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-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 TVfrom 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

(ωp/∆)2~102 ~103

COMBINED MOTT - HUBBARD STATE

2 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)

Us0 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.

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’

22inex

First theory: S.B. & N.Kirova 1981 - Combined Peierls state ,

Baptizing: E.Mele and M.Rice, 1982

R

R’

22inex

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

e

eQ

1tan2

eeQ 1tan

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 HF.

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 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.

Proof for spontaneous dimerization through the existence of solitons

(AB)x Conducting Polymers Organic Crystals

build-in

nonequivalence of sites nonequivalence of bonds

spontaneous

Peierls bonds alternation Wigner-Mott-Hubbard charge localization

Lifting of the inversion symmetry, leading to electric polarization, hence the ferroelectricity.

22inex

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-CHx 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)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.

PF6

AsF6

SbF6

ReO4

SCN

PF6

SbF6AsF6ReO4

Dielectric anomaly (T) in (TMTTF)2X, 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 Tc:above - 105K and below - 97K.

T- dependence of relaxation

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

Low frequency shoulder - only at T<Tc :

pinning of FE domain walls ?

Dow we see the motion of FE solitons ? Yes at T<Tc

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 assuredTMTSF – metallic compounds, Mott and CO are present fluctuationally

Low T

collective mode or exciton= two kinks bound state

optically activephonon of FE state

Eg=2 - pair of free kinks.

very narrowDrude peak –It is a metal !