Results

The current and voltage data acquired for a Josephson junction in the absence of a magnetic field, at a temperature of 4.2K is shown below.

Fig.5: Plot of the voltage-current characteristic observed for the junction as the current rose form -40mA to 40mA. The effect of hysteresis has been circled in red.

Fig.6: Plot of the voltage-current characteristic observed for the junction as the current decreased form 40mA to -40mA. The effect of hysteresis has been circled in red.

The effects of hysteresis are clearly observed from the above graph. Whenever the current approaches the critical current from a region of finite voltage, a smoothening out of the theoretical discontinuity is observed. This effect causes an apparent overestimation of the critical current. In light of this, the critical current can only be correctly determined by examining the transition as the current approaches the critical current from a zero voltage regime. From Figs. 5 & 6, the critical currents we measured were (27.97

0.08) mA and (-34.190.08) mA, respectively. From measurements of the ohmic regions of the above figures, the normal state resistance was found to be

where T is the temperature of the junction, k is Boltzmann’s constant, Δ is the binding energy of a Cooper pair, or, the energy difference between electrons in a Cooper pair and two unpaired electrons, and R_n is the resistance of the superconductor in its normal, or non-superconducting, state. An approximation of

for T below the critical temperature of the superconductor is given by [10]

where and [11]. Using Eq. 8 for T=4.2 K, = 9.25 K, and as given above, a theoretical value for the current was obtained of (9.70.2) mA.