1. Nonlinear optical study of ferroelectric organic conductors
International Research School and Workshop on Electronic Crystals ECRYS-2011
August 19, 2011
Nonlinear optical study of ferroelectric organic conductors
Kaoru Yamamoto
Institute for Molecular Science (Japan)

2. Collaborators Dr. Sergiy Boyko Univ. Ontario Inst. Tech, CAN
SHG measurements
Dr. Aneta A. Kowalska Institute for Mol. Science (JSPS Fellow)
Ferroelectric Domain Observation
Dr. Chikako Nakano Institute for Mol. Science
Single Crystal Preparations
Prof. Kyuya Yakushi Toyota RIKEN, Japan
Prof. Shinichiro Iwai Tohoku Univ., Japan
SHG Measurements
Prof. Nobuyuki Nishi Nagoya Inst. Tech.

3. Outline
0. Introduction to Electron FerroElectricity (FE) 1. Fano-like dip-shape signal (overtone of molecular vib) in IR spectrum of CO systems 2. FE CO revealed by Second-Harmonic Generation (SHG) in α-(ET)2I3 3. Ferroelectric domain observation by SHG interferometry

4. 0. Introduction Classification of FEs in terms of source of P
Ionic Polarization
Dipolar Polarization
Electronic Polarization p
e.g. NaNO2
Nad, Monceau, Brazovskii, PRL, 2001
e.g. BaTiO4
Fe2O4: N. Ikeda et al., Nature, 2005

5. 1. Fano-like dip-shape signal in IR spectrum of CO systems

6. Optical conductivity spectrum of θ-(ET)2RbZn(SCN)4
Mol. and Charge arrangements in θ-RbZn Salt
M. Watanabe et al., JPSJ 2004
K.Yamamoto et al., Phys. Rev. B, 65, (2002).

7. Isotope Shift Measurements for θ-(ET)2RbZn(SCN)4
Optical Conductivity of several CO systems

8. Anharmonic Electron-Molecular Vibration (EMV) Coupling in CO Cluster Model
Diatomic Dimer Model
Adiabatic Potential
M.J. Rice, SSC, 1979.

9. Calculation of Dynamic Electric Susceptibility: Higher-order perturbation effect of H’emv
M. J. Rice, Solid State Commun. 31, 93 (1979).
実際に行ったことは，

10. Comparison of Experiment and Calculation
Calculation Results
Comparison of Experiment and Calculation
K. Yamamoto et al., to appear in PRB

11. Relation between Anharmonic EMV Coupling and NLO
Dip-shape signal: vibrational overtone activated by higher-order effect of the emv coupling  Are there any physical properties connected with the overtone?
Formal equivalence between Q- and F
Higher-order perturbation of H’emv  Overtone (Anharmonicity)
Higher-order perturbation of H’F  Nonlinear Optical Properties?

12. 2. Second-Harmonic Generation in α-(ET)2I3

13. Two-Dimensional 3/4 Filled Complex: α-(ET)2I3
Molecular Arrangement and Charge Ordering
Metal-Insulator Trans. (=CO)
K. Bender et al., MCLC, ’84
Nonlinear Conductivity M. Dressel et al., J. Phys. I France, ’94
Charge Ordering H. Seo, C. Hotta, F. Fukuyama, Chem.Rev. ’04
Super Conductivity under uniaxial pressure N. Tajima et al., JPSJ, ’02
Zero-gap (Dirac-cone) state
A. Kobayashi, S. Katayama, Y. Suzumura, Sci. Technol. Adv. Mater., ’09
N. Tajima et al., JPSJ, ’06
Persistent Photoconductivity
N. Tajima et al., JPSJ, ’05
Photo-Induced Phase-Transition S. Iwai et al., PRL, ’07
S. Katayama, A. Kobayashi, Y. Suzumura, JPSJ (2002)
Space grp.: P-1 Z = 2, (4xET mols: A,A’,B,C)
P-1 -> P1 (T<TCO).
T. Kakiuchi, H. Sawa et al., JPSJ, 2007.

14. Physical Properties of α-(ET)2I3
built-in alternation in overlapping

16. Semi-Transparent Region in Abs Spectrum of Organic Conductors

17. Temperature Dependence of SHGχ ij
(2) (2 w j ; i , )
for l (
)=1.4 m
(Relative to BBO)
K. Yamamoto et al., JPSJ, 2008

19. 3. Domain observation by means of SHG interferometry

20. Visualization of FE Domains by SHG Interferometry
このようにコンセプトは理解しやすいが，電荷自由度が凍結する電荷秩序転移は金属の相転移。
金属が強誘電体になるということは，直感的には受け入れがたく，また検証も困難。

21. SHG Interference Image of Ferroelectric Domains
(> TCO)
(< TCO)
Transmission Image
SHG image splits into bright and dark regions for T < TCO
→ Generation of ferroelectric domains
Growth of large domains → P is cancelled by residual charge carriers
K. Yamamoto et al., APL, 2010.

22. Constructive and Destructive Interference of SHG
K. Yamamoto et al., APL, 2010.

23. Variation of Domain Structureb
Domain walls are shifted when crystal is annealed above TCO
→ Domains are mobile!! (though we have not succeeded in control by electric fields)

24. Summary 1. Dip-shape anomaly in IR spectrum:
▬ assigned to the overtone of molecular vibrations
▬ The activation is attributed to the anharmonic emv coupling associated with charge disproportionation
2. Activation of SHG along with CO in α-(ET)2I3
▬ verifies our hypothesis derived from the study of the overtone
▬ unambiguous proof of the generation of spontaneous polarization
3. Observation of SHG interference in α-(ET)2I3
▬ Ferroelectric domains are visualized for the first time
▬ Large domains: P is screened by residual charge carriers
▬ Mobility of domain walls is demonstrated

25. Temperature Dependence of SHG: (TMTTF)2SbF6
1mm
Nad, Monceau, Brazovskii, PRL, 2001

26. Concept of “Electronic FEs”
Uniform Chain
Centric
+ CO (N-I transition)
Centric
+ Bond Ordering
Non-centric
(e.g. TTF-CA)
Dimeric Chain
Centric +
Charge Ordering
Non-centric
(TMTTF)2X: P. Monceau et al., PRL 2001

28. Pump-Probe Measurement of SHG
cf. TTF-CA (organic ferroelectric)
a-(BEDT-TTF)2I3
T. Luty et al., Europhys. Lett., 2002.
K. Yamamoto et al., JPSJ 2008
Interplay of Charge and Lattice
Pure-Electronic

29. Comparison of Crystal Structure
α-(BEDT-TTF)2I3
α’-(BEDT-TTF)2IBr2
Triclinic P-1, Z=2 (4xBEDT-TTF in unit cell)

31. Physical Properties of -(ET)2I3 and ’-(ET)2IBr2
α-(BEDT-TTF)2I3
α’-(BEDT-TTF)2IBr2
K. Bender et al., MCLC 1984 Tr
Tsipn
30K
alternating Heisenberg (S = 1/2) J1=106 K, J1/J2=0.35, N/NA=0.89
B. Rothaemel et al. PRB 1986
Y. Yue et al.,　JPSJ, 2009
TSHG
K. Y. et al., JPSJ, 20081
206K
(N.A. Fortune et al., SSC, 1991)

32. Toward Characteristics of Electronic FEs
変位型強誘電体も電子強誘電体も安定状態は同等
我々は，電荷秩序という電子相転移で，確かに強誘電的な分極が現れることを確認したが
それほど明らかな違いはない。
両者の違いは分極状態へ至る過程にある
したがって，電子型強誘電体の真の特徴が現れるのは，分極相と無分極相が拮抗した絶妙な状態。
揺らぎの大きい。
単純にバンド幅を広げ，電荷秩序を不安定化するだけでない工夫。
例えば，幾何学的フラストレーションが強い系を探すのは一計。

33. Iex2 Spot size: d = 7.1 mm (x5 objective, l=1.55 mm)
Obj. Lens
a-(ET)2I2Br
Iex2 x5
x20
x10
T=150 K
Spot size: d = 7.1 mm (x5 objective, l=1.55 mm)
Laser: l=1.55 mm, t=100 fs, Rep.=20 MHz
Estimated excitation density for Iex = 500 mW:
Power: 1.28 kW/cm2
Energy: 64 mJ / cm2