NONLINEAR OPTICAL STUDY OF FERROELECTRIC ORGANIC CONDUCTORS

Kaoru YamamotoInstitute for Molecular Science (Japan)

International Research School and Workshop on Electronic Crystals ECRYS-2011

August 19, 2011

Collaborators

Prof. Kyuya YakushiToyota RIKEN, Japan

Prof. Shinichiro IwaiTohoku Univ., Japan

SHG Measurements

Prof. Nobuyuki NishiNagoya Inst. Tech.

SHG Measurements

Dr. Sergiy BoykoUniv. 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

Outline0. 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

e.g. NaNO2

p

e.g. BaTiO4

Fe2O4: N. Ikeda et al., Nature, 2005

Nad, Monceau, Brazovskii, PRL, 2001

+ +-Ba2+ Ti4+ O2-

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

Optical conductivity spectrum of θ-(ET)2RbZn(SCN)4

Mol. and Charge arrangementsin θ-RbZn Salt

K.Yamamoto et al., Phys. Rev. B, 65, 085110 (2002).

1000 2000WAVENUMBER (cm-1)

3000 40000

0

0

00000

800

400

RT 200K

TCO~190K 180K

160K 130K

100K

50 T=14 K

OPTI

CAL C

ONDU

CTIV

ITY

(S/cm

)

Eex

C=C

str.

1000 2000WAVENUMBER (cm-1)

3000 40000

0

0

00000

800

400

RT 200K

TCO~190K 180K

160K 130K

100K

50 T=14 K

OPTI

CAL C

ONDU

CTIV

ITY

(S/cm

)

Eex

?

C=C

str.

1000 2000WAVENUMBER (cm-1)

3000 40000

0

0

00000

800

400

RT 200K

TCO~190K 180K

160K 130K

100K

50 T=14 K

OPTI

CAL C

ONDU

CTIV

ITY

(S/cm

)

Eex

M. Watanabe et al., JPSJ 2004

　q-(ET)2TlZn(SCN)4

a-(ET)2I3

a’-(ET)2IBr2

q-(BDT-TTP)2Cu(NCS)2

b’’-(ET)(TCNQ)

1000 2000 3000 4000

Opt

ical

Con

duct

ivity

(arb

.u.)

Wavenumber (cm

q-(ET)2TlZn(SCN)4

a-(ET)2I3

a’-(ET)2IBr2

q-(BDT-TTP)2Cu(NCS)2

b’’-(ET)(TCNQ)

1000 2000 3000 4000

Opt

ical

Con

duct

ivity

(arb

.u.)

Wavenumber (cm

0

300

1500 2000 2500 3000

0

100

200

Opt

ical

Con

duct

ivity

(S/c

m)

E//a

13C

12C

Opt

ical

Con

duct

ivity

(S/c

m)

0

300

600

E//c

Wavenumber (cm-1)

13C

1500 2000 2500 3000

0

100

200

(b)

(c)12C

S

S

S

S

S

S

S

S

Optical Conductivity of several CO systems 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

2

gelec 2 2

g

2 g2

n

n n

n ni

- -

vib emvelec ( ) ( )FH H H t H tH

2

g g

2 2g

2( )

22n n

Qn n

Qgi

- -

222g g

2 2eg g

2( )

2n n

QQn n

QgE i

-

- -

12

total elec elec eleceg

( ) ( 2) ( ) 1 ( ) ( ) ( ) ( )2 Q QQ

g geaE

-

- -

M. J. Rice, Solid State Commun. 31, 93 (1979).

emv ( ) ( )

( ) 2 ( )F

H t g nQ t

H t ea n F t

-

-

Calculation Results

1000 1500 2000 2500 3000 3500 4000

100

200

300

400

(cm-1)

s (a

rb. u

.)

}

n3(ag)(Fundamental)

t = 180 meVg = 180 meV

K. Yamamoto et al., to appear in PRB

Comparison of Experiment and Calculation

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?

~FH nFemvH nQ -

Higher-order perturbation of H’emv Overtone (Anharmonicity)

Higher-order perturbation of H’F Nonlinear Optical Properties?

Formal equivalence between Q- and F

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

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

Stack I Stack II Stack I

AC

Ca

b

B

A

A’

Space grp.: P-1Z = 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

TEMPERATURE (K)

10 -2

100

102

104

106

-5-4-3-2-1012

0 50 100 150 200 250 300

B. Rothaemel et al. PRB 1986

K. Bender et al., MCLC 1984

CpS

800

1000

1200

1400

(N.A. Fortune et al., SSC, 1991)

0

1.0

2.0

3.0

4.0

5.0

S = 82% Rln2

b

Stack II

CA’

built-in alternationin overlapping

1000 2000 3000 4000

OP

TIC

AL

CO

ND

UC

TIV

ITY

(arb

. u.)

WAVENUMBER (cm -1)

300 200

TCO~135 K 150

130 136

120

604.8 K

100

C=C s

tr.

IR Spectrum of α-ET2I3

Eex //

Semi-Transparent Region inAbs Spectrum of Organic Conductors

0 5000 10000 15000 20000 250000

500

1000

1500

2000

5000 2000 1000 600 500 400WAVELENGTH(nm)

CT

Intramol.

OPTIC

ALCO

NDUC

TIVITY

(S/cm

)

WAVENUMBER(cm-1)

Eex // aE // b

I3-

ex

1400 7

a

bO

A

A'

B

C

Temperature Dependence of SHG

K. Yamamoto et al., JPSJ, 2008

i,j a,a a,b b,a b,b χij(2) 21 8.5 44 31

(Relative to BBO)

0 50 100 150 2000

1.0

2.0

3.0

4.0

SH IN

TENS

ITY (a

rb.u

.)

TEMPERATURE (K)

TCO=135 K

Excitation (w):1400 nm SHG (2w): 700 nm

128 130 132 134 136 1380

1.0

0.5

140T (K)

0.1 0.3 0.5 0.8 1.0 µJ

Pulse Energy

138 140128

SingleCrystal

a

b 0.5 mm

χij(2)(2 j ; i , i ) for l()=1.4 mm

+e+e+e+e +e+e

+e+e +e+e

C

BA

A’

Stack I Stack II Stack I

C

BA

A’

a

b

+e+e+e+e

+e+e +e+e

C

BA

A’

Electric Dipole

3. Domain observation by means of SHG interferometry

Visualization of FE Domains by SHG Interferometry

Sample [α-(BEDT-TTF)2I3]

Dipole Momen

Sample [α-(BEDT-TTF)2I3]

Excitation (ω) Dipole Moment SHG (2ω

Sample [α-(BEDT-TTF)2I3]

Excitation (ω) Dipole Moment SHG (2ω)There is a phase shift between

SH waves fromdifferentdomains

Reference(Single Domain)

Excitation (ω)

SHG Contrast Image

Interference of SH Lights

Reference(Single Domain)

Sample [α-(BEDT-TTF)2I3]

Excitation (ω) Dipole Moment SHG (2ω)There is a phase shift between

SH waves fromdifferentdomains

Chopper

fs Er-dopedFiber Laser

PCControlledCryostat &Stage

ObjectiveLens

ScanningMirrors

DCPreamp

Lock-in

PC

Filters

CooledPMT

SapphireCell

SHG Interference Image of Ferroelectric Domains

0.5 mm

Eex//a-axisb

a

Transmission Image

T=100 K

K. Yamamoto et al., APL, 2010.

• 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

T=140 K

100 μm

SH IN

TENS

ITY

b

a

(> TCO

) (< TCO

)

Constructive and Destructive Interference of SHG

100μm

SHIN

TENS

ITY

ba

40 60 80 100 120 140 160

0.5

0

1.0

SHIn

tens

ity(ar

b.un

its)

Temperature (K)K. Yamamoto et al., APL, 2010.

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)

200 μm

a

b

Summary1. 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

Nad, Monceau, Brazovskii, PRL, 2001

1mm

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)2X: P. Monceau et al., PRL 2001

- ---

- ---

- ---

Cold fingerCopper

Sapphiresubstrate

Acrylic resinEpoxy resin

Specimen

LBOAluminium

ca. 0.5 mm

Fundamental Beam

Pump-Probe Measurement of SHGcf. TTF-CA (organic ferroelectric)

-- - -

K. Yamamoto et al., JPSJ 2008

Time delay (ps)0 10 20 30

-0.6

-0.4

-0.2

0

Pulse Width=100 fs

Pulse ProfileSHG

Exponential Decay

pumpprobe

delay

SHG COIn

sulat

orPh

oto-

Induc

edMe

tal

a-(BEDT-TTF)2I3

Interplay of Charge and Lattice Pure-Electronic

T. Luty et al., Europhys. Lett., 2002.

Comparison of Crystal Structure

α-(BEDT-TTF)2I3 α’-(BEDT-TTF)2IBr2

Triclinic P-1, Z=2 (4xBEDT-TTF in unit cell)

C

BA

A’

B

B’

A

A’a

b

a

b

Stack II Stack I Stack IIStack I Stack II Stack I

Physical Properties of a-(ET)2I3 and a’-(ET)2IBr2

206K

Tsipn

TSHG

30K

alternating Heisenberg (S = 1/2)J1=106 K, J1/J2=0.35, N/NA=0.89

T

Y. Yue et al., 　 JPSJ, 2009

α-(BEDT-TTF)2I3 α’-(BEDT-TTF)2IBr2

10-1

102

105

108

0.5

1.0

1.5

2.0

2.5

00.51.01.52.02.5

0 50 100 150 200 250 3000

200

400

600

800

SHG

Cp

TEMPERATURE (K)

∆Cp

-3em

u/mo

00.51.01.52.02.5

SHG

Cp

TEMPERATURE (K)

10 -2

100

102

104

106

-5-4-3-2-1012

(10-

4 emu

/mol)

0 50 100 150 200 250 300600

800

1000

1200

1400

K. Bender et al., MCLC 1984

B. Rothaemel et al. PRB 1986

K. Y. et al., JPSJ, 20081

(N.A. Fortune et al., SSC, 1991)

Toward Characteristics of Electronic FEs

- ---

- ---

)))

)))

1.000

10.000

100.000

0.010 0.100 1.000 10.000 100.000L aser P ower (mW)

SH In

tens

ity (a

rb. u.

) x5Obj. Lens

x10x20

-20000

20000

60000

100000

140000

0 50 100 150 200 250 300T (K)

SH

Inte

nsity

(ar

b. u

.)

a-(ET)2I2Br

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

Iex2