Diapositive 1

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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.
• 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.

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Conducting polymers todays 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
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• Ferroelectricity is a rising demand in
  fundamental and applied solid state physics.
• RD 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 e10.
Plastic ferrroelectric is necessary in medical
imaging low weight compatibility of acoustic
impedances with biological tissues.
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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 !
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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 (?p/?)2102
? 103
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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
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Spontaneous Us can change sign between
different FE domains. Domain boundary Us?-Us -
the phase soliton ?f -2? non integer charge
q-2?/? per chain.
alpha- solitons - walls between domains of
opposite FE polarizations
On-chain conducting particles above
TFE. Macroscopic walls below TFE (do not
conduct, but determine the FE depolarization
dynamics.
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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.
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First theory S.B. N.Kirova 1981 - Combined
Peierls state , Baptizing
E.Mele and M.Rice, 1982
R
R

• 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

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Solitons with fractional charge
S0
S1/2
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.
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Our allusion of early 80s (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.
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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 !

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

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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.
• 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.

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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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?' - 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 ? is still very big. No hysterezis, a
pure mono-domain initial FE susceptibility.
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Dow we see the motion of FE solitons ? Yes at
TltTc
Frequency dependence of imaginary part of
permittivity 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, and the
activation law at low T friction of FE domain
walls by charge carriers
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parasite intra -molecular modes
gap 2??
Drude peaks
log scale linear scale
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
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Optical Conductivity, ETH group
Do we see the solitons in optics ?
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