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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.

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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-Hubbardphase 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
Conducting polymers: today’s 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


  • 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.


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 !


conterion ferroelectric ?

= 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 ferroelectric ?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)

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


Instructions of the fe design combined symmetry breaking

Instructions of the FE design: Combined symmetry breaking. ferroelectric ?

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.

  • Keepthe 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 ferroelectric ?∆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

R’

R


Solitons with fractional charge: ferroelectric ?

S=0

S=1/2

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.


Our allusion of early 80’s ferroelectric ? : (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.


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.


LESSONS and PERSPECTIVES solitons

  • 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.


WARNINGS solitons

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.


AsF6 solitons

SbF6

AsF6

ReO4

PF6

SbF6

ReO4

PF6

SCN

′ -

linear scale

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 at solitonsT<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 ?

T- dependenceofrelaxation

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


Do we see the solitons in optics ? solitons

Optical Conductivity, ETH group

collective mode or exciton

= two kinks bound state

Eg=2 - pair of free kinks.

very narrow

Drude peak –

It is a metal !

optically active

phonon of FE state

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


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