Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y19: Charge Density Wave Order |
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Sponsoring Units: DCMP Chair: David Hawthorn, University of Waterloo Room: 321 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y19.00001: Specific heat studies of the chiral phase transition in charge ordered 1T-TiSe$_{2}$ Xu Luo, J.-P. Castellan, S. Rosenkranz, R. Osborn, Q. Li, G. Karapetrov, J.P.C. Ruff, U. Welp, J. Van Wezel We use high-resolution steady-state ac-micro-calorimetry to investigate the transition of 1T-TiSe$_{2}$ into the charge-ordered state. A mean-field like step of $\sim$ 0.4 J/molK in the specific heat C(T) near 193 K signals the transition into the commensurate CDW state. Upon further cooling, C(T) varies linearly in temperature until near 180 K a clear break in the slope of C(T) by 13mJ/molK$^{2}$ and possibly a small step indicate a second phase transition. Comparisons with theoretical predictions based on the Ginzburg-Landau free energy, with resistivity measurements, and with x-ray diffraction indicate that, at this transition, the commensurate CDW state changes into a helically ordered state along the crystal c-axis. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y19.00002: Electronic Properties of Cu$_x$TiSe$_2$ Single Crystals Petra Husanikova, Jan Fedor, Jan Derer, Vladimir Cambel, Goran Karapetrov We investigate the normal state and superconducting properties of 1T-TiSe$_2$ family of single crystals intercalated with different level of copper content. Magnetoresistance and Hall effect data indicate that 1T-TiSe$_2$ is a compensated narrow band-gap semiconductor or semimetal with small number of electron and hole carriers. We compare the influence of copper intercalant and titanium interstiatials on the temperature evolution of charge density waves via resistivity and Hall effect measurements. Our findings indicate that the origin of the charge density waves in 1T-TiSe$_2$ is due to the combination of exciton and Jahn-Teller mechanisms. At higher copper concentrations we investigate the superconducting properties of Cu$_x$TiSe$_2$ in overdoped regime and find that the system is a single-gap strongly type-II superconductor with in-plane Ginzburg-Landau parameter reaching 50. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y19.00003: The effect of dimensionality on the charge-density-wave phase in layered dichalcogenides Darshana Wickramaratne, Pradyumna Goli, Alexander Balandin, Roger Lake Transition-metal dichalcogenides exhibit a variety of conducting phases, which includes a charge-density wave state (CDW). Exfoliation of these layered materials allows the effect of dimensionality on the CDW state to be studied. CDW collective states are currently being considered as an alternative state variable for information processing [1]. 2H-TaSe$_{2}$ and 1T-TiSe$_{2}$ are examples of layered transition metal dichalcogenides that undergo a CDW transition. Our recent experiments demonstrated an increase in the CDW transition temperature of TiSe$_{2}$ with a decrease in film thickness [1]. This increase in temperature was attributed to the negative coefficient of the CDW transition temperature-pressure relationship. Here we present a density-functional theory investigation of the CDW instability in bulk, single and few-layer 1T-TiSe$_{2}$ and other layered dichalcogenide materials. The effect of the film thickness on the atomic structure, electronic structure, electron-phonon coupling and the CDW transition temperature will be discussed for each material. \\[4pt] [1] Goli, P., Khan, J., Wickramaratne, D., Lake, R. K., \& Balandin, A. A. (2012). Charge Density Waves in Exfoliated Films of Van der Waals Materials: Evolution of Raman Spectrum in TiSe$_{2}$. Nano Letters. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y19.00004: Charge-Density Wave and Superconducting Dome in TiSe$_2$ from Electron- Phonon Interaction Matteo Calandra, Mauri Francesco At low temperature TiSe2 undergoes a charge-density wave instability. Superconductivity is stabilized either by pressure or by Cu intercalation. We show $[1]$ that the pressure phase diagram of TiSe2 is well described byfirst-principles calculations. At pressures smaller than 4 GPa charge-density wave ordering occurs, in agreement with experiments. At larger pressures the disappearing of the charge-density wave is due to a stiffening of the short-range force constants and not to the variation of nesting with pressure. This suggests a common origin of the charge density waves instability in transition metal dichalcogenides, as also demonstrated in previous works by first principles calculations on bulk and few layers NbSe2 $[2]$. In TiSe2, the behavior of Tc as a function of pressure is entirely determined by the electron-phonon interaction without need of invoking excitonic mechanisms. Our work demonstrates that phase diagrams with competing orders and a superconducting dome are also obtained in the framework of the electron-phonon interaction.\\ $[1]$ M. Calandra and F. Mauri, PRL 106, 196406 (2011)\\ $[2]$ M. Calandra, I. I. Mazin and F. Mauri, PRB 80, 241108(R) (2009) [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y19.00005: A quantum phase transition from triangular to stripe charge order in NbSe$_2$ Eric Hudson, Anjan Soumyanarayanan, M. M. Yee, Yang He, D. J. Rahn, K. Rossnagel, Jasper van Wezel, M.R. Norman, Jennifer E. Hoffman We use scanning tunneling microscopy to reveal a previously unknown unidirectional (stripe) charge density wave (CDW) smoothly interfacing with the familiar tridirectional (triangular) CDW on the surface of the stoichiometric superconductor NbSe$_2$. Our low temperature measurements rule out thermal fluctuations, and point to local strain as the tuning parameter for this quantum phase transition. We use this discovery, in conjunction with bandstructure calculations, to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe$_2$. First, the 15$\%$ wavelength difference between the two CDWs demonstrates that Fermi surface nesting plays a minor role in determining the CDW wavevectors in NbSe$_2$. Second, we disentangle a $\Delta \sim$12 meV particle-hole asymmetric CDW gap from a spectrum dominated by collective modes, resolving a longstanding debate regarding anomalous gaps previously observed by STM and ARPES. Our results highlight the importance of local strain in governing phase transitions and competing phenomena, and suggest a new direction of inquiry for resolving similarly longstanding debates in cuprate superconductors and other strongly correlated materials. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y19.00006: Role of impurities in the charge density wave state of transition metal dichalcogenides Junichi Okamoto, Andrew Millis Motivated by recent scanning tunneling microscope (STM) measurements of NbSe$_2$ which revealed the formation of charge density wave (CDW) droplets around impurities even at temperatures of the order of three times the transition temperature [1], we present a theory of impurity-induced CDW formation, and examine its consequences for the thermodynamic phase transition and low temperature ordered phases. Our fits to the STM measurements suggest that the CDW is strongly pinned by impurities, so that a standard theory predicts that even at lowest temperature the material should be in the disordered phase. We present a new theoretical picture explaining how to reconcile the experimental observation of a sharp transition with the strong pinning. [1] S. P. Chockalingam $et$ $al.$ (submitted to PNAS) [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y19.00007: Electronic Structure and Charge-Density Wave Instabilities in Monolayers of Transition Metal Dichalcogenides Pierre Darancet, Andrew J. Millis, Chris A. Marianetti Transition metal dichalcogenides (TMDC) are layered materials displaying a variety of charge-density wave (CDW) instabilities and complex phase diagrams for group IV \& V transition metals. Recent progress in mechanical exfoliation and device fabrication now allow for electrical characterization and gating of individual, 3-atom thick layers [1] of TMDCs, providing new probes of the complex many-body interactions arising in these compounds. In this talk, I will present our investigations using density functional and dynamical mean-field theory regarding the electronic structure and electronic correlations arising in distorted monolayers, bilayers, and trilayers of octahedral group V TMDCs. We will examine the importance of doping, crystal fields, and many-body interactions, and their influence on the transport and optical properties of these materials upon distortion. [1] K. S. Novoselov et al., PNAS 102, 10451 (2005) . [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y19.00008: Role of Disorder in Atomic Scale Onset of Charge Density Waves Erick Andrade, Carlos Arguello, Ethan Rosenthal, Subbaiah Chockalingam, Luiyan Zhao, Christopher Gutierrez, Woo Chung, Wencen Jin, Po-Chun Yeh, Tonica Valla, Rafael Fernandes, Shuang Jia, Richard Osgood, Andrew Millis, Robert Cava, Abhay Pasupathy How does strong disorder affect the electronic states of complex electronic materials? This question is of relevance to many quantum materials such as the cuprates and pnictides, where interesting electronic phases like superconductivity only arise in strongly disordered samples. The study of these materials is complicated by the presence of multiple electronic phases, which obscures the interpretation of local spectroscopic measurements. To gain insight into this problem, we study 2H-NbSe$_{2}$, a relatively simple material with a 2D charge density wave ground state. To tune the disorder in the sample, we use sulfur substitution to go from weak (in pristine NbSe$_{2})$ to strong disorder (in NbSe$_{2-x}$S$_{x})$. We use variable-temperature scanning tunneling microscopy and spectroscopy to visualize the electronic structure in real space. Strong changes in the local electronic spectrum are observed with the introduction of disorder, with a pseudogap appearing in the local density of states. We also observe strong changes in the quasiparticle interference from spectroscopic images. We will discuss the interpretation of quasiparticle interference in the limit of strong disorder, and its relevance to existing measurements in the cuprates and pnictides. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y19.00009: Charge Density Wave Disproportionation in Pd(III)-containing PdTeI Patrick Cottingham, John Sheckelton, David Miller, James Neilson, Tyrel McQueen Exotic electronic properties in strongly correlated materials often emerge from the interplay of structure and charge. In most Pd$^{3+}$-containing materials, Pd$^{3+}$ statically disproportionates into Pd$^{2+}$ (d$^{8}$) and Pd$^{4+}$ (d$^{6}$) with square planar and octahedral geometries, respectively. However, high-resolution diffraction data acquired for PdTeI indicate exclusively octahedral coordination of the Pd species within this compound. Temperature-dependent electrical resistivity measurements of this material performed in our lab show a hysteresis between T$_{CDW1}$ $\sim$ 120 K and T$_{CDW2}$ $\sim$ 50 K, indicative of a first-order phase transition. The most likely origin of this anomaly is the formation of a CDW involving partial, dynamic charge disproportionation of Pd$^{3+}$. In addition, low-temperature diffraction data show a broadening of Bragg peaks on cooling which is indicative of strain or of disorder concomitant with disproportion. In this presentation the temperature dependencies of the magnetic susceptibility, heat capacity, and electronic properties of PdTeI will be discussed in the context of CDW formation. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y19.00010: Short-range CDW correlations in Co$_{x}$NbSe$_{2}$ and Mn$_{x}$NbSe$_{2}$ J. Lee, R. Di Capua, G. Karapetrov, T. Nishizaki, N. Kobayashi, M. Iavarone Scanning tunneling microscopy and transport measurements were performed on NbSe$_{2}$ and Co- and Mn-intercalated NbSe$_{2}$ single crystals, to address the effect of disorder induced on the CDW structure by the effect of intercalation. We find that the CDW transition at T$_{CDW}$=33 K in the pure compound is accompanied by a small anomaly in resistivity, a strong non linearity of the Hall effect, with a sign reversal occurring at CDW transition, and high magnetoresistance in agreement with previous reports. The system remains metallic below the CDW transition. Upon increase of disorder the anomaly in resistivity moves at a lower temperature and eventually disappears for higher doping levels. By increasing the disorder also the magnetoresistance decreases and the Hall effect does not show any sign reversal. STM measurements on a pure sample reveal that CDW phase is long-range ordered below T$_{CDW}$. For doped samples short range CDW correlations dominate a large part of the phase diagram. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y19.00011: Optical Excitation Spectrum in Ni- and Cu-doped ZrTe$_3$ Chiara Mirri, Adam Dusza, Leonardo Degiorgi, Cedomir Petrovic We report on an optical study performed on Cu$_x$ZrTe$_3$ and Ni$_x$ZrTe$_3$ single crystals. ZrTe$_3$ was previously found to display a BCS-like CDW-gap opening in the optical spectra along the direction orthogonal to the Zr-chains and to undergo a filamentary superconducting transition below a T$_c$ of about 2 K. The intercalation by Cu and Ni between the ZrTe$_3$ layers partially fills the CDW gap and induces bulk superconductivity coexisting with the CDW state below T$_c$. Here we show the effect of Ni and Cu intercalation on the reflectivity and optical conductivity above and below the CDW phase-transition temperature. Furthermore, we analyze the optical spectral weight, providing equivalent information in both compounds about the partial gapping of the Fermi surface and the overall redistribution of spectral weight across the CDW phase transition. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y19.00012: Interplay between electron-electron and electron-lattice interactions in the RTe3 compounds Alexander Kemper, H.-M. Eiter, M. Lavagnini, R. Hackl, E.A. Nowadnick, T.P. Devereaux, J.-H. Chu, J.G. Analytis, I.R. Fisher, L. Degiorgi Charge and spin density waves, periodic modulations of the electron and magnetization densities, respectively, are among the most abundant and non-trivial low-temperature ordered phases in condensed matter. The ordering direction is widely believed to result from the Fermi surface topology. However, several recent studies indicate that this common view needs to be supplemented. Here, we show how an enhanced electron-lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge density wave (CDW) system ErTe3. We show how the electron-phonon coupling in the vicinity of band degeneracy points is strongly enhanced, leading to a CDW direction that is different from that determined by first-principles calculations. This combination of electron-electron and electron-lattice interactions may be generally relevant for driving phase transitions in other broken-symmetry ground states. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y19.00013: Charge density wave formation in multi-band systems Rudi Hackl, Hans-Martin Eiter, Michela Lavagnini, Elizabeth A. Nowadnick, Alexander F. Kemper, Thomas P. Devereaux, Jiun-Haw Chu, James G. Analytis, Ian R. Fisher, Leonardo Degiorgi Charge and spin density waves are among the most abundant low-temperature ordered phases in condensed matter. The Fermi surface topology is widely believed to determine the ordering direction. However, several recent experimental and theoretical studies show that nesting is only one out of various other driving forces behind these instabilities. We use Raman scattering to demonstrate in which way an enhanced electron-lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge density wave (CDW) system ErTe$_3$ and other rare-earth tri-tellurides. In our joint experimental and theoretical study we exploit the symmetry properties of the electron-photon and electron-phonon coupling vertex and establish a relation between the selection rules of the electronic light scattering spectra and the enhanced electron-phonon coupling in the vicinity of band degeneracy points. The proposal shown here for CDW formation, may be of more general relevance in multi-band systems for driving phase transitions into other broken-symmetry ground states. For example, the iron-based superconductors exhibit a similar phenomenology close to the intersection points of the backfolded electron bands. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y19.00014: Charge-density wave transitions of rare-earth tritellurides investigated by femtosecond electron crystallography Tzong-Ru Han, Zhensheng Tao, Subhendra D. Mahanti, Kiseok Chang, Chong-Yu Ruan, Christos D. Malliakas, Mercouri G. Kanatzidis The electron-phonon mechanism that gives rise to various charge-ordered systems depends on the topology of the Fermi surface that is subjective to the influence of hybridization, nesting, and electron correlation at low dimensions. Rare-earth tritellurides are ideal systems to investigate the two-dimensional charge-density density wave (CDW) formation as both nesting and hybridization are at play to select the unidirectional CDW at different temperatures. Using fs electron crystallography, we investigate the noncooperative suppression of the structural order parameters following ultrafast electronic quenching and correlate electronic and ionic evolutions based on a framework of three-temperature model and nonisotropic fluctuational analysis. We show that a joint consideration of the couplings between the lattice phonons, the CDW collective modes, and the corresponding electronic subsystem is required to account for the various novel structural dynamics features. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y19.00015: 1/f noise anomalies in nanoribbons of charge density wave materials Zhenzhong Shi, Adam Stabile, Peter M. Marley, Sarbajit Banerjee, Ganapathy Sambandamurthy Charge density wave (CDW) as an ordered form of matter has attracted attention for many decades. Below a critical temperature (T$_{P}$), CDW materials undergo a Peierls transition and enter the CDW ground state, where the energy is minimized by a collectively pinning mechanism. Under a moderate electric field, CDWs can be depinned and they start sliding. An onset of a large broad band noise (BBN) has been observed in bulk CDW materials as a signature of this depinning process. We report low frequency conductance fluctuation (1/f noise) measurements on single nanoribbon devices of single-crystalline NbSe$_{3}$, across both Peierls transitions. In the CDW state, a non-monotonic behavior in the noise magnitude was observed when approaching the threshold electric field for depinning: while increasing voltage from the zero-bias limit, the magnitude of BBN first decreases before increasing sharply near the threshold voltage. This unusually large BBN magnitude and the non-monotonic behavior below the depinning threshold suggest some inherent instability that could be suppressed by a small bias field, and is clearly different from results from bulk materials. Transport and noise studies from individual nanoribbons of NbSe$_{3}$, Ta-doped NbSe$_{3}$ and o-TaS$_{3}$ will be presented. [Preview Abstract] |
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