Point Contact Josephson Junctions Research Articles

Monitoring the NSL Standard of EMF Using the AC Josephson Effect

The techniques used for the monitoring of the NSL standard of emf using the ac Josephson effect are reported. The standard has been monitored since 29th May 1970 and the reported measurements span a period of 482 days. These measurements have predominantly used niobium point contact Josephson junctions operated at the 1 mV level.The mean of the set of standard cells comprising the NSL standard has been determined in terms of a `Josephson frequency'. The value of this frequency at the time of the NBS-NSL volt transfer was 492.590 64 THz and a value of 2 e/h = 483.593 80 ± 0.1 ppm THz/VNSL has been obtained for this period.

The Josephson junction as a frequency analyser and mixer of submillimetre radiation sources

In an extension of earlier work, the response of niobium-niobium point-contact Josephson junctions to radiation from hydrogen cyanide and deuterium cyanide lasers has been studied. Constant voltage steps corresponding to the 805, 891 and 964 GHz lines from a HCN laser have been observed as well as steps corresponding to the frequency mixtures of all these lines. In addition, subharmonic and harmonics of `mixed-frequency' steps have also been observed. The first observation of Josephson step structure corresponding to the 1540 and 1578 GHz emissions of a DCN laser is reported.

A simple point-contact Josephson junction for use in the millimetre and submillimetre wavelength regions

A very simple arrangement for producing 'point-contact' Josephson junctions is described. The devices are adjusted before cooling to liquid helium temperatures, and are small enough to operate in a helium storage dewar with an 18 mm diameter neck. They cannot, however, be thermally cycled without some change in electrical characteristics. Mechanical stability is much superior to the fully adjustable types of point-contacts normally used for radiation studies. Some preliminary results on response to frequencies up to 1 THz are reported.

Enhanced-Mode Radiation Detection by Superconducting Films

In the course of investigating radiant-energy detection by high-resistance superconducting films [≈10 KΩ/square (□)] an unexpected enhanced mode of detection was observed at microwave frequencies. This mode was observed in addition to and differed significantly from the expected thermal or bolometer mode of detection. Since that time detailed measurements have been made of the dependence of the enhanced-mode response on bias current, incident power, temperature, and sample resistance; these are reported here. Correlation of the characteristics of the enhanced mode with the nonlinear VI (voltage-current) characteristics of the film is shown. This suggests that the enhanced mode is the result of a nonlinear response to currents induced in the film by the incident radiation, which explains the short wavelength cutoff associated with the enhanced mode. Physical mechanisms which might cause this response are considered and fabrication techniques for these films are presented in detail. Since the initial report, enhanced-mode responses have been observed with a responsivity per square (r=104 V/W) which is thousands of times greater than the peak bolometer responsivity. At the operating point, the film has a lower output impedance [<50Ω/square(□)] and responds with a time constant less than 0.1 nsec as estimated from heterodyning measurements at 9.4 GHz. At present, enhanced-mode devices with an active area of 1 cm2 can achieve a noise equivalent power of less than 10−13 W Hz−1/2. This compares quite favorably with other millimeter wave detectors such as InSb free carrier detectors, doped Ge detectors, and superconducting point-contact Josephson junction devices.

Superconducting Microwave Mixers Utilizing Josephson Junctions

Microwave superconducting mixers operating at 10–13 GHz utilizing Josephson junctions have been investigated analytically and experimentally. Measurements on mixer sensitivity were carried out at 13 GHz using waveguide-mounted experimental point-contact Josephson junctions at 4.2 K. Results showed sensitivities of −90 dBm/MHz (10−18 W/Hz) which included mismatch losses encountered in the waveguide structure. The Josephson junction mixing mechanism was analyzed on the basis of small-signal modulation of the staircase like dc I-V characteristics. It was shown that a small-signal microwave excitation results in an ac lateral shift of the regions between steps, such that: (a) Optimum Josephson mixing occurs halfway between steps. (b) Mixing occurs between all steps with maximum conversion efficiency between the zero and first step. (c) Predicted low conversion loss and possibly internal gain for a matched mixer. The analytical model was simulated by a digital computer and the behavior of the Josephson mixer predicted for various conditions of excitation. The model chosen for the analysis and the computer simulation is based on the simplest assumptions concerning bulk-formed Josephson junctions. In the laboratory an external local oscillator set 60 MHz below the signal frequency was used to induce the well-known step structure in the dc I-V characteristic of the point-contact Josephson junction. Mixing of the Lo and signal was found to occur between constant voltage steps in agreement with both analysis and computer simulation. The correctness of the model is demonstrated by the agreement between the theory and experiment.

Maximal Output PowEr of Point-Contact Josephson Junctions

A simple model is considered in order to describe point-contact Josephson junctions as oscillators. The maximal output power of these generators into a matched load is determined as a function of frequency.

Detection of vortex motion in superconducting samples by means of a Josephson junction

An original method is presented which allows to study the vortex dynamics in one particular point of a superconducting sample. The vortex motion is detected by means of a point contact Josephson junction placed in the point under consideration, the critical d.c. Josephson current of the junction being modulated by the passage of vortices. The results of two types of experiments are in reasonably good agreement with theoretical predictions.

REGENERATIVE JOSEPHSON EFFECT DETECTOR FOR FAR-INFRARED RADIATION

Experimental evidence is presented for feedback-narrowed far-infrared response of a point contact Josephson junction which is strongly coupled to a resonant cavity. The observed response shows high sensitivity (NEP ≲ 10−14W/Hz) and frequency selectivity (Q ≳ 103) and so is of value as a tunable narrow-band detector of millimeter and submillimeter wave radiation. The Werthamer-Shapiro theory of the nonlinear coupling of junction current to a resonant cavity has been extended to include the effect of applied radiation at the cavity resonance frequency and is shown to account for the feedback narrowing.

Quantum Phase Noise in Superconductors: Temperature Dependence Measured by Quantum Interference Techniques with Point-Contact Josephson Junctions

Thermodynamic fluctuations limit the ultimate performance of superconducting devices of practical interest, devices such as the quantum interference magnetometer and the Josephson junction radiation detector. We report direct observations of thermodynamically driven quantum phase fluctuations near the superconducting critical temperature. The experiments used point-contact quantum interference techniques with niobium points on a single-crystal tin substrate. Near the superconducting transition of tin, TC=3.72°K, the niobium points served as probes to measure fluctuations in the phase of the superconducting order parameter within the tin crystal. The temperature was slowly increased, beginning well below TC for tin, where the quantum phase was stable. Quantum phase fluctuations were first observed approximately 5 mdeg below TC. As the temperature continued to increase through this 5 mdeg range near TC, the characteristic frequency of the phase fluctuations increased 6 orders of magnitude. At TC a discontinuous transition to a resistive state gave rise to a finite voltage difference in the tin with the associated driven quantum phase precession. Thermodynamic Johnson noise voltages developed across this resistance of order 10−10Ω observably broadened the precession line. We will present the detailed temperature dependence of the frequency power spectrum of quantum phase fluctuations below and above TC.

Millimeter-Wave Mixing with Josephson Junctions

Experiments are reported in which two millimeter-wave signals incident on point-contact Josephson junctions produced changes in the junction dc voltage versus current characteristic and an intermediate frequency output whose amplitude depended sensitively on both junction bias and applied power. Equations are derived, based on Josephson's phenomenological equations, for the Josephson current in a junction exposed to two applied rf signals. When the applied signals differ appreciably in frequency, additional constant-voltage steps in the $V\ensuremath{-}I$ curve are predicted which are spaced at the difference frequency. These steps have been observed in experiments employing sources at 64 and 72 Gc/sec. Results of mixing experiments utilizing two sources nearly equal in frequency are reported at 23 and at 72 Gc/sec. In this case the two waves beat together and are equivalent in their effect to a single signal amplitude modulated at the difference frequency. Also explained on the same basis are experiments in which the third harmonic of a signal at 24 Gc/sec mixed with a signal at 72 Gc/sec. These results demonstrate the existence of the Josephson mixing mechanism as opposed to classical nonlinear mixing, and they show that it is operative at microwave and millimeter-wave frequencies over a wide range of power.

Far Infrared Response of Point-Contact Josephson Junctions

We have measured spectral-response curves of superconducting point contacts suggesting the existence of frequency-dependent Josephson current amplitudes peaking in the vicinity of the energy gap. These point-contact Josephson junctions respond sensitively and with high speed to millimeter and submillimeter radiation. Radiation emitted to free space by one such junction was detected by another junction.