Nonlinear Electrical Conductivity, Current Oscillation and Its Control in Halogen-Bridged Nickel(III) Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 86

high-resistance branch of the I - V curve to around 400 V. At this voltage, the sample
shows a sudden change in resistance to low resistance state, which is upper branch
of the I - V plot. After reaching the low-resistance branch, the current begins to
decrease along the low-resistance branch and jump down to the high-resistance
branch. After this, the above procedures are repeated. In this behavior, the trajectory
is not exactly coincident with the I - V curve. Particularly, in the low resistance state,
the inconsistency is conspicuous. The resistivity in low-resistance state is depen-
dent on the history of current flow. It may indicate that the Joule heating affects the
resistivity in the low resistance state to some extent.

6.4 Control of the Current Oscillation

Because the current oscillation is assisted by the external circuit, the period of the
oscillation can be controlled by the circuit constant of the external circuit. The period
of the current oscillation is calculated using a simple model and the nonlinear circuit
theory and described as shown below [ 6 ]. The total period is a sum of the transient
times within the high-resistance state

t high and low-resistance state t low

t low ¼ CR 1 ln E 1 V low
E 1 V high

(6.1)

and

t high ¼ CR 2 ln E 2 V high
E 2 V low

(6.2)

Here R 1 ; E 1 , R 2 , and E 2 are defined as follows:

R 1 ¼ðR L R a Þ=ðR L þ R a Þ (6.3)

E 1 ¼ R a V b =ðR L þ R a Þ (6.4)

R 2 ¼ðR L R g Þ=ðR L þ R g Þ (6.5)

E 2 ¼ðR g V b þ V x 0 R L Þ=ðR L þ R g Þ (6.6)

V low ( V high ) is the switching voltage from the low (high) to high (low) resistance state.
R a and R g are the differential resistance of high-resistance and low-resistance branch in
I - V plot. V x 0 is the x -intercept of the extrapolated line of the low-resistance branch.
Equations ( 6.1 )and( 6.2 ) indicate that the period tð¼ t low þ t high Þ canbecontrolledby

the external parameters, C , V b ,and R L .
In Fig. 6.5a , we show the C dependence of time evolution of the current
for Ni-Br measured at 90 K. With no capacitance (open) and small capacitance