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

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

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.

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 ! Can we have pure organic, particularly, polymer ferroelectric ?

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 (ω p /∆) 2 ~10 2  ~10 3

COMBINED MOTT - HUBBARD STATE 2 types of dimerization  Site dimerization : H U s =-U s cos 2  (spontaneous) Bond dimerization :H U b =-U b sin 2  (build-in) H U = -U s cos 2  -U b sin 2  = -Ucos (2  -2  ) U s  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

Spontaneous U s can change sign between different FE domains. Domain boundary U s  -U s - the phase soliton Δφ =-2  non integer charge q=-2  /  per chain. φ =-  φ=  alpha- solitons - walls between domains of opposite FE polarizations On-chain conducting particles above T FE. Macroscopic walls below T FE (do not conduct, but determine the FE depolarization dynamics.

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.  Keep the double degeneracy to get a ferroelectric. Bonds are polar because of site dimerization Dipoles are not compensated if bonds are also dimerized.

Joint effect of extrinsic ∆ ex and intrinsic ∆ in contributions 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 Combined Peierls state, Baptizing: E.Mele and M.Rice, 1982 R R’

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. S=0 S=1/2 Solitons with fractional charge:

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.

Optical results by Z.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. Proof for spontaneous dimerization through the existence of solitons

(AB) x Conducting PolymersOrganic Crystals build-in nonequivalence of sitesnonequivalence of bonds spontaneous Peierls bonds alternationWigner-Mott-Hubbard charge localization Lifting of the inversion symmetry, leading to electric polarization, hence the ferroelectricity.

LESSONS and PERSPECTIVES   -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  - 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-CH x will serve duties of re-polarization walls.

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) 2 X cases are ferroelectrics. 4. ……. ………………… High-Tc superconductivity was discovered leading by a “false idea” of looking for a vicinity of ferroelectric oxide conductors.

PF6 AsF6 SbF6 ReO4 SCN PF6 SbF6 AsF6 ReO4 Dielectric anomaly  (T) in (TMTTF) 2 X, after Nad & Monceau Left: at f=1MHz in semi 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.  ′ - linear scale

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

parasite intra - molecular modes Drude peaks log scale linear scale gap 2Δ ρ Comparison of optical absorption in two subfamilies: Mott insulator TMTTF (upper plots) and a “metal” TMTSF (lower plots) Degiorgi group, ETH, 1998 Notice the identity of static (TMTTF case) and fluctuational (TMTSF case) Mott states – both the result and the tool

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