Diapositive 1

1
Perspectives for the ferroelectricity in
p-conjugated systems FROM 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

2

• 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
3
 In the beginning was the Word, and without
him was not anything made that was made 
He laid his fingers on my earsAnd they were
filled with roaring soundI 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'
The Profet. Vrubel illustration to verses by
Pushkin
4

• 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).

5
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
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Microwave conductivity and dieiectric constant of
tetramethyltetrathiafulvalene salts (TMTTF)2X,
X-SCN, Re04, SbF6 H. 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)
7

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

Felix Nad, Pierre Monceau, S. Brazovskii - PRL
2001 Stuart Brown et al NMR group of UCLA -
PRL 2001 Review S. B. cond-mat/0606009
Springer series 2008 T Earlier 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 20th 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 cobaltideshigh-T
crossover line becomes a true transition.
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(TMTCF)2X, 1980 Bechgaard, Jerome Black and
white SC- superconductivity AF- AFM SDW SP-
Spin-Peierls LL- Luttinger liquid MI- Mott
insulator Red line TFE 2000s
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 - TFE?150K is 10 times above 3D
electronic transitions.
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conterion dopant X
Molecule TMTTF or TMTSF
Built-in dimerization of bonds - counterions
against each second pair of molecules )
Spontaneous symmetry breaking displacements of
counterions, nonequivqlence of sites
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 (?p/?)2102
giving even a background ? 103
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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 types HU -Uscos 2? -Ubsin 2?
-Ucos (2?-2?) Us?0 ? ? ?0 ? phase ? mean
displacement of all electronsshifts 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
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?' - linear scale
Dielectric anomaly ??(T) in (TMTTF)2X, after Nad
Monceau Left at f1MHz in semi logarithmic
scale Right at f100 kHz in linear
scale Anti-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.
12
T dependence of the inverse of the real part
1/?' at f100Hz, XPF6,AsF6,SbF6,ReO4 1/ ?'
C(T-T0) Typically C104/T0 Clt2Cgt -- exact
Landau theory ! complication PF6
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.
13
Frequency dependence of imaginary part of e
Comparison of the e''(f) curves at two
temperatures near Tc above - 105K and below -
97K.
Low frequency shoulder - only at TltTc pinning
of FE domain walls ?
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
Landau-Khalatnikov critical relaxation
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Ferroelectricity imprinting Inversion symmetry
lifting tested by nonlinear optics.
Second harmonic generation ?(?)1400nm
K. Yamamoto et al (2008)
a-(ET)2I3 - should be a metal, actually a
correlated (Mott) insulator, below T140K it
becomes a Wigner Crystal
Problem of identification of thefrozen
polarization through anomalous optical
activity - lack of inversion summetry
And what's about the metallic state of the Se
sub-family? Can the Charge ordering or even
Ferroelectricity be hidden there ?
15
Log scales of frequency
Optical Conductivity ?(?) absorption The peak
due to either 2? of pairs of kinks production
or Eg - optical absorption edge (exciton bound
kinkantikink) Notice identity of static (TMTTF
case) and fluctuational (TMTSF case) Mott states
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CONCLUSIONS for ORGANICS Discovery of the
ferroelectric anomaly (Nad, Monceau) and of
the charge disproportionation (S. Brown et
al) call for a revaluation of the phase
diagram of the (TMTCF)2X compounds and return
the attention to the interplay of electronic
and structural properties (S.B. and V.
Yakovenko 1985). Theory versions Charge
Disproportionation (Seo and Fukuyama) Combined
Mott-Hubbard state FE (S.B.).Links4KF
anomalies (Kagoshima, Pouget et al), Wigner Xs
(Kanoda et al)extentions to the BEDT family
charge segregation in transition metals oxides,
stripes Main challenges Hidden existence of
CD/FE in the metallic Se subfamilyOptical
identification of gaps and soft modesSolitons
via conductivity, optics, NMR.FE hysteresis,
relaxation, domains.
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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
e100. Plastic ferrroelectrics are necessary in
medical imaging shaping, also need a low
weight for compatibility of acoustic impedances
with biological tissues.
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Ferroelectrics are available mostly in the
inorganic world. Can we have organic only,
particularly polymer only ferroelectric ?

• 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.

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 todays applications
Tsukuba, LED TV.
LED display and microelectronic chip made by
Phillips Research Lab
Ferroelectricity in conjugated polymers?

• Where does the confidence come?
• What may be a scale of effects ?
• Proved by the success in organic conducting
  crystals.

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But was organic supercondictivity the only
promised land? Not quite some of the profets
visions actually imply a spontaneous electric
polarization, hence they are FERROELECTRIC.
Rphenyl
Later popular drawing (Sci. Am.) It must be a
ferroelectric if R?H
Drawing from the PRB 1964 It is a pyroelectric if
N?H
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Instructions of the FE design Combined symmetry
breaking.

• Lift the inversion symmetry, remove the mirror
  symmetry,
• do not leave a glide plane.
• Keep the double degeneracy to get a ferroelectric.

• Realization conjugated polymers of the (AB)x
  type
• modified polyacetylene (CRCR)x

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
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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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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 !
Not a polaron, but spin soliton ?
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Particular interest in developing of
ferroelectric p-conjugated systems electronic
ferroelectrics

• Manipulations of charged solitons by electric
  field. Their spectral features arrive in optics
• Gigantic e, easy repolarization fast response
• 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).
• High and fast nonlinearity ?2 for the optical
  mixing , second harmonic generation

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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.
• 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.
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Diatomic (C2RR) chain (AB)x polymer
Solitonic intra-gap states
Special experimental advantage 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.
S0
?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
S1/2
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What does conduct in these narrow gap
ferroelectric semiconductors ? NOT the electrons
!
PF6 AsF6 SbF6
Conductance G , normalized to RT - Ahrenius plot
Log G(1/T). Gaps for thermal activation ? range
within 500-2000K.
1KHz (TMTTF)2X
Contrarily to normal semiconductors - no gap in
spin susceptibility ?(T) stays flat as for
metal. Clearlest example for conduction by
charged spinless solitons - holons .
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For a given Us the ground state is still doubly
degenerate between ? ? and ? ??. HU -Ucos
(2?-2?) It allows for phase ? solitons, i.e.
holons with the charge e.
Purely on-chain solitons, exist as conducting
quasiparticles both above and below the TFE.
Spontaneous Us itself can change sign between
different FE domains. Then electronic system
must also adjust its ground state from ? to -?.
Hence the domain boundary Us?-Us requires for
the phase soliton of the increment ? -2? which
will concentrate the non integer charge q-2?/?
per chain.
alpha- solitons are walls between domains of
opposite FE polarizations
They are on-chain conducting particles only above
TFE. Below TFE they aggregate into macroscopic
walls. They do not conduct any more, but
determine the FE depolarization dynamics.