applied to the sample by attaching a magnetized steel wire to the center of the sample. The torsional strain in the crystal was measured by placing the sample in an RF cavity in a small, variable magnetic field. We have found that twisting the sample by a few degrees can have large effects on the induced strain: i) twisting can change the magnitude and dynamics of the VITS; ii) in some cases, twisting can change the direction of the VITS. The latter effect suggests that the VITS is caused by dislocation lines in the crystal causing transverse gradients in the CDW phase. As these gradients compress and dilate with alternating applied voltage, they can cause torsional strains in the crystal. A puzzle, however, is what causes the voltage-induced torsional strain to be so slow (time constants $\sim $ 1 sec near the depinning threshold). [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A22.00010: High Field Proton NMR Studies of Single Crystal Per$_{2}$Pt[mnt]$_{2}$ Elizabeth L. Green, J.S. Brooks, P.L. Kuhns, A.P. Reyes, S. Brown, M. Almeida Per$_{2}$Pt[mnt]$_{2}$ is a quasi-one-dimensional organic conductor that has been studied for over thirty years. It consists of perylene and Pt[mnt]$_{2 }$chains that undergo a spin-peierls (SP)-charge density wave (CDW) transition below 8 K. The phase diagram has previously been mapped out up to 42 T using transport measurements. The work we present here is the first single crystal NMR experiment, primarily focusing on the physics of the localized moment present on the platinum site. By measuring relaxation rates and spectra at high fields, up to 26 T, and low temperatures, down to 1.5 K, we were able to map out the SP phase boundary. Preliminary results indicate that the SP transition occurs at a lower temperature than the CDW boundary determined from transport, suggesting that the lattice instability on the perylene drives the dimerization of the platinum moment. Our ultimate goal is to use NMR spectra to observe the platinum moment in the field induced charge density wave (FICDW) state in the range 25-42 T. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A22.00011: Spin frustration and charge ordering in TMTTF salts Kazuyoshi Yoshimi, Hitoshi Seo, Shoji Ishibashi, Stuart Brown Quasi-one-dimensional organic conductors (TMTTF)$_2$X salts exhibit various types of phase transitions such as magnetic ordering, charge ordering (CO), and superconducting transitions. Among them, (TMTTF)$_2$SbF$_6$ shows a peculiar behavior under pressure: a cooperative reduction of CO and anti-ferromagnetic (AF) phase transition temperatures by the application of pressure has been reported by NMR measurements [1]. This result naively does not coincide with the case for typical CO transitions, where CO suppresses the tendency toward magnetic ordering due to decrease of the effective spin exchange coupling. To explain this behavior, we investigate a 1/4-filled quasi-one-dimensional extended Hubbard model with Coulomb interactions and inter-chain hopping which causes spin frustration between the dimers on the one-dimensional chains. By numerical exact diagonalization method, we find that CO relaxes spin frustration and enhances two-dimensionality which stabilizes AF ordering. To compare our results with experiments, we determine the hopping parameters by first principles band calculation for several TMTTF salts and discuss the relation between spin frustration and CO.\\[0pt] [1] W. Yu et al., Phys. Rev. B. 70 121101 (2004). [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A22.00012: The Paired Electron Crystal: order from frustration in the quarter-filled band S. Dayal, R.T. Clay, H. Li, S. Mazumdar The effect of lattice frustration on two dimensional (2D) quantum spin models and the 2D half-filled Hubbard model has been intensively studied in order to understand the connections between antiferromagnetism (AFM), valence-bond ordered states, candidate spin-liquid states, and unconventional superconductivity. For several classes of unconventional superconductors, including the organic charge-transfer solids and superconducting spinels such as LiTi$_2$O$_4$, the correct starting point is however the quarter-filled rather than $\frac{1}{2}$-filled band. We present a study of the effect of frustration on the 2D $\frac{1}{4}$-filled interacting band. We demonstrate that in addition to the well known AFM state occurring with lattice dimerization, and Wigner crystal (WC) state, a paired insulating state occurs in the frustrated region of the phase diagram. This paired electron crystal (PEC) state has coexisting charge order and bond order and a spin-gap due to the formation of nearest-neighbor singlets in the pairs. We investigate fully the phase diagram, including effects of varying the strength of on-site and nearest-neighbor Coulomb interactions as well as electron phonon coupling strength. We present the full phase diagram showing the extent of AFM, PEC and WC phases. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A22.00013: The Paired Electron Crystal in quarter-filled organic superconductors R.T. Clay, S. Dayal, H. Li, S. Mazumdar In the 2D organic superconductors the underlying carrier density in the conducting layers is 1/2 electron per molecule. Because molecules often occur in dimer pairs, an effective model is frequently used with one electron per dimer. With strong electron-electron correlations, this effective model describes the occurrence of antiferromagnetism. Because of lattice frustration, $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ and other organics have been suggested to have spin liquid ground states. Recent experiments however have found strong lattice effects at low temperature in this material and raised uncertainty whether excitations are gapped or gapless. We argue that to resolve these issues one must go beyond the effective dimer model and instead start from the underlying 1/4-filled band. We have recently shown that in 2D 1/4-filled strongly correlated systems a commensurate insulating state forms that we have termed a Paired Electron Crystal (PEC). While in the antiferromagnetic state charge densities are uniform within a dimer, in the spin-gapped PEC state dimer charges become unequal and pairs of charge-rich sites are separated by pairs of charge-poor sites. We review the PEC concept and explain how it can provide a unified theoretical view of the 2D organics. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A22.00014: Oscillating magnetothermopower in a Q2D organic conductor Danica Krstovska The beating oscillations of the interlayer thermopower with a large amplitude on both the magnetic field magnitude and an angle between the normal to the Q2D layer plane and the magnetic field are shown to occur when the cyclotron energy is comparable with the interlayer transfer integral. It is found that, in a Q2D organic conductor with a simple slightly warped FS, the amplitude of the quantum oscillations of the interlayer thermopower substantially exceeds the amplitude of its classical part due to the presence of features of the DoS of the charge carriers when their energy spectrum is quantized. The semi-classical Boltzmann theory predicts that the position of the beats in the magnetic oscillations of the interlayer thermopower are shifted with respect of those in the interlayer magnetoresistance. The shift is even bigger with increasing magnetic field. It might be expected that the difference between the beats in the interlayer thermopower and Shubnikov de Haass angular oscillations is not magnetic field dependent. However, experiments will be necessary for more detailed analysis of the magnetothermopower in Q2D organic metals to be made. [Preview Abstract] |
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