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)

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.

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

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

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

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, 085110 (2002).

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

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

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，

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

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?

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

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.

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

Semi-Transparent Region in Abs Spectrum of Organic Conductors

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

3. Domain observation by means of SHG interferometry

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。

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.

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

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

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

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

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

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

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

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)

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。

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