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Modeling of the bending of an electroactive pseudo trilayer based on PEDOT, a semiconductor polymer
Authors:
Mireille Tixier,
Joël Pouget
Abstract:
Electroactive polymers (EAP) are smart materials that can be used as actuators, sensors or energy harvesters in many fields. We had previously studied an ionic metal-polymer composites (IPMC), which consists in an ionic polymer film such as Nafion saturated with water and coated on both sides with a thin layer of metal acting as electrodes. This system bends when it is subject to an electric field…
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Electroactive polymers (EAP) are smart materials that can be used as actuators, sensors or energy harvesters in many fields. We had previously studied an ionic metal-polymer composites (IPMC), which consists in an ionic polymer film such as Nafion saturated with water and coated on both sides with a thin layer of metal acting as electrodes. This system bends when it is subject to an electric field orthogonal to the film and can thus be used as an actuator. Conversely, the deflection of the film generates a potential difference between the electrodes ; the same system can therefore be used as a sensor.We have developed a ''continuous medium'' model for this system. The thermodynamics of linear irreversible processes had enabled us to establish its constitutive equations.We are currently interested in a system of close properties based on PEDOT, a semiconductor EAP. The central part of the device consists in two interpenetrating polymers playing the role of an ions reservoir. The PEDOT is polymerized on each side and forms an interpenetrating network with the two other polymers. A pseudo trilayer is obtained, the two outer layers containing the PEDOT acting as electrodes. It is then saturated with an ionic liquid. When the blade thus obtained is placed in an electric field orthogonal to its faces, the PEDOT undergoes a reduction reaction (or dedoping) on the side of the negative electrode, which attracts cations from the central part and therefore swells ; the blade ultimately bends towards the positive electrode.We have first adapted our model to this two-components system : the cations on the one hand, and the three polymers and the anions on the other hand. We have written its balance equations and thermodynamic relations first at the microscopic scale for each phase, then at the macroscopic scale for the whole material using an averaging technique. The thermodynamics of linear irreversible processes then provides its constitutive relations : a Kelvin - Voigt type stress-strain relation and generalized Fourier's and Darcy's laws. The equations obtained were applied to the case of a cantilevered blade subject to a continuous potential difference at constant temperature. The numerical resolution of the equations system enabled us to draw the profiles of the different quantities, which are very steep functions near the electrodes. We also evaluated the tip displacement and the force that must be exerted on the free end of the beam to prevent its displacement (blocking force). The results obtained are in good agreement with the experimental data published in the literature.
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Submitted 3 April, 2023;
originally announced May 2023.
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Electron-hole response function of transition metal trichalcogenides NbSe$_3$ and monoclinic-TaS$_3$
Authors:
Bogdan Guster,
Miguel Pruneda,
Pablo Ordejón,
Enric Canadell,
Jean-Paul Pouget
Abstract:
NbSe$_3$ and monoclinic-TaS$_3$ ($m$-TaS$_3$) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave (CDW) Peierls transitions at T$_{P_1}$ and T$_{P_2}$. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their electron-hole Lindhard response function. As a result of stronger inter-chain interac…
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NbSe$_3$ and monoclinic-TaS$_3$ ($m$-TaS$_3$) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave (CDW) Peierls transitions at T$_{P_1}$ and T$_{P_2}$. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their electron-hole Lindhard response function. As a result of stronger inter-chain interactions the Fermi surface (FS) and Lindhard function of NbSe$_3$ are considerably more complex than those for $m$-TaS$_3$; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the Lindhard response for NbSe$_3$ are found with the (0$a$*, 0$c$*) and (1/2$a$*, 1/2$c$*) transverse components at T$_{P_1}$ and T$_{P_2}$, respectively, whereas the second maximum is not observed for $m$-TaS$_3$ at T$_{P2}$. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transverse $a$* components in NbSe$_3$. For the transverse coupling along $c$* in NbSe$_3$ and along both $a$* and $c$* for $m$-TaS$_3$, one must take into account the strongest inter-chain Coulomb coupling mechanism. Phonon spectrum calculations show the formation of a giant 2$k_F$ Kohn anomaly in $m$-TaS$_3$. All these results support the weak coupling scenario for the Peierls transition of transition metal trichalcogenides.
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Submitted 15 December, 2020; v1 submitted 12 December, 2020;
originally announced December 2020.
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Probing the Ionic Dielectric Constant Contribution in the Ferroelectric Phase of the Fabre-Salts
Authors:
Mariano de Souza,
Lucas Squillante,
Cesar Sônego,
Paulo Menegasso,
Pascale Foury-Leylekian,
Jean-Paul Pouget
Abstract:
In strongly correlated organic materials it has been pointed out that charge-ordering could also achieve electronic ferroelectricity at the same critical temperature $T_{co}$. A prototype of such phenomenon are the quasi-one dimensional (TMTTF)$_2X$ Fabre-salts. However, the stabilization of a long-range ferroelectric ground-state below $T_{co}$ requires the break of inversion symmetry, which shou…
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In strongly correlated organic materials it has been pointed out that charge-ordering could also achieve electronic ferroelectricity at the same critical temperature $T_{co}$. A prototype of such phenomenon are the quasi-one dimensional (TMTTF)$_2X$ Fabre-salts. However, the stabilization of a long-range ferroelectric ground-state below $T_{co}$ requires the break of inversion symmetry, which should be accompanied by a lattice deformation. In this work we investigate the role of the monovalent counter-anion $X$ in such mechanism. For this purpose, we measured the quasi-static dielectric constant along the $c^{*}$-axis direction, where layers formed by donors and anions alternate. Our findings show that the ionic charge contribution is three orders of magnitude lower than the intra-stack electronic response. The $c^{*}$ dielectric constant ($ε'_{c^*}$) probes directly the charge response of the monovalent anion $X$, since the anion mobility in the structure should help to stabilize the ferroelectric ground-state. Furthermore, our $ε'_{c^*}$ measurements %conjugated with earlier investigations of the $c^*$ lattice thermal expansion, show that the dielectric response is thermally broaden below $T_{co}$ if the ferroelectric transition occurs in the temperature range where the anion movement begin to freeze in their methyl groups cavity. In the extreme case of the PF$_6$-H$_{12}$ salt, where $T_{co}$ occurs at the freezing point, a relaxor-type ferroelectricity is observed. Also, because of the slow kinetics of the anion sub-lattice, global hysteresis effects and reduction of the charge response upon successive cycling are observed. In this context, we propose that anions control the order-disorder or relaxation character of the ferroelectric transition of the Fabre-salts.
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Submitted 2 January, 2018;
originally announced January 2018.
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Resonant inelastic x-ray scattering probes the electron-phonon coupling in the spin-liquid kappa-(BEDT-TTF)2Cu2(CN)3
Authors:
V. Ilakovac,
S. Carniato,
P. Foury-Leylekian,
S. Tomic,
J. -P. Pouget,
P. Lazic,
Y. Joly,
K. Miyagawa,
K. Kanoda,
A. Nicolaou
Abstract:
Resonant inelastic x-ray scattering at the N K edge reveals clearly resolved harmonics of the anion plane vibrations in the kappa-(BEDT-TTF)2Cu2(CN)3 spin-liquid insulator. Tuning the incoming light energy at the K edge of two distinct N sites permits to excite different sets of phonon modes. Cyanide CN stretching mode is selected at the edge of the ordered N sites which are more strongly connecte…
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Resonant inelastic x-ray scattering at the N K edge reveals clearly resolved harmonics of the anion plane vibrations in the kappa-(BEDT-TTF)2Cu2(CN)3 spin-liquid insulator. Tuning the incoming light energy at the K edge of two distinct N sites permits to excite different sets of phonon modes. Cyanide CN stretching mode is selected at the edge of the ordered N sites which are more strongly connected to the BEDT-TTF molecules, while positionally disordered N sites show multi-mode excitation. Combining measurements with calculations on an anion plane cluster permits to estimate the sitedependent electron-phonon coupling of the modes related to nitrogen excitation.
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Submitted 6 October, 2017;
originally announced December 2017.
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Constitutive equations for an electro-active polymer
Authors:
Mireille Tixier,
Joël Pouget
Abstract:
Ionic electro-active polymers (E.A.P.) can be used as sensors or actuators. For this purpose, a thin film of polyelectrolyte is saturated with a solvent and sandwiched between two platinum electrodes. The solvent causes a complete dissociation of the polymer and the release of small cations. The application of an electric field across the thickness results in the bending of the strip and vice vers…
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Ionic electro-active polymers (E.A.P.) can be used as sensors or actuators. For this purpose, a thin film of polyelectrolyte is saturated with a solvent and sandwiched between two platinum electrodes. The solvent causes a complete dissociation of the polymer and the release of small cations. The application of an electric field across the thickness results in the bending of the strip and vice versa. The material is modelled by a two-phase continuous medium. The solid phase, constituted by the polymer backbone inlaid with anions, is depicted as a deformable porous media. The liquid phase is composed of the free cations and the solvent (usually water). We used a coarse grain model. The conservation laws of this system have been established in a previous work. The entropy balance law and the thermodynamic relations are first written for each phase, then for the complete material using a statistical average technique and the material derivative concept. One deduces the entropy production. Identifying generalized forces and fluxes provides the constitutive equations of the whole system : the stress-strain relations which satisfy a Kelvin-Voigt model, generalized Fourier's and Darcy's laws and the Nernst-Planck equation.
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Submitted 27 June, 2017;
originally announced June 2017.
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Conservation laws of an electro-active polymer
Authors:
Mireille Tixier,
Joël Pouget
Abstract:
Ionic electro-active polymers (E.A.P.) is an active material consisting in a polyelectrolyte (for example Nafion). Such material is usually used as thin film sandwiched between two platinum electrodes. The polymer undergoes large bending motions when an electric field is applied across the thickness. Conversely, a voltage can be detected between both electrodes when the polymer is suddenly bent. T…
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Ionic electro-active polymers (E.A.P.) is an active material consisting in a polyelectrolyte (for example Nafion). Such material is usually used as thin film sandwiched between two platinum electrodes. The polymer undergoes large bending motions when an electric field is applied across the thickness. Conversely, a voltage can be detected between both electrodes when the polymer is suddenly bent. The solvent-saturated polymer is fully dissociated, releasing cations of small size. We used a continuous medium approach. The material is modelled by the coexistence of two phases; it can be considered as a porous medium where the deformable solid phase is the polymer backbone with fixed anions; the electrolyte phase is made of a solvent (usually water) with free cations. The microscale conservation laws of mass, linear momentum and energy and the Maxwell's equations are first written for each phase. The physical quantities linked to the interfaces are deduced. The use of an average technique applied to the two-phase medium finally leads to an Eulerian formulation of the conservation laws of the complete material. Macroscale equations relative to each phase provides exchanges through the interfaces. An analysis of the balance equations of kinetic, potential and internal energy highlights the 2 Mireille Tixier, Jo{ë}l Pouget phenomena responsible of the conversion of one kind of energy into another, especially the dissipative ones : viscous frictions and Joule effect.
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Submitted 27 June, 2017;
originally announced June 2017.
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Evolution of coherent collective modes through consecutive CDW transitions in (PO$_{2}$)$_{4}$(WO$_{3}$)$_{12}$ mono-phosphate tungsten bronze
Authors:
L. Stojchevska,
M. Borovšak,
P. Foury - Leylekian,
J. -P. Pouget,
T. Mertelj,
D. Mihailovic
Abstract:
All optical femtosecond relaxation dynamics in a single crystal of mono-phosphate tungsten bronze (PO$_{2}$)$_{4}$(WO$_{3}$)$_{2m}$ with alternate stacking m=6 of WO$_{3}$ layers was studied through the three consequent charge density wave (CDW) transitions. Several transient coherent collective modes associated to the different CDW transitions were observed and analyzed in the framework of the ti…
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All optical femtosecond relaxation dynamics in a single crystal of mono-phosphate tungsten bronze (PO$_{2}$)$_{4}$(WO$_{3}$)$_{2m}$ with alternate stacking m=6 of WO$_{3}$ layers was studied through the three consequent charge density wave (CDW) transitions. Several transient coherent collective modes associated to the different CDW transitions were observed and analyzed in the framework of the time dependent Ginzburg-Landau theory. Remarkably, the interference of the modes leads to an apparent rectification effect in the transient reflectivity response. A saturation of the coherent-mode amplitudes with increasing pump fluence well below the CDWs destruction threshold fluence indicates a decoupling of the electronic and lattice parts of the order parameter under strong optical drive.
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Submitted 18 April, 2017;
originally announced April 2017.
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Charge, spin and lattice effects in the spin-Peierls ground state of MEM(TCNQ)$_2$
Authors:
M. Poirier,
M. de Lafontaine,
C. Bourbonnais,
J. -P. Pouget
Abstract:
We report an investigation of charge, spin and lattice effects in the spin-Peierls state of the organic compound MEM(TCNQ)$_2$. The 16.5 GHz dielectric function along the chain axis shows an enhancement below the spin-Peierls transition temperature near 18 K consistent with the charge coupling to the elastic strain involved in the transition. The velocity of two elastic modes perpendicular to the…
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We report an investigation of charge, spin and lattice effects in the spin-Peierls state of the organic compound MEM(TCNQ)$_2$. The 16.5 GHz dielectric function along the chain axis shows an enhancement below the spin-Peierls transition temperature near 18 K consistent with the charge coupling to the elastic strain involved in the transition. The velocity of two elastic modes perpendicular to the chain axis presents anomalies at the transition which can be explained with a Landau free energy model including a linear-quadratic coupling energy term between the appropriate elastic strain $e$ and the spin-Peierls magnetic gap $Δ_q$. The analysis of the dielectric and elastic features aims toward an order parameter with an associated critical exponent $β\sim$ 0.36, which is similar to the three-dimensional behavior seen in other spin-Peierls materials. All these effects studied in a magnetic field up to 18 Teslas appear also compatible with a mean-field model of a quasi-one-dimensional spin-Peierls system.
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Submitted 1 November, 2013;
originally announced November 2013.
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Charge-Ordering Transition in (TMTTF)$_2$X Explored via Dilatometry
Authors:
Mariano de Souza,
Jean-Paul Pouget
Abstract:
Charge-ordering phenomena have been highly topical over the last few years. A phase transition towards a charge ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one dimensional organic charge-transfer salts (TMTTF)$_2$X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anio…
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Charge-ordering phenomena have been highly topical over the last few years. A phase transition towards a charge ordered state has been observed experimentally in several classes of materials. Among them, many studies have been devoted to the family of quasi-one dimensional organic charge-transfer salts (TMTTF)$_2$X, where (TMTTF) stands for tetramethyltetrathiafulvalene and X for a monovalent anion (X = PF$_6$, AsF$_6$ and SbF$_6$). However, the relationship between the electron localization phenomena and the role of the lattice distortion in stabilizing the charge-ordering pattern is poorly documented in the literature. Here we present a brief overview of selected literature results with emphasis placed on recent thermal expansion experiments probing the charge-ordering transition of these salts.
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Submitted 31 July, 2013; v1 submitted 26 July, 2013;
originally announced July 2013.
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Low temperature structural effects in the (TMTSF)$_2$PF$_6$ and AsF$_6$ Bechgaard salts
Authors:
P. Foury-Leylekian,
S. Petit,
I. Mirebeau,
G. Andre,
M. de Souza,
M. Lang,
E. Ressouche,
A. Moradpour,
J. -P. Pouget
Abstract:
We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$AsF$_6$ (TMTSF : tetramethyl-tetraselenafulvalene). The 4 K neutron scattering structure refinement of the fully deuterated (TMTSF)$_2$PF$_6$-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared…
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We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$AsF$_6$ (TMTSF : tetramethyl-tetraselenafulvalene). The 4 K neutron scattering structure refinement of the fully deuterated (TMTSF)$_2$PF$_6$-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF)$_2$PF$_6$-H12 salt previously determined at the same temperature. Surprisingly it is found that deuteration corresponds to the application of a negative pressure of 5 x 10$^2$ MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF$_6$ and AsF$_6$ salts. Two different thermal behaviors have been distinguished. Low-Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies θ$_E$ = 8.3 K and θ$_E$ = 6.7 K for the PF$_6$-D12 and AsF$_6$-D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large-Bragg-angle measurements evidence an unexpected structural change around 55 K which probably corresponds to the linkage of the anions to the methyl groups via the formation of F...D-CD2 bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF)$_2$PF$_6$ is dominated by the librational motion of the PF$_6$ units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF6 librations: θ$_E$ = 50 K and θ$_E$ = 76 K for the PF$_6$-D12 and PF$_6$-H12 salts, respectively.
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Submitted 31 July, 2013; v1 submitted 15 May, 2013;
originally announced May 2013.
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Magneto-elastic coupling in the spin-Peierls ground state of hydrogenated and deuterated (TMTTF)$_2$PF$_6$ salts
Authors:
Mario Poirier,
Alexandre Langlois,
Claude Bourbonnais,
Pascale Foury-Leylekian,
Alec Moradpour,
Jean-Paul Pouget
Abstract:
We report an ultrasonic study of the magneto-elastic coupling of the hydrogenated and deuterated (TMTTF)$_2$PF$_6$ organic salts. For both salts the temperature dependence of the longitudinal velocity along the c* axis displays a monotonic stiffening of the $C_{33}$ compressibility modulus upon cooling. Below the characteristic temperature scale 40 K the modulus stiffening becomes markedly enhance…
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We report an ultrasonic study of the magneto-elastic coupling of the hydrogenated and deuterated (TMTTF)$_2$PF$_6$ organic salts. For both salts the temperature dependence of the longitudinal velocity along the c* axis displays a monotonic stiffening of the $C_{33}$ compressibility modulus upon cooling. Below the characteristic temperature scale 40 K the modulus stiffening becomes markedly enhanced, in concomitance with the reduction of spin degrees of freedom previously seen in magnetic measurements as low dimensional precursors of the spin-Peierls transition. The magneto-elastic coupling appears to be much weaker in the hydrogenated salt due to the highly inhomogeneous elastic behavior induced by the proximity of the charge ordering transition to the spin-Peierls phase. For the deuterated salt, an important anomaly in the ultrasound velocity is observed below the spin-Peierls transition temperature $T_{\rm SP}$ in agreement with scaling of the elastic deformation with the spin-Peierls order parameter. In spite of the weakly inhomogeneous character of the spin-Peierls phase transition, the magnetic field dependence of $T_{\rm SP}$ is well captured with the mean-field prediction for the lattice distorted Heisenberg spin chain.
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Submitted 26 July, 2012;
originally announced July 2012.
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Inhomogeneous superconductivity in organic conductors: role of disorder and magnetic field
Authors:
Sonia Haddad,
Samia Charfi-Kaddour,
Jean-Paul Pouget
Abstract:
Several experimental studies have shown the presence of spatially inhomogeneous phase coexistence of superconducting and non superconducting domains in low dimensional organic superconductors. The superconducting properties of these systems are found to be strongly dependent on the amount of disorder introduced in the sample regardless of its origin. The suppression of the superconducting transiti…
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Several experimental studies have shown the presence of spatially inhomogeneous phase coexistence of superconducting and non superconducting domains in low dimensional organic superconductors. The superconducting properties of these systems are found to be strongly dependent on the amount of disorder introduced in the sample regardless of its origin. The suppression of the superconducting transition temperature $T_c$ shows clear discrepancy with the result expected from the Abrikosov-Gor'kov law giving the behavior of $T_c$ with impurities. Based on the time dependent Ginzburg-Landau theory, we derive a model to account for the striking feature of $T_c$ in organic superconductors for different types of disorder by considering the segregated texture of the system. We show that the calculated $T_c$ quantitatively agrees with experiments. We also focus on the role of superconducting fluctuations on the upper critical fields $H_{c2}$ of layered superconductors showing slab structure where superconducting domains are sandwiched by non-superconducting regions. We found that $H_{c2}$ may be strongly enhanced by such fluctuations.
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Submitted 12 October, 2011;
originally announced October 2011.
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Magnetism and Charge ordering in TMTTF$_2$-PF$_6$ organic crystals
Authors:
Gianluca Giovannetti,
Sanjeev Kumar,
Jean-Paul Pouget,
Massimo Capone
Abstract:
Using a combination of Density Functional Theory, mean-field analysis and exact diagonalization calculations we reveal the emergence of a dimerized charge ordered state in TMTTF$_2$-PF$_6$ organic crystal. The interplay between charge and spin order leads to a rich phase diagram. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase, which has been observed…
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Using a combination of Density Functional Theory, mean-field analysis and exact diagonalization calculations we reveal the emergence of a dimerized charge ordered state in TMTTF$_2$-PF$_6$ organic crystal. The interplay between charge and spin order leads to a rich phase diagram. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase, which has been observed experimentally. The tendency to the dimerization is magnetically driven revealing TMTTF$_2$-PF$_6$ as a multiferroic material.
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Submitted 1 April, 2011;
originally announced April 2011.
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Ground State of the Quasi-1D \bvs\ resolved by Resonant Magnetic X-ray Scattering
Authors:
Ph. Leininger,
V. Ilakovac,
Y. Joly,
E. Schierle,
E. Weschke,
O. Bunau,
H. Berger,
J. -P. Pouget,
P. Foury-Leylekian
Abstract:
Resonant-magnetic x-ray scattering (RMXS) near the vanadium $L_{2,3}$-absorption edges has been used to investigate the low temperature magnetic structure of high quality \bvs\ single crystals. Below $T_N$ = 31 K, the strong resonance revealed a triple-incommensurate magnetic ordering at wave vector (0.226 0.226 $ξ$) in the hexagonal notation, with $ξ$ = 0.033. The simulations of the experimental…
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Resonant-magnetic x-ray scattering (RMXS) near the vanadium $L_{2,3}$-absorption edges has been used to investigate the low temperature magnetic structure of high quality \bvs\ single crystals. Below $T_N$ = 31 K, the strong resonance revealed a triple-incommensurate magnetic ordering at wave vector (0.226 0.226 $ξ$) in the hexagonal notation, with $ξ$ = 0.033. The simulations of the experimental RMXS spectra with a time-dependent density functional theory indicate an antiferromagnetic order with the spins polarized along $a$ in the monoclinic structure.
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Submitted 17 February, 2011;
originally announced February 2011.
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Exploring the Charge-Ordering Transition in (TMTTF)$_2$X via Thermal Expansion Measurements
Authors:
Mariano de Souza,
Daniel Hofmann,
Pascale Foury-Leylekian,
Alec Moradpour,
Jean-Paul Pouget,
Michael Lang
Abstract:
We report results of high-resolution measurements of the \emph{c$^*$}-axis expansivity ($α_{c^{*}}$) at the charge-ordering (CO) transition for the quasi-1D (TMTTF)$_{2}$X compounds with X = SbF$_6$ and Br and make a comparison with previous results for the X = PF$_6$ and AsF$_6$ salts. For X = SbF$_6$, due to the screening of the long-range Coulomb forces, a sharp $λ$-type anomaly is observed a…
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We report results of high-resolution measurements of the \emph{c$^*$}-axis expansivity ($α_{c^{*}}$) at the charge-ordering (CO) transition for the quasi-1D (TMTTF)$_{2}$X compounds with X = SbF$_6$ and Br and make a comparison with previous results for the X = PF$_6$ and AsF$_6$ salts. For X = SbF$_6$, due to the screening of the long-range Coulomb forces, a sharp $λ$-type anomaly is observed at $T_{CO}$, which contrasts with the step-like mean-field anomaly at $T_{CO}$ for PF$_6$ and AsF$_6$, where CO occurs in the Mott-Hubbard charge-localized regime. For the latter two salts, a negative contribution to $α_{c^{*}}$ is observed above $T_{CO}$. This feature is assigned to the anions' rigid-unit modes, which become inactive for $T$ $<$ $T_{CO}$. Our $α_{c^{*}}$ results for the X = Br salt, where such rigid-unit modes are absent, reveal no traces of such negative contribution, confirming the model based on the anions' rigid-unit modes for the X = PF$_6$ and AsF$_6$ salts.
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Submitted 23 September, 2009;
originally announced September 2009.
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Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)$_2$PF$_6$ salts
Authors:
Alexandre Langlois,
Mario Poirier,
Claude Bourbonnais,
Pascale Foury-Leylekian,
Alec Moradpour,
Jean-Paul Pouget
Abstract:
We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)$_2$PF$_6$. The temperature behavior of the dielectric function is consistent with short-range polar order whose relaxation time decreases rapidly below the charge ordering temperature. If this transition has more a relaxor character in the hydrogenated salt, charge ordering is strengthened…
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We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)$_2$PF$_6$. The temperature behavior of the dielectric function is consistent with short-range polar order whose relaxation time decreases rapidly below the charge ordering temperature. If this transition has more a relaxor character in the hydrogenated salt, charge ordering is strengthened in the deuterated one where the transition temperature has increased by more than thirty percent. Anomalies in the dielectric function are also observed in the spin-Peierls ground state revealing some intricate lattice effects in a temperature range where both phases coexist. The variation of the spin-Peierls ordering temperature under magnetic field appears to follow a mean-field prediction despite the presence of spin-Peierls fluctuations over a very wide temperature range in the charge ordered state of these salts.
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Submitted 23 July, 2009;
originally announced July 2009.
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Evidence for Lattice Effects at the Charge-Ordering Transition in (TMTTF)$_2$X
Authors:
M. de Souza,
P. Foury-Leylekian,
A. Moradpour,
J. -P. Pouget,
M. Lang
Abstract:
High-resolution thermal expansion measurements have been performed for exploring the mysterious "structureless transition" in (TMTTF)$_{2}$X (X = PF$_{6}$ and AsF$_{6}$), where charge ordering at $T_{CO}$ coincides with the onset of ferroelectric order. Particularly distinct lattice effects are found at $T_{CO}$ in the uniaxial expansivity along the interstack $\textbf{\textit{c*}}$-direction. W…
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High-resolution thermal expansion measurements have been performed for exploring the mysterious "structureless transition" in (TMTTF)$_{2}$X (X = PF$_{6}$ and AsF$_{6}$), where charge ordering at $T_{CO}$ coincides with the onset of ferroelectric order. Particularly distinct lattice effects are found at $T_{CO}$ in the uniaxial expansivity along the interstack $\textbf{\textit{c*}}$-direction. We propose a scheme involving a charge modulation along the TMTTF stacks and its coupling to displacements of the counteranions X$^{-}$. These anion shifts, which lift the inversion symmetry enabling ferroelectric order to develop, determine the 3D charge pattern without ambiguity. Evidence is found for another anomaly for both materials at $T_{int}$ $\simeq$ 0.6 $\cdot$ $T_{CO}$ indicative of a phase transition related to the charge ordering.
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Submitted 24 November, 2008; v1 submitted 4 July, 2008;
originally announced July 2008.
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Structural aspects of the metal-insulator transition in BaVS3
Authors:
S. Fagot,
P. Foury-Leylekian,
S. Ravy,
J. P. Pouget,
M. Anne,
G. Popov,
M. V. Lobanov,
M. Greenblatt
Abstract:
A sequence of structural transitions occurring in the quasi-one-dimensional (1D) 3d1 system BaVS3 at low temperature was investigated by high resolution synchrotron X-ray diffraction. The orthorhombic Cmc21 structure of the intermediate-temperature (70K<T<240K) phase was confirmed. A model for the low-T (T<70K) k=(1 0 1/2)O superstructure (with Im symmetry) is proposed and refined. The formation…
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A sequence of structural transitions occurring in the quasi-one-dimensional (1D) 3d1 system BaVS3 at low temperature was investigated by high resolution synchrotron X-ray diffraction. The orthorhombic Cmc21 structure of the intermediate-temperature (70K<T<240K) phase was confirmed. A model for the low-T (T<70K) k=(1 0 1/2)O superstructure (with Im symmetry) is proposed and refined. The formation of the superstructure is associated with the stabilization of a mixed bond order / charge density wave.
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Submitted 5 October, 2004;
originally announced October 2004.
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X-Ray Study of the Density Wave Instability of alpha-(BEDT-TTF)2MHg(SCN)4 with M=K and Rb
Authors:
P. Foury-Leylekian,
S. Ravy,
J. -P. Pouget,
H. Muller
Abstract:
We present an X-ray diffraction study of the quasi-2D conductors a-(BEDTTTF)2MHg(SCN), with M=K and Rb. They exhibit a phase transition of the density wave type at TDW=8-10K and 12-13K respectively, evidenced by magnetoresistivity, specific heat, NMR and Hall constant measurements. The structural study shows the presence of satellite reflections already at ambient temperature. The related modula…
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We present an X-ray diffraction study of the quasi-2D conductors a-(BEDTTTF)2MHg(SCN), with M=K and Rb. They exhibit a phase transition of the density wave type at TDW=8-10K and 12-13K respectively, evidenced by magnetoresistivity, specific heat, NMR and Hall constant measurements. The structural study shows the presence of satellite reflections already at ambient temperature. The related modulation is incommensurate with multiple harmonics. For some of the compounds studied, the intensity of the satellite reflections strongly increases below TDW. According to Fermi surface (FS) calculations, the wave vector of the structural modulation achieves a quite good nesting of the global FS. This suggests a coupling of the modulation with the electronic degrees of freedom leading to a charge density wave ground state.
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Submitted 3 March, 2003;
originally announced March 2003.
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Soliton Dynamics in a 2D Lattice Model with Nonlinear Interactions
Authors:
T. Ioannidou,
J. Pouget,
E. Aifantis
Abstract:
This paper is concerned with a lattice model which is suited to square-rectangle transformations characterized by two strain components. The microscopic model involves nonlinear and competing interactions, which play a key role in the stability of soliton solutions and emerge from interactions as a function of particle pairs and noncentral type or bending forces. Special attention is devoted to…
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This paper is concerned with a lattice model which is suited to square-rectangle transformations characterized by two strain components. The microscopic model involves nonlinear and competing interactions, which play a key role in the stability of soliton solutions and emerge from interactions as a function of particle pairs and noncentral type or bending forces. Special attention is devoted to the continuum approximation of the two-dimensional discrete system with the view of including the leading discreteness effects at the continuum description. The long time evolution of the localized structures is governed by an asymptotic integrable equation of the Kadomtsev-Petviashvili I type which allows the explicit construction of moving multi-solitons on the lattice. Numerical simulation performed at the discrete system investigate the stability and dynamics of multi-soliton in the lattice space.
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Submitted 26 November, 2002;
originally announced November 2002.
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Friedel Oscillations and Charge-density Waves Pinning in Quasi-one-dimensional Conductors: An X-ray Access
Authors:
S. Rouziere,
S. Ravy,
J. -P. Pouget,
S. Brazovskii
Abstract:
We present an x-ray diffraction study of the Vanadium-doped blue bronze K0.3(Mo0.972V0.028)O3. At low temperature, we have observed both an intensity asymmetry of the +-2kF satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that the profile asymmetry is due to Friedel oscillation around the V substituant and that the intensity asym…
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We present an x-ray diffraction study of the Vanadium-doped blue bronze K0.3(Mo0.972V0.028)O3. At low temperature, we have observed both an intensity asymmetry of the +-2kF satellite reflections relative to the pure compound, and a profile asymmetry of each satellite reflections. We show that the profile asymmetry is due to Friedel oscillation around the V substituant and that the intensity asymmetry is related to the charge density wave (CDW) pinning. These two effects, intensity and profile asymmetries, gives for the first time access to the local properties of CDW in disordered systems, including the pinning and even the phase shift of FOs.
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Submitted 19 June, 2000; v1 submitted 13 January, 2000;
originally announced January 2000.
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X-Ray Diffraction from Pinned Charge Density Waves
Authors:
S. Rouziere,
S. Ravy,
S. Brazovskii,
J. -P. Pouget
Abstract:
We present an x-ray study of doped charge density waves systems. When a 2k_f-charge density wave is strongly pinned to impurities, an interference effect gives rise to an asymmetry between the intensities of the +2k_f and -2k_f satellite reflections. Moreover, profile asymmetry of the satellite reflections indicates the existence of Friedel oscillations (FOs) around the defects. We have studied…
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We present an x-ray study of doped charge density waves systems. When a 2k_f-charge density wave is strongly pinned to impurities, an interference effect gives rise to an asymmetry between the intensities of the +2k_f and -2k_f satellite reflections. Moreover, profile asymmetry of the satellite reflections indicates the existence of Friedel oscillations (FOs) around the defects. We have studied these effects in V- and W-doped blue bronzes. A syncrotron radiation study of the V-doped blue bronze clearly reveals the presence of FO around the V atoms.
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Submitted 6 November, 1999;
originally announced November 1999.