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the direct evidence for the complete release of the bridging halogen displacements,
that is, for the complete CDW to MH transition within the photoinduced domains.
From these discussions, the CDW-MH transition dynamics by the weak excita-
tion can be schematically shown as Fig.
5.19
. By the photoirradiation, a large 1D
MH domain (ca. 70 Pt
3+
sites) is formed from a CT excited state within the temporal
resolution (i). Subsequently, the equilibrium positions of the I ions come on the
midpoints between the neighboring Pt ions (ii). At the same time, the coherent
vibration of the I ions is generated as
ð
i
Þ!ð
ii
Þ!ð
iii
Þ!ð
ii
Þ!ð
i
Þ!ð
ii
Þ
. The
photogenerated MH domain returns to the CDW ground state with ca. 20 ps.
5.5 Summary
In this chapter, we have reviewed the investigations of PIPTs in the three types of
MX-chain compounds. Under the influence of the strong electron-electron and
electron-lattice interactions, materials exhibit characteristic PIPTs, some of which
are driven in an ultrafast (ps or sub ps) time scale.
In Sect.
5.3
, a photoswitching from an insulator to a metal in a 1D Mott insulator
of the bromine-bridged Ni-chain compound has been detailed. The increase of the
excitation density changes the photoproduct from the midgap state to the metallic
state. A most significant feature of this PIPT is that the response time is quite fast.
The photoinduced metallic state is generated within the temporal resolution
(
180 fs) and recovers to the insulator with a time constant of 0.5 ps. This is the
first observation of the photoinduced Mott transition in the strongly correlated
electron systems.
In Sect.
5.4
, PIPTs in the CDW state of the halogen-bridged palladium-chain
compound, [Pd(chxn
Þ
2
Br]Br
2
, and platinum-chain compound, [Pt(chxn
Þ
2
I]I
2,
have
Þ
2
Br]Br
2
is located near the CDW-MH phase boundary,
and it was demonstrated that the irradiation of a fs laser pulse causes the instanta-
neous formation of a 1D Mott-Hubbard domain composed of ca. 20 Pd sites
without alternation of bridging-Br displacements. This result demonstrates the
important role of intersite Coulomb repulsion in the phase transition and in the
stabilization of the CDW state. The formation of the MH domain is followed by a
coherent oscillation with a period of 360 fs, attributable to the relaxation of the
bridging-Br displacements coupled with the charge transfer between neighboring
Pd ions. In [Pt(chxn
been discussed. [Pd(chxn
Þ
2
I]I
2
,
which is also located near the CDW-MH phase boundary,
the PIPT from the CDW phase to MH phase is found by means of the femtosecond
pump-probe reflection spectroscopy. The results have demonstrated the high effi-
ciency of the photoinduced CDW-MH transition in which ca. 70 Pt sites are
converted to MH state per photon. Such large conversion efficiency makes possible
the complete and transient transition from CDW to MH phase. In the
photogenerated MH domains, the bridging I ions come on the midpoints between
the neighboring Pt ions. With increase in the excitation density, the low-energy
spectral weight of absorption is increased, suggesting that the CDW-MH transition
is completed and the transition to the metallic state occurs.
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