Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 54

complicated interactions among charge, spin, and lattice degree of freedom and the
time scale will be dominated by the magnitudes of the interactions, such as on-site
or intersite electron-electron (e-e) Coulomb interaction, spin-spin (s-s) interac-
tion, electron-lattice (e-l) interaction, and spin-lattice (s-l) interaction. By obser-
ving the dynamics of each degree of freedom separately by means of ultrafast
snapshots, we will be able to characterize the interactions dominating PIPTs, which
cannot be accessed in steady-state measurements and then be able to clarify the
mechanisms of PIPTs.
From the viewpoint of applications, ultrafast PIPTs are expected to be new
mechanisms for Tbit/s-class switching devices. For this purpose, it is important to
utilize a purely electronic transition with no structural changes. In this sense, some
material systems in which their electronic structures and physical properties are
dominated by electron correlations are good targets. Some of such materials indeed
show photoinduced changes of electronic states and their recovery within a few
picoseconds. These phenomena are expected to be utilized as future all-optical
switching devices. Another advantage of PIPTs for applications is that transport,
and magnetic properties as well as optical properties can be considerably modulated
by photoexcitation. This enables us to construct new switching or memory devices.
For the developments of such devices using PIPTs, discriminations of the dynamics
of different degrees of freedom are also important.
A key strategy toward realizing such PIPTs is the exploration of 1D systems. Since
1D systems have simple electronic structures as compared with 2D and 3D systems,
they will provide good opportunities for us to discuss the mechanism and dynamics of
PIPTs in detail. In addition, 1D systems essentially include instabilities inherent to
e-l and s-l interactions as well as the e-e interaction, and sometimes produce
characteristic phase transitions by lowering temperature or applying pressure.
Under the influence of these interactions, a small density of photoexcitations will
be able to stimulate instability of electronic states, and then drastic PIPTs may be
observed. In this sense, the halogen (X)-bridged transition metal (M) compound (the
MX-chain compound) focused on here, which are prototypical 1D systems with
strong e-e and e-l interactions, are good targets for realizing characteristic PIPTs.
In this chapter, we review several kinds of PIPTs observed in the MX-chain
compound (1) a photoinduced insulator-metal transition in the bromine-bridged
nickel-chain compound [ 23 ], (2) a photoinduced charge-density-wave (CDW) to
Mott-Hubbard phase transitions, and (3) a photoinduced insulator-metal transition
in the halogen-bridged palladium-chain [ 40 ] and platinum-chain compounds [ 25 ].
Here, we will briefly introduce the compounds discussed in this chapter and
summarize the concepts of the PIPTs they exhibit. The first example is a photoin-
duced insulator-metal transition (or a photoinduced Mott transition) in a bromine-
bridged nickel chain compound [ 23 ],

cyclohexa-
nediamine), which consists of nickel (Ni 3+ )-bromine (Br ) chains [ 41 ]. The crystal
and electronic structures of this compound are detailed in Chap. 4 . This compound
has an unpaired electron in the 3d orbital of each Ni site forming a half-filled 1D
electronic state. It is, however, not a metal due to the large Coulomb repulsion ( U )
among 3d electrons of Ni overcoming the electron transfer energy. Electrons are

[Ni(chxn) 2 Br]Br 2 (chxn ¼