Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T46: Charge Density Waves |
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Sponsoring Units: DCMP Chair: Makariy Tanatar, Ames National Laboratory Room: Mile High Ballroom 4E |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T46.00001: Theory of charge-density-wave non-contact nanofriction Erio Tosatti, Franco Pellegrini, Giuseppe E. Santoro Bulk dissipation caused by charge-density-wave (CDW) voltage-induced depinning and sliding is a classic subject. We present a local, nanoscale mechanism describing the occurrence of distance-dependent dissipation in the dynamics of an atomic force microscope tip oscillating over the surface of a CDW material. A mechanical tip hysteresis is predicted in correspondence to localized 2 slips of the CDW phase, giving rise to large tip dissipation peaks at selected distances. Results of static and dynamic numerical simulations of the tip-surface interaction are believed to be relevant to recent experiments on the layer compound NbSe . [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T46.00002: Probing Charge Density Wave Dynamics using Coherent X-ray Scattering A. Palmer, T.F. Rosenbaum, Yejun Feng, A. Sandy X-ray photon correlation spectroscopy (XPCS) is a coherent x-ray scattering technique able to detect equilibrium fluctuations of ordered states. We examine the equilibrium fluctuations of the two-dimensional incommensurate charge density wave in NbSe2 at the thermal approach to the transition from below. The temporal correlation of these fluctuations contains information about CDW dynamics. The CDW scattering intensity in this experiment is much lower than traditional XPCS measurements, necessitating the development of a new methodology to extract dynamics information from weak scattering signals. Generalization of this method may make possible the extraction of scaling relations near quantum critical points. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T46.00003: Experimental evidence for a Bragg glass density wave phase in a transition-metal dichalcogenide Jun-ichi Okamoto, Carlos Arguello, Ethan Rosental, Abhay Pasupathy, Andrew Millis We show that the spatial dependence of current-voltage characteristics obtained by scanning tunneling microscopy indicates that the charge density wave occurring in NbSe$_2$ is subject to locally strong pinning arising from a non-negligible density of impurities. However, on the length scales accessible in this experiment, the material is found to be in a Bragg glass phase where dislocations and anti-dislocations occur in bound pairs; free dislocations are not observed. We present calculations based on a Landau theory which explain how strong local modulations may produce only a weak long range effect on the CDW phase. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T46.00004: Strong-coupling charge order in NbSe$_2$ Felix Flicker, Jasper van Wezel The emergence of charge density wave (CDW) order in NbSe$_2$ has been surrounded by controversy ever since its discovery several decades ago. Because of the absence of any clear nesting in the Fermi surface, various alternative driving forces for CDW formation have been suggested, from nested saddle points to phonon-driven scenarios. Recently, the availability of high-precision experimental data has raised additional questions: different experimental techniques observe different electronic gap sizes, the gap itself has been reported to be asymmetric and centered well above the Fermi energy, and unexpected local fluctuations of the charge order have been observed far above the critical temperature. We resolve all of these seemingly conflicting observations in a model that takes into account both the electronic structure and the strong, momentum-dependent, coupling to phonon modes. We show that this model can explain the recent controversial observations by scattering and ARPES experiments as well as by local probes like scanning tunneling spectroscopy. This model provides for the first time a consistent description of the entire range of experimental results and presents a complete picture of the CDW in NbSe$_2$ as a prototypical example of strong-coupling charge order. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T46.00005: Analyzing topological defects in disordered charge density waves in transition-metal dichalcogenides TaSe$_2$ and TaS$_2$ using scanning tunneling microscopy Danielle Schaper, Kyle McElroy, Eduardo Calleja, Jixia Dai, Lijun Li, Wenjian Lu, Yuping Sun, Xiangde Zhu Charged ordered states are becoming a common feature in the phase diagrams of correlated materials. In many cased there are indications that doping controlled quantum critical points between the CO state and others are related to interesting properties including superconductivity. An interesting test case is the ordered 2D CDW found in the transition metal dichalcogenides. We performed an analytical study on the dichalcogenides tantalum disulfide (TaS$_2$) and tantalum diselenide (TaSe$_2$) to observe how CDWs present in the material can be melted as disorder is introduced into the system via copper doping. Data was taken using a scanning tunneling microscope (STM) below the transition to the CDW state, both with and without copper dopants added. The resulting topographs were then analyzed to investigate the relationship between the phase and the amplitude of the disordered CDW. We found that the copper doping caused disorder in the CDW state characterized by phase wanderings and 2$\pi$ phase winding ``point defects'' in the CDW not present in the undoped parent compound. The locations of these point defects and windings were, in turn, found to have the characteristics of topological defects. Implications for studies of other disordered CO states seen in STM will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T46.00006: Charge-density waves competitions in 1T-TaS$_{2}$ and ErTe$_{3}$ investigated by femtosecond electron crystallography Faran Zhou, Tzong-Ru Han, Zhensheng Tao, Jenni Portman, Dat Do, Subhendra Mahanti, Phillip Duxbury, Chong-Yu Ruan, Christos Malliakas, Mercouri Kanatzidis Competitions between different lattice- and charge-ordered states in two-dimensional materials can lead to strongly first order phase transitions. In 1T-TaS$_{2}$, the phase transitions are primarily driven by strong electron correlations and Fermi surface nesting, but between the Mott insulating ground state and the high-temperature incommensurate charge-density wave (CDW) there exists a near-commensurate phase characterized by unique domain structures, where their long-range coherence and pseudo-gap property are currently under debates. Using femtosecond electron crystallography, we resolved the domain proliferation dynamics and the distinctly different characters of electronic phase transitions and CDW restructuring. We also compare our results with the CDW competitions in the weakly correlated system ErTe$_{3}$. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T46.00007: Role of the electron phonon coupling in pump-probe experiments on 1T-TaS$_2$ Jenni Portman, Bin Hwang, Dat Thanh Do, Faran Zhou, Tzong-Ru Terry Han, Chong-Yu Ruan, S.D. Mahanti, Phillip Duxbury 1T-TaS$_2$ is a transition-metal layered compound that shows unique electronic properties and phase transitions, including charge density wave (CDW) formation. By performing pump-probe experiments, the CDW state can be suppressed and its recovery can be investigated as a function of time delay and laser parameters. We perform density functional theory calculations of the band structure of 1T-TaS$_2$ to explain the microscopic quantum mechanical origin of this behaviour. By calculating the phonon band structure and the phonon-electron coupling, we quantify the contribution of lattice relaxation at various laser frequencies. We also perform calculations of the absorption spectrum using time-dependent DFT and show how an interplay of the phonon and electron degrees of freedom can explain the different timescales and structural properties involved in the response of CDW materials to laser excitations. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T46.00008: Finite size effects in electrical transport and noise measurements in mesoscopic NbSe$_{3}$ nanobeams Ali Alsaqqa, Shi Zhenzhong, Sujay Singh, David Wilson, Katie Farley, Sarbajit Banerjee, G. Sambandamurthy NbSe$_{3}$ is a transition metal trichalcogenide system exhibiting two charge density wave (CDW) transitions at 59 K and 141 K. At temperatures below the transition, the CDW state is pinned by residual disorder and a finite electric field can depin and slide the CDW. In this study, individual nanobeams of single-crystalline NbSe$_{3}$ are used in a muli-terminal device configuration to study the effects of finite length (from few $\mu$m to hundreds of $\mu$m) on the physical properties near the CDW transitions. Transport and ultra low frequency (less than 1 Hz) noise measurements are carried out as functions of temperature, device length and electric field across the thermal-driven CDW transitions and across electric-field induced depinning of the CDW state. The dependence of the depinning threshold electric field on device length is found to be different than in bulk samples thereby underlying the presence of finite size effects. The dependence on device length of the noise magnitude across the CDW transition will also be presented and the implications of these results in understanding the pinning/depinning transition in finite size samples of NbSe$_3$ will be discussed. The work is supported by NSF DMR 0847324. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T46.00009: Low frequency noise behavior in mesoscopic charge density wave conductors of o-TaS$_{3}$ and NbSe$_{3}$ Zhenzhong Shi, Sujay Singh, Katie Farley, Peter Marley, Sarbajit Banerjee, G. Sambandamurthy In quasi-one dimensional materials, charge density waves (CDW) often form as a result of an instability of the Fermi surface below a critical temperature (T$_{P}$). In the presence of disorder, CDW is pinned. As a result, fully gaped materials like o-TaS$_{3}$ exhibit an insulator-like behavior below T$_{P}$ and partially gaped materials like NbSe$_{3}$ show mixed signatures of both CDW and ungapped quasi-particles. A sufficient dc electric field can depin and slide the CDW. CDW phase fluctuation and phase slippage in the pinned state can be detected as resistance noise in an appropriate frequency window. Herein, results from electrical transport and low frequency noise measurements on single crystalline o-TaS$_{3}$ nanoribbons will be presented and compared with results on single-crystalline NbSe$_{3}$ nanoribbons. Interesting features in the differential conductance measurements across the electric field-driven depinning transitions in the nanoscale samples are observed. The noise magnitude, in the CDW pinned state, shows a non-monotonic dependence on driving electric field in NbSe$_3$ whereas in o-TaS$_{3}$ a monotonic dependence is observed. Results will be discussed in light of the differences in these materials and any possible finite size effects. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T46.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T46.00011: Raman study of KNi$_2$Se$_2$ and KNi$_2$S$_2$: an origin of re-entrant transition in KNi$_2$Se$_2$ Natalia Drichko, James Neilson, Tyrel McQueen The unusual phenomena of an increase in symmetry upon cooling due to a re-entrant transition can be associated with electronic correlations. In KNi$_2$Se$_2$ our vibrational Raman spectroscopy study identifies regular Ni-atoms displacements, which disappear below approximately 50 K resulting in an increase of symmetry of the unit cell. At low temperatures heavy fermion behavior with m$_{eff}$ of about 20m$_e$ is observed [1]. To find the origin of this untypical high-temperature behavior, we compare our results on KNi$_2$Se$_2$ with that of the sister-compound KNi$_2$S$_2$ [2], where Raman spectroscopy does not observe clear evidence of the high-temperature symmetry breaking, but the heavy fermion effect is still present. \\[4pt] [1] J. R. Neilson et al. Phys. Rev. B (2012), 86, 054512.\\[0pt] [2] J. R. Neilson et al. Phys. Rev. B (2013), 87, 045124 [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T46.00012: Local increase of symmetry on cooling in KNi$_2$Se$_2$ James Neilson, Natalia Drichko, Anna Llobet, Mahalingham Balasubramanian, Matthew Suchomel, Tyrel McQueen Materials with the ThCr$_2$Si$_2$-type structure host myriad examples of many-body physics, including high-temperature superconductivity and heavy fermion behavior. In these compounds, the emergence of the collective electronic state frequently occurs near a magnetic instability, suggesting that magnetic fluctuations underlie the electronic phenomena. I will provide evidence for similar many-body physics in the structurally related, but non-magnetic compound, KNi2Se2. KNi$_2$Se$_2$ exhibits an increase of symmetry on cooling below $T\le 50$ K, as observed by Raman spectroscopy and high-resolution synchrotron x-ray diffraction. X-ray absorption spectroscopy confirms that the symmetry increase is due to changes in nickel-nickel interactions and suppression of charge density wave fluctuations. Density functional theory calculations reveal a zone-boundary lattice instability that provides a model of the room-temperature x-ray pair distribution function data, but fails to describe the higher local symmetry observed for $T\le 50$ K. Together, these results support many-body correlation effects as drivers for the unusual heavy fermion electronic ground state in KNi$_2$Se$_2$. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T46.00013: Coexistence of Chiral Charge Density Wave and Superconductivity in Cu$_x$TiSe$_2$ G. Karapetrov, P. Husanikova, V. Cambel, P. Szabo, P. Samuely, J. Fedor, M. Iavarone We investigate bulk superconducting properties and atomic scale scanning tunneling microscopy and spectroscopy in Cu$_x$TiSe$_2$. We map the vortex phase diagram and find unusually broad vortex liquid regime for such a low-$T_c$ superconductor. STM measurements reveal coexistence of chiral charge density wave and superconductivity. We find that the amplitude of charge density wave modulation is strongly suppressed with respect to strongly underdoped case ($x<0.06$) with the chiral domain size remaining the same. Superconductivity exhibits BCS character at variety of dopings with $2\Delta/kT_c\sim 3.6\div3.7$ indicating an intermediate coupling strength. Application of the external magnetic field introduces the Abrikosov vortex lattice that is weakly pinned. The size of the vortex core extracted from vortex images corresponds to the one extracted from the upper critical field. Our results suggest that, if charge density wave quantum critical point exist, it should be well above the optimal copper concentration of x=0.08. [Preview Abstract] |
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