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.
FERROELECTRICITY: FROM ORGANIC CONDUCTORS TO CONDUCTING POLYMERS
N. Kirova &S. Brazovski
CNRS - Orsay, France
disproportionation in quasi 1d organic conductors.
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
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.
Can we have pure organic, particularly, polymer ferroelectric ?
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 !
= dopant X
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 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)
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
do not leave a glide plane.
Bonds are polar because of site dimerization Dipoles are not compensated if bonds are also dimerized.
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
Solitons with fractional charge:
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.
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.
Proof for spontaneous dimerization through the existence of solitons
Optical results byZ.V. Vardeny group:
Still a missing link : no idea was to check for the Ferroelectricity:
To be tried ? and discovered !
Proved by success in organic conducting crystals.
LESSONS and PERSPECTIVES
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.
13. High-Tc superconductivity was discovered leading by a “false idea” of looking for a vicinity of ferroelectric oxide conductors.
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.
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 ?
time for the main peak:
Critical slowing down near Tc,
and the activation law at low T –
friction of FE domain walls by
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.
Drude peak –
It is a metal !
phonon of FE state
Illustrative interpretation of optics on TMTSF in terms of firm expectations for CO/FE state in TMTTF's
TMTTF – compounds found in the Mott state, charge ordering is assured
TMTSF – metallic compounds, Mott and CO are present fluctuationally