Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C17: Charge Density Waves: Low Dimensional and Organic Conductors |
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Sponsoring Units: DCMP Chair: Eva Andrei, Rutgers University Room: 316 |
Monday, March 14, 2016 2:30PM - 2:42PM |
C17.00001: Enhancement of charge ordering by dynamic electron-phonon interaction Andrej Singer, Eric Fullerton, Oleg Shpyrko Symmetry breaking and emergence of order is one of the most fascinating phenomena in condensed matter physics and leads to a plethora of intriguing ground states such as in antiferromagnets, Mott insulators, superconductors, and density-wave systems. Exploiting non-equilibrium dynamics of matter following ultrafast external excitation can provide even more striking routes to symmetry-lowered, ordered states, for instance, by accessing hidden equilibrium states in the free-energy landscape or dynamic stabilization of non-equilibrium states. This is remarkable because ultrafast excitation typically creates disorder, reduces the order parameter, and raises the symmetry. Here, we demonstrate for the case of antiferromagnetic chromium that moderate photo-excitation can transiently enhance the charge-density-wave (CDW) order by up to 30{\%} above its equilibrium value, while strong excitation leads to an oscillating, large-amplitude CDW state that persists above the equilibrium transition temperature. Both effects result from dynamic electron-phonon interaction, which provides an efficient mechanism to selectively transform a broad excitation of the electronic order into a well defined, long-lived coherent lattice vibration. This mechanism may be exploited to transiently enhance the order parameter in other systems with coupled electronic and lattice orders. The data was collected at the x-ray free electron laser LCLS at SLAC. [Preview Abstract] |
Monday, March 14, 2016 2:42PM - 2:54PM |
C17.00002: Transport studies in the incommensurate charge density wave series RTe$_3$ Philip Walmsley, Simon Aeschlimann, Paula Giraldo Gallo, Ian Fisher The quasi-2D rare-earth tritelluride compounds (RTe$_3$: R=La-Tm) are a model series in which to study incommensurate charge density waves (iCDWs), with the interplay between Fermi surface nesting and electron-phonon coupling forming an open and lively area of research. The slight orthorhombicity in the 2D Te bilayer that forms the Fermi surface favors a single-domain unidirectional iCDW along the c-axis, with a second, perpendicular unidirectional iCDW forming at lower temperatures in the heavier members (R=Tb-Tm). It remains unclear how the lower temperature iCDW disappears with rare earth substitution (chemical pressure) and whether there is an associated quantum phase transition. We present recent transport measurements that study the evolution of the two iCDWs as they are tuned across the enormous phase-space offered by these compounds, with a particular focus on the in-plane anisotropy and Fermi-surface geometry. [Preview Abstract] |
Monday, March 14, 2016 2:54PM - 3:06PM |
C17.00003: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 3:06PM - 3:18PM |
C17.00004: Temperature-Dependent Studies of Charge Density Wave States in TbTe$_{\mathrm{3}}$ Michael Boyer, Aaron Kraft, Ling Fu, Bishnu Sharma, Ian Fisher We use temperature-dependent scanning tunneling microscopy (STM) to study charge density wave (CDW) states in TbTe$_{\mathrm{3}}$. TbTe$_{\mathrm{3}}$ undergoes a bulk CDW transition near 335 K, though x-ray data shows evidence for CDW fluctuations up to 363 K.[1] Our STM measurements characterize the well-established, long-range, unidirectional CDW state (q$_{\mathrm{cdw}}$ $=$ 0.71 c*) at 300 K. Our temperature-dependent measurements above T$_{\mathrm{CDW}}$ show evidence for localized static CDW order which is consistent with x-ray detection of CDW fluctuations above T$_{\mathrm{CDW}}$. Surprisingly, we also find evidence for localized static order associated with a second CDW along the a-axis, a CDW state which never establishes long-range coherence in the bulk of TbTe$_{\mathrm{3}}$ even at low temperatures. [1] Ru et al., PRB 77, 035114 (2008). [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:30PM |
C17.00005: Doping-induced Charge-Density-Wave Atsushi Nomura, Kazuhiko Yamaya, Shigeru Takayanagi, Koichi Ichimura, Toru Matsuura, Satoshi Tanda Doping is a useful method for searching new characters in solids, as we can see in the discoveries of impurity semiconductors and high-temperature superconductors. If a Charge-Density-Wave (CDW) is induced in materials which do not exhibit a CDW, new CDW properties might be brought there. TaSe$_{3}$ exhibits no CDW transition but a superconductivity transition at about 2 K while it has a quasi-one-dimensional chain structure as well as typical CDW conductors, NbSe$_{3}$, TaS$_{3}$, and NbS$_{3}$. Therefore, TaSe$_{3}$ is one of the suitable materials for the induction of a CDW by doping, and we tried to induce a CDW in TaSe$_{3}$ by doping Cu. Cu concentration was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The high Cu concentration was consistent with the high value of residual resistance ($R(4.5\, \mathrm{K})/(R(280\, \mathrm{K})-R(4.5\, \mathrm{K}))$). Single-crystal X-ray diffraction pattern (XRD) showed an expansion of the $c$-axis in Cu-doped TaSe$_{3}$. The temperature dependence of the resistivity showed the anomaly at 80-100 K in Cu-doped TaSe$_{3}$, which was never observed in pure TaSe$_{3}$. These results suggest that the Cu-doping induces a CDW. We will discuss the relation between the resistivity anomaly and superconductivity. [Preview Abstract] |
Monday, March 14, 2016 3:30PM - 3:42PM |
C17.00006: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 3:42PM - 3:54PM |
C17.00007: Observation of an Excitonic Sound Wave in TiSe2 with meV-resolution EELS Anshul Kogar, Melinda Rak, Sean Vig, Ali Husain, Young Il Joe, Peter Abbamonte The charge density wave (CDW) in TiSe2 has been attributed to an excitonic instability by many authors. In a conventional CDW material, there exists a soft phonon at the transition temperature, which, below the transition temperature, gives way to the phase and amplitude collective excitations of the CDW order parameter. In TiSe2, a soft phonon has indeed been observed with inelastic X-ray scattering. In the 1960s, though, W. Kohn predicted that one of the signatures of an excitonic instability would be the presence of a soft electronic excitation which similarly gives way to an excitonic sound mode in the condensed phase. In this talk, I will present data showing that the TiSe2 exhibits a collective excitation consistent with the excitonic sound wave prediction, which emerges out of the normal phase plasmon. This provides strong evidence that excitonic correlations play a role in the CDW formation in TiSe2. Hence, a more nuanced view of the charge density wave transition in TiSe2 is needed where both excitonic effects and electron-phonon coupling must be taken into consideration. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C17.00008: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 4:06PM - 4:18PM |
C17.00009: Decay of Bloch oscillations in the charge-density-wave ordered phase of an all electronic charge density wave state Oleg Matveev, Andrij Shvaika, Thomas Devereaux, James Freericks The charge-density-wave phase of the Falicov-Kimball model displays a number of anomalous behavior including the appearance of subgap density of states as the temperature increases. These subgap states should have a significant impact on transport properties, particularly the nonlinear response of the system to a large dc electric field. Using the Kadanoff-Baym-Keldysh formalism, we employ nonequilibrium dynamical mean-field theory to exactly solve for this nonlinear response. We examine both the current and the order parameter of the conduction electrons as the ordered system is driven by a dc electric field. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C17.00010: Ground state in \kappa-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Br studied by Raman Spectroscopy and Heat Capacity measurements N. Hassan, S. A. Turunova, E.I. Zhilyaeva, R.N. Lyubovskaya, N. Drichko Quasi-two-dimensional organic conductor \kappa-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Br is a Mott insulator ($T$_{c}$\approx 100K$) on a triangular lattice which makes it a potential spin liquid candidate. To elucidate its magnetic ground state we study heat capacity and Raman response of single crystals of this material. Our low temperature heat capacity measurements suggest a presence of a linear term in the temperature dependence, which might indicate the existence of gapless spinons. Vibrational Raman response indicates a presence of charge order fluctuations in the insulating state. The low-frequency Raman response is discussed in terms of fluctuations of “paired electron crystal’’ state [1]. [1] S. Dayal, R. T. Clay, H. Li, and S. Mazumdar, Phys. Rev. B 83, 245106 (2011) [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C17.00011: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 4:42PM - 4:54PM |
C17.00012: Raman scattering in an anisotropic triangular spin lattice system Hideo Kishida, Yuto Nakamura, Kazushi Mizukoshi, Yukihiro Yoshida, Gunzi Saito Spin-disordered quantum phases in an anisotropic triangular spin lattice system, $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$B(CN)$_{\mathrm{4}}$, were recently reported [1]. In this compound, the ratio of the two transfer integrals, ${t}'/t$, reaches 1.44 at 298 K and 1.80 at 100 K. Its optical conductivity in the infrared region is anisotropic. The temperature dependence of the optical anisotropy correlates with that of ${t}'/t$. From the experimentally evaluated optical anisotropy, we expect that the values of ${t}'/t$ are larger than 1.80 in the lower temperature region. For this compound, we observe the polarization-dependent broad Raman scattering signals below 600 cm$^{\mathrm{-1}}$ at 10 K. In such a wavenumber region, we have observed the magnetic Raman signals in triangular spin lattice systems such as $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$X [2] and ${\beta }'$-type Pd(dmit)$_{\mathrm{2}}$ salts [3]. By comparison with them, we discuss the origin of the Raman signals observed for $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$B(CN)$_{\mathrm{4}}$. [1] Y. Yoshida \textit{et al.}, Nat. Phys. \textbf{11}, 679 (2015). [2] Y. Nakamura \textit{et al.}, J. Phys. Soc. Jpn. \textbf{83}, 074708 (2014). [3] Y. Nakamura\textit{ et al.}, J. Phys. Soc. Jpn. \textbf{84}, 044715 (2015). [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C17.00013: Strong light-field effects in correlated oraganic conductors. Shinichiro Iwai, Yohei Kawakami, Yota Naitoh, Hirotake Itoh, Sumio Ishihara, Kenji Yonemitsu Optical responses of organic conductors have attracted much attentions, because they exhibit ultrafast solid-state phase transitions in the conducting and/or dielectric natures upon photo-excitations. In this decade, photoinduced melting of correlated insulators with clear charge gap have been extensively investigated. On the other hand, optical rsponses of correlated metal has not been studied well. Here, we describe a charge localization induced by the 9.3 MV/cm instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor alpha- (bis[ethylenedithio]-tetrathiafulvelene)2I3. A large reflectivity change of 30 percent and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the 10 percent reduction of t driven by the strong, high-frequency electric field. Furthermore, the contribution of Coulomb repulsion will be discussed on the basis of the polarization dependence of the pump light and the theory. [Preview Abstract] |
Monday, March 14, 2016 5:06PM - 5:18PM |
C17.00014: Dielectric Properties of Organic Charge-Transfer Salts J. K. H. Fischer, P. Lunkenheimer, S. Krohns, R. S. Manna, B. Hartmann, H. Schubert, M. Lang, J. M\"uller, J. A. Schlueter, C. M\'{e}zi\`{e}re, P. Batail, A. Loidl The BEDT-TTF-based charge-transfer salts have attracted considerable attention due to their often intriguing dielectric properties. An example is $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. It was recently found to exhibit multiferroicity, for which a new electric-dipole driven mechanism was proposed [1]. The polar moment in this system was suggested to arise from the dimerization of the BEDT-TTF molecules, combined with charge order. Another interesting recent example is $\alpha$-(BEDT-TTF)$_2$I$_3$, which shows the signature of relaxor-ferroelectric behavior [2]. Here, we will present an overview of the dielectric properties of the above systems and provide new results on $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, which also seems to show relaxor-ferroelectric behavior in its charge-ordered state. In addition, we present measurements of $\delta$-(EDT-TTF-CONMe$_2$)$_2$Br. This compound lacks dimerization, but exhibits charge order already at room temperature. $[1]$ P. Lunkenheimer \textit{et al.}, Nat. Mater. \textbf{11}, 755 (2012). $[2]$ P. Lunkenheimer \textit{et al.}, Phys. Rev. B \textbf{91}, 245132 (2015). [Preview Abstract] |
Monday, March 14, 2016 5:18PM - 5:30PM |
C17.00015: Anisotropic transport and structure of single-crystal molybdenum bronze, Li$_{0.33}$MoO$_{3}$ Saeed Moshfeghyeganeh, Joshua L. Cohn, John J. Neumeier We present transport measurements (resistivity, thermopower, thermal conductivity) on single crystals of the quasi-one-dimensional (Q1D), small-gap semiconductor$^a$ Li$_{0.33}$MoO$_{3}$ in the temperature range 150-500 K. The Q1D character of this material is reflected in $T=300$~K resistivity ratios, $\rho_c$:$\rho_a$:$\rho_{b^*}\simeq$~1:20:180, and extreme anisotropy in the Seebeck coefficient within the $a-c$ planes, $S_c-S_a\simeq 250\ \mu$V/K. A weak structural anomaly near $T_s=355$~K (0.001\AA\ expansions along $c^*$ and $b^*$ directions, comparable contraction along $a^*$) is identified in the temperature-dependent lattice constants from x-ray diffraction, and is coincident with changes in the transport coefficients. Analysis of the transport data at $T>T_s$ shows that an intrinsic semiconductor model can be applied to explain transport along the most conducting $c$ axis, but along $a$ and $b^*$ the transport is better described by a non-adiabatic, small-polaron picture. \vskip .1in \noindent $^a$ B. T. Collins {\it et al.}, J. Sol. St. Chem. {\bf 76}, 319 (1988). [Preview Abstract] |
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