Expanding Possibilities of π electronsin Molecular Conductors
Reizo KATO (RIKEN)

Molecular p-systems: Clean, Soft, Clear• Small in Quantity and Size, Fragile• High Quality Measurements: Sensitivity, Accuracy, Resolution• Multiple Extreme Conditions:
Strong Magnetic Field, High Pressure, Low Temperature

SiO2

p-frustration system(EtMe3P[Pd(dmit)2]2 )Aup-FET(k-(ET)2Cu[N(CN)2]Br / SiO2)

Exotic Properties of p Electrons
Based on Various Degrees of Freedom
in a Mott system with a quasi-triangular lattice(Cation)

Pd2Cation
Me4N
Me4PMe4As
Me4Sb

EtMe3N
EtMe3P
EtMe3As
EtMe3Sb

Layer 1a

Et2Me2N
Et2Me2P
Et2Me2As
Et2Me2Sb

Layer 2co
CationCationDimer• Spin
• Orbital (HOMO-LUMO interplay)• Charge• LatticeHOMO-LUMO Level Crossing in the [Pd(dmit)2]2 Dimer
Strong dimerization: [Pd(dmit)2]2

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

Small energy gap
ΔE between
DEHOMO
andLUMO

HOMODimer

Mott insulator
at ambient pressure

The stronger dimerization
provides the larger gap.
E. Canadell, et al., Solid State Commun. 75 (1990) 633

Electronic Structure of (Cation)[Pd(dmit)2]2Electrons
move
fromofdimer
to dimerAt ambientpressure,
mostPd(dmit)
2 salts behaveapproximation)
as Mott (Tight-binding
insulators whereone spin is localized on each dimer.
Electron

Magnetic frustration?

[Pd(dmit)2]2-aE(k) = 2.0 [t’ cos ka+ t cos k(a-b)+ t cos kb]

ott’b

Effective on-site Coulomb
energy on the dimer

At
Transfer integral

A >> t ≥ t’

M. Tamura and R. Kato,JPSJ, 73, 3108 (2004)

A/W
Correlation parameter

prevents long-range
antiferromagnetic order
Deviation from the regulartriangular lattice（ t’/t ）Band width W

Frustration parameter

Mott Insulator with a Quasi-Triangular Latticeb’-(Cation)[Pd(dmit)2]2

T/K

Me4As

EtMe3As
Et2Me2P
Et2Me2As

Me4Sb

Antiferromagnetic
Insulator

EtMe3Sb

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

8× 10-4

Et2Me2Sb[Pd(dmit)2]2
c / cm3 mol-1(Frustrated)
Paramagnetic
Insulator

Me4P

Magnetic susceptibility

EtMe3Sb

6Me4P4TN2002 Dimer- →Dimer 0 + Dimer 2(Charge Ordering)
100200300T/K

The antiferromagnetic ordering is not
Et2Me2Sb the only way to remove the frustration

Valence bond (VB) formation-Transitions towards a singlet stateEt2Me2Sb

EtMe3P (Triclinic)

CO(ChargeOrdering)

EtMe3P (Monoclinic)

VBS(Valence BondSolid

CO + VBS-0.5-1-0.5-1-0.5-0.5-1-0.50-0.5

00
M. Tamura et al, CPL,411 (2005) 133

VB0-1
M. Tamura et al, JPSJ,
75 (2006) 093701

VB

Pressure-induced metallic and superconducting statesb’-(Cation)[Pd(dmit)2]2

Me4P

T/K

Me4As(Frustrated)
Paramagnetic
Insulator

Uni-axial Strain (//b)
Me4As
Et2Me2As

EtMe3As
Et2Me2P
Et2Me2As

Me4Sb

Antiferromagnetic
Insulator

EtMe3Sb

Et2Me2Sb

Tc = 6 K

Pump-probe time resolved spectroscopy
for Et2Me2Sb[Pd(dmit)2]2 (TCO = 70 K)
Koshihara Group(TIT, ERATO)
Probe: 1.31 eV (E//a)
The lifetime of the photo-induced
state is short within 20ps.

Pump: 1.58 eV (E//a)

0.80-2

Excitation intensity

50 K

2.3 × 1020 / cm30.6DR/R

Reflectivity-1DR/R = (R – R50K) / R50K

1.1 × 1020 / cm3
5.6 × 1019 / cm3
2.8 × 1019 / cm30.4

1.4 × 1019 / cm3

0.2
0.0

Photon energy (eV)010

Delay Time (ps)20

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

The valence bond solid phase neighbors
the superconductor and metal phases
a=4.08, b=4.10, c=4.12, d=4.15 kbar
Y. Shimizu et al, Phys. Rev. Lett., 99 (2007) 256403

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

SiO2Auk-(BEDT-TTF)2Cu[N(CN)2]Br

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

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

Expanding Possibilities of πIMSS
electrons
forin Molecular Conductors
Reizo KATO (RIKEN)“Advanced Tools” inspire “Advanced Sciences”.

SiO2

p-frustration system
Bulk crystal

Au

p-FET
Interface

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)