Superconducting Technology Assessment - nitrd

INTRODUCTION TO SUPERCONDUCTOR
SINGLE FLUX QUANTUM CIRCUITRY
The Josephson junction (JJ) is the intrinsic switching device in superconductor electronics. Structurally a JJ is a
thin-film sandwich of two superconducting films separated by a very thin, non-superconducting, barrier material.
For the junctions of interest here, the superconductors are Nb and the barrier material is a dielectric, aluminum
oxide. The barrier is sufficiently thin, ~1nm, that both normal electrons and superconducting electron pairs can
tunnel through the barrier.
Electrically, a tunnel junction between normal metals is equivalent to an ohmic resistor shunted by a capacitor.
If the metals are superconductors, however, electron pairs can tunnel at the same rate as normal electrons. Only
pairs can tunnel at zero voltage, while both pairs and normal electrons tunnel if a voltage appears across the junction.
Pair tunneling gives rise to a third parallel channel, resulting in unique electrodynamics and highly-nonlinear
current-voltage characteristics.
I c R d
Current
Figure 1. Equivalent circuit of Josephson junctions. I C represents the nonlinear switch.
JJs are characterized by the equivalent circuit shown in Figure 1, where I C is the critical current (maximum current
at zero voltage) of the junction, C is the capacitance, and R d is the resistance. C is capacitance of the junction.
R d is the parallel combination of the internal junction resistance and any external shunt. Low-inductance shunts are
commonly employed in RSFQ circuits to control junction properties. The intrinsic junction resistance is very voltage
dependent as shown in Figure 2a. It is very large compared with typical external shunts for voltages below the energy
gap, V g ~2.7 mV for niobium (Nb), and very small at the gap voltage. Above the gap voltage, the resistance
becomes ohmic, with a value R N, the resistance of the junction in the non-superconducting state. The product I CR N
is approximately V g.
C
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Voltage
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