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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
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
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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