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5.4 Photoinduced Phase Transition in CDW State of Halogen-
Bridged Palladium-Chain and Platinum-Chain Compounds
In this section, we report another typical example of PIPTs in MX-chain
compounds, that is, the photoinduced transition from the CDW state to the
Mott-Hubbard state observed in the bromine-bridged Pd compound. In this photo-
induced transition, the valences of Pd ions are changed just after the photo-
irradiation. Subsequently, the bridging halogen displacements are released, giving
rise to the generation of coherent oscillations of bridging halogen ions. In the
iodine-bridged platinum compound with more itinerant electronic states, the
CDW ground state can be converted not only to the Mott-Hubbard state but also
to the metallic state by strong photoexcitations. Dynamical aspects of these
transitions are discussed in detail.
5.4.1 Ultrafast Photoinduced Transition from Charge-Density-
Wave State to Mott-Hubbard State in Bromine-Bridged
Palladium-Chain Compound
In this subsection, we discuss the CDW to MH transition, in [Pd(chxn)
2
Br]Br
2
,
shows the polarized reflectivity spectrum of [Pd(chxn)
2
Br]Br
2
with the CDW state
for the light polarization (
E
i
) oriented parallel to the chain axis
b
(i.e.,
E
i
//
b
). The
reflectivity spectrum of
[Pd
0.84
Ni
0.16
(chxn)
2
Br]Br
2
in the MH state is also
shown for comparison [
47
], and the corresponding
spectra obtained by the
Kramers-Kronig transformation (KKT) of the
R
spectra are shown in Fig.
5.10b
.
The broad peak at ca. 0.7 eV in [Pd(chxn)
2
Br]Br
2
is due to the charge-transfer (CT)
transition,
s
Pd
2
þ
;
Pd
4
þ
Þ!ð
Pd
3
þ
;
Pd
3
þ
Þ
ð
(see Fig.
5.10b
). The
s
spectrum for
Br
2
exhibits a peak structure at ca. 0.55 eV attributable to
the Mott-gap transition,
Pd
0
:
84
Ni
0
:
16
ð
chxn
Þ
2
Br
Pd
3
þ
;
Pd
3
þ
Þ!ð
Pd
2
þ
;
Pd
4
þ
Þ
(Fig.
5.10b
)[
47
]. Consid-
ering the Ni concentration, we call in the following these two compounds, [Pd
(chxn)
2
Br]Br
2
and [Pd
0.84
Ni
0.16
(chxn)
2
Br]Br
2
,as
x ¼
ð
0.16 compounds,
respectively. Figure
5.10e
shows the transient reflectivity change (
0 and
x ¼
D
R
) for
E
i
//
b
using the 1.55-eV pump with polarization parallel to
b
. The averaged excitation
photon density
x
ph
within the absorption depth (450
˚
) is 0.025 photon (ph.)/Pd.
Here,
x
ph
was evaluated from the absorption coefficient (2.23
10
5
cm
1
) and the
reflection loss (61 %) of the pump light. Immediately after the photoexcitation
(
t
d
¼
0.25 ps), the reflectivity at 0.8-2 eV decreases and the reflectivity below
0.8 eV increased. The spectral shape of the transient
D
R
is similar to the differential
spectrum, i.e.,
0) in Fig.
5.10c
, indicating that the MH
state is generated by the photoexcitation in a CDW background.
D
R ¼ R
(
x ¼
0.16)-
R
(
x ¼
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