Direct measurement of the Josephson plasma resonance frequency from I-V characteristics

The speed of Josephson circuits such as those required for Rapid Single Flux Quantum logic is limited by the Josephson plasma frequency, omega(p), which (with few exceptions) increases monotonically with increasing critical current density, J(c). Here we describe a new technique for directly measuring omega(p), using the current-voltage characteristics of a simple structure based on externally-shunted, series-connected tunnel junctions. We present both experimental and theoretical demonstrations of the technique and describe its application to the determination of the junction capacitance, a property poorly characterized for high-J(c) junctions.