Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 57

Variable
delay stage

Optical
chopper

pump

τ d

Ti : sapphire
regenerative
amplifier

OPA

sample

OPA

probe

Trigger

Photo detector
(signal)

Photo detector
(reference)

OPA
= Optical Parametric Amplifier

Boxcar
Integrator

A/D
converter

PC

Trigger

Fig. 5.2 Schematic of experimental setup for femtosecond pump-probe reflection measurement

5.3 Photoinduced Phase Transition in Mott-Insulator State
of Halogen-Bridged Nickel-Chain Compound

Since the discovery of high- T C superconductivity, doping-induced insulator
(I)-metal (M) transitions or filling-control Mott transitions in 3d transition-metal
compounds have been attracting much attention. In most undoped 3d transition-
metal oxides, electrons are localized on atomic sites due to the large on-site
Coulomb repulsion energy U , forming antiferromagnetic insulators (Mott
insulators). Their electronic and magnetic properties can, however, be modified to
a large extent by chemical doping [ 50 - 52 ]. The high- T C superconductivity that
emerges in the hole- or electron-doped layer-structured cuprates is the most drastic
example. Photoexcitation is another effective method to create carriers in materials
[ 53 ]. In this section, we describe the photoinduced I-M transition of the bromine-
bridged Ni-chain compound, Ni(chxn

Br 2 , which is a prototypical 1D Mott
insulator. Photocarrier doping by using a 130-femtosecond laser pulse on the Ni
chain induces a marked change of the electronic structure. When the photoexcita-
tion density exceeds 0.1 per Ni site, a Drude-like high-reflection band emerges in
the IR region, indicating the formation of a metallic state. Ultrafast dynamics of the
photoinduced metallic state will be discussed on the basis of the results of temporal
and doping-density dependence of the reflectivity spectra.
Before the detailed discussion on the photoinduced I-M transition in the Ni-Br
chain compound, we will review the studies of the filling-control Mott transition in
the 2D cuprates, as a prototypical example of the I-M transition in strongly
correlated electron systems. In a typical undoped cuprate, La 2 CuO 4 , the crystal
structure of which is shown in Fig. 5.3a , hole carries can be introduced to the
CuO plane (the ab plane) by substituting La 3+ ions with Sr 2+ ions [ 50 , 54 ]. As the

Þ 2 Br