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Ferroelectricity in organic low-dimensional 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

 

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Electronic ferroelectricity3

for a review: S. Ishihara, J. Phys. Soc. Jpn 79, 011010, 2010

Ferroelectricity induced by charge ordering4

J.Van der Brick and D. I. KhomskiiJ. Phys.: Condens. Matter 20, 434217, 2008

S. IshiharaJ. Phys. Soc. Jpn 79, 011010, 2010

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Ground state phase diagram of the 1D extended Hubbard model at 1/4 filling on the plane of U/t and V/t

S. Ejima et al.Europhys. Lett. 70, 051009, 2006

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

Shibata et al. Tsuchiizu et al. 2001

1/4 filled band

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Acknowledgements for samples

J. M. FabreLaboratoire de Chimie Organique, Montpellier, France

MT. Nakamura and K. FurukawaInstitute for Molecular Science, Okazaki, Japan

H. MüellerESRF , Grenoble, France

H. M. YamamotoRiken, Saitama, Japan

K. YamamotoInstitute for Molecular Science, Okazaki, Japan

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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. FabreJ. Phys. IV 9, Pr10, 1999

ECRYS 2008

8Charge disproportionation

D.S. Chow et al. Phys. Rev. Lett. 85 (2000) 1698

C13 NMR spectra for (TMTTF)2AsF6

Spectral splitting (~charge disproportionation order parameter)versus temperature

NMR measurements in an external field of 9T (fre 96.4 MHz)

Below TCO, doubling of the spectral line due totwo inequivalent molecules with unequal electron densities Charge disproportionation : 3:1 from T1

-1 measurements

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 behaviour.

9AC conductivity of (TMTTF)2AsF6

T dependence of the conductance

T dependence of the real part of the dielectricpermittivity, ’ at 100 et 300kHz, 1,3 and 10MHz

100kHz

Nad et al.: J. Phys.:Cond. Matter, 12(2000)L435

10Real part of dielectric constant of (TMTTF)2X salts

’ = ImG/

ReO4

SbF6

AsF6

PF6

1- For all anions: at T≈ T, there is no anomaly2- 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

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Imaginary part of the permittivity of(TMTTF)2AsF6

T > TCO = 101 K T <TCO

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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

Motion of domain walls

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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 disproportionationon 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

A ’ = ---------- T- TCO

1/ ’ (T) is close to be linearRatio AL / AH (AL at T TCO

AH at T>TCO)

in CSA: AL / AH ≈ 2

in ReO4 AL / AH ≈ 1.5

ReO4SbF6AsF6

PF

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Phys. Rev. Lett. 86 (2001) 4081

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(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 dimerizationas in -(ET)2X compounds, a singleelectron occupy the bonding state of each dimer

half-filled bandand to the insulating state due tothe effect of U[ called a dimer-Mott state -Hotta et al.: Chem. Rev. 114 (2004)]

Structures of and - (BEDT-TTF)2RbZn(SCN)4 and -(BEDT-TTF)2I3

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Y. Tanaka and K. YonemitsuJ. Phys. Soc. Jpn 79, 024712, 2010

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Stripe phases

Different spatial patterns of stripe phasesare 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

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Metal-insulating phase transition in -(BEDT-TTF)2I3

Abrupt phase transition at T=135.1K of first orderTransition slightly hysteretic in specific heat with latent heat[Fortune et al. Solid St. Comm 77 (1991) 265]

Change of intensity of Bragg peak (for reflectionsonly with an odd index with the a-component)(Nogami et al. Synth. Metals 16 (1986) 367And 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 diffractionfrom Heidmann etal.: Solid St. Comm. 84 (1992) 711

Dimerization of stacks I along the a axisof Peierls type

19Charge ordering from NMR in -(BEDT-TTF)2I3

from Takano et al. J. Phys. Chem. Solids 62 (2001) 393

13C-NMR 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 spectroscopyWojciechowski etal. Phys. Rev. B67 (2003) 224105

Charge disproportionation20

T. Kakiuchi et al. J. Phys. Soc. Jpn 76, 113702, 2997

Horizontal stripe structure

CD already above CO transition

From infrared spectroscopy, NMR, and x-ray

At room temperature:A=A’= 0.60B=0.68C=0.44

In the CO state:A=0.81A’= 0.26B=0.74C=0. 23

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ac conductivity in -(BEDT-TTF)2I3

Conductivity

Dielectric constant at 2MHz (sample 1)

Dielectric constant (sample 2)

Drop of ’ below TMI

Structural transition (dimerization)

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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)

-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

Low temperature dielectric response in -(BEDT-TTF)2I3

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Frequency dependence of the real (ε') and imaginary (ε'') part of the dielectricfunction in α-(BEDT-TTF)2I3 for E // [1¯10]. Below 75 K, two dielectric relaxation modes are observed

T. Ivek et al.Phys. Rev. B83, 165128, 2011

Optical second harmonic generation 24

K. Yamamoto et al.J. Phys. Soc. Jpn, 77, 074709, 2008

Activation of the even-order nonlinear optical phenomenom signifies the lack of inversion symmetry

Observation of ferroelectric domains by SHG interferometry

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K. Yamamoto et al.Appl. Phys. Lett. 96, 122901, 2010

a)Transmission imageSH images at 140K (b) and 50K (c). d) SH image after annealing above Tco and slow cooling at T=50Ke) SH intensity versus T

Photoexcitation in -(BEDT-TTF)2I3

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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 polarization)

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 pulseand 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

Photoinduced variation of reflectivity and second harmonic generation (SHG) as the function of the delay time between pump and probe pulses. T=100K.

K. Yamamoto et al.J. Phys. Soc. Jpn, 77, 074709, 2008

See also:Y. Kawakami et al. Phys. Rev. Lett. 105, 246402, 2010Y. Tanaka and K. Yonemitsu, J. Pjys. Soc. Jpn. 79, 024712, 2010H. Nakaya et al. Phys. Rev. B81, 155111, 2010

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

28Charge ordering from NMR in - (ET)2RbZn(SCN)4

from Miyagawa et al. Phys. Rev. B62 (2000) R7679

13C-NMR with H=8T normal to the conducting layers

With T decrease, the spectra become broadenedindicating a continuous molecule to moleculedistribution 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 spectroscopyK. Yamamoto et al. Phys. Rev. B65 (2002) 085110

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AC conductivity in - (BEDT-TTF)2RbZn(SCN)4

3 4 5 6 7

10-4

10-2

100

G/G

o

1000/T (K-1

)

1502002503000

2x104

4x104

'

( )Temperature K

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The ’ growth above TMI may indicate the polarizability of the charge disproportionation seen in NMRThe 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)

-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 divergencenear TMI

’(T) jumps down to a small magnitude sharply below TMI

-the same jump up of ’ in heating

Effect of cooling rate on the CO in - (ET)2RbZn(SCN)4

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F. Nad, PM and H.M. YamamotoPhys. Rev. B76, 205101, 2007

1) conductivity

Effect of cooling rate on the CO in - (ET)2RbZn(SCN)4

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2) Dielectric permittivity

F. Nad, PM and H.M. YamamotoPhys. Rev. B76, 205101, 2007

Glass-like state in - (BEDT-TTF)2CsZn(SCN)4 33

F. Nad, PM and H.M. YamamotoJ. Phys.: Conden. Matter 20, 485211, 2008

Neutral-ionic transition34

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

Dielectric response TTF-p-cloranil (TTF-CA)35

H. Okamoto et al.Phys. Rev. B43, 8224, 1991

Ferroelectric domains 36

H. Kishida et al.Phys. Rev. B80, 205201, 2009

The domain wall (DW) is located betweenthe rightward polarized domain (IA) and the leftward one (IB)

The direction of spontaneous polarizations, Ps, in IA and IB are oppositewhile the electric-field-induced polarizations, P, in IA and IB are parallelThese changes in polarization are accounted for the change in the charge transfer,

Electroreflectance microscopy

SHG in TTF-CA37

Luty et al.Europhys. Lett. 59, 619, 2002

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)

TTF-p-bromanil TTF-BA 38

F. Kagawa et al.Nature Phys. 6, 169, 2010

TTF and BA molecules are almost ionic. The D+A- stacks can be regarded as a 1D Heisenberg chain with spin 1/2.

Magnetoelectric coupling in TTF-BA 39

F. Kagawa et al.Nature Phys. 6, 169, 2010