Microstructure of NiCu multilayers electrodeposited from a citrate electrolyte

Superconducting (S) thin film superlattices composed of Nb and a normal-metal spacer (N) have been extensively utilized in Josephson junctions given their favorable surface roughness compared to Nb films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence lengths of $S/N$ superlattices using polarized neutron reflectometry and electrical transport. Despite the normal-metal spacer layers being only approximately 8% of the total superlattice thickness, we surprisingly find that the introduction of these thin N spacers between S layers leads to a dramatic increase in the measured London penetration depth compared to that of a single Nb film of comparable thickness. Using the measured values for the effective in- and out-of-plane coherence lengths, we quantify the induced anisotropy of the superlattice samples and compare to a single Nb film sample. From these results, we find that the superlattices behave similarly to layered 2D superconductors.

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Distortions to the penetration depth and coherence length of superconductor/normal-metal superlattices

A new class of materials known as compositionally modulated alloys (CMA) or artificially modulated multilayers has generated an ever-increasing enthusiasm due to their unique properties. This paper reviews the recent developments in the field of electrodeposition techniques for the production of CMA, which offers many advantages over other methods. It also discusses the structure of CMA and their salient properties such as corrosion resistance, magnetic and mechanical properties, which differ to those obtainable in normal metallurgical alloys. Some of their interesting applications are also discussed.

Electrodeposition of compositionally modulated alloys: An overview

Vertically stacked Josephson tunnel junctions with a high number of junctions (up to n=20) were realized using a fabrication process based on the Nb/Al technology. An experimental study aimed toward their potential applications was made. First, we report on a series of DC measurements, with the main purpose of studying the quality of the I-V curve as a function of the number of barriers in the stack; we found that the junction quality slowly becomes worse as we increase this number. In fact, some thermal effects appear, which result in an increase of the subgap current and a decrease of the width of the current jump at the total gap voltage. Further, we found a low frequency voltage noise on devices with n>10 that depends on the junction area and on the substrate nature. Later, we discuss some RF properties of stacked junctions irradiated with a microwave signal. These measurements show that they seem to behave independently. Once again, the thermal effects drastically influence the response to an external RF drive and seem to limit the use for applications of these devices.

Fabrication and properties of multiple vertically stacked Josephson tunnel junctions

Niobium nitride/aluminium nitride multilayers have been fabricated using dc reactive magnetron sputter deposition. By careful optimization a suitable set of deposition conditions has been achieved under which both high quality superconducting and insulating layers may be fabricated. This has allowed automation of the deposition process to fabricate multilayers by simple computer control of substrate position and shuttering. Deposition onto a range of single crystal and amorphous substrates has given a range of different crystal structures. We have measured the transport properties of these multilayers as a function of superconductor and insulator layer thickness in order to study the interlayer coupling, and have fabricated stacked superconductor-insulator-superconductor junctions.

Niobium nitride/aluminium nitride superconductor/insulator multilayers and tunnel junctions

An engineered structure is proposed that can alleviate quasi-particle recombination losses via the existence of a phononic band gap that overlaps the 2-Delta energy of phonons produced during recombination of quasi-particles. Attention is given to a 1D Kronig-Penny model for phonons normally incident to the layers of a multilayered superconducting tunnel junction as an idealized example. A device with a high density of Bragg resonances is identified as desirable; both Nb/Si and NbN/SiN superlattices have been produced, with the latter having generally superior performance.

Role of engineered materials in superconducting tunnel-junction x-ray detectors: suppression of quasi-particle recombination losses via a phononic bandgap

Superconducting tunnel junctions have a potential, statistically limited, energy resolution on the order of eV's. The best results to date, however, have been an order of magnitude worse than this and required operating temperatures on the order of 0.1 K. Niobium based junctions operating at approximately 1 K have shown X-ray detection capabilities, but have only achieved energy resolutions on the order of 100 eV's at best. Several mechanisms, including quasiparticle self trapping, loss of `hot' excitations, quasiparticle recombination, and loss of `cold' excitations, have been proposed to explain the degradation of energy resolution in these devices. We will present a design concept for an X-ray detector, along with recent experimental and computer modeling results, based on a 1D superlattice of superconducting tunnel junctions. This multilayered superconducting tunnel junction design has the potential for alleviating many of the potential resolution degrading mechanisms while operating in the 1 K temperature range. In addition, the possibility of engineering the device to improve the signal to noise ratio of the output and to control the transport of phonons in the structure will be discussed.

High-energy-resolution x-ray detection using multilayered superconducting tunnel junctions

Superconducting tunnel junction X‐ray detectors have potential theoretical energy resolutions on the order of eV's. Such high energy resolutions would be of great use for astrophysical observations in the discrimination of spectral lines in hot plasmas and the determination of the Doppler shifts of these lines. Current results for these devices, however, yield energy resolutions more than an order of magnitude worse than the theoretical limit. In addition, low efficiency and breakdown under thermal cycling are also common problems. We propose a new approach based on multiple (10's to 100's) layers of vertically stacked tunnel junctions. This approach holds promise for the alleviation of many of the mechanisms that have been suggested as leading to the degradation of energy resolution and providing built in redundancy against barrier shorts. It should also be possible to make use of different materials which would yield higher operating temperatures and more robust devices. Also, the artificial 1‐...

Multilayered superconducting tunnel junction x‐ray detectors

A high resolution transmission electron microscopy study of multilayer films prepared by magnetron sputtering shows that the morphology of the growing interface in a-Si/Nb and a-SiN/NbN multilayers is remarkably uniform and smooth; this is in contrast to the polycrystalline AlN/NbN multilayers grown under similar conditions, which exhibit columnar grain structure with rough interfaces. For proper sputtering parameters, the amorphous layers seem to periodically restore a relatively smooth initial interface condition for the successive Nb (or NbN) layer growth, consequently interrupting the tendency toward increased roughness due to mechanisms such as columnar growth. Artificial multilayers having very flat interfaces could stimulate applications based on multilayer Josephson junctions.

High quality a -Si/Nb and a -SiN/NbN artificial multilayers for Josephson applications

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Role of engineered materials in superconducting tunnel-junction x-ray detectors: suppression of quasi-particle recombination losses via a phononic bandgap

High-energy-resolution x-ray detection using multilayered superconducting tunnel junctions

Multilayered superconducting tunnel junction x‐ray detectors

High quality a -Si/Nb and a -SiN/NbN artificial multilayers for Josephson applications