Josephson Junctions: Oscillators and Broadband Spectrometers at Frequencies up to 1 THz
Doctoral thesis, 1994
This thesis is devoted to some selected applications of the Josephson junction oscillator. The initial work was performed using low-Tc Nb/PbBi tunnel junctions, while the later parts were done using the recently developed high-Tc YBa2Cu3O7 (YBCO) Josephson junctions. The latter junctions have raised the hope for Josephson oscillator applications with substantially increased operation frequencies and temperatures.
In the first part of this thesis the use of inductor-resistor (LR) shunted tunnel junctions in arrays of phase locked Josephson junction oscillators was investigated. The LR shunt is designed to resonate out the intrinsic tunnel junction capacitance at the operation frequency. We show that LR shunted tunnel junctions have important advantages compared to the commonly used resistively shunted tunnel junctions; i.e. narrower linewidths, higher impedance of the junctions and that they can be optimized also at frequencies higher than about 100 GHz. Design rules for stable, non-chaotic oscillations with optimized output power were developed. Measurements on a non- optimized circuit show that oscillator arrays of LR shunted tunnel junctions can generate substantial power, ~10 nW to a 300 .OMEGA. load at 390 GHz from a 10 junction array.
In the second part the phase locking of YBCO Josephson junctions was clearly demonstrated for the first time. Mutual phase locking of two junctions was observed at frequencies between 0.2 and 1 THz, and was only limited by the bandwidth of the coupling circuit. The locking was in good agreement with simulations based on the resistively shunted junction model. The interaction of a single Josephson junction with a 5 junction oscillator array was also measured at frequencies up to about 350 GHz, but was substantially reduced by the high losses of the coupling circuit. The YBCO Josephson junctions were found to be interesting candidates for microwave generation at frequencies approaching 1 THz and oscillator arrays to be mainly limited by the high losses of the coupling circuits at these frequencies.
In the third part the interaction of YBCO Josephson junctions with integrated resonators were studied for the first time. Using a novel method, Josephson Broadband Spectroscopy (JOBS), the high frequency properties of the resonators were measured with a bandwidth of up to 1 THz and in the temperature range 4-77 K. The YBCO surface resistance (9 m.OMEGA. at 4 K and 100 GHz) was found to scale as f2 and had a temperature dependence in good agreement with the two-fluid model, modified with 18% non-pairing charge carriers. The YBCO London penetration depth was frequency independent (460 nm at 4 K) and had a temperature dependence in agreement with the two-fluid model.