Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X12: Charge Density Wave and Charge Order |
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Sponsoring Units: DCMP Chair: Eduardo Fradkin, University of Illinois at Urbana-Champaign Room: Morial Convention Center 203 |
Friday, March 14, 2008 8:00AM - 8:12AM |
X12.00001: Fermi Surface Evolution Across Multiple CDW Transitions in RTe$_{3}$ R.G. Moore, V. Brouet, J. Laverock, S. Dugdale, R. He, N. Ru, I.R. Fisher, Z.-X. Shen The Fermi surface (FS) evolution across multiple charge density wave (CDW) transitions is investigated using angle-resolved photoemission spectroscopy. Low temperature measurements reveal two incommensurate CDW gaps created by perpendicular FS nesting vectors. A larger gap ($\sim $250 meV) arising from a CDW with q$_{CDW}\sim $0.7c* is in good agreement with the expected trend determined from light rare earth members of the bi-layer family of rare earth tritelluride compounds (RTe$_{3})$. A second, smaller gap ($\sim $50 meV) is due to a second CDW with q$_{CDW}\sim $0.7a* never before seen in other RTe$_{3}$ compounds. The temperature dependence of the FS and the two CDW gaps is characterized. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X12.00002: Momentum and time dependent electronic dynamics in the CDW compound TbTe3 Felix Schmitt, Patrick Kirchmann, Laurenz Rettig, Marcel Krenz, Nancy Ru, Jun-Haw Chu, Rob Moore, Uwe Bovensiepen, Martin Wolf, Ian Fisher, Zhi-Xun Shen The rare earth tritellurides RTe3 are charge-density-wave (CDW) systems in which the coupling strength can be tuned by substituting different rare earths. By using ultrashort IR pump and UV probe laser pulses, we have performed time resolved angular resolved electron photoemission spectroscopy (trARPES) on TbTe3 and have observed the time-resolved closing of the CDW gap in the electronic band structure. To our knowledge, this is the first time-resolved study showing momentum dependent charge dynamics. We will also talk about other results on RTe3 compounds. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X12.00003: Phonon Softening and Displacement Pattern in Commensurate Charge Density Wave in 2H-TaSe$_{2}$ Tom Berlijn, Wei-Guo Yin, Wei Ku The formation of the commensurate charge density wave (CCDW) phase of 2H-TaSe$_{2}$ is investigated via a first-principles study of the phonon spectrum. Both the linear response and the frozen phonon approach reveal strong softening of the $\Sigma_{1}$ branch at the CCDW wave vector (2$\Gamma $M/3), as observed by inelastic neutron scattering[1]. Furthermore, to resolve the disagreement between currently proposed patterns by neutron scattering[1], electron diffraction[2] and TDPAC[3], the size and the pattern of the CCDW displacement are evaluated by an $ab initio$ total energy calculation. The role of nesting and electron-phonon coupling in the miscroscopic origin of the instability will be addressed. [1] D.E. Moncton, J.D. Axe and F.J. DiSalvo, Phys. Rev. B \textbf{16}, 801 (1977), [2] D.M. Bird, S. McKernan and J.W. Steeds, J. Phys. C \textbf{18} 499 (1985), [3] T. Butz, S. Saibene and A. Lerf, J. Phys. C \textrm{19} 2675 (1986). [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X12.00004: Quantum and classical mode softening near the charge-density-wave/superconductor transition of Cu$_{x}$TiSe$_{2}$: Raman spectroscopic studies Minjung Kim, Harini Barath, S.L. Cooper, P. Abbamonte, E. Fradkin, E. Morosan, R.J. Cava We report temperature- and x-dependent Raman studies of the charge density wave (CDW) amplitude modes in Cu$_{x}$TiSe$_{2}$, which allow us to study the temperature- and x-dependence of the soft mode in this system. Among the key results: we find that the A$_{1g}$ amplitude mode exhibits identical power law scaling with the reduced temperature, p=T/T$_{CDW}$, and the reduced Cu content, p=x/x$_{c}$, i.e., $\omega _{o}\sim $(1-p)$^{0.15}$, suggesting that mode softening is independent of the control parameter used to approach the CDW transition; we provide evidence that x-dependent mode softening originates from the expansion of the lattice, which leads to a x-dependent reduction of the electron-phonon coupling constant; and we infer from our x-dependent mode softening results the presence of a quantum critical point, x$_{c}$(T=0)$\sim $0.07, within the superconducting phase of Cu$_{x}$TiSe$_{2}$. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X12.00005: Insulating charge density wave for a half-filled SU(N) Hubbard model with an attractive on-site interaction in one dimension Jize Zhao, Kazuo Ueda, Xiaoqun Wang We study a one-dimensional SU(N) Hubbard model with an attractive on-site interaction and $N>2$ at half-filling on the bipartite lattice using density-matrix renormalization-group method and a perturbation theory. We find that the ground state of the SU(N) Hubbard model is a charge density wave state with two-fold degeneracy. All the excitations are found to be gapful, resulting in an insulating ground state, in contrast to that in the SU(2) case. Moreover, the charge gap is equal to the Cooperon gap, which behaves as $-2Nt^2/(N-1)U$ in the strong coupling regime. However, the spin gap $\Delta_{s}$ and the quasiparticle gap $\Delta_{1}$ as well open exponentially in the weak coupling region, while in the strong coupling region, they linearly depend on $U$ such that $\Delta_{s}\sim -U(N-1)$ and $\Delta_{1}\sim -U(N-1)/2$. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X12.00006: Spin-soliton excitations from the Bond-Charge-Density Wave spin-Peierls state in the $\frac{1}{4}$-filled band R.T. Clay, R.P. Hardikar, S. Mazumdar The spin-Peierls (SP) transition is commonly discussed in the context of the $\frac{1}{2}$-filled band where the SP state is bond-dimerized. Spin excitations from the SP state generate spin solitons in pairs with opposite-phased bond alternation in between the solitons. As thermal excitations generate additional solitons, oppositely-phased regions overlap and ultimately give way to the uniform phase. Throughout the process the charges on the sites remain uniform. This simple description has to break down in $\frac{1}{4}$-filled band systems where the SP state is a Bond-Charge-Density Wave (BCDW) state with coexisting bond and charge-tetramerization. At $\frac{1}{4}$-filling spin excitations are necessarily accompanied by changes in site charges. We prove the highly interesting result that site charges here can change locally in two different ways, leading to two different kinds of spin solitons. Which kind of soliton dominates is parameter-dependent. The two kinds of solitons promote two different high temperature states, which are either bond-dimerized or charge-dimerized. We discuss experimental consequences of our work. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X12.00007: Measurements of the effect of charge density wave fluctuations on the $^{87}$Rb spin echo decay rate in Rb$_{0.30}$MoO$_{3}$. W.G. Clark, Guoqing Wu, S.E. Brown Measurements and interpretation of the spin echo decay rate (1/$T_{2})$ for the $^{87}$Rb quadrupolar split satellite lines in the charge density wave (CDW) system Rb$_{0.30}$MoO$_{3}$ (blue bronze) are reported over the temperature ($T)$ range 80 K to 250 K at several alignments of the magnetic field (\textbf{\textit{B}} = 9.00 T). In contrast to the central transition, where the contribution of CDW fluctuations to 1/$T_{2}$ is very small, they provide a large contribution to 1/$T_{2}$ for the satellite lines below the CDW transition at $T_{CDW} \quad \approx $ 180 K. This sensitivity to CDW fluctuations shows that 1/$T_{2}$ of the satellite lines should be a much more sensitive probe of both electric field driven and thermal motions of the CDW than the effects of density wave motion on the central transition, which has been used for prior work on this topic. A major goal of our future work is to exploit this property to obtain detailed measurements of electric field driven and thermal motion of the CDW in this material over a wide temperature range. This work has been supported by NSF Grants DMR-0334869 (WGC) and 0520552 (SEB). [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X12.00008: Electro-Optic Search for Critical Divergence of the Charge-Density-Wave Diffusion Constant at the Onset of Depinning in Blue Bronze J.W. Brill, L. Ladino, E.G. Bittle, M. Uddin We have used electro-reflectance measurements to study charge-density-wave (CDW) polarization dynamics at voltages near the CDW depinning onset (V$_{on})$ in the quasi-one-dimensional conductor blue bronze. For low voltages, where the phase-slip rate is low, it is expected that the polarization relaxation time should be inversely proportional to the CDW diffusion constant, which is expected to diverge at V$_{on}$. At T = 78 K, we observe saturation of the relaxation time at low voltages, suggesting that we are in this low phase-slip, ``elastic'' limit and allowing us to estimate the non-critical value of the CDW diffusion constant D($\infty )$ $\sim $ 0.02cm$^{2}$/s, consistent with the measured phason velocity. At other temperatures, the relaxation time increases with decreasing voltage even at the lowest voltages we could measure, indicating we are still dominated by phase-slip. In no case, however, do we observe the expected ``critical speeding up'', setting an upper limit on the critical region of (V/V$_{on}$ -1)$_{critical} <$ 0.06. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X12.00009: Position and Current Dependence of Charge-Density-Wave Polarization Dynamics L. Ladino, J.W. Brill We have studied the frequency and position dependence of charge-density-wave (CDW) polarization by simulating the response to square-waves of variable amplitude and frequency using parameters appropriate for niobium triselenide at T = 90 K, in its upper CDW state. For these simulations, we have numerically solved the phase-slip augmented diffusion model introduced by Adelman \textit{et al} (Phys. Rev. B \underline {53}, 1833 (1996)) for time domain studies. At each position in the sample, the frequency dependence was fit to a modified harmonic oscillator expression and the position and current dependence of the fitting parameters determined. In particular, both the delay time (1/resonant frequency) and relaxation time decrease with increasing current (and phase-slip rate) and increase with distance from the contact, with the delay time vanishing adjacent to the contact, as experimentally observed with electro-optic measurements in blue bronze. No decay of the polarization at long times is observed however, in contrast to electro-optic results. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X12.00010: Thermoelectric studies of charge density wave dynamics. Ross McDonald, Neil Harrison, John Singleton The conventional pyroelectric effect is intimately connected to the symmetry, or rather lack of center of symmetry, of the material. Although the experiments we discuss involve studies of low symmetry materials, the pyroelectric currents observed are of an entirely new origin. Systems with broken-translational-symmetry phases that incorporate orbital quantization can exhibit significant departures from thermodynamic equilibrium due to a change in magnetic induction. For charge density wave systems, this metastable state consists of a balance between the density-wave pinning force and the Lorentz force on the extended currents due to the drift of cyclotron orbits. In this way the density wave pinning potential plays a similar role to the edge potential in a two-dimensional electron gas, leading to a large Hall angle and quantization of the Hall resistance. A thermal perturbation that reduces the pinning potential returns the system towards thermal equilibrium, via a phason avalanche orthogonal to the sample surface. The observation of this new form of pyroelectric effect in the high magnetic field phase (B~$>$~30~T) of the organic charge transfer salt $\alpha $-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$, thus provides a measure of the phason thermopower. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X12.00011: Super-Crystalline CDW Phase in Organic Conductor (Per)$_2$Pt(mnt)$_2$ Si Wu, Andrei Lebed We suggest a model [1,2], where phase transitions between the Peierls and Super-Crystalline [or soliton wall superlattice (SWS)] charge-density-wave (CDW) phases occur in a magnetic field. The model accounts for peculiarities of an electron spectrum in a quasi-one-dimensional (Q1D) conductor (Per)$_2$Pt (mnt)$_2$ . In particular, we show that the Pauli spin- splitting effects improve the nesting properties of a realistic Q1D electron spectrum, and, therefore, a high resistance Peierls CDW phase is stabilized in high magnetic fields. In low and very high magnetic fields, a periodic SWS (or Super- Crystalline) phase is found to be a ground state. We discuss [3] possible experimental investigations of the theoretically predicted phase transitions in (Per)$_2$Pt(mnt)$_2$ to discover a unique SWS phase.\newline \newline [1] A.G. Lebed and Si Wu, Phys. Rev. Lett. 99, 026402 (2007). \newline [2] A.G. Lebed and Si Wu, JETP Lett. 86, 135 (2007). \newline [3] Si Wu and A.G. Lebed, Phys. Rev. B, submitted (2008). [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X12.00012: Temperature dependence of charge-ordering in (TMTCF)$_2$X, C = S, Se Sumit Mazumdar, Rahul Hardikar, R. Torsten Clay Quasi one-dimensional 1/4-filled band charge transfer solids (CTS) undergo two distinct phase transitions as temperature decreases. At high temperature ($\approx$ 100K) these materials undergo a 4k$_F$ (period- two) charge or bond ordering transition. At low temperature, these CTS undergo a magnetic transition to either a spin-Peierls (SP) or anti-ferromagnetic (AFM) ground state, both of which coexist with charge-order (CO). Understanding the relationship between the high and low temperature CO states is a key problem here. We show that (i) the critical nearest neighbor Coulomb interaction V that drives the high temperature Wigner crystal CO is spin-dependent; (ii) as a consequence, for intermediate values of V, there occurs a transition from the Wigner cystal CO to a Bond-Charge-Density wave (BCDW) can occur as temperature decreases. This transition is consistent with recent NMR observations of a charge redistribution occurring simultaneously with the SP state. Our theory is able to explain the competition between the Wigner crystal and SP phases, as well as the occurrence of two difference AFM phases. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X12.00013: Infrared investigation of the charge ordering pattern in the organic spin ladder candidate (DTTTF)$_2$Cu(mnt)$_2$ J.L. Musfeldt, S. Brown, S. Mazumdar, R.T. Clay, M. Mas-Torrent, C. Rovira, J.C. Dias, R.T. Henriques, M. Almeida We measured the variable temperature infrared response of the spin ladder candidate (DTTTF)$_2$Cu(mnt)$_2$ in order to distinguish between two competing ladder models, rectangular versus zigzag, proposed for this family of materials. The distortion along the stack direction below 235 K is consistent with a doubling along $b$ through the metal-insulator transition. While this would agree with either of the ladder models, the concomitant transverse distortion rules out the rectangular ladder model and supports the zigzag scenario. A mode analysis provides the microscopic basis for this distortion and an estimate for the degree of charge ordering. Intramolecular distortions within the DTTTF building block molecule also give rise to on-site charge asymmetry. [Preview Abstract] |
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