Expanding Possibilities of π electronsin Molecular Conductors

Reizo KATO (RIKEN)

• Small in Quantity and Size, Fragile

: Clean, Soft, Clear

• Multiple Extreme Conditions: Strong Magnetic Field, High Pressure, Low Temperature

Molecular p-systems

p-frustration system(EtMe3P[Pd(dmit)2]2 )

p-FET(k-(ET)2Cu[N(CN)2]Br / SiO2)

• High Quality Measurements: Sensitivity, Accuracy, Resolution

Au SiO2

• Spin • Charge• Orbital (HOMO-LUMO interplay) • Lattice

Pd(Cation)2

Exotic Properties of p ElectronsBased on Various Degrees of Freedom

in a Mott system with a quasi-triangular lattice

Layer 1 Layer 2 a

o c Cation Cation

Dimer

Cation

Et2Me2SbEtMe3SbMe4SbEt2Me2AsEtMe3AsMe4AsEt2Me2PEtMe3PMe4PEt2Me2NEtMe3NMe4N

HOMO

LUMO

DE

HOMO-LUMO Level Crossing in the [Pd(dmit)2]2 Dimer

Strong dimerization: [Pd(dmit)2]2

Small energy gapΔE betweenHOMO andLUMO

E. Canadell, et al., Solid State Commun. 75 (1990) 633

Conduction Band· Two-dimensional· Half-filled· Narrow

Mott insulatorat ambient pressure

DimerThe stronger dimerizationprovides the larger gap.

>A > t ≥ t’Transfer integral

Electrons move from dimer to dimer (Tight-binding approximation)

Band width W

Deviation from the regulartriangular lattice（ t’/t ）

Electron

A / W Correlation parameter

Effective on-site Coulombenergy on the dimerM. Tamura and R. Kato, JPSJ, 73, 3108 (2004)

Frustration parameter

At ambient pressure, most of Pd(dmit)2 salts behave as Mott insulators where one spin is localized on each dimer.

Electronic Structure of (Cation)[Pd(dmit)2]2

o b

at’ t

tA

Magnetic frustration

?

prevents long-rangeantiferromagnetic order

[Pd(dmit)2]2-

E(k) = 2.0 [t’ cos ka + t cos k(a-b) + t cos kb]

EtMe3Sb

Me4P

Me4As

EtMe3As

Me4SbEt2Me2AsEt2Me2P

b’-(Cation)[Pd(dmit)2]2

The antiferromagnetic ordering is not the only way to remove the frustration Gapless Spin Liquid

T. Itou et al., PRB, 77 (2008) 104413

Antiferromagnetic Insulator

(Frustrated)ParamagneticInsulator

T / K

Magnetic susceptibility

EtMe3Sb

Me4P

100 200 3000

2

4

6

8× 10-4

T / Kc

/ cm

3 mol-1

0

TN

Et2Me2Sb[Pd(dmit)2]2

Mott Insulator with a Quasi-Triangular Lattice

2 Dimer- → Dimer 0 + Dimer 2- (Charge Ordering)

Et2Me2Sb

Valence bond (VB) formation-Transitions towards a singlet state-

CO + VBS

0

-1

0

-1

-0.5

-0.5VB

Et2Me2Sb EtMe3P (Triclinic) EtMe3P (Monoclinic)

CO(Charge Ordering)

-1

-1

00

-0.5

-0.5

M. Tamura et al, CPL, 411 (2005) 133

VBS(Valence Bond Solid

-0.5

-0.5VB

M. Tamura et al, JPSJ, 75 (2006) 093701 　　

EtMe3Sb

Me4P

Me4As

EtMe3As

Me4SbEt2Me2AsEt2Me2P

Et2Me2Sb

b’-(Cation)[Pd(dmit)2]2

Antiferromagnetic Insulator

(Frustrated)ParamagneticInsulator

T / K

Pressure-induced metallic and superconducting states

Tc = 6 K

Et2Me2As

Uni-axial Strain (//b)Me4As

Photon energy (eV)

Ref

lect

ivity

0

-2

-1

Pump: 1.58 eV (E//a)Probe: 1.31 eV (E//a)

0 10 200.0

0.2

0.4

0.6

0.8

DR/R

Delay Time (ps)

50 K

1.4 × 1019 / cm3

2.8 × 1019 / cm3

5.6 × 1019 / cm3

1.1 × 1020 / cm3

2.3 × 1020 / cm3

DR/R = (R – R50K) / R50K

Pump-probe time resolved spectroscopyfor Et2Me2Sb[Pd(dmit)2]2 (TCO = 70 K)

The lifetime of the photo-induced state is short within 20ps.

Koshihara Group(TIT, ERATO)

Excitation intensity

EtMe3P[Pd(dmit)2]2 (Monoclinic): Superconductivity and Valence Bond Solid

Y. Shimizu et al, Phys. Rev. Lett., 99 (2007) 256403

The valence bond solid phase neighbors the superconductor and metal phases

VBS transition

a=4.08, b=4.10, c=4.12, d=4.15 kbar

k-(BEDT-TTF)2Cu[N(CN)2]Br

• n-type• m = 94 cm2 / Vs• On / Off ratio > 107

Y. Kawasugi et al, Appl. Phys. Lett., 92, 243508 (2008).

Field effect transistor (FET) behavior ofπ electrons in an organic Mott insulator

Au

SiO2

Organic Mott FET

Expanding Possibilities of π electronsin Molecular Conductors

Reizo KATO (RIKEN)

“Advanced Tools” inspire “Advanced Sciences”.

IMSSfor

In collaboration withA. Tajima, Y. Kawasugi, M. Hosoda, N. Tajima, H. M. Yamamoto, K. Kubo, Y. Ishii,A. Nakao (KEK), A. Fukaya (IHI), T. Yamamoto (Osaka Univ.), Y. Shimizu (NagoyaUniv.), M. Tamura (Tokyo Univ. of Science), K. Tsukagoshi (AIST)

p-frustration systemBulk crystal

p-FETInterface

Au SiO2