Rf-Induced Effects in Superconducting Tunnel Junctions

The addition of a new term in the conventional Hamiltonian is used to study photon-assisted tunneling for superconducting junctions containing active barrier impurities. For a sufficiently sharp molecular resonance, the impingent radiation can cause a large enhancement of the $n=1$ Dayem and Martin current step or a significant alteration of the dc Josephson current. The important mechanism is capture of a photon by a molecule followed by electron tunneling through the barrier.

We have studied on the quasiparticles (QPs) injection effects for Nb/Al/AlOx/Al/Nb tunnel junctions at 0.4 K. A device structure was a stack of an injector Josephson junction (JJ) and a detector JJ. At a low temperature, there is small number of thermally excited QPs and we expected to observe easily the QPs injection effect of the detector JJ using the injector JJ. In this study, the structure of the Nb/Al/AlOx/Al/Nb tunnel junctions was the same for X-ray detectors using superconducting tunnel junctions. For constant-QPs current injection, we expected a small-signal current gain exceed the unity. We used pulsed-QPs current with changing pulse duration. For long duration, I-V curves for the detection junctions were sum of original current and QP current. On the other hands, for short duration time, the I-V curves showed different shapes from those for the long duration time.

We report the first experimental results obtained by Low Temperature Scanning Electron Microscopy (LTSEM) on dynamic states of Josephson tunnel junctions. The spatially resolved measurements dealing with self-resonant modes (Fiskesteps) revealed information about the distribution of the rf-magnetic field amplitudes inside the Josephson junction. We imaged one- and two-dimensional cavity-modes inside a square junction. By coupling a Josephson element phase-coherently to a microwave field we imaged the distribution of the maximum current density of the generated constant-voltage steps (Shapiro-steps). For junction designs used as modern voltage standards we measured homogeneously distributed step-current densities up to the fourth step. By irradiating superconducting tunnel junctions with microwaves photon-assisted tunneling of quasiparticles takes place. Using LTSEM we imaged the standing microwave pattern inside the junction. As a result, the wavelength of the microwave inside a single tunnel junction and the wavelength inside a superconducting stripline is displayed directly. The spatial resolution achieved by LTSEM is about 1–2 μm.

The Riedel singularity in thin-film aluminum tunnel junctions was measured from temperature behavior of the zero voltage step and the RF-induced Josephson steps at around the gap frequency. From these experiments the author derived a value of the imaginary gap parameter to be about 1% of the gap parameter value at the gap edge when the temperature was 0.7 of the transition temperature. The author believes that, using magnetic field for tuned superconductor-insulator-superconductor (SIS) mixers with relatively large area, the Riedel singularity should not play an important role for frequencies around the gap frequency at finite temperatures. >

We have measured the spectral density of the 1/f voltage noise in current-biased resistively shunted Josephson tunnel junctions and dc SQUIDs. A theory in which fluctuations in the temperature give rise to fluctuations in the critical current and hence in the voltage predicts the magnitude of the noise quite accurately for junctions with areas of about 2 × 104 µm2, but significantly overestimates the noise for junctions with areas of about 6 µm2. DC SQUIDs fabricated from these two types of junctions exhibit substantially more 1/f voltage noise than would be predicted from a model in which the noise arises from critical current fluctuations in the junctions. This result was confirmed by an experiment involving two different bias current and flux modulation schemes, which demonstrated that the predominant 1/f voltage noise arises not from critical current fluctuations, but from some unknown source that can be regarded as an apparent 1/f flux noise. Measurements on five different configurations of dc SQUIDs fabricated with thin-film tunnel junctions and with widely varying areas, inductances, and junction capacitances show that the spectral density of the 1/f equivalent flux noise is roughly constant, within a factor of three of (10−10/f)ϕ 20 Hz−1. It is emphasized that 1/f flux noise may not be the predominant source of 1/f noise in SQUIDS fabricated with other technologies.

Superconducting tunnel junctions, which are also known as superconductor-insulator-superconductor (SIS) junctions, are widely used in coherent detection (heterodyne mixing) of electromagnetic radiation at millimeter and submillimeter wavelengths to achieve high spectral resolution (<i>&Delta;&nu;/&nu;</i><10^-6). With extremely low dark current and high quantum efficiency, superconducting tunnel junctions are also promising for THz photon detection. Besides the merit of high detection sensitivity, THz photon detectors with superconducting tunnel junctions can be operated at relatively high temperatures (above 0.3 K) and don't need rather complicated filtering systems by simply incorporating a superconducting resonant circuit with superconducting tunnel junctions. This paper mainly investigates the characteristics of THz photon detectors with Nb and NbN-based superconducting tunnel junctions. The MAR effect and associated shot noise are addressed. Some analytical and preliminary experimental results are presented.

Electronic noise of superconducting tunnel junction detectors

In this paper, we calculate power of electromagnetic wave emitted to open space by a terahertz (THz) source based on a Josephson flux-flow oscillator (FFO). The FFO is integrated with a transmitting slot antenna and a superconductor-insulator-superconductor (SIS) tunnel junction on a single chip, providing a frequency-tunable signal in a wide range of 200 - 700 GHz to open space. For non-calibrated power measurements, we used a highly sensitive silicon bolometer at 4.2 K. A small part of output FFO power is branched to the on-chip SIS-mixer and was estimated by absolute value by means of the Tien and Gordon photon-assisted tunneling theory. Using the absolute value, a calibrated power emitted to open space was estimated in the whole operating range. A power of from fractions to a few μW was obtained with a maximum of about 3 μW at 330 GHz.

We demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlOx/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.

The Quasiparticle Trapping Transistor (QuaTraTran, QTT) was proposed by Booth et al. as a new, multi-purpose superconducting electronic device for the amplification of low-level electrical signals at low temperatures. In this paper we consider its application as a cold 'pre-preamplifier' for signals coming from a superconducting tunnel junction (STJ) used as a soft X-ray detector. The mplification of signals in an STJ itself relies on multiple tunneling of quasiparticles, which has a large associated noise when compared to the Fano noise. In the soft X-ray domain (i.e for photon energies < 1 keV), tunnel noise is the limiting factor for the energy resolution. The amplification in a QTT, on the other hand, depends on the down-conversion of quasiparticles at the superconducting gap of a superconducting metal to the Fermi level when they enter a normal metal. The charge created in the absorber layer tunnels through an asymmetric SIS' junction with a high tunnel probability in one direction only. This SIS' junction is in direct contact with a NIS junction, together forming the QTT. We investigate the prospects of applying QTT read-out to a strip detector, and its possible advantages over a design based on STJs. The goal is to improve on the energy resolution of STJs, while retaining the advantages of the STJ approach, such as its application in imaging detectors and single photon detection. Estimates are made of influences on the noise of the SIS part of the QTT, the down-conversion process, thermal quasiparticles and recombination and thermal phonons.

A model is proposed to theoretically study photon-assisted tunneling in superconducting junctions with active molecular impurities in the barrier. A simple calculation is performed to demonstrate the method. Temperature Green's functions are employed so that it is possible to include known results on strong coupling superconductors while extending theory to include molecular-impurity effects.

Superconducting tunnel junctions as energy resolving single photon detectors

We focus on a recently experimentally realized scenario of normal-metal-insulator-superconductor tunnel junctions coupled to a superconducting resonator. We develop a first-principles theory to describe the effect of photon-assisted electron tunneling on the quantum state of the resonator. Our results are in very good quantitative agreement with the previous experiments on refrigeration and heating of the resonator using the photon-assisted tunneling, thus providing a stringent verification of the developed theory. Importantly, our results provide simple analytical estimates of the voltage-tunable coupling strength and temperature of the thermal reservoir formed by the photon-assisted tunneling. Consequently, they are used to introduce optimization principles for initialization of quantum devices using such a quantum-circuit refrigerator. Thanks to the first-principles nature of our approach, extension of the theory to the full spectrum of quantum electric devices seems plausible.

The method of experimental determination of the Fano factor in superconducting materials is based on usage of two sensor signals of a distributed readout imaging detector (DROID). In detector of this type a photon is absorbed in a superconducting layer with two superconducting tunnel junctions (STJs) at the ends. In this work it is shown, that the correlation between the two STJ signals depends on the fundamental combination of the Fano factor and the effective energy of quasiparticle formation in the superconducting material of DROID. The suggested formula for this fundamental combination depends on the energy of a primary particle, on the mean values of STJs’ signal amplitudes, and on the variances of the sum and the difference of the STJs’ signals. As this fundamental combination depends on the energy of a primary particle and on the particle interaction point, then it should depend on the local physical properties of the absorber. Independent determination of the effective energy of quasiparticle formation allows one to determine the value of the Fano factor. The important advantage of proposed method is its independence from the STJs’ electronic gains and noise.

Experiments are reported in which high frequency phonons, both thermally distributed and monochromatic, are generated and detected on a sapphire single crystal substrate at temperatures between 0.3 °K and 1 °K. Inelastic processes in the system (revealed by using superconducting tunnel junctions as high pass detectors of the monochromatic phonons) and the thermal bonding of the sapphire to a copper block are investigated.