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NONLINEAR OPTICAL STUDY OF FERROELECTRIC ORGANIC CONDUCTORS Kaoru Yamamoto Institute for Molecular Science (Japan) International Research School and Workshop.

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Presentation on theme: "NONLINEAR OPTICAL STUDY OF FERROELECTRIC ORGANIC CONDUCTORS Kaoru Yamamoto Institute for Molecular Science (Japan) International Research School and Workshop."— Presentation transcript:

1 NONLINEAR OPTICAL STUDY OF FERROELECTRIC ORGANIC CONDUCTORS Kaoru Yamamoto Institute for Molecular Science (Japan) International Research School and Workshop on Electronic Crystals ECRYS-2011 August 19, 2011

2 Collaborators Prof. Kyuya Yakushi Toyota RIKEN, Japan Prof. Shinichiro Iwai Tohoku Univ., Japan SHG Measurements Prof. Nobuyuki Nishi Nagoya Inst. Tech. SHG Measurements 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

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) 2 I 3 3. Ferroelectric domain observation by SHG interferometry

4 0. Introduction Classification of FEs in terms of source of P Ionic Polarization Dipolar PolarizationElectronic Polarization e.g. NaNO 2 p e.g. BaTiO 4 Fe 2 O 4 : N. Ikeda et al., Nature, 2005 Nad, Monceau, Brazovskii, PRL, 2001

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

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

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

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).

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

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? Higher-order perturbation of H’ emv  Overtone (Anharmonicity) Higher-order perturbation of H’ F  Nonlinear Optical Properties? Formal equivalence between Q  and F

12 2. Second-Harmonic Generation in α-(ET) 2 I 3

13 Two-Dimensional 3/4 Filled Complex: α-(ET)2I3 Molecular Arrangement and Charge Ordering S. Katayama, A. Kobayashi, Y. Suzumura, JPSJ (2002) 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 Space grp.: P-1 Z = 2, (4xET mols: A,A’,B,C) P-1 -> P1 (T

14 Physical Properties of α-(ET) 2 I 3 built-in alternation in overlapping

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16 Semi-Transparent Region in Abs Spectrum of Organic Conductors

17 Temperature Dependence of SHG K. Yamamoto et al., JPSJ, 2008 (Relative to BBO) χ ij (2) (2  j ;  i, i )for (  )=1.4  m

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19 3. Domain observation by means of SHG interferometry

20 Visualization of FE Domains by SHG Interferometry

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

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

23 Variation of Domain Structure Domain walls are shifted when crystal is annealed above T CO → Domains are mobile!! (though we have not succeeded in control by electric fields) a b

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) 2 I 3 ▬ 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) 2 I 3 ▬ 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) 2 SbF 6 Nad, Monceau, Brazovskii, PRL, mm

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

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

29 Comparison of Crystal Structure α-(BEDT-TTF) 2 I 3 α’-(BEDT-TTF) 2 IBr 2 Triclinic P-1, Z=2 (4xBEDT-TTF in unit cell)

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31 Physical Properties of  -(ET) 2 I 3 and  ’-(ET) 2 IBr 2 206K T sipn T SHG 30K alternating Heisenberg (S = 1/2) J1=106 K, J1/J2=0.35, N/N A =0.89 TT Y. Yue et al., JPSJ, 2009 α-(BEDT-TTF) 2 I 3 α’-(BEDT-TTF) 2 IBr 2 K. Bender et al., MCLC 1984 B. Rothaemel et al. PRB 1986 K. Y. et al., JPSJ, (N.A. Fortune et al., SSC, 1991)

32 Toward Characteristics of Electronic FEs

33 x5 Obj. Lens x10 x20 a-(ET) 2 I 2 Br T=150 K Spot size: d = 7.1  m (x5 objective, =1.55  m) Laser: =1.55  m, t=100 fs, Rep.=20 MHz Estimated excitation density for I ex = 500  W:  Power: 1.28 kW/cm 2  Energy: 64  J / cm 2 I ex 2


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