Microstrip coupling techniques applied to thin-film Josephson junctions at microwave frequencies

Three different schemes for coupling to low impedance Josephson devices have been investigated. They all employ superconducting thin-film microstrip circuit techniques. The schemes are: (i) a quarterwave stepped impedance transformer, (ii) a microstrip resonator, (iii) an adjustable impedance transformer in inverted microstrip. Using single microbridges to probe the performance we found that the most primising scheme in terms of coupling efficiency and useful bandwidth was the adjustable inverted microstrip transformer.     

Quantum dynamics of the charge in Josephson tunnel junctions

It is shown that (quasi) charge of a Josephson tunnel junction can be considered as a macroscopic quantum degree of freedom. In the adiabatic limit we derive the effective action of the tunnel junction in the quasicharge representation and calculate the quantum decay rate of the charge states.     

Quantum fluctuations and quantum dynamics of small Josephson junctions

The quantum properties of small Josephson junctions with both linear Ohmic and quasiparticle (cosine) dissipation are investigated by means of a real-time path-integral technique. Quantum diffusion of the quasicharge and quantum corrections to the classical conductivity are analyzed. The current-voltage characteristics and the effect of voltage steps are studied. It is shown that coherent voltage oscillations may exist not only for small, but also for large values of the junction tunneling conductivity. For not very small values of the quasiparticle conductivity of the junction the frequency of these oscillations is twice the Bloch oscillation frequency. The quantum dynamics of the phase difference across the junction is studied for a wide region of parameters. It is shown that linear and cosine dissipation mechanisms cause the presence of two dissipative phase transitions, which are qualitatively different from each other in many aspects.     

Modeling input chains of a microwave amplifier with Josephson junctions

An analytical model for a SQUID amplifier with double Josephson junctions is proposed, with an equivalent scheme representing a long transmission line loaded on the junctions via a mutual inductance that varies along the line. The results of analytical calculations agree with the results of numerical simulations. An analysis shows that load matching can be reached provided that the electrical length of a microstrip coil with an idle-run end is approximately equal to a quarter of the wavelength in the transmission line.     

Critical current 0-pi transition in designed Josephson Quantum Dot junctions

We report on quantum dot based Josephson junctions designed specifically for measuring the supercurrent. From high-accuracy fitting of the current-voltage characteristics, we determine the full magnitude of the supercurrent (critical current). Strong gate modulation of the critical current is observed through several consecutive Coulomb blockade oscillations. The critical current crosses zero close to, but not at, resonance due to the so-called 0-pi transition in agreement with a simple theoretical model.     

Memory and logic circuits using semiconductor-barrier Josephson junctions

Theoretical and experimental studies on the use of semiconductor-barrier Josephson junctions in switching circuits are reported. This work includes memory loops as well as latching and nonlatching logic circuits. It has been found previously that the switching time of a single junction (67 ps) is comparable with a similar oxide-barrier junction and that the Q of the junction is considerably lower than the oxide-barrier counterpart, as was predicted theoretically, so cavity resonance effects are nearly absent. The memory loop switching time measured is comparable with those employing oxide-barrier junction. The current experimental work on logic circuits is using semiconductor-barrier junctions as they can be made to give the desired values of the McCumber parameter βcrequired to achieve nonlatching operation. In all cases the junctions used are Pb-Te-Pb sandwich structures.     

Spin-filter Josephson junctions

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years. Of particular interest has been the realization of so called π-junctions with a built-in phase difference, and induced triplet pairing. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (I(F)) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation and an unconventional temperature dependence of the critical current I(c), difficulties with the few known I(F) materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of I(c)is strongly modified by the I(F). In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the I(F) barrier can be modified by magnetic inhomogeneity in the barrier.     

Automatic material characterization at microwave frequencies

An instrument was assembled to measure complex permittivity at microwave frequencies in an efficient, inexpensive manner. It bridges the gap between traditional, tedious methods of characterization and expensive vector network analyzer techniques. A microcomputer is used to control the operation of a scalar network analyzer. Measurements of the magnitude of the reflection and transmission coefficients are automatically made as a function of frequency and temperature over a range of 2-18 GHz. A theory is developed covering the different regimes of operation: the thin sample, the multiple reflecting sample, and the high insertion loss sample. Sample thickness criteria for accurate measurements are reported. Results are presented for dielectric and semiconductor samples     

Microwave wideband tunable oscillators using coherent arrays of Josephson junctions

Coherent microwave radiation from arrays containing up to 100 indium microbridges has been directly detected. As expected the power scales as N²and the linewidth as 1/N for an array containing N junctions. Design criteria and fabrication methods for these arrays are discussed.     

Frequency-selective Josephson detector: Power dynamic range at subterahertz frequencies

The power dynamic range of an YBa₂Cu₃O_7−x frequency-selective Josephson detector for the Hilbert-transform spectroscopy was experimentally studied in the subterahertz frequency range. At a temperature of 80 K, the dynamic range of 2×10⁵ was achieved at a noise equivalent power of 8×10⁻¹⁵ W/Hz^1/2.     

Electrical properties of films at microwave frequencies

A technique suitable for the measurement of electrical properties of films/sheets at microwave frequencies has been described. Groups of materials are identified for which the absence of any kind of electrodes in these microwave measurements can be utilised with advantage. Results are presented for a variety of materials such as metal films, superionic conductors, semiconductors and insulating polymers.     

Quantum interference filters based on oxide superconductor junctions for microwave applications

We have studied superconducting quantum interference filters (SQIFs) based on bicrystal neodymium gallate substrates, which can be used in the microwave frequency range. The characteristics of a serial SQIF have been compared for the first time with those of a single superconducting quantum interference device (SQUID) and a chain of serially connected SQUIDs with equal areas of superconducting loops. The regime of SQIF operation with a voltage-flux (V-Φ) characteristic determined by the magnetic-field dependence of the critical current in the Josephson junction has been analyzed. It is shown that the output noise of a SQIF measured with a cooled amplifier in the 1–2 GHz range is determined by the slope of the V-Φ characteristic. The influence of a spread in the parameters of Josephson junctions in the SQIF on the integral V-Φ characteristic of the whole structure is considered.     

Magnetic field patterns in Josephson junctions

The critical currents of Pb/Ag/Pb Josephson junctions have been measured as a function of magnetic field and temperature in order to study the types of magnetic field distributions that occur in superconductor-normal metal-super-conductor (SNS) junctions. A study is made of the quantum interference regime near the critical temperature where the junction is filled with vortices and the crossover to Meissner-like behavior at low temperature occurs. Special emphasis is placed on the changes in effective area of the junction as the London and Josephson penetration depths change with temperature. A mapping of the crossover regime in the temperature-magnetic field plane is given.This work was supported by the office of Basic Energy Sciences of the Department of Energy.     

Two-particle tunneling current in Josephson junctions

The contribution to the tunneling current of a Josephson junction from the Two-particle tunneling, to the 2nd order approximation in the barrier transparency, is investigated. Expressions for the current onset amplitudes corresponding to eV = _1,2 are given together with the full expressions for the voltage and the temperature dependencies of the two-particles current. The theory has ben developed to take also into account corrections due to depairing mechanisms, which lead to the smearing of the current singularities. Introducing a depairing parameter , which accounts for the probability of these processes, I–V curves in the voltage region of the onset of single and two particle current are obtained. It is shown that, though having the same functional dependence, spreading occurs over a voltage range of different widths. In particular, it is shown that the width of the single-particle structure is twice larger the one for the two-particle. A careful investigation of the I–V curves in the region 2 -eV is presented and some aspects of the interesting voltage region near ¦₁-₂¦ is discussed.     

Dielectric measurements on PWB materials at microwave frequencies

In quest of finding new substrate for printed wiring board (PWB) having low dielectric constant, we have made PSF/PMMA blends and evaluated the dielectric parameters at 8.92 GHz frequency and at 35°C temperature. Incorporating PMMA in PSF matrix results in reduced dielectric constant than that of pure PSF. The dielectric parameters of pure PMMA and PSF films of different thicknesses have also been obtained at microwave frequencies. We have used dielectric data at microwave frequencies as a tool to evaluate optical constants, absorption index ‘K’ and refractive index ‘n’. The blends of PSF/PMMA may be used as base materials for PWBs.