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FERROELECTRICITY: FROM ORGANIC CONDUCTORS TO CONDUCTING POLYMERS

FERROELECTRICITY: FROM ORGANIC CONDUCTORS TO CONDUCTING POLYMERS. N. Kirova & S. Brazovski CNRS - Orsay, France. Conducting polymers 1978-2008: electrical conduction and optical activity. Modern requests for ferroelectric applications and materials.

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FERROELECTRICITY: FROM ORGANIC CONDUCTORS TO CONDUCTING POLYMERS

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  1. FERROELECTRICITY: FROM ORGANIC CONDUCTORS TO CONDUCTING POLYMERS N. Kirova &S. Brazovski CNRS - Orsay, France • Conducting polymers 1978-2008: electrical conduction and optical activity. • Modern requests for ferroelectric applications and materials. • Ferroelectric Mott-Hubbardphase and charge disproportionation in quasi 1d organic conductors. • Existing structural ferroelectricity in a saturated polymer. • Expectations of the electronic ferroelectricity in conjugated modified polyenes.

  2. Conducting polymers: today’s applications Tsukuba, LED TV from a polymer. Polymeic LED display and microelectronic chip made by Phillips Research Lab “Polymers? Everything is clear, just applications are left.”heard on 2007

  3. Ferroelectricity is a rising demand in fundamental and applied solid state physics. • R&D include: • Active gate materials and electric RAM in microelectronics, • Capacitors in portable WiFi communicators, • Electro-Optical-Acoustic modulators, • Electro-Mechanical actuators, • Transducers and Sensors in medical imaging. Request for plasticity – polymer-ceramic composites but weakening responses – effective ε~10. Plastic ferrroelectric is necessary in medical imaging – low weight :compatibility of acoustic impedances with biological tissues.

  4. Can we have pure organic, particularly, polymer ferroelectric ? Saturated polymer: Poly(vinylidene flouride) PVDF : ferroelectric and pyroelectric,efficient piezoelectric if poled – quenched under a high voltage. Helps in very costly applications - ultrasonic transducers.Unique as long stretching actuator. But conjugated polymers? Can we mobilize their fast pi-electrons? First go for help to organic conductors !

  5. conterion = dopant X Molecule TMTTF or TMTSF Displacements of ions X. Collinear arrows – ferroelectricity. Alternating arrows – anti-ferroelectricity. A single stack is polarized in any case. Redistribution of electronic density, amplification of polarizability by (ωp/∆)2~102 ~103

  6. COMBINED MOTT - HUBBARD STATE2 types of dimerization  Site dimerization :HUs=-Us cos 2 (spontaneous) Bond dimerization:HUb=-Ub sin 2 (build-in) HU= -Uscos 2 -Ubsin 2 = -Ucos (2-2) Us00  shifts from  =0 to  =  - the gigantic FE polarization. From a single stack to a crystal: Macroscopic FerroElectric ground state: the same  for all stacks Anti-FE state: the sign of  alternates

  7. Instructions of the FE design: Combined symmetry breaking. Realization: conjugated polymers of the (AB)x type: modified polyacetylene (CRCR’)x • Lift the inversion symmetry, remove the mirror symmetry, do not leave a glide plane. • Keepthe double degeneracy to get a ferroelectric. Bonds are polar because of site dimerization Dipoles are not compensated if bonds are also dimerized.

  8. Joint effect of extrinsic ∆ex and intrinsic ∆incontributions to dimerization gap ∆. ∆ex comes from the build-in site dimerization – non-equivalence of sites A and B. ∆in comes from spontaneous dimerization of bonds, the Peierls effect. ∆in WILL NOT be spontaneously generated – it is a threshold effect -if ∆ex already exceeds the wanted optimal Peierls gap. Chemistry precaution: make a small difference of ligands R and R’ First theory: S.B. & N.Kirova 1981 - Combined Peierls state , Baptizing: E.Mele and M.Rice, 1982 R’ R

  9. Solitons with fractional charge: S=0 S=1/2 Special experimental advantage: an ac electric field alternates polarization by commuting the bond ordering patterns, i.e. moving charged solitons. Through solitons’ spectral features it opens a special tool of electro-optical interference.

  10. Our allusion of early 80’s : (CHCF)x - vaguely reported to exist; it may not generate bonds dimeriszation: strong effect of substitution HF. Actual success: in 1999 from Kyoto-Osaka-Utah team. By today – complete optical characterization, indirect proof for spontaneous bonds dimerization via spectral signatures of solitons. “Accidental” origin of the success to get the Peierls effect of bonds dimerization: weak difference or radicals – only by a distant side group.Small site dimerisation gap provoke to add the bond dimerisation gap.

  11. Proof for spontaneous dimerization through the existence of solitons Optical results byZ.V. Vardeny group: Soliton feature, Absorption, Luminescence, Dynamics Still a missing link : no idea was to check for the Ferroelectricity: To be tried ? and discovered ! • Where does the confidence come? • What may be a scale of effects ? Proved by success in organic conducting crystals.

  12. LESSONS and PERSPECTIVES • p-conjugated systems can support the electronic ferroelectricity. • Effect is registered and interpreted in two families of organic crystalline conductors (quasi 1D and quasi 2D). • Mechanism is well understood as combined collective effects of Mott (S.B. 2001) or Peierls (N.K.&S.B. 1981) types. • An example of a must_be_ferroelectric polyene has been already studied (Vardeny et al). • The design is symmetrically defined and can be previewed. Cases of low temperature phases should not be overlooked. • Conductivity and/or optical activity of p-conjugated systems will add more functionality to their ferroelectric states. • Polarizability of chains can allow to manipulate morphology (existing hybrids of polymers and liquid crystals (K. Akagi - Kyoto). • SSH solitons of trans-CHx will serve duties of re-polarization walls.

  13. WARNINGS 1. Ferroelectric transition in organic conductors was weakly observed, but missed to be identified, for 15 years before its clarification. 2. Success was due to a synthesis of methods coming from a. experimental techniques for sliding Charge Density Waves, b. materials from organic metals, c. ideas from theory of conjugated polymers. 3. Theory guides only towards a single chain polarization. The bulk arrangement may be also anti-ferroelectric – still interesting while less spectacular. Empirical reason for optimism: majority of (TMTTF)2X cases are ferroelectrics. 4. ……. ………………….. 13. High-Tc superconductivity was discovered leading by a “false idea” of looking for a vicinity of ferroelectric oxide conductors.

  14. AsF6 SbF6 AsF6 ReO4 PF6 SbF6 ReO4 PF6 SCN ′ - linear scale Dielectric anomaly (T) in (TMTTF)2X, after Nad & Monceau Left: at f=1MHz insemi logarithmic scale | Right:at f=100 kHz in linear scale Anti-FE case of SCN shows only a kink as it should be; is still very big. No hysterezis, a pure mono-domain “initial” FE susceptibility.

  15. Dow we see the motion of FE solitons ? Yes atT<Tc Frequency dependence of imaginary part of permittivityε′′ Comparison of the ε′′(f) curves at two temperatures near Tc: above - 105K and below - 97K. Low frequency shoulder - only at T<Tc : pinning of FE domain walls ? T- dependenceofrelaxation time for the main peak: Critical slowing down near Tc, and the activation law at low T – friction of FE domain walls by charge carriers

  16. Do we see the solitons in optics ? Optical Conductivity, ETH group collective mode or exciton = two kinks bound state Eg=2 - pair of free kinks. very narrow Drude peak – It is a metal ! optically active phonon of FE state Low T Illustrative interpretation of optics on TMTSF in terms of firm expectations for CO/FE state in TMTTF's Vocabulary : TMTTF – compounds found in the Mott state, charge ordering is assured TMTSF – metallic compounds, Mott and CO are present fluctuationally

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