Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S46: Organic Conductors and Related Topics |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Claude Bourbonnais, Universite de Sherbrooke Room: Mile High Ballroom 4E |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S46.00001: The role of electron-phonon interaction in a magnetically driven mechanism for superconductivity Claude Bourbonnais, Hassan Bakrim We use the renormalization group method to examine the effect of phonon mediated interaction on d-wave superconductivity driven by spin fluctuations in a quasi-one-dimensional electron system. The influence of a tight-binding electron-phonon interaction on the spin-density-wave and d-wave superconducting instability lines is calculated alongside its effect on the amplitude of spin correlations in the normal phase for arbitrary phonon frequency and antinesting of the Fermi surface. We show the existence of a positive isotope effect for spin-density-wave and d-wave superconducting critical temperatures that scales with the antinesting distance from quantum critical point where the two instabilities merge. We also study the electron-phonon strengthening of spin fluctuations at the origin of extended quantum criticality in the metallic phase above superconductivity. The impact of our results on quasi-one-dimensional organic conductors like the Bechgaard salts where a Peierls distortion is absent and superconductivity emerges near a spin-density-wave state under pressure is emphasized. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S46.00002: Ultrafast phase transitions in (TMTTF)2AsF6 and alpha-(ET)2I3 driven by infrared 1.5 cycle CEP stabilized pulse Shinichiro Iwai, Takahiro Ishikawa, Tuto Sagae, Hirotake Itoh, Kaoru Yamamoto, Takahiko Sasaki Recent progress of several 10 fs laser enables us to capture the coherent dynamics of the correlated electrons leading to the photoinduced phase transition. In this study, coherent 18 fs oscillations of the correlated CO electrons were captured in the 1D chain salts (TMTTF)2AsF6 and in layered organic salts alpha-(ET)2I3 by using the 1.5 optical-cycle (7 fs carrier envelope phase (CEP) stabilized) IR(1.7 micron) pulse. We observed the build up time of 50 fs for the coherent oscillation in CO and metallic phases. In CO phase, photoinduced CO melting is driven by the electron oscillation in (TMTTF)2AsF6. In the metallic phase of alpha-(ET)2I3, the coherent oscillation of the correlated charge triggers the metal to insulator transition which decays within 100 fs. Such ultrafast metal to insulator transtion can be detected only for the strong excitation condition. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S46.00003: Understanding the interplay between charge, spin and phonons in the spectral properties of the 1D Hubbard-Holstein model Mohammad Soltanieh-ha, Alberto Nocera, Adrian Feiguin We present an analytical construction to calculate the spectral functions of the Hubbard-Holstein model in the limit of strong electron-phonon coupling, and in the limit of U $\rightarrow \infty$ . We argue that in this limit, the phonons only couple to the charge, and not the spin. The resulting spectral function can be understood as a convolution of three contributions, originating from the charge, the spin, and the phonons, in a similar fashion as the large U limit of the Hubbard chain. We support the analytical results with extensive Density Matrix Renormalization Group simulations. We recognize and interpret the signatures of the three contributions in the final spectrum and we discuss their experimental implications. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S46.00004: Raman scattering study of electric-field-induced change of charge distribution in BEDT-TTF dimer compounds Hideo Kishida, Yuma Hattori, Satoshi Iguchi, Takahiko Sasaki, Shinichiro Iwai, Hiromi Taniguchi Some BEDT-TTF dimer-Mott insulators show novel dielectric properties resulting from the positional degree of freedom for charge within a dimer. We have performed the microscopic Raman spectroscopy of a dimer-Mott insulator, ${\beta}'$-(BEDT-TTF)$_{2}$ICl$_{2}$. The charge sensitive vibrational mode is a single peak without electric field, while, by applying electric field beyond a threshold value, two side peaks appear on the low-wavenumber and high-wavenumber sides of the original position. This implies that an imbalance of charge within a dimer occurs. Moreover, we measured the electric-field dependence, the positional dependence and the temperature dependence of the electric-field-induced Raman signals. The positions of the side peaks are not so largely influenced by the electric-field intensity. The side-peak intensity shows a significant positional dependence. We will discuss the nature of the electric-field-induced change of the charge distribution in BEDT-TTF dimer-Mott insulators. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S46.00005: Ultrafast photo-excited dynamics in two-dimensional charge ordered systems Hiroshi Hashimoto, Hiroaki Matsueda, Hitoshi Seo, Sumio Ishihara Charge-order (CO) is one of the central subjects in strongly correlated electron system such as transition metal oxides and organic salts. Recently the photo-induced non-equilibrium states of the CO states have been studied in both theories and experiments intensively. The femto-second spectroscopies show a variety of exotic phenomena, for example ultra-fast insulator-metal transition accompanied by melting of CO. In order to clarify ultra-fast photo-induced phenomena in CO materials, we study theoretically a real-time dynamics of a two-dimensional spinless fermion model by using the exact-diagonalization method in finite size clusters. This is the simplest theoretical model to describe CO and its melting. We calculate the real time-dependence of the charge-correlation function and other several physical quantities. We find reduction of the charge correlation for an initial CO pattern, and emergence of the correlation for other types of COs', that is, a photo-induced CO phase transition. Transient stripe-type CO correlations strongly depend on the light polarization of the pump photon. We identify mechanism of these exotic photo-induced phenomena, and discuss its implications to other correlated electron model and real materials. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S46.00006: NMR studies of phase transition form a metallic state to a Dirac-electron state in the organic system, $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ Kazuya Miyagawa, Michihiro Hirata, Kyohei Ishikawa, Tomotaka Taniguchi, Masafumi Tamura, Kazushi Kanoda The Dirac electron phase is realized in the bulk organic systems, $\theta$ and $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$. The bulky nature of the system allows one to study the Dirac electrons in the spin degrees of freedom by means of NMR (K. Miyagawa et al. JPSJ {\textbf 79}, 063703 (2010)). Moreover, in $\theta$-(BEDT-TTF)$_{2}$I$_{3}$, the Dirac electron phase neighbors a metallic (superconducting) phase in a pressure-temperature phase diagram. To clarify how the Dirac phase emerges from the metallic state, we performed $^{13}$C NMR measurements for this material at ambient and under pressures. The angular dependence of NMR spectra demonstrates that all the molecules are equivalent (Hirata et al. PRB {\textbf 85}, 195146, (2012)). The temperature dependences of Knight shift and spin-lattice relaxation rate, 1/$T_{1}$, hold the Korringa relation, which signifies metallicity under pressures before the transition to the Dirac phase. However, after the system undergoes a transition to the Dirac electron state, the NMR spectral shape becomes complicated indicating a structural phase transition. The analysis of the angular dependence of the NMR spectra shows the molecular arrangement changes from theta to alpha type. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S46.00007: Effects of Charge Fluctuations on Massless Dirac Fermions in Organic Conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ Taro Kanao, Hiroyasu Matsuura, Masao Ogata A quasi-two-dimensional organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ has attracted much interest both for its charge ordering (CO) transition and for its remarkable transport properties at high pressure which appear when the transition suppressed. The latter has been revealed to be due to massless Dirac fermions (MDFs) at the Fermi energy, by measurements such as transport, NMR, and specific heat. Recently, the MDF phase has been re-examined, and some behaviors beyond non-interacting MDFs has been reported. In transport measurements, the resistivity shows a logarithmic increase at low temperatures. Also, NMR measurements show deviations from the non-interacting behaviors. The cause of these behaviors has not been clarified. Since the transition between the MDF phase and the CO phase is almost continuous, charge fluctuations are important there. As a cause of the behaviors above, we investigate effects of charge fluctuations using a minimal model for this system. We analyze this model by a self-consistent renormalization (SCR) theory, which can deal with effects of fluctuations precisely. On this basis, electric resistivity (or damping rate), specific heat, and one-particle density of states are calculated. Relevance to the experimental facts is discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S46.00008: Phase Diagram of the 2D Fermionic Ring Exchange Only Model Katharine Hyatt, Bryan K. Clark, Matthew P.A. Fisher There has been significant interest in understanding non-Fermi liquid phases. Recently, DMRG studies on 2-leg ladders have suggested the presence of such a phase (the $d$-wave metal) in the $t-J-K$ model on a 2D square lattice, where $K$ is a nearest neighbor ring exchange term. The fermion sign problem generically prevents Monte Carlo studies of this model on larger systems. However, in the $t=J=0$ limit, the Hamiltonian becomes sign free. Using Green's function Monte Carlo, we investigate the phase diagram of this ring-exchange only fermionic model as a function of density. In this talk, we report our findings. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S46.00009: Polarization dependence of wide-range Raman scattering spectra in $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ Yuto Nakamura, Naoki Yoneyama, Takahiko Sasaki, Arao Nakamura, Hideo Kishida We measured Raman scattering spectra of $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$, which is a quantum spin-liquid candidate, over a wavenumber range, 25-3200 cm$^{-1}$. This compound shows a relaxor-like dielectric response that could originate in the charge disproportionation within each BEDT-TTF dimer. The Raman spectra are composed of not only sharp vibrational modes but some broad structures. One broad component is observed below 700 cm$^{-1}$ in cross polarization configuration. It is assigned to a magnetic excitation and has a significant intensity at as low as 25 cm$^{-1}$ at 10 K, which results from a spin frustration effect. Moreover, some of the vibrational Raman lines below 120 cm$^{-1}$, which are assigned to intermolecular vibration, show a similar temperature dependence with the dielectric constant. This might indicate that the charge instability leading to the relaxor-like dielectric behaviors affects the Raman signals for molecular motions. We will discuss the magnetic excitation and the charge disproportionation in $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ in terms of wide-range Raman scattering spectra. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S46.00010: Effect of randomness on dielectric property in dimer-type organic salts Makoto Naka, Sumio Ishihara Electronic ferroelectricity is known as phenomena where electric polarization is attributed to charge order without inversion symmetry. Organic material kappa-(ET)$_{2}$Cu$_{2}$(CN)$_{3\, }$is a candidate of the electronic ferroelectric material. The ET molecule dimers are arranged on a two dimensional triangular lattice. Recently, a dielectric anomaly is experimentally observed around 30K. An origin of this anomaly is supposed to be electronic dipoles generated by asymmetric electronic charge distribution in the ET dimers. The observed broad peak structure and dispersion in the dielectric constant suggest that a relaxor-like state is realized in this material. Motivated by these experimental results, we study theoretically effects of randomness on dielectric properties in a dimer-Mott insulator. We analyze an low-energy effective model where electric dipoles inside the ET molecules interact with each other under random electric field. This model is analyzed by using the cluster mean-field approximation. With increasing the random field, phase transition occurs from a ferroelectric charge ordered phase to a charge glass phase. This transition is first- (second-) order in low (high) temperature. A tricritical point exists on the transition line. In the charge glass phase near the tricritical point, the dielectric susceptibility shows broad peak structure. We also find a spin-charge glass phase due to the random electric field and spin-charge coupling. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S46.00011: Flow equation approach to one-body and many-body localization Victor Quito, Paraj Bhattacharjee, David Pekker, Gil Refael We study one-body and many-body localization using the flow equation technique applied to spin-1/2 Hamiltonians. This technique, first introduced by Wegner, allows us to exact diagonalize interacting systems by solving a set of first-order differential equations for coupling constants. Besides, by the flow of individual operators we also compute physical properties, such as correlation and localization lengths, by looking at the flow of probability distributions of couplings in the Hilbert space. As a first example, we analyze the one-body localization problem written in terms of spins, the disordered XY model with a random transverse field. We compare the results obtained in the flow equation approach with the diagonalization in the fermionic language. For the many-body problem, we investigate the physical properties of the disordered XXZ Hamiltonian with a random transverse field in the z-direction. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S46.00012: Many body localization in a quantum Ising model: A numerical study Jonas Kjall, Jens Bardarson, Frank Pollmann Closed correlated quantum systems with disorder can experience many-body localization. These systems do not thermalize and the properties of the individual finite energy eigenstates become important. Recently, Huse et. al. concluded that eigenstates with broken symmetry order the quantum system, even at energy densities where the corresponding thermal system is disordered. We perform a detailed exact diagonalization study of a random Ising chain with a short ranged interaction between the excitations. We find signatures of the three predicted localized phases. One is paramagnetic and the two others have a broken Z2 symmetry with spin-glass order. These last two can further be distinguished by spectral properties. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S46.00013: Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons Dmitri Efremov, Joseph Betouras, Andrey Chubukov We discuss the form of the damping of magnetic excitations in a metal near a ferromagnetic instability. The paramagnon theory predicts that the damping term should have the Landau form $\gamma(q,\omega) \propto \omega/v_F q$. However, the experiments on uranium metallic compounds UGe$_2$ and UCoGe showed non-Landau damping $\gamma (q,\omega) \propto \omega/\Gamma$, with $\Gamma = const$ for small $q$. It would violate the spin conservation in systems with one type of fermions. Recently it has been conjectured that this non-Landau damping can arise due to the presence of both localized and itinerant electrons in these materials, with ferromagnetism involving predominantly localized spins. We present microscopic analysis of the damping of near-critical localized excitations due to interaction with itinerant carriers. We show explicitly how the presence of two types of electrons breaks the cancelation between the contributions to $\Gamma $ from self-energy and vertex correction insertions into the spin polarization bubble. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S46.00014: Finite size scaling in crossover among random matrix ensembles describing interacting lattice systems Subroto Mukerjee, Ranjan Modak We study the crossover among different random matrix ensembles [Poissonian, Gaussian Orthogonal Ensemble (GOE), Gaussian Unitary Ensemble (GUE) and Gaussian Symplectic Ensemble (GSE)] realized in different microscopic models of interacting and disordered lattice systems.. We find that the perturbation causing the crossover among the different ensembles scales to zero with system size as a power law with an exponent that depends on the ensembles between which the crossover takes place. This exponent is independent of microscopic details of the perturbation. We also find that the crossover from the Poissonian ensemble to the other three is dominated by the Poissonian to GOE crossover which introduces level repulsion while the crossover from GOE to GUE or GOE to GSE associated with symmetry breaking introduces a subdominant contribution. We also conjecture that the exponent is dependent on whether the system contains interactions among the elementary degrees of freedom or not and is independent of the dimensionality of the system. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700