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Fig. 4.13 Optical conductivity spectra with the light polarization (
E
i
)// the
b
axis (chain axis) at
room temperature for [Ni
1
x
Pd
x
(chxn)
2
Br]Br
2
(
x ¼
1), which has been established to be due to an intervalence charge transfer
(IVCT) transition from the occupied Pd(II) 4d
z
2
band to the unoccupied Pd(IV) 4d
z
2
band.
4.3.5 Local Electronic Structure of [Ni
1
x
Pd
x
(chxn)
2
Br]Br
2
Measured by Using STM
Scanning tunneling microscopy (STM) is a powerful tool for studying local elec-
tronic structures in these chain complexes. We performed STM measurements on
[Ni
1
x
Pd
x
(chxn)
2
Br]Br
2
in order to clarify the local electronic states [
11
].
Figure
4.14a
shows an STM image of [Ni(chxn)
2
Br]Br
2
in the range
200
200
˚
2
. Bright spots in the image were observed every 5
7
˚
2
. The Ni
Ni distances along the
b
(1D chain) and
c
axes were 5.16 and 7.12
˚
[
12
],
respectively, meaning that the spots reflect the periodicity of the [Ni(chxn)
2
] units
in the
bc
plane. In the STM image of [Pd(chxn)
2
Br]Br
2
, on the other hand, bright
spots were observed every 10
7
˚
2
, as shown in Fig.
4.14b
.ThePd
Pd
distances along the
b
(1D chains) and
c
axes were 5.29 and 7.07
˚
, respectively
[
2
], indicating that these spots reflect the twofold periodicity of the valence struc-
ture of the CDW state of [Pd(chxn)
2
Br]Br
2
. The phase of the CDW state is nearly
aligned in the
bc
plane, which is consistent with the X-ray diffuse scattering study.
Here we discuss the origin of the bright spots. Figure
4.15
shows band structure of
[Ni(chxn)
2
Br]Br
2
and [Pd(chxn)
2
Br]Br
2
. The valence and conduction bands of [Ni
(chxn)
2
Br]Br
2
are composed of the
p
z
band of the bridging Br
ions and the d
z
2
orbitals of the Ni ions, respectively [
16
]. On the other hand, the valence and
conduction bands of [Pd(chxn)
2
Br]Br
2
are the d
z
2
orbitals of the Pd(II) and Pd(IV)
ions, respectively. When STM is performed with a positive sample bias, a tunnel
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