Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D38: Charge Density Waves |
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Sponsoring Units: DCMP Chair: Peter Abbamonte, University of Illinois at Urbana-Champaign Room: F149 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D38.00001: Systematic study of doping effect on the electronic structure of Cu$_{x}$NbSe$_{2}$ Y.K. Kim, Y.Y. Koh, W.S. Jung, G.R. Han, S.R. Park, C.S. Leem, Chul Kim, D.J. Song, W.S. Kyung, H.Y. Choi, L.X. Yang, C. He, F. Chen, D.L. Feng, C. Kim NbSe$_{2}$ is the one of the most studied transition metal dichalcogenide(TMD) materials. It has two phases : superconducting and charge density wave(CDW) phases. The superconductivity in NbSe$_{2}$ is known to be a conventional BCS superconductor. On the other hand, CDW mechanism in NbSe$_{2}$ cannot be explained by the conventional Fermi surface nesting scenario. To find out the exact mechanism of the CDW in NbSe$_{2}$, we try to change the electronic structure of NbSe$_{2}$ by Cu doping. In this presentation, we report the electronic structure studies of the Cu$_{x}$NbSe$_{2}$(x=0, 0.02, 0.04, 0.06, 0.08) measured by angle resolved photoemission spectroscopy (ARPES). We indeed observe doping effects on the electronic structure. The main effect is a Fermi surface topology change, which in turn modifies the nesting condition for CDW phase. To see the nesting condition more clearly, we performed auto-correlation analysis of the data. In addition, we studied the doping and temperature effects on the band renormalization and kink energy. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D38.00002: Dynamics of the hysteretic voltage-induced torsional strain in tantalum trisulfide associated with charge-density-wave depinning J.W. Brill, J. Nichols, C. Sandamali Weerasooriya Crystals of orthorhombic tantalum trisulfide twist (by $\sim $ 1 degree) when voltages near the charge-density-wave depinning threshold are applied. We have studied how this hysteretic voltage-induced torsional strain depends on square-wave and triangle-wave voltages of different frequencies and amplitudes. The strains are measured by placing the sample, with a wire glued to the center as a transducer, in a radio frequency cavity and measuring the modulated response of the cavity. From the triangle waves, we map out the time dependence of the hysteresis loops, and find that the hysteresis loops broaden for waves with periods less than 30 seconds. The square-wave response shows that the dynamic response to positive and negative voltages can differ. The overall frequency dependence is relaxational, but with multiple relaxation times which typically decrease with increasing voltage. The detailed dynamic response is sample dependent, suggesting that it depends on interactions of the CDW with sample defects. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D38.00003: Inelastic X-ray Investigation of the Phonon Softening in NbSe$_{2}$ Frank Weber, Stephan Rosenkranz, John-Paul Castellan, Ray Osborn, Dmitry Reznik, Roland Hott, Rolf Heid, Ayman Said We investigated the phonon softening in the charge density wave compound NbSe$_{2}$ using the high-resolution hard inelastic x-ray scattering beamline 30-ID-C at the Advanced Photon Source, Argonne National Laboratory. The two lowest $\Sigma _{1}$ phonon branches were measured from the zone center $\Gamma $ to the M point at temperatures between 250 K and 7 K across the CDW transition at T$_{CDW}$ = 33 K. Density functional calculations for the lattice dynamical properties which predict an extended phonon breakdown are used to analyze the detailed nature of the softening phonon branch. Work supported by US DOE BES-DMS DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D38.00004: Synchrotron X-ray diffraction study of the CDW correlations in Cu$_x$TiSe$_2$ S. Rosenkranz, J.P. Castellan, F. Weber, G. Karapetrov, R. Osborn Charge density wave order in layered chalcogenides has been studied for several decades, but the nature of the instability driving the transition is still a subject of debate. For Cu$_x$TiSe$_2$, it was reported that the suppression of CDW with Cu doping results in superconductivity, with a phase diagram strongly reminiscent of copper oxide and heavy fermion superconductors. It is however not understood whether or how the order parameters are coupled in this or any other chalcogenide compounds where CDW and superconductivity can coexist. Here, we present our synchrotron X-ray diffraction measurements of the CDW order parameter and the diffuse scattering associated with CDW fluctuations on either side of the alleged quantum phase transition of Cu$_x$TiSe$_2$. We will discus the behavior of the critical exponents and the lengths of the CDW correlations as a function of temperature and doping. Our results indicate that CDW order exists at high doping levels, above the optimal doping for superconductivity, where bulk probes reported absence of CDW formation. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D38.00005: Superconductivity and Excitonic Charge Order Jasper van Wezel, Paul Nahai-Williamson, Siddarth Saxena It was suggested four decades ago that excitons as well as phonons could mediate superconductivity and that the temperature limits usually imposed by phonons could thus be avoided. In practice this form of excitonic pairing turned out to be elusive, because phonon softening typically causes a structural instability to occur before excitonic superconductivity has a chance to arise. Upon suppression of this CDW order however, superconductivity once again has an opportunity to materialise, as has recently been observed in for example pure TiSe$_2$ under pressure. It is unclear what role is played by the excitons in such an environment of critical structural fluctuations, and whether they can have any effect on the pairing or indeed T$_{\mathrm{C}}$. Here we introduce a theoretical model to study the ways in which SC, CDW and excitonic order compete, coexist and even cooperate. Applying the model to TiSe$_2$, we show that the hitherto elusive mechanism driving its CDW transition is a combination of excitonic and Jahn-Teller effects, and that under pressure it is likely to display an unusual type of superconductivity mediated by combinations of excitons and phonons. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D38.00006: Quantum phase transition in 1T-TiSe$_{2}$ studied with high pressure x-ray scattering Y.I. Joe, J.P. Reed, J.C.T. Lee, X. Chen, S.L. Cooper, S. Yuan, W. Turner, P. Abbamonte, K.D. Finkelstein The dichalcogenides exhibit a close competition between charge density wave (CDW) and superconducting (SC) orders. This competition resembles that between ordered stripes and superconductivity in High T$_{c}$ superconductors. In particular, the CDW in 1T-TiSe$_{2}$ has been shown to be suppressed, and SC stabilized, by intercalation of Cu. Raman studies have shown that the CDW order is also suppressed by hydrostatic pressure. Here, we present a detailed study of the suppression of the CDW in 1T-TiSe$_{2}$ with high-pressure x-ray scattering. We found that pressure drives the system to a quantum critical point at P${_c}\sim 4GPa$. The transition was found to be sharp along the temperature axis, but broadened along the pressure axis, suggesting the quantum phase transition is more sensitive to disorder than the classical one. [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D38.00007: Asymmetry of the energy dependence of the charge density wave modulation of {$2H$-NbSe$_2$} Christian Lupien, Behnaz Behmand Theoretical predictions of unconventional charge density waves (CDW) such as pair density waves have a characteristic symmetry of the energy dependence of the local density of state (LDOS) modulations. The scanning tunneling microscopy (STM) and spectroscopy (STS) techniques obtain information related to the LDOS and could extract the symmetry but setpoint effects prevent a direct identification of the theoretical energy symmetry. We have investigated these effects in the conventional CDW compound $2H$-NbSe$_2$ with very low temperature STM/STS. We observed the expected setpoint effects on the LDOS modulation and describe ways to identify it. We also obtain that the CDW modulation in $2H$-NbSe$_2$ seems to be present over a large energy range (up to 140 meV). [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D38.00008: Charge Density Wave Pinning Dynamics in TaS$_2$ J.-D. Su, J. Mohanty, A. R. Sandy, O. G. Shpyrko, M. Sutton Using x-ray photon correlation spectroscopy, we study the dynamic properties of charge density waves (CDWs) in pure and doped 1$T$-TaS$_2$. Using coherent x-ray diffraction, speckles are observed in the first-order impurity-broadened CDW satellite peak. Time dependence of the speckle pattern is observed after a temperature jump (typically 10K). Surprisingly, the speckle pattern displays non-equilibrium relaxation behavior with a characteristic timescale progressively growing with increasing time after the temperature change. We describe a model in which the slow dynamics is associated with rearrangements of the CDW phase under the influence of the randomly distributed pinning sites. We compare our results to descriptions provided by transport and thermodynamic measurements. [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D38.00009: From Mott state to superconductivity in 1T-TaS$_2$ Balazs Sipos, Anna Kusmartseva, Helmuth Berger, Laszlo Forro, Eduard Tutis 1T-TaS$_2$ is a layered transition metal dichalcogenide (TMD). It exhibits a series of charge-density wave phases, including the only reported Mott phase in TMD material. Under high pressure a series of low temperature electronic states appears: the Mott phase melts into a textured CDW; superconductivity develops within the CDW state with $T_c \sim 5$ K, and survives to very high pressures, insensitive to subsequent disappearance of the CDW state and, surprisingly, also the strong changes in the normal state. This is also the first pristine 1T-TMD compound showing superconductivity. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D38.00010: Charge-density-wave and superconducting instabilities in TaS$_2$ and TaSe$_2$ Yizhi Ge, Amy Liu At ambient pressure, 1T-TaS$_2$ and 1T-TaSe$_2$ undergo similar charge-density-wave transitions at low temperature. Recent experiments have further found that 1T-TaS$_2$ becomes superconducting under pressure. Here we present density-functional theory investigations of the CDW and superconducting instabilities in these two materials. The calculations, which accurately capture the instability of the undistorted 1T structure at zero pressure, confirm that pressure suppresses the CDW instability in both materials. The calculated electron-phonon coupling constants in the compressed phases show strong wave-vector dependence and are large enough to suggest that both materials should have superconducting phases under pressure. The relationship between the CDW instability, Fermi surface topology, and electron-phonon coupling will also be discussed. [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D38.00011: Novel Mechanism for the CDW phase transitions in TaSe$_{2}$ Yue Cao, Zhe Sun, Qiang Wang, Helmuth Berger, Daniel Dessau In this talk, we will report our recent study of the classical charge density wave material TaSe$_{2}$. The rich variety of features observed, e.g. partial Fermi surface nesting, inhomogeneous gap opening, pseudogap and soft bosonic modes, suggest the profound underlying physics. Several nesting schemes are put forward in this talk, including the dog bone band nesting with itself, the dog bone with the Gamma pocket and the Gamma pocket with the K pocket. We employ a novel band-by-band technique to analyze the characteristic correlation functions for the CDW phase. We will compare the different nesting possibilities to reveal the driving force behind the normal -- incommensurate CDW -- commensurate CDW phase transitions. Moreover, this discussion will help to improve our understanding of the CDW and of the nesting picture in general. [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 4:54PM |
D38.00012: Raman scattering evidence for a cascade-like evolution of the charge-density-wave collective amplitude mode M. Lavagnini, H.-M. Eiter, L. Tassini, B. Muschler, R. Hackl, R. Monnier, J.-H. Chu, I.R. Fisher, L. Degiorgi The two-dimensional rare-earth tri-tellurides undergo a unidirectional charge-density-wave (CDW) transition at high temperature and, for the heaviest members of the series, a bidirectional one at low temperature. Raman scattering experiments as a function of temperature on DyTe$_3$ and on LaTe$_3$ at 6 GPa provide a clear-cut evidence for the emergence of the respective collective CDW amplitude excitations. In the unidirectional CDW phase, we surprisingly discover that the amplitude mode develops as a succession of two mean-field, BCS-like transitions in different temperature ranges. [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D38.00013: Temperature dependence of the charge-density-wave gap in the rare-earth tritelluride compounds F. Pfuner, M. Lavagnini, J.-H. Chu, I.R. Fisher, L. Degiorgi The layered rare-earth tritellurides $R$Te$_3$ ($R$= Er and Ho) host an unidirectional, incommensurate charge-density-wave (CDW) transition at $T_{CDW1}\sim$ 265 and 280 K and a further transition to a bidirectional CDW state at $T_{CDW2}\sim$ 160 and 120 K for the Er and Ho compound, respectively. We present optical reflectivity data collected as a function of temperature over a very broad energy interval, ranging from the far-infrared up to the ultraviolet. We extract the temperature dependence of the CDW gap and compare it with our previous results on the whole rare-earth series ($R$=La, Ce, Pr, Nd, Sm, Gd, Tb and Dy) as a function of chemical and externally applied pressure. We provide clear-cut evidence that upon destroying the CDW state with increasing temperature and pressure there is a progressive closing of the CDW gap excitation. [Preview Abstract] |
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