Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P20: Charge Density Waves |
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Sponsoring Units: DCMP Chair: Kim Modic, Max Planck Institute Room: 280 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P20.00001: Charge Order Along Mirror Twin Boundaries in MoSe$_2$ Felix Flicker, Adolfo Grushin, Sebastian Wickenburg, Sara Barja, Alexander Weber-Bargioni One-dimensional systems generically exhibit charge ordering, in which the lattice develops a periodic modulation, through the Peierls instability. The argument assumes an infinite system size. What happens as this assumption is relaxed? Using atomically-resolved scanning tunneling microscopy and atomic force microscopy on semiconducting monolayers of molybdenum diselenide (MoSe$_2$) we see conducting edge states along one-dimensional mirror twin boundaries. Remarkably, these edge states, which are approximately thirty to sixty atoms in length, develop their own energy gap via charge ordering. Self-consistently solving a Hubbard model we demonstrate that these systems exactly at the crossover from charge ordering to quantum well physics, demonstrating features of both simultaneously. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P20.00002: Superconductivity in S-doped TiSe2 Gilberto de la Peña Muñoz, Sangjun Lee, Anshul Kogar, David Casavant, Kannan Lu, Xiaolan Sun, Yizhi Fang, Peter Abbamonte, Gregory MacDougall We have observed superconductivity in resistivity measurements of sulfur-doped TiSe\textunderscore (2-x)S\textunderscore x. We studied the SC phase boundary as a function of S-substitution using X-ray scattering to characterize the charge density wave (CDW) phase in these samples as a function of doping and temperature. A charge density wave peak persists in the highest doped sample studied, at x $=$ 1.09. Our temperature vs. doping phase diagram shows that the SC dome, located at around x $=$ 0.15, is well within the boundary of the charge ordered phase, indicating that the dome is not associated with an amplitude quantum critical point. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P20.00003: Superlattice Formation and Charge Density Waves in TiSe$_{\mathrm{2-x}}$Te$_{\mathrm{x}}$ Aaron Wegner, Junjie Yang, Despina Louca Charge density waves (CDW) are spatial modulations of electron density that are accompanied by a periodic lattice distortion that creates a superlattice. Many layered structures such as transition metal dichalcogenides undergo temperature dependent CDW transitions. TiSe$_{\mathrm{2}}$ has a much studied but still not completely understood transition below 200K. The origin of charge density waves (CDW) in TiSe$_{\mathrm{2}}$ is controversial, with exciton formation and an indirect Jahn Teller effect as leading candidates to be the driving force behind the transition. Neutron scattering experiments were carried out at NOMAD at Oak Ridge National Lab and BT-1 at NIST Center for Neutron Research to investigate the effect of tellurium doping on the CDW transition in TiSe$_{\mathrm{2-x}}$Te$_{\mathrm{x}}$. Finding the composition dependence of the transition is instrumental in understanding what drives the phase change. Using pair distribution function analysis of the local structure, peak splitting shows that the nominal hexagonal unit cell is insufficient to describe the high temperature phase and a monoclinic cell must be used. Rietveld refinement of BT-1 data shows that phase change continues to occur with increased concentrations of Te. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P20.00004: Interplay between charge density wave formation and superconductivity in palladium-intercalated ErTe$_3$ Joshua Straquadine, Ian Fisher The family of layered rare-earth tritellurides are model systems for studying the interplay between incommensurate CDW states and superconductivity. At ambient pressure, ErTe$_3$ exhibits two sequential CDW transitions, each of which partially gaps portions of the Fermi surface. Here we show, via a combination of diffraction and transport measurements, that Pd intercalation uniformly suppresses both CDW transitions, eventually giving rise to superconductivity for $x>0.035$. The resulting phase diagram, and the variation of intensity of the superlattice peaks as a function of temperature and Pd concentration, indicate that disorder induced by the Pd intercalation plays an important role in this behavior. We suggest that this material might be a useful analog of other more complex systems that appear to exhibit short-range CDW correlations and superconductivity in their phase diagrams, possibly including the cuprate high-temperature superconductors. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P20.00005: Surface Strain and Multiple Charge Density Wave States in TbTe$_{\mathrm{3}}$ Ling Fu, Aaron Kraft, Bishnu Sharma, Manoj Singh, Philip Walmsley, Ian Fisher, Michael Boyer We present our near-room temperature scanning tunneling microscopy (STM) measurements on TbTe$_{\mathrm{3}}$. Recent x-ray measurements and DFT calculations [1] indicate that anisotropic lattice strain energy is important in the establishment of a unidirectional CDW along the $c$-axis over the $a$-axis in the bulk below $\sim $336 K. In our STM measurements [2], we detect spatially separated perpendicular unidirectional CDWs established along both the $a-$ and $c-$crystal axes with no directional preference for the in-plane crystal axes ($a$ or $c)$. In addition we find regions where these two unidirectional orders coexist leading to observed bidirectional order. Our measurements indicate that the surface Te layer exposed on cleaving is only weakly coupled to the bulk and that strain variations across the surface drive the specific CDW order(s) observed. Our work suggests that similar mechanisms for CDW formation in the bulk are also involved at the surface. [1] Moore et al., PRB, 93, 024304, 2016. [2] Fu et al., PRB, 94, 205101, 2016. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P20.00006: ARPES study of CDW energy gap in 2H-TaS2 Utpal Chatterjee, Junjing Zhao, Kapila Wijayaratne, Mercouri Kanatzidis We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) study of the electronic structure of 2H-TaS2, a canonical Charge Density Wave (CDW) system. We have mapped its entire Fermi surface, from which it is evident that CDW order in 2H-TaS2 is not due to Fermi surface nesting. Similar conclusion has been reached for 2H-NbSe2 in a number of recent studies. Our measurements have detected particle-hole asymmetric~CDW energy gap around the entire K-centric Fermi surface, however, no energy gap was detected around the Gamma-centric Fermi surface. Furthermore, our temperature-dependent measurement shows that the CDW energy gap persists even above the CDW transition temperature, which is similar to what has been observed in case of~2H-NbSe2. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P20.00007: Out-of-plane dependence of the CDW and superconducting gap in bulk 2{\it H}-NbSe$_2$ revealed by phonon spectroscopy S. Rosenkranz, A.H. Said, F. Weber, R. Heid In 2{\it H}-NbSe$_2$, the CDW energy gap below $T_{CDW}$=33 K opens only on a small part of the Fermi surface in the ($h,k$) plane, which explains why superconductivity can emerge in this system at lower temperature $T_C$=7.2K. However, the interplay between these competing states and their relative evolution upon intercalation or applied pressure remains puzzling. We will discuss our high-energy-resolution inelastic x-ray scattering investigation of the CDW soft phonon mode upon entering the superconducting state. From the observed changes in the phonon lineshapes, we demonstrate that the superconducting gap exhibits an out-of-plane dependence. Reversely, our data imply that the CDW energy gap is strongly localized along $k_z$, a result that could not be obtained with surface techniques, such as ARPES or STM. This confinement of the CDW gap to a very small momentum region, both within as well as out-of the plane, explains the rather low competition and easy coexistence of CDW order and superconductivity in 2{\it H}-NbSe$_2$. Our results further provide a microscopic explanation of the decrease/increase of $T_{C}$/$T_{CDW}$ in single-layer NbSe$_2$ as compared to the bulk system. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P20.00008: Hard X-ray photoelectron spectroscopy of the charge-density wave system Ce$_{\mathrm{3}}$Co$_{\mathrm{4}}$Sn$_{\mathrm{13}}$ A Singh, Y Y Chin, H Y Haung, Y F Liao, K D Tsuei, H J Lin, C N Kuo, H F Liu, C S Lue, C T Chen, D J Huang, A Chainani We study the electronic structure of the charge density wave (CDW ; T$_{\mathrm{CDW}}$ \textasciitilde 155 K) system Ce$_{\mathrm{3}}$Co$_{\mathrm{4}}$Sn$_{\mathrm{13}}$[1,2] using bulk-sensitive Hard X-ray photoelectron spectroscopy (HAXPES ; h$\nu $ \textasciitilde 6.480 keV). We measure the core-levels and valence band spectra of Ce$_{\mathrm{3}}$Co$_{\mathrm{4}}$Sn$_{\mathrm{13}}$ above and below the CDW transition, at T $=$ 170 K and T $=$ 20 K, respectively. While the Ce 3d core-levels show negligible changes, the Co 2p and Sn 3p core-levels show a small but finite increase in the peak-widths and binding energies in the CDW phase. The behavior of the Ce 3d and Co 2p core levels are consistent with a recent x-ray fluorescence study at the Ce L$_{\mathrm{3}}$-edge and Co K-edge[2]. Based on known atomic cross-sections, the HAXPES valence band spectra are not sensitive to Ce 4f partial density of states but show a feature at about 1.5 eV binding energy, which is attributed to Co 3d partial density of states. The valence band spectra show a clear Fermi edge even in the CDW phase, consistent with the metallic behavior observed by electrical resistivity[1]. The results indicate very weak charge-disproportionation in Ce$_{\mathrm{3}}$Co$_{\mathrm{4}}$Sn$_{\mathrm{13}}$ across the CDW transition. References : [1] C. S. Lue et al. Phys. Rev. B 85, 205120 (2012). [2] Y. Otomo et al. Phys . Rev. B 94, 075109 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P20.00009: XAFS Spectroscopy of Structurally Tunable Charge Density Wave Systems Nathan Turner, Sarah Kim, Matthew Marcus, Sirine Fakra, James Brozik, Susan Dexheimer Mixed-valence halide-bridged transition metal linear chain (MX) complexes have proven to be excellent model systems for investigating a range of phenomena inherent to correlated electron systems owing to their structural tunability. In these materials, the nature of the ground electronic state is determined by competing electron-phonon and electron-electron interactions, which can be systematically tuned by varying the metal ion (M) and the halide ion (X) that comprise the linear chain structure. Platinum-halide materials exhibit a Peierls distortion, with alternating Pt-halide bond lengths, and fractional charge states on alternating Pt ions in the chain, giving rise to a charge density wave ground state. Varying the halide controls the strength of the charge density wave and the Peierls distortion. We present oriented Pt LIII EXAFS spectra and FEFF9 modeling of a series of PtX materials with X = Cl, Br, and I. We find distinct photoelectron threshold values for the two inequivalent Pt ions in each of the mixed-valence chains, and find that the difference in threshold values varies with the amplitude of the charge density wave. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P20.00010: Simulations of Photo-induced Nonequilibrium Dynamics in Charge Density Wave Materials Linghua Zhu, Tsezar F. Seman, Michel Van Veenendaal, Ken Ahn We present the results of our simulations for nonequilibrium dynamic of lattice distortion and electronics state in charge density wave materials initiated by optical pulses. We use a two-dimensional AB$_{2}$ square lattice as a model for the charge density wave systems. The coherent dynamics of B ions is considered through the Newtonian equation. The dynamics of electron distribution on A sites due to scattering with other electrons and incoherent phonons is described through the Boltzmann equation. The effect of optical pump is modeled as electron excitations in the initial state. We analyze the coupled dynamics of charge density wave order parameter, coherent lattice distortions and electronic state in photo-induced non-equilibrium states through computational simulations and compare with recent experimental results. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P20.00011: Quantum Time Crystal By Decoherence in Incommensurate Charge Density Wave Ring Keiji Nakatsugawa, Toshiyuki Fujii, Satoshi Tanda A quantum time crystal (QTC)\footnote{F. Wilczek. PRL {\bf109}:160401, (2012)}$^,$\footnote{ T. Li et al. PRL, {\bf109}:163001, (2012)} is a novel quantum mechanical ground state which breaks time-translation symmetry $\mathcal T$. $\mathcal T$ of a system with thermodynamic (infinite-volume) limit is expected to become discrete by spontaneous $\mathcal T$ breaking$^1$, but the possibility to realize such system is still under debate \footnote{P. Bruno. PRL, {\bf111}:070402, (2013)}$^,$\footnote{H. Watanabe and M. Oshikawa. PRL, {\bf114}:251603, (2015)}. Meanwhile, Volovik\footnote{G. E. Volovik. JETP Lett., {\bf98}:491, (2013)} proposes the possibility of an effective QTC, that is, a QTC with a finite size in time. In this presentation we show that $\mathcal T$ of a finite-size system becomes discrete by a ``decoherence measurement", which is best described using an incommensurate charge density wave (ICDW). We use a ring-shaped ICDW threaded by a fluctuating magnetic flux to model an effective QTC with a finite size in space and time: $\mathcal T$ becomes discrete by measuring the ground state of an ICDW ring and the lifetime of the QTC is extended using decoherence\footnote{A. Caldeira and A. Leggett. Physica A, {\bf121}:587, (1983)}. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P20.00012: Ising tricriticality in the extended Hubbard model with bond dimerization Holger Fehske, Satoshi Ejima, Florian Lange, Fabian H. L. Essler We explore the quantum phase transition between Peierls and charge-density-wave insulating states in the one-dimensional, half-filled, extended Hubbard model with explicit bond dimerization. We show that the critical line of the continuous Ising transition terminates at a tricritical point, belonging to the universality class of the tricritical Ising model with central charge c=7/10. Above this point, the quantum phase transition becomes first order. Employing a numerical matrix-product-state based (infinite) density-matrix renormalization group method we determine the ground-state phase diagram, the spin and two-particle charge excitations gaps, and the entanglement properties of the model with high precision. Performing a bosonization analysis we can derive a field description of the transition region in terms of a triple sine-Gordon model. This allows us to derive field theory predictions for the power-law (exponential) decay of the density-density (spin-spin) and bond-order-wave correlation functions, which are found to be in excellent agreement with our numerical results. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P20.00013: Charge density wave order near the non-superconductor to superconductor phase transition in YBa$_2$Cu$_3$O$_{6+x}$ David Hawthorn, Andrew Achkar, Christopher McMahon, Martin Zweibler, Feizhou He, Ronny Sutarto, Ruixing Liang, Doug Bonn, Walter Hardy, Jochen Geck Using resonant x-ray scattering, we extend previous studies of the doping dependence of charge density wave (CDW) order in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) to the heavily underdoped regime. We find that CDW order persists down to low doping ($x=0.335, p = 0.06$), close to the non-superconductor to superconductor phase transition and within a regime where static incommensurate spin density wave (SDW) order has been observed. Notably, at this low doping, the CDW order is unidirectional, propagating along the $b$ axis but not along the $a$ axis. Moreover, we identify a reduction of the $a$-axis correlation length, $\xi_a$, relative to the $b$ axis correlation length, $\xi_b$, at low doping, indicative of a unidirectional character to CDW order in YBCO. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P20.00014: Dynamical scaling of quantum spin liquid in the triangular lattice $\kappa$-(ET)$_2$Ag$_2$(CN)$_3$ Yasuhiro Shimizu, Takaaki Hiramatsu, Mitsuhiko Maesato, Akihiro Otsuka, Hideki Yamochi, Akihiro Ono, Masayuki Itoh, Makoto Yoshida, Masashi Takigawa, Yukihiro Yoshida, Gunzi Saito The static and dynamic spin susceptibilities of quantum spin liquid are investigated in an organic Mott insulator $\kappa$-(ET)$_2$Ag$_2$(CN)$_3$ with a spin-1/2 triangular lattice. The application of negative chemical pressure to $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ allows for extensive tuning of antiferromagnetic exchange coupling, with $J/k_{\rm B} = 175 - 310$ K, under hydrostatic pressure. Based on $^{13}$C nuclear magnetic resonance measurements under pressure, we uncover universal scaling in the static and dynamic spin susceptibilities down to low temperatures $\sim 0.1k_{\rm B}T/J$. The low-lying spin excitations persists in the nuclear spin-lattice relaxation rate and specific heat coefficient, consistent with the presence of gapless low-lying excitations. The results demonstrate fundamental finite-temperature properties of quantum spin liquid in a wide parameter range. [Preview Abstract] |
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