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

Prof. Kyuya Yakushi

Dr. Sergiy Boyko
Univ. Ontario Inst. Tech, CAN
Toyota RIKEN, Japan


Prof. Shinichiro Iwai
Tohoku Univ., Japan



Dr. Aneta A. Kowalska
Institute for Mol. Science
SHG Measurements
(JSPS Fellow)
Prof. Nobuyuki Nishi

Nagoya Inst. Tech.

SHG Measurements
SHG measurements

Ferroelectric Domain
Observation
Dr. Chikako Nakano
Institute for Mol. Science

Single Crystal Preparations
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
p
Ba2+
Ti4+
O2-
e.g.
NaNO2
Electronic Polarization
+
-
Dipolar Polarization
+
Ionic Polarization
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
M. Watanabe et al., JPSJ 2004
800
C=C str.
Mol. and Charge arrangements
in θ-RbZn Salt
OPTICAL CONDUCTIVITY (S/cm)
Optical conductivity spectrum of θ-(ET)2RbZn(SCN)4
Eex // a-axis
?
400
T=14 K
50
0
100K
0
130K
0
160K
0
0
180K
0
TCO~190K
0
200K
0
RT
1000
2000
3000
4000
WAVENUMBER (cm-1)
K.Yamamoto et al., Phys. Rev. B, 65, 085110 (2002).
Optical Conductivity of several CO systems
q-(ET)2TlZn(SCN)4
Optical Conductivity (arb. u.)
b’’-(ET)(TCNQ)
q-(BDT-TTP)2Cu(NCS)2
a-(ET)2I3
a’-(ET)2IBr2
1000
2000
3000
Wavenumber (cm-1)
4000
Isotope Shift Measurements
for θ-(ET)2RbZn(SCN)4
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
 (t )  H F (t )
H  Helec  H vib  Hemv
 (t ) g n Q- (t )

 H emv


 H F (t )   -ea 2   n F (t )
M. J. Rice, Solid State Commun. 31, 93 (1979).


g
g 
 total ( )  ( -ea 2) elec ( ) 1 Q ( )  elec ( ) 
QQ ( )  elec ( ) 


E
2
eg


2
 elec    
n
2ng n  n g
2
ng 2 -  2 - 2i
 g  2ng  n Q-  g
Q ( )   

2
2
2  ng -   2i
n 
 g 2
QQ ( )    Eeg
n 
-1
2
2
 2ng  n Q-  g

2
2
 ng -   2i
2
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?

Hemv
 nQ-
H F ~  nF
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
Stack I
A
A’
Stack II
Stack I
C
B
a
A
C
b
Space grp.: P-1
Z = 2, (4xET mols: A,A’,B,C)
S. Katayama, A. Kobayashi,
Y. Suzumura, JPSJ (2002)
P-1 -> P1 (T<TCO).
 T. Kakiuchi, H. Sawa et al., JPSJ, 2007.
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

Physical Properties of α-(ET)2I3
10 6
K. Bender et al., MCLC 1984
Stack II
10 4
10 2
10 0
10 -2
2
1
0
-1
-2
-3
-4
-5
b
B. Rothaemel et al. PRB 1986
built-in alternation
in overlapping
Cp
1400
1200
1000
800
(N.A. Fortune et al., SSC, 1991)
5.0
A’
C
S
4.0
S = 82% Rln2
3.0
2.0
(94% Rln2 by N.A. Fortune
et al., SSC 1991)
1.0
0
0
50
100
150
200
TEMPERATURE (K)
250
300
C=C str.
IR Spectrum of α-ET2I3
OPTICA L CONDUCTIVIT Y (arb. u.)
Eex // b-axis
200
300
TCO~135 K
150
136
130
120
100
60
4.8 K
1000
2000
3000
4000
-1
WAVENUMBER (cm )
Semi-Transparent Region in
Abs Spectrum of Organic Conductors
WAVELENGTH (nm)
5000
2000
1000
600
500
400
a
B
A
C
A'
O
b
700
Eex // a
Eex // b
2000
1400
OPTICAL CONDUCTIVITY(S/cm)
-
I3
1500
1000
Intramol.
500
CT
0
0
5000
10000
15000
-1
WAVENUMBER(cm )
20000
25000
Temperature Dependence of SHG
Pulse Energy
0.1
0.3
0.5
0.8
1.0 µJ
1.0
4.0
0.5
b
0.5 mm
SH INTENSITY (arb. u.)
a
3.0
0
128
2.0
130
132
134 136
T (K)
138
140
Single
Crysta
l
TCO=135 K
1.0
Excitation (w):1400 nm
SHG (2w): 700 nm
0
0
50
100
150
200
TEMPERATURE (K)
χij(2) (2 j ;  i, i) for l ()=1.4 mm
(Relative to BBO)
i,j
χij(2)
a,a a,b b,a b,b
21 8.5 44 31
K. Yamamoto et al., JPSJ, 2008
Stack I
Stack II
Stack I
+e
+e
A
+e
B
+e
A
A
B
B
+e
A’
+e
C
+e
Electric Dipole
A’
A’
+e
C
a
b
+e
C
3. Domain observation by means of SHG interferometry
Visualization of FE Domains by SHG Interferometry
Excitation (ω) Dipole Moment
SHG (2ω)
There is a phase shift between
SH waves from different domains
Sample [α-(BEDT-TTF)22I33]
Reference
(Single Domain)
Interference of SH Lights
SHG Contrast Image
Excitation (ω)
Scanning
Mirrors
PCControlled
Cryostat & Stage
Cooled PMT
fs Er-doped
Fiber Laser
Chopper
Objective
Lens
Filters
DCPreamp
Lock-in
Sapphire Cell
Reference
(Single Domain)
PC
SHG Interference Image of Ferroelectric Domains
T=140 K(> TCO)
T=100 K(< TCO)
SH INTENSITY
Transmission Image
a
b
Eex// a-axis
0.5 mm
a
b
100 μm
• 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
0.5
SH INTENSITY
SH Intensity (arb. units)
1.0
a
b
100 μm
0
40
60
80
100
120
140
160
Temperature (K)
K. Yamamoto et al., APL, 2010.
Variation of Domain Structure
a
b
200 μm
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
Sap p h ire
su b strate
Acrylic resin
Ep oxy resin
Sp ecim en
Fu n d am en tal Beam
SHG Beam
ca. 0.5 m m
Alu m in iu m
Cold fi n g er
Cop p er
LBO
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
Stack I
Stack II
α’-(BEDT-TTF)2IBr2
Stack II
Stack I
Stack I
A
A
a
B
B
A’
A’
C
b
a
B’
b
Triclinic P-1, Z=2 (4xBEDT-TTF in unit cell)
Stack II
Physical Properties of a-(ET)2I3 and a’-(ET)2IBr2
α-(BEDT-TTF)2I3
α’-(BEDT-TTF)2IBr2
10 6
K. Bender et al., MCLC 1984
10 4
T
10 2
(10 -4 emu/mol)
10 0
Tsipn
10 -2
2
1
0
-1
-2
-3
-4
-5
2.5
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
SHG
2.0
1.5
TSHG
1.0
K. Y. et al., JPSJ, 20081
0.5
0
Cp
1400
206K
1200
1000
800
(N.A. Fortune et al., SSC, 1991)
600
0
50
100
150
200
TEMPERATURE (K)
250
300
Toward Characteristics of Electronic FEs
100.000
Obj. Lens
140000
SH Intensity (arb. u.)
SH Intensity (arb. u.)
a-(ET)2I2Br
100000
60000
20000
Iex2
50
100
150
200
250
10.000
T=150 K
300
T (K)



1.000
0.010
0.100
1.000
10.000
Laser Power (mW)
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
x20
x10
-20000
0
x5
100.000