We have experimentally investigated the quasiparticle shot noise in NbN/MgO/NbN superconductor -insulator -superconductor tunnel junctions. The observed shot noise is significantly larger than theoretically expected. We attribute this to the occurrence of multiple Andreev reflection processes in pinholes present in the MgO barrier. This mechanism causes the current to flow in large charge quanta (Andreev clusters), with a voltage dependent average value of m ≈ 1 + 2∆ eV times the electron charge. Because of this charge enhancement effect, the shot noise is increased by the factor m.PACS numbers: 72.70.+m, 74.50.+r, 74.80.Fp A dc current I flowing through a vacuum tube or a tunnel junction generates shot noise, time dependent fluctuations of the current due to the discreteness of charge carriers. Shot noise studies provide information on the nature of conduction not obtainable by conductance studies, e.g. the electric charge of carriers or the degree of correlation in the conducting system. For an uncorrelated system in which the electrons do not interact, the passage of carriers can be described by a Poisson distribution. The spectral density of current fluctuations S I then equals full shot noise: S I = 2qI = P Poisson for zero frequency and temperature 1 . The charge quantum q is normally the electron charge e.In superconductor -normal metal (SN) systems Andreev reflection occurs, causing an effective charge to be transferred of 2e. Due to this doubling of the charge, the shot noise in such a system is predicted to have a maximum of twice the Poisson noise 2-4 . More recently, giant shot noise in the supercurrent is predicted in a singlechannel superconductor -normal metal -superconductor (SNS) point contact 5 which is attributed to transport of large charge quanta (q ≫ e) at finite voltages caused by Multiple Andreev Reflection (MAR)6 . Observation of enhanced charge quanta in SN or SNS structures requires a combination of conductance and shot noise measurements. Despite extensive theoretical work, experimental results in this field are rare. A recent experiment 7 is performed on a NbN/c/Nb structure in which c is assumed to be a Nb constriction with a length of 7 nm and a diameter of 15 nm. At 9.5 K the structure acts like an NS interface but doubled shot noise is not observed. The predicted giant supercurrent shot noise is not observed either (at 4.2 K).From an applied point of view, shot noise in superconducting structures is of interest since this noise forms a major limitation to the sensitivity of NbN/MgO/NbN Superconductor-Insulator-Superconductor (SIS) THz radiation detectors 8 . For these reasons, we have investigated quasiparticle current transport and shot noise in an SNS structure in which the quasiparticle current is carried by MAR. Anticipating the experimental shot noise results presented in this paper we demonstrate that in a system in which multiple Andreev reflections occurs the quasiparticle shot noise at V < 2∆/e has a maximum value given by S I = (1 +

The need for extremely sensitive heterodyne receivers in the astronomical community has created strong efforts to develop appropriate frequency mixers. Nb/Al-Al x O y -Nb tunnel junctions with Nb matching circuits lead to best results up to approximately 700 GHz, the energy-gap frequency of Nb. For higher frequencies the Nb in the matching circuit becomes lossy and is replaced beyond 800 GHz by Al (which operates in the normal conducting state). The gap frequency of NbN is as high as 1.2 THz, but NbN technology is not yet mature. Practical films still have substantial radio frequency losses and the barrier of NbN tunnel junctions is too leaky. More recently, NbTiN, for which the gap frequency is also about 1.2 THz, was found to have very low losses, and is therefore a good choice for tuning circuits. New interesting tunnel junctions for mixers around 1 THz are NbTiN-MgO-NbTiN and Nb/Al-AlN x -NbTiN. Excellent performance from about 1 THz up to several terahertz can be expected from hot-electron transition-edge bolometer mixers. They consist of NbN or Nb microbridges, they do not need matching circuits and their frequency limit is not determined by the gap frequency.

Abstract. Superconducting GHz electronics can be improved by variable tuning circuits. We present a low temperature (< 150°C) process for the fabrication of niobium (Nb) thin film air bridges as variable capacitors, which can be integrated in Nb superconducting electronics. These elements can be applied for on-chip adjustment of filters, resonators and tuning circuits. Measurements and calculations of the electrostatic actuation of the bridges will be compared.

This letter reports a low noise submillimeter-wave mixer using NbN tunnel junctions integrated in Nb matching circuits. The double side band receiver noise temperature was 245 K at 345 GHz. Plasma conditions for NbN film deposition on quartz substrates at room temperature are created by using a second Nb target as a selective nitrogen pump. Electrodes for tunnel junctions with critical temperatures above 15 K and normal state resistivities in the range from 130 to 160 μΩ cm were obtained. This permits integrating NbN junctions into normal metal or non-NbN superconducting matching circuits, which is of great interest for THz mixers.