Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J11: Correlated Organic Conductors |
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Sponsoring Units: DMP Chair: Claude Bourbonnais, Universite de Sherbrooke Room: Colorado Convention Center Korbel 1F |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J11.00001: Mixed spin-charge solitons and thermodynamics of (TMTTF)$_2$X Sumit Mazumdar, R. Torsten Clay, R. P. Hardikar The (TMTTF)$_2$X salts are quasi-one-dimensional materials that undergo two phase transitions as the temperature is lowered from 300 K under ambient pressure. The high temperature transition at T$_{CO}$ $\sim$ 100 K is to a charge-ordered (CO) state. The low temperature transition is often to a spin-Peierls (SP) state that appears at T$_{SP}$ $\sim$ 10 K, and that competes with the CO state. We have investigated the thermodynamics of these systems within an extended Hubbard Hamiltonian that includes (a) on-site and nearest neighbor Coulomb interactions, and (b) bond- and site-coupled quantum phonons. From calculations of charge, bond and spin-susceptibilities we are able to explain the transition from the CO to the SP state. The CO state corresponds to the charge occupancy scheme ...1010... (where `1' and `0' denote charge-rich and charge-poor sites respectively), while the SP state has charge occupancy ...1100.... The transition from the CO to the SP phase as temperature is lowered is driven by spin effects: At high temperatures, high-spin states dominate the free energy, and favor the ...1010... CO configuration. At low temperatures, spin singlet states dominate the free energy and instead favor a singlet SP state with the ...1100... charge pattern. In the temperature region T$_{SP} < T < T_{CO}$ there occur mixed spin-charge solitons that are domain walls between the ...1010... and ...1100... charge patterns. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J11.00002: Finite-Temperature Phase Transitions in Quasi-One-Dimensional Molecular Conductors Hitoshi Seo, Yukitoshi Motome, Takeo Kato Phase transitions to symmetry-broken states in quarter-filled quasi-one-dimensional molecular conductors, such as DCNQI$_2X$, TMTTF$_2X$, and EDO-TTF$_2X$, are studied theoretically. We consider extended Hubbard chains with on-site, intra-chain, and inter-chain Coulomb interactions, coupled to the lattice degree of freedom by Peierls-type and Holstein-type electron-lattice interactions. We apply the numerical quantum transfer-matrix method to an effective one-dimensional model, treating the inter-chain term within mean-field approximation. Finite-temperature properties are investigated for the charge ordering, the dimer-type Mott transition (bond dimerization), and the spin-Peierls transition (bond tetramerization), by computing the temperature dependences of the order parameters together with those of the charge and spin susceptibilities. A coexistent state of charge order and bond dimerization exhibiting dielectricity is predicted in a certain parameter range, even when intrinsic dimerization is absent. Reference: H. Seo, Y. Motome and T. Kato, preprint(cond-mat/0611499) to be published in J. Phys. Soc. Jpn. {\bf 76} (2007) No. 1. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J11.00003: Soliton Wall Superlattice Phase in Organic Conductor (Per)$_{2}$Pt(mnt)$_{2}$ in a Magnetic Field Si Wu, Andrei Lebed We suggest a model [1] to explain the appearance of a high resistance high magnetic field charge-density-wave (CDW) phase, discovered in quasi-one-dimensional (Q1D) organic conductor (Per)$_{2}$Pt(mnt)$_{2}$. In particular, we show that the Pauli spin-splitting effects improve the nesting properties of a realistic Q1D electron spectrum, and, therefore, a high resistance Peierls CDW phase is stabilized in high magnetic fields. In intermediate and very high magnetic fields, a periodic soliton wall superlattice (SWS) phase is found to be a ground state. We suggest to study the predicted phase transitions between the Peierls and SWS CDW phases to discover a unique SWS state. [1] A.G. Lebed and Si Wu, Physical Review Letters, submitted (2006). [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J11.00004: Nature of superconducting state in the new phase of (TMTSF)$_{2}$PF$_{6}$ under pressure Lev Gor'kov, Pavel Grigoriev The unusual phase has been recently observed in the organic material (TMTSF)$_{2}$PF$_{6}$, where superconductivity coexists with spin-density wave in the pressure interval p$_{c1}<$p$<$p$_{c}$ below the first order transition into superconducting or normal metal phase. Assuming that the coexistence takes place on the microscopic scale, we consider the properties of the intermediate phase. We show that the new superconducting state inside spin-density wave phase just above p$_{c1}$ must bear a triplet pairing. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J11.00005: Angular magnetoresistance oscillations in quasi-one-dimensional organic conductors in the presence of a crystal superstructure Anand Banerjee, Victor Yakovenko Crystal superstructures, produced by anion ordering in the quasi-one-dimensional organic conductors $\rm(TMTSF)_2ReO_4$ and $\rm(TMTSF)_2ClO_4$, modify electron spectra in these materials and generate effective tunneling amplitudes between distant chains. These amplitudes cause multiple peaks in the interlayer conductivity for the magnetic field orientations along the rational crystallographic directions (the Lebed magic angles). The different wave vectors of anion ordering in $\rm(TMTSF)_2ReO_4$ and $\rm(TMTSF)_2ClO_4$ result in the odd and even Lebed angles, as observed experimentally.\\ Reference: cond-mat/0608317. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J11.00006: Angular and Temperature Dependent $^{77}$Se NMR in the Metallic and Field-Induced Spin Density Wave State in (TMTSF)$_{2}$ClO$_{4}$ Lloyd Lumata, Phil Kuhns, Arneil Reyes, James Brooks We present an exploratory investigation of the NMR pulse-power and magnetic field direction dependence of the $^{77}$Se NMR line shapes and relaxation rates in the metallic and field-induced spin density wave (FISDW) state of the quasi-one-dimensional organic conductor (TMTSF)$_{2}$ClO$_{4}$. By reducing the integrated NMR pulse power (via width and/or pulse height), the limitations of the enhancement factor below the FISDW transition are overcome, and the $^{77}$Se spin-lattice relaxation rate 1/T$_{1}$ can be measured in both the metallic and FISDW states vs. temperature and field direction. Our results on the temperature dependence of 1/T$_{1 }$in the vicinity of the FISDW transition, and also a description of the temperature and field direction dependence of the NMR spectra, will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J11.00007: Superconductivity in (TMTSF)$_2$ClO$_4$ probed by $^{77}$Se NMR J. Shinagawa, Y. Kurosaki, S. E. Brown, D. Jerome, J. B. Christensen, K. Bechgaard Superconductivity in the Bechgaard salts (TMTSF)$_2$X, with X=PF$_6$, ClO$_4$, survives well beyond the paramagnetic limit set by the transition temperature $T_c\approx$1K. As a result, it has been hypothesized that the spin pairing is triplet. We report on measurements of the $^{77}$Se Knight shift and spin-lattice relaxation rate $T_1^{-1}$, conducted {\it in situ} with interlayer resistivity, deep within the superconducting state of (TMTSF)$_2$ClO$_4$. At fields $H_0\approx$10kOe aligned along the $\mathbf{a}-$ and $\mathbf{b'}-$axes, the Knight shift reveals a decrease in spin susceptibility $\chi_s$ that is likely consistent with singlet pairing. The field dependence of $T_1^{-1}$ at temperatures $T\ll T_c$ exhibits a very sharply-defined increase at a field $H_s\approx$15kOe. For $H_0>H_s$, $T_1^{-1}$ is close to the normal state value, even though $H_{c2}\gg H_s$ and $R_{zz}=0$ to within experimental uncertainty. We discuss the implications for interpreting the results as evidence for a crossover, or a phase transition within the superconducting state. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J11.00008: Angular Dependent Magnetoresistance Measurements of (Per)$_{2}$Au(mnt)$_{2}$ Under Pressure D. Graf, E.S. Choi, J.S. Brooks, M. Almeida The quasi-one-dimensional organic conductor (Per)$_{2}$Au(mnt)$_{2}$ has a charge density wave (CDW) ground state when cooled below a transition temperature of T$_{CDW} \quad \sim $ 12 K under ambient pressure. The CDW state is largely suppressed by applying a pressure of $\sim $ 6 kbar, as shown by a dramatic increase in low temperature conductivity where the behavior remains slightly activated, providing evidence of a mixed CDW-metal state. Oscillations under pressure are observed in the magnetoresistance (MR) which agree well with band structure estimates of the Fermi surface and are explained by Stark quantum interference. The angular dependence of the MR oscillations has been studied using the continuous rotation of a pressure cell in constant magnetic fields, aligned with the crystallographic planes of the sample. The results will be discussed within the context of known MR angular effects (i.e. Lebed or Danner-Kang-Chaikin oscillations) as well as the inhomogenous CDW-metal state which may exist in this pressure range. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J11.00009: Phase diagram of pressure-induced superconductor $\beta $-(BDA-TTP)$_{2}$\textit{MX}$_{4}$ ($M$=Fe, Ga and $X$=Cl, Br) with localized magnetic moments E.S. Choi, D. Graf, T. Tokumoto, J.S. Brooks, Jun-ichi Yamada We have investigated transport and magnetization properties of $\beta $-(BDA-TTP)$_{2}$\textit{MX}$_{4}$ ($M$=Fe, Ga and $X$=Cl, Br) as a function of pressure, temperature and magnetic field. The title material undergoes metal-insulator transitions above 100 K at ambient pressure. The insulating phase is suppressed with pressure and superconductivity eventually appears above $P_{c}$= 4.5 kbar ($X$=Cl) and 13 kbar ($X$=Br). The general temperature-pressure (\textit{TP}) phase diagram is similar each other, while higher pressure is required for $X$=Br compounds to suppress the insulating state and induce the superconductivity. Pressure dependent DC magnetization studies on $\beta $-(BDA-TTP)$_{2}$FeCl$_{4}$ compound revealed that the AFM ordering persist well above $P_{c}$. In spite of similarity of phase diagram between $M$=Fe and $M$=Ga compounds, magnetoresistance results show distinct behaviors, which indicates the magnetic interaction with the conduction electrons are still effective. The comparison between $X$=Cl and $X$=Br compounds suggests the anion-size effect rather than the existence of localized magnetic moments plays more important role in determining the ground state. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J11.00010: High field ESR study of the \textit{pi}-$d$ interaction effect in beta-(BDA-TTP)$_{2}$MCl$_{4 }$(M=Fe, Ga) Takahisa Tokumoto, J. vanTol, L.-C. Brunel, E.S. Choi, J.S. Brooks, T. Kaihatsu, H. Akutsu, J. Yamada Novel magnetic organic conductors with \textit{pi}-$d$ interaction have commanded attention since the discovery of field induced superconductivity. One of them, beta-(BDA-TTP)$_{2}$FeCl$_{4}$, has alternating donor molecules and quasi 2$D$ electrical properties. Previous studies of electrical and magnetic properties show an M-I transition at 120K and an AF transition at $T_{N}$=8.5K, suggesting an exchange interaction between the conduction electrons and the Fe$^{3+} \quad d$-electrons. The properties of beta-(BDA-TTP)$_{2}$GaCl$_{4}$ are similar with exception of the absence of the AF transition, which is apparently due to the absence of \textit{pi}-$d$ exchange interaction. We report angular/temperature dependent 240GHz quasi optical ESR measurements on both compounds to probe the magnetic properties. The Ga compound signals follow the donor molecule structure, and show no magnetic order at any temperature. The Fe compound signals are quite different from the Ga compound, and exhibit AF behavior below $T_{N}$. The difference of Fe and Ga compounds will be discussed in terms of the interaction between localized and itinerant magnetic moments. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J11.00011: Metastable domains at the pressure induced neutral-ionic transition of TTF-CA Zoltan Soos, Anna Painelli Tetrathiafulvalene-Chloranil (TTF-CA) is the prototypical organic charge transfer (CT) salt whose neutral-ionic and dimerization (Peierls) transitions have been studied on cooling or under pressure. Volume changes switch the ground state from a band insulator with a fractional CT from TTF to CA of $\rho \sim$ 0.3 in a regular stack to a Mott insulator with $\rho >$ 0.5 in a dimerized stack. TTF-CA spectra indicate electron-vibration coupling to both lattice (e-ph) and molecular (e-mv) modes that lead to competing instabilities. Near the metallic point of the rigid system, a one-dimensional adiabatic Hubbard model with linear e-ph and e-mv coupling leads to metastable domains with different $\rho$, $\rho'$ that are thermally accessible at 300 K over a limited bistability range. The transition of TTF-CA single crystals at 1 GPa indicates a pressure range with two resolved $\rho$, $\rho'$. The model also describes the first order transition at 81 K at ambient pressure and generates anharmonic potential energy surfaces. These quantum transitions are driven by volume changes without contributions from electronic excited states. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J11.00012: Mapping the temperature-dependent quasiparticle scattering rate over the Fermi surface of an organic superconductor John Singleton, Paul Goddard, Arzhang Ardavan, Stephen Blundell, Ross McDonald, Stan Tozer, Amalia Coldea, John Schlueter The interlayer magnetoresistance $\rho_{zz}$ of the organic metal $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ is studied in fields of up to 45~T and at temperatures $T$ from 0.5~K to 30~K. The peak in $\rho_{zz}$ seen in in-plane fields, a definitive signature of interlayer coherence, remains to $T$s exceeding the Anderson criterion for incoherent transport by a factor $\sim 30$. Angle-dependent magnetoresistance oscillations are modeled using an approach based on field-induced quasiparticle paths on a 3D Fermi surface, to yield the $T$ dependence of the scattering rate $\tau^{-1}$. The results suggest that $\tau^{-1}$ does not vary strongly over the Fermi surface, and that it has a $T^2$ dependence due to electron-electron scattering. These findings are contrasted with recent experiments on cuprates, and their implications for models of organic superconductivity (e.g. FLEX) are discussed. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J11.00013: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J11.00014: Coexisting fluctuations of charge ordering in quasi-2D organic conductors, $\theta$-(ET)$_2X$ Masafumi Udagawa, Yukitoshi Motome Charge ordering is a crystallization of electrons driven by strong electronic correlations, and is one of the central issues in organic conductors. In particular, the charge ordering in 1/4-filled quasi-2D materials, $\theta$-(ET)$_2X$, attracts much attention since the anisotropic triangular lattice structure enables us to study the stability of charge order under the geometrical frustration and quantum fluctuation. A systematic phase diagram was obtained for the anion $X$ which controls the frustration, and surprisingly, an unusual coexistence of charge fluctuations with different wave numbers was observed in the quantum critical regime where the transition temperature goes to zero. Some exotic phenomena such as the strongly non-linear I-V characteristics and large magneto-resistence are observed in this regime, possibly induced by the anomalous properties of the charge degree of freedom. Here we theoretically study how the charge order and its fluctuations develop in the frustrated systems by applying the random-phase approximation to the extended Hubbard model with electron-phonon couplings. We successfully reproduce the coexisting charge fluctuations as well as the phase diagram. The coexistence originates from the competition between the kinetic energy and the Coulomb repulsion in the intermediate correlation regime, and is characteristic of the charge degree of freedom. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J11.00015: Anisotropy of the competing superconducting and magnetic states in quasi-2D organic conductor $\kappa $-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br: An elastic investigation D. Fournier, M. Poirier, K.D. Truong Ultrasonic measurements performed on the quasi-2D organic conductor $\kappa $-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br reveal a phase separation between superconductivity and magnetism in the vicinity of the Mott transition line. We report here longitudinal (L) and transverse (T) ultrasonic velocity measurements propagating perpendicularly to the highly conducting planes; a magnetic field up to 18 Tesla could be applied along the same direction to differentiate the superconducting phase from the magnetic one. The huge velocity dip observed between 30 and 40 K and associated to a compressibility increase driven by the electronic degrees of freedom is not observed for T-waves polarized along [001]; this implies that only magnetic fluctuations associated to 1D sheets of the Fermi surface can couple to the ultrasonic waves. Around T$_{c}$= 12 K, both the temperature profile and the amplitude of the elastic anomalies are highly dependent on the wave polarization. A magnetic field investigation of these anomalies not only establishes the anisotropic character of the superconducting anomaly, but it reveals also the onset of a magnetic transition below 15 K over the same temperature range as the superconducting one. These anomalies likely favor a multi-component superconducting order parameter. [Preview Abstract] |
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