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Fig. 6.2 Current densities
are plotted versus electric
field for (a) [Ni(chxn) 2 Br]Br 2
and (b) [Ni(chxn) 2 Cl]Cl 2 .
The original data shown in (a)
and (b) are from [ 6 ] and [ 11 ],
respectively
a
[Ni(chxn) 2 Br]Br 2
80
0.8
60
0.6
90K
110K
125K
150K
175K
300K
40
0.4
20
0.2
0
0
0
1
2
Electric field (kV/cm)
b
[Ni(chxn) 2 Cl]Cl 2
0.03
400
300
0.02
180K
160K
140K
120K
100K
200
0.01
100
0
0
0
10
20
30
40
Electric field (kV/cm)
temperature phase. With increase of the temperature, the dimerization disappears. As
the NDR can be observed only in the low temperature phase, the large change of the
resistivity is assigned to the reduction of the degree of the dimerization [ 12 ]. On the
other hand, in the halogen-bridged nickel(III) compounds, the NDR disappears
between 150 and 300 K, though they show no temperature-induced phase transitions
at such temperature region. Therefore, the NDR in halogen-bridged nickel(III)
compounds cannot be assigned to the temperature-induced phase transitions. This
is in contrast to the reported NDR phenomena. Another class of one-dimensional
Mott insulator, Sr 2 CuO 3 ,alsoshowsNDR[ 15 ], but it also shows no temperature-
induced phase transition. As for this compound, the breakdown of the Mott-insulating
states is proposed. Similar mechanism might be a possible origin for the NDR in
halogen-bridged nickel(III) compounds.
6.3 The Current Oscillation
The spontaneous current oscillation based on the nonlinear-conducting behaviors
has been discussed for long years [ 2 , 3 , 16 , 17 ]. One of the strategies and examples
to achieve the spontaneous current oscillation using the NDR behaviors of organic
compounds was proposed in [ 6 ]. According to the proposed method,
the
 
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