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Perspectives for the ferroelectricity in π-conjugated systemsFROM ORGANIC CONDUCTORS TO CONDUCTING POLYMERS

S. Brazovski & N. Kirova CNRS - Orsay, France

Ferroelectric Mott-Hubbard phase and charge disproportionation in quasi 1d organic conductors.

Modern requests for plastic ferroelectrics. Reality: Existing structural ferroelectricity in a saturated polymer.

Expectations: electronic ferroelectricity in conjugated modified polyenes.

Hypothesis: zigzag edge states in graphene

I.F. Shchegolev

Memorial Volume Common Trends in Synthetic Metals and High-Tc Superconductors.

Journal de Physique I December 1996

56 state-of-the-art articles on Organic Conductors, related low dimensional systems and High- Tc materials.

Examples of this volume reviews relevant to this talk:

Structural Aspects of the Bechgaard Salts and Related CompoundsJ.P. Pouget and S. Ravy

Investigations of Organic Conductors by the Shchegolev MethodH.W. Helberg and M. Dressel

The Profet. Vrubel illustration to verses by Pushkin

He laid his fingers on my earsAnd they were filled with roaring sound:I heard the music of the spheres,The flight of Angels through the skies,The beasts that crept beneath the sea,The heady uprush of the vine;…………………..'Rise, prophet, rise, and hear, and see,And let my works be seen and heard'

« In the beginning was the Word, … and without him was not anything made that was made »

His very first publication and the first review:

I.F.Shchegolev Signal/noise ratio of NMR-regenerative detector (1958).I.F.Shchegolev Investigation of electron structure of metals by NMR-technique (1962).

-Something special that the founders of Synthetic Metals were coming from the NMR-(Heeger, Jerome, Schegolev). The same for the later comers and for the today core (Berthier and Coulon, Clark and Brown, Takahashi and Kanoda) – unless another source : X-ray (Shibaeva, Comes and Pouget, Kagoshima). NMR as a common tool and hence a bridge between physics and chemistry as the essence of our science?

Indeed, a fate is driving to the future destiny, unforeseen yet:

Yu.S.Karimov, I.F.Shchegolev Magnetic properties of ferrocene polymers (1962).

and now firmly on the path:

E.B.Yagubskii, M.L.Chidekel, I.F.Shchegolev, L.I.Buravov, R.B.Lubovskii, V.B.Stryukov Investigation in potential organic superconductors field. I. Painting complexes with 7,7,8,8-tetracyanquinodimethane (1968).

Moving to the subject:

Shchegolev path of anomalous dielectric susceptibilities.

"You have been looking for superconductors and discovered the superdielectric"

- Peotr L. Kapitza comment to the early discovery of the Schegolev group.

L.I.Buravov, M.L.Khidekel, I.F.Shchegolev, E.B.Yagubskii (1970)Superconductivity and dielectric constant of highly conductive complexes of TCQM

And then spread over the community :

Microwave conductivity and dieiectric constant of tetramethyltetrathiafulvalene salts [(TMTTF)2X, X-SCN, Re04, SbF6H. H. S. Javadi, R. Laversanne, and A. J. Epstein Phys. Rev. B (1988)

"Microwave measurements were performed using a cavity perturbation technique [15] which has been a successful tool in earlier studies of metal-insulator transitions, Peierls gaps, CDW and spin-density waves (SDW's) and their ground-state excitations" …[15] L. Buravov and I. F. Shchegolev, Prib. Tekh. Eksp. 2, 171 (1970)

These high-T anomalies have been seen also by other methods (C.Coulon et al, mid 80's)but NOT by X-rays (till just 2009 – J.P. Pouget talk), hence the name (incorrect today) "structure-less transitions", then unexplained and abandoned, and the amnesia till 2000's.In today's retrospective, this is the charge ordering phase transition which shows up, amusingly, as a ferro- or anti-ferro-electricity (ReO4 or SCN cases)

FERROELECTRIC MOTT-HUBBARD PHASE and CHARGE DISPROPORTIONATION in QUASI 1D ORGANIC CONDUCTORS

Felix Nad, Pierre Monceau, S. Brazovskii - PRL 2001Stuart Brown et al: NMR group of UCLA - PRL 2001Review : S. B. cond-mat/0606009 & Springer series 2008TEarlier theory and experimental hints on the Charge Ordering:Seo and Fukuyama, Kanoda et al (late 90's)

Intrigues for organic conductors and beyond:20 years old mystery of “structureless” phase transitions;revaluation of their basic phase diagram just at the 20’th anniversary

(1980) of the superconductivity discovery;synthesis of lost relationships in synthetic metals : organic conductors, conjugated polymers, charge density waves.Link to High-Tc world :

recent discovery of charge ordering in cobaltides;high-T crossover line becomes a true transition.

(TMTCF)2X, 1980 – Bechgaard, Jerome Black and white:SC- superconductivityAF- AFM = SDWSP- Spin-PeierlsLL- Luttinger liquidMI- Mott insulatorRed line TFE – 2000’s revolution:Structurless transitions (Coulon et al 1985) = Ferroelectricity (Monceau et al)= Charge disproportionation (Brown et al)

Resolving the mystery of structureless transitions: Coulon et all, 1985

Gigantic anomaly in permitivity of (T) (Nad et al, Grenoble-Moscow)Charge Orderin seen by NMR (Brown - UCLA, Fujiyama -IMS).

Views and interpretations:FerroElectric Mott-Hubbard state,mixed site/bond 4KF CDW, nonsymmetrically pinned Wigner crystal,charge ordering = disproportionation

Facility to see Solitons:Purely 1D regime for electrons - TFE150K is 10 times above 3D electronic transitions.

conterion= dopant X

Molecule TMTTFor TMTSF

Arrows show displacements of ions X.They follow and stabilize the electronic charge disproportionation. Collinear arrows – ferroelectricity.Alternating arrows – anti-ferroelectricity.A single stack is polarized in any case.

Major polarization comes from redistribution of electronic density,hence amplification of polarizability by a factor of (ωp/∆)2~102

giving even a background ~103

Built-in dimerization of bonds - counterions against each second pair of molecules )

Spontaneous symmetry breaking – displacements of counterions,nonequivqlence of sites

COMBINED MOTT - HUBBARD STATE2 types of dimerization

2 interfering sources for two-fold commensurability 2 contributions to the Umklapp interaction:

Site dimerization : HUs=-Us cos 2 (spontaneous)

Bond dimerization : HUb=-Ub sin 2 (build-in)

At presence of both site and bond typesHU= -Uscos 2 -Ubsin 2 = -Ucos (2-2)

Us0 0 phase = “mean displacement of all electrons”shifts from =0 to = , hence the gigantic FE polarization.

From a single stack to a crystal: Macroscopic FerroElectric ground state if the same is chosen for all stacks, Anti-FE state if the sign of alternates - both cases are observed

PF6

AsF6

SbF6

ReO4

SCN

PF6

SbF6AsF6ReO4

Dielectric anomaly (T) in (TMTTF)2X, after Nad & MonceauLeft: at f=1MHz in semi logarithmic scale | Right: at f=100 kHz in linear scaleAnti-FE case of SCN shows only a kink as it should be. Smoothened anomaly in PF6 correlates with its weak frequency dispersion - Mabe FE domain walls and hidden hysteresis ?. Other cases - pure mono-domain “initial” FE susceptibility.

′ -linear scale

Clear cut fitting of the anomaly in (T) to the Curie law prooves the least expected case of the ferroelectric phase. Even more curiously, it is the ferroelectric version of the Mott-Hubbard state and of the Charge Disproportionation.

T dependence of the inverse of the real part : 1/' at f=100Hz,

X=PF6,AsF6,SbF6,ReO4

1/ ' =C(T-T0)

Typically C~104/T0

C<=2C> --

exact Landau theory !

complication: PF6

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Frequency dependence of imaginary part of 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,

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 ?

Landau-Khalatnikovcritical relaxation

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Second harmonic generation λ(ω)=1400nmK. Yamamoto et al (2008)

Problem of identification of thefrozen polarization:through anomalous optical activity- lack of inversion summetry

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2

8EEW

E3 may exist only in case of inversion symmetry breaking

α-(ET)2I3 - should be a metal, actually a correlated (Mott) insulator, below T≈140K it becomes a Wigner Crystal

Ferroelectricity imprinting:Inversion symmetry lifting tested by nonlinear optics.

And what's about the metallic state of the Se sub-family?Can the Charge ordering or even Ferroelectricity be hidden there ?

Optical Conductivity () = absorption The peak due to either 2Δ of pairs of kinks production orEg - optical absorption edge (exciton = bound kink+antikink)Notice identity of static (TMTTF case)and fluctuational (TMTSF case) Mott states

Log scalesof frequency

parasite intra-molecular modes

Drude peak –TMTSF is a metal !

Mott gap 2Δρ in TMTTF

emerges only whenit is as low as in Br

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Ferroelectricity is a rising demand in

fundamental and applied solid state physics. 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 work today

but with weak responses – effective ε~100.

Plastic ferrroelectrics are necessary in medical imaging – shaping,

also need a low weight for compatibility of acoustic impedances with

biological tissues.

One ferroelectric saturated polymer does exist - Poly(vinylidene flouride) PVDF :• ferroelectric and pyroelectric, • efficient piezoelectric if poled – quenched under a high voltage.

Light, flexible, non-toxic, cheap to produce Helps in very costly applications• ultrasonic transducers.• hydrophone probes, sonar equipment• unique as long stretching actuator.

Ferroelectrics are available mostly in the inorganic world.Can we have organic only, particularly polymer only ferroelectric ?

PVDF structural unit, ferroelectric β-phase (trans-polymer)

Can we go wider, diversely, and may be better with conjugated polymers?Can we mobilize their fast pi-electrons to make a better job than common ions?

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Conducting polymers: today’s applications

LED display and microelectronic chip made by Phillips Research Lab

Tsukuba, LED TV.

Ferroelectricity in conjugated polymers?

Where does the confidence come? What may be a scale of effects ?

Proved by the success in organic conducting crystals.

But was “organic supercondictivity” the only promised land?Not quite : some of the profet’s visions actually imply a spontaneous electric polarization, hence they are FERROELECTRIC.

Drawing from the PRB 1964It is a pyroelectric if N≠H

Later popular drawing (Sci. Am.)It must be a ferroelectric if R≠H

R=phenyl

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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.

Precausion: – At presence of bonds dimerization, the Charge Ordering is a threshold effect

∆bonds WILL NOT be spontaneously generated -

if ∆sites already exceeds the wanted optimal Peierls gap ∆0

Chemistry precaution: make a small difference of ligands R and R’

220 sitesbonds

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The necessary polymer does exist which has not been realized:

Di-Substituted Poly-Acethylene, since 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 provokes to add the

bond dimerisation gap.

Still a missing link : no idea was to check for the Ferroelectricity:To be tried ? and discovered !

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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 !

Proof for spontaneous dimerization through the existence of solitons

Not a polaron, but spin soliton ?

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Particular interest in developing of ferroelectric π-conjugated systems“electronic ferroelectrics”:

Manipulations of charged solitons by electric field. Their spectral features arrive in optics

Gigantic ε, easy repolarization – fast responseConductivity 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).High and fast nonlinearity : χ2 for the optical mixing , second harmonic generation

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2

8EEW

E3 may exist only in case of inversion symmetry breaking

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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.

Solitons will serve duties of re-polarization walls.

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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.