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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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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.

  • 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.


Conducting polymers today s applications

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


Ferroelectricity from organic conductors to conducting polymers

  • 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.


Ferroelectricity from organic conductors to conducting polymers

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 !


Ferroelectricity from organic conductors to conducting polymers

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


Ferroelectricity from organic conductors to conducting polymers

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


Instructions of the fe design combined symmetry breaking

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.


Ferroelectricity from organic conductors to conducting polymers

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


Ferroelectricity from organic conductors to conducting polymers

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.


Ferroelectricity from organic conductors to conducting polymers

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.


Ferroelectricity from organic conductors to conducting polymers

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.


Ferroelectricity from organic conductors to conducting polymers

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.


Ferroelectricity from organic conductors to conducting polymers

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.


Ferroelectricity from organic conductors to conducting polymers

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.


Ferroelectricity from organic conductors to conducting polymers

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


Ferroelectricity from organic conductors to conducting polymers

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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