1
Ferroelectricity in organic lowdimensional systems
Pierre Monceau*
Institut Neel, CNRS and University Joseph Fourier,
Grenoble, France
* in collaboration with Felix Nad, Institut Kotel’nikov of
Radioengineering and Electronics, RAS, Moscow, Russia
2
• Ferroelectricity is defined by the appearance of a
macroscopic electric polarization and its reversibility by
applying an external field
• For some ferroelectric materials, electron degrees of
freedom and/or electronic interactions directly give rise
to a macroscopic electric polarization and a ferroelectric
transition

electronic ferroelectricity
Electronic ferroelectricity
for a review: S. Ishihara, J. Phys. Soc. Jpn 79, 011010, 2010
3
4
Ferroelectricity induced by charge ordering
J.Van der Brick and D. I. Khomskii
J. Phys.: Condens. Matter 20, 434217,
2008
S. Ishihara
J. Phys. Soc. Jpn 79, 011010, 2010
5
1/4 filled band
Shibata et al. Tsuchiizu et al. 2001
The mean field approximation of the 1D Hubbard
model show that when V exceeds a critical value,
Vc, , charge disproportionation occurs among sites
with alternating « charge rich » and « charge poor »
sites (Seo and Fukuyama 1997).
With D ≠ 0 numerical calculations on the plane U
and V for a fixed D where the metallic phase
at D = 0 is replaced by the Mott insulating phase,
and a phase with Wigner crystal-type CD is
still present in the large U and V region
S. Ejima et al.
Europhys. Lett. 70, 051009, 2006
Ground state phase diagram of the 1D extended
Hubbard model at 1/4 filling on the plane of U/t
and V/t
6
Acknowledgements for samples
J. M. Fabre
Laboratoire de Chimie Organique, Montpellier, France
MT. Nakamura and K. Furukawa
Institute for Molecular Science, Okazaki, Japan
H. Müeller
ESRF , Grenoble, France
H. M. Yamamoto
Riken, Saitama, Japan
K. Yamamoto
Institute for Molecular Science, Okazaki, Japan
7
ECRYS 2008
Temperature dependence of the real part of the dielectric permittivity '
measured at 1MHz for (TMTTF)2Br and (TMTTF)2PF6
F. Nad, PM and J-M. Fabre
J. Phys. IV 9, Pr10, 1999
Charge disproportionation
8
C13 NMR spectra for (TMTTF)2AsF6 NMR measurements in an external field of 9T (fre 96.4 MHz)
Below TCO, doubling of the spectral line due to
two inequivalent molecules with unequal electron
densities Charge disproportionation : 3:1 from T1-1
measurements
Spectral splitting (~charge disproportionation order
parameter)versus temperature
D.S. Chow et al. Phys. Rev. Lett. 85 (2000) 1698
At high temperatures the unit cell consists of two equivalent TMTTF molecules related by inversion
about the counterion.
The breaking of the inversion symmetry within the unit cell below TCO, and the spontaneous dipole
moment associated with the charge imbalance on the two molecules yield the ferroelectric behaviou
AC conductivity of (TMTTF)2AsF6
9
100kHz
T dependence of the conductance
T dependence of the real part of the dielectric
permittivity, ’ at 100 et 300kHz, 1,3 and 10MHz
Nad et al.: J. Phys.:Cond. Matter, 12(2000)L435
Real part of dielectric constant of (TMTTF)2X salts
10
’ = ImG/
AsF6
SbF6
ReO4
PF6
1- For all anions: at T≈ T, there is no anomaly
2- for CSA and ReO4 anions, ’diverges at TCO. Huge magnitudes of ’ :
2.106 for AsF6, 5.105 for ReO4
Anion ordering
11
F. Nad and P.M.
J.Phys. Soc. Jpn. 75, 051005, 2006
12
Imaginary part of the permittivity of(TMTTF)2AsF6
T > TCO = 101 K
T <TCO
Motion of domain walls
Frequency of the maximum in ’’ the same at T=97 K and T=105 K (TCO = 101K)
The slow relaxation processes involved in the shoulder of ’’ may correspond to the
motion of the domain wall structure developped in the ferroelectric state
Freezing of the ferroelectric domain structure below 90K = TCO - 10K
13
14
Ferroelectric character
The ferroelectric state is triggered by the uniform shift of anions yielding a macroscopic
ferroelectric polarization which is gigantically amplified by the charge disproportionation
on the molecular stacks ( S. Brazovski and T. Nattermann, Adv. In Phys. 53, 177, 2004)
CSA and ReO4 salts show at TCO a
second order phase transition
described by the Curie law
’ =
PF
6
AsF6
SbF6
ReO4
A
--------- T- TCO 
1/ ’ (T) is close to be linear
Ratio AL / AH
(AL at T TCO
AH at T>TCO)
in CSA: AL / AH ≈ 2
Phys. Rev. Lett. 86 (2001) 4081
in ReO4 AL / AH ≈ 1.5
(ET)2X compounds
Mott insulator in the case of half-filled band, due to strong interactions
between electrons with strong U between neighoring sites
If U is large, it is more favourable to localize the particules on the lattice sites
to minimize the repulsion and the system is an insulator
In presence of strong dimerization
as in -(ET)2X compounds, a single
electron occupy the bonding state
of each dimer
half-filled band
and to the insulating state due to
the effect of U
[ called a dimer-Mott state Hotta et al.: Chem. Rev. 114 (2004)]
15
Structures of and - (BEDT-TTF)2RbZn(SCN)4 and -16
(BEDT-TTF)2I3
Y. Tanaka and K. Yonemitsu
J. Phys. Soc. Jpn 79, 024712, 2010
Stripe phases
17
Different spatial patterns of stripe phases
are stabilizeddepending on the anisotropy
of the transfer integrals tc and tp and of
the values of intersite Coulomb energies
along the stacking direction Vc and along
the bonds in the transverse direction Vp
Seo: J. Phys. Soc. Japan 69 (2000) 805
18
Metal-insulating phase transition in -(BEDT-TTF)2I3
Abrupt phase transition at T=135.1K of first order
Transition slightly hysteretic in specific heat with
latent heat[Fortune et al. Solid St. Comm 77 (1991) 265]
Change of intensity of Bragg peak (for reflections
only with an odd index with the a-component)
(Nogami et al. Synth. Metals 16 (1986) 367
And T. Kakiuchi et al. J. Phys. Soc. Jpn. 76, 113702, 2007)

Relative change of the sample length along a, b and c*
Measured by capacitive dilatometry et x-ray diffraction
from Heidmann etal.: Solid St. Comm. 84 (1992) 711
Dimerization of stacks I along the a axis
of Peierls type
Charge ordering from NMR in -(BEDT-TTF)2I3
13C-NMR
19
spectra at different temperatures below the
M-I transition, the spectra consist of two Pake
doublets. The positions of the two doublets have
differentT dependences.
That indicates two differently charged BEDT-TTF
molecules below the transition:
Also from Raman spectroscopy
Wojciechowski etal. Phys. Rev. B67
(2003) 224105
from Takano et al. J. Phys. Chem. Solids 62 (2001) 393
Charge disproportionation
Horizontal stripe structure
CD already above CO transition
From infrared spectroscopy, NMR, and x-ray
At room temperature:
A=A’= 0.60
B=0.68
C=0.44
In the CO state:
A=0.81
A’= 0.26
B=0.74
C=0. 23
T. Kakiuchi et al.
J. Phys. Soc. Jpn 76, 113702, 2997
20
ac conductivity in -(BEDT-TTF)2I3
21
Dielectric constant at 2MHz (sample 1)
Conductivity
Drop of ’ below TMI

Structural transition (dimerization)
Dielectric constant (sample 2)
22
-3 orders of magnitude jump of conductivity at TMI with hysteresis Tcooling=199K, Theating=204.5K
-charge gap below TMI = 1900K much larger than in (TMTTF)2AsF6 with =350K
-’(T) shows a smooth monotonic increase from room temperature, more sharp,
closed to divergence near TMI
-’(T) jumps down to a small magnitude sharply below TMI
-the same jump up of ’ in heating

First order transition
The ’ growth above TMI may indicate the polarizability of the charge disproportionation seen in
NMR. The jump below TMI is associated with the 2c superstructure and the large charge gap

Role essential of the structural transition in the metal-insulating transition (posssibly alter the
symmetry and the magnitude of the transfer integrals relative to V)
Low temperature dielectric response
in -(BEDT-TTF)2I3
Frequency dependence of the real (ε') and imaginary (ε'') part of the dielectric
function in α-(BEDT-TTF)2I3 for E // [1¯10].
Below 75 K, two dielectric relaxation modes are observed
23
T. Ivek et al.
Phys. Rev. B83, 165128, 2011
Optical second harmonic generation
Activation of the even-order nonlinear optical phenomenom
signifies the lack of inversion symmetry
K. Yamamoto et al.
J. Phys. Soc. Jpn, 77, 074709, 2008
24
Observation of ferroelectric domains by SHG
interferometry
a)Transmission image
SH images at 140K (b) and 50K (c).
d) SH image after annealing above Tco
and slow cooling at T=50K
e) SH intensity versus T
K. Yamamoto et al.
Appl. Phys. Lett. 96, 122901, 2010
25
Photoexcitation in -(BEDT-TTF)2I3
26
For ordinary feroelectrics, polarization is induced by lattice modulation.
In -(BEDT-TTF)2I3 , the crystal shows a monotonic lattice shrinkage without substantial
displacement of molecules (structural modulation makes a minor contribution to the polarizatio
The polarization is mainly due to the modulation of the electron distribution caused by CO.
The large value of SHGsignal indicates the growth of large polar domains.
Then, if polarization originates from electron phenomena,
fast response to external perturbations is expected.
Pump-probe measurement of the SHG : stimulation of the CO by strong pumping pulse
and recorded the induced variation of the SHG with a weaker probing pulse
Melting of the CO
See also the suppression of the SHG femtosecond signal in TTF-CA
Femtosecond photoresponse
27
The time required for CO melting and generation of the
metallic state is 15fs. at 20K. The early stage photo-induced
dynamics is driven by the electron response.
Additional slower (300fs) growth indicates interplay between
electron oscillations and vibrations
K. Yamamoto et al.
J. Phys. Soc. Jpn, 77, 074709, 2008
See also:Y. Kawakami et al. Phys. Rev. Lett. 105, 246402, 2010
Y. Tanaka and K. Yonemitsu, J. Pjys. Soc. Jpn. 79, 024712, 2010
H. Nakaya et al. Phys. Rev. B81, 155111, 2010
Photoinduced variation of reflectivity and second harmonic
generation (SHG) as the function of the delay time
between pump and probe pulses. T=100K.
Charge ordering from NMR in - (ET)2RbZn(SCN)4
28
13C-NMR
with H=8T normal to the conducting layers
With T decrease, the spectra become broadened
indicating a continuous molecule to molecule
distribution of Knight shift.
Below 195K, the spectrum consits of a broad line (A)
(with a much larger relaxation rate)and a paket doublet
(line B).

Separation of the BEDT-TTF into two inequivalent
molecules
Also from vibrational spectroscopy
K. Yamamoto et al. Phys. Rev. B65 (2002) 085110
from Miyagawa et al. Phys. Rev. B62 (2000) R7679
29
AC conductivity in - (BEDT-TTF)2RbZn(SCN)4
0
G/Go
10
-2
10
4
4x10
'
-4
10
3
4
5
6
-1
1000/T (K )
7
2x104
0
150
200
250
Temperature(K)
300
30
-3 orders of magnitude jump of conductivity at TMI
with hysteresis Tcooling=199K, Theating=204.5K
-charge gap below TMI = 1900K much larger than
in (TMTTF)2AsF6 with =350K
-’(T) shows a smooth monotonic increase from
room temperature, more sharp, closed to divergence
near TMI
-’(T) jumps down to a small magnitude sharply below TMI
-the same jump up of ’ in heating
The ’ growth above TMI may indicate the polarizability of the charge disproportionation
seen in NMR
The jump below TMI is associated with the 2c superstructure and the large charge gap

Role essential of the structural transition in the metal-insulating transition (posssibly
alter the symmetry
and the magnitude of the transfer integrals relative to V)
31
Effect of cooling rate on the CO in - (ET)2RbZn(SCN)4
1) conductivity
F. Nad, PM and H.M. Yamamoto
Phys. Rev. B76, 205101, 2007
32
Effect of cooling rate on the CO in - (ET)2RbZn(SCN)4
2) Dielectric permittivity
F. Nad, PM and H.M. Yamamoto
Phys. Rev. B76, 205101, 2007
33
Glass-like state in - (BEDT-TTF)2CsZn(SCN)4
F. Nad, PM and H.M. Yamamoto
J. Phys.: Conden. Matter 20, 485211, 2008
Neutral-ionic transition
The neutral-ionic transition is caused by the energy gain of the long range Coulomb interaction
overcoming the effective ionization energy of DA pairs.
There is a finite transfer energy between the D and A molecules in the mixe-stack CT compounds,
the degree of CT () between D and A is not equal to 0 and 1.
 is 0.3 and 0.7 between the N and I phases , respectively.
In the I phase, each molecule has spin S=1/2, constituting 1D spin chains,
which are dimerized due to the spin-Peierls mechanism.
Neutral chains
Ionic chains
34
35
Dielectric response TTF-p-cloranil (TTF-CA)
H. Okamoto et al.
Phys. Rev. B43, 8224, 1991
Ferroelectric domains
36
Electroreflectance microscopy
The domain wall (DW) is located between
the rightward polarized domain (IA) and the leftward one (IB)
The direction of spontaneous polarizations, Ps, in IA and IB are opposite
while the electric-field-induced polarizations, P, in IA and IB are parallel
These changes in polarization are accounted for the change in the charge transfer, 
H. Kishida et al.
Phys. Rev. B80, 205201, 2009
37
SHG in TTF-CA
Generation dynamics of metastable N phase fraction converted from the stable
ferroelectric I phase at 77K.
The second harmonic generation (SHG) disappears within 100ps.
The amount of converted N phase starts to increase around 100ps after the excitation
light pulse - flash illumination 1230fs - (60% at around 500ps delay time)
Luty et al.
Europhys. Lett. 59, 619, 2002
TTF-p-bromanil
TTF-BA
38
TTF and BA molecules are almost ionic. The
D+A- stacks can be regarded as a 1D
Heisenberg chain with spin 1/2.
F. Kagawa et al.
Nature Phys. 6, 169, 2010
39
Magnetoelectric coupling in TTF-BA
F. Kagawa et al.
Nature Phys. 6, 169, 2010