Nonlinear optical study of ferroelectric organic conductors

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

2. Collaborators • Dr. SergiyBoykoUniv. Ontario Inst. Tech, CAN • SHG measurements • Dr. Aneta A. KowalskaInstitute for Mol. Science(JSPS Fellow) • Ferroelectric Domain Observation • Dr. Chikako Nakano Institute for Mol. Science • Single Crystal Preparations • Prof. KyuyaYakushiToyota RIKEN, Japan • Prof. Shinichiro IwaiTohoku Univ., Japan • SHG Measurements • Prof. Nobuyuki NishiNagoya Inst. Tech. • SHG Measurements

3. Outline 0. Introduction to ElectronFerroElectricity(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. IntroductionClassification 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 arrangementsin θ-RbZn Salt M. Watanabe et al., JPSJ 2004 K.Yamamotoet al., Phys. Rev. B, 65, 085110 (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 Adiabatic Potential Diatomic Dimer Model 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).

10. 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 perturbationof 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 underuniaxial 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-1Z = 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 alternationin overlapping

16. Semi-Transparent Region inAbs Spectrum of Organic Conductors

17. (2) w w w l w m χ (2 ; , ) for ( )=1.4 m ij j i i Temperature Dependence of SHG (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

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. a b Variation of Domain Structure 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 a-(BEDT-TTF)2I3 cf. TTF-CA (organic ferroelectric) 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)2I3and ’-(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 Y. Yue et al.,JPSJ, 2009 B. Rothaemel et al. PRB 1986 TSHG K. Y. et al., JPSJ, 20081 206K (N.A. Fortune et al., SSC, 1991)

32. Toward Characteristics of Electronic FEs

33. Obj. Lens Iex2 a-(ET)2I2Br 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 forIex = 500 mW: • Power: 1.28 kW/cm2 • Energy: 64 mJ / cm2