IcRn Product Research Articles

Bicrystalline Grain Boundary and Hybrid SNS Junctions Based on Ba-122 Thin Films

To investigate the properties of iron-based superconductors, we examined different kinds of Josephson junctions using Co-doped Ba-122 thin films. Based on bicrystalline SrTiO3 substrates we prepared grain boundary (GB) junctions, which showed clear Josephson effects. We present electrical measurements at varying temperatures and magnetic fields. Furthermore, we prepared hybrid junctions using thin film technique. Confined by a Ba-122 base electrode (S) and a PbIn counter electrode (S') two different geometries of hybrid junctions are formed: a planar SNS' junction with a gold barrier (N) in between and an edge-type junction with an interface-engineered barrier. All three kinds of junctions showed asymmetric resistively shunted junction-like (RSJ) behavior with ICRN products of 20.2 /mu V (GB junctions), 18.4 /mu V (planar SNS' junctions) and 12.3 /mu V (edge-type junctions), respectively. An excess current could be observed at all junctions.

Characterization of the reliability and uniformity of an anodization-free fabrication process for high-quality Nb/Al–AlOx/Nb Josephson junctions

We have developed a reliable and reproducible fabrication process for high-quality Nb/Al–AlOx/Nb Josephson junctions that completely avoids anodization techniques, that are typically used to define the junction area, to electrically insulate the base electrode as well as the sidewalls of the counter-electrode and to protect the tunnel barrier. Hence, this process is well suited for the fabrication of electrically floating junction-based devices such as non-hysteretic rf-SQUIDs. Josephson junctions of various sizes have been produced and characterized at 4.2 K. We found that our junctions have a high quality, which is confirmed by measured gap voltages Vg and Ic Rn products up to 2.9 and 1.8 mV and on-wafer average values of the resistance ratio Rsg/Rn above 30 in most cases. Here, Rsg and Rn denote the subgap and the normal state resistance of a Josephson junction. Although the uniformity of the properties of the Josephson junctions across a wafer is high, we observe some systematic variations of the critical current density and the gap voltage over an entire wafer. These variations are most likely to be attributed to residual stress in the Nb films as well as the surface roughness of the Nb base electrode.

High-quality epitaxial NbN/AlN/NbN tunnel junctions with a wide range of current density

We have developed high-quality epitaxial NbN/AlN/NbN Josephson tunnel junctions with a wide range of current density Jc. The junctions show excellent tunneling properties with a large gap voltage of 5.6 mV and a large IcRN product of 3.5 mV. The quality factor Rsg/RN is about 60 for the junctions with a Jc of 2.2 A/cm2, and above 10 for the junctions with a Jc of 25 kA/cm2. The crystal structures across the junction barrier are investigated using x-ray diffraction and cross-sectional scanning transmission electron microscopy, and demonstrate epitaxial growth of the NbN/AlN/NbN trilayers for the wide range of Jc.

Magnetic Josephson Junctions With Superconducting Interlayer for Cryogenic Memory

We investigate a Magnetic Josephson Junction (MJJ) - a superconducting device with ferromagnetic barrier for a scalable high-density cryogenic memory compatible with energy-efficient single flux quantum (SFQ) circuits. The superconductor-insulator-superconductor-ferromagnet-superconductor (SIS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> FS) MJJs are analyzed both experimentally and theoretically. We found that the properties of SIS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> FS junctions fall into two distinct classes based on the thickness of S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> layer. We fabricate Nb-Al/AlOx-Nb-PdFe-Nb SIS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> FS MJJs using a co-processing approach with a combination of HYPRES and ISSP fabrication processes. The resultant SIS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> FS structure with thin superconducting S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> -layer is substantially affected by the ferromagnetic layer as a whole. We fabricate these type of junctions to reach the device compatibility with conventional SIS junctions used for superconducting SFQ electronics to ensure a seamless integration of MJJ-based circuits and SIS JJ-based ultra-fast digital SFQ circuits. We report experimental results for MJJs, demonstrating their applicability for superconducting memory and digital circuits. These MJJs exhibit <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> product only ~ 30% lower than that of conventional SIS junctions co-produced in the same fabrication. Analytical calculations for these SIS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> FS structures are in a good agreement with the experiment. We discuss application of MJJ devices for memory and programmable logic circuits.

Ferromagnetic Josephson switching device with high characteristic voltage

We develop a fast magnetic Josephson junction (MJJ)—a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient single flux quantum (SFQ) circuits. We present experimental results for superconductor-insulator-ferromagnet-superconductor (SIFS) MJJs with high characteristic voltage IcRn of &amp;gt;700 μV proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ IcRn product is only ∼30% lower than that of conventional junction co-produced in the same process, allowing for integration of MJJ-based and SIS-based ultra-fast digital SFQ circuits operating at tens of gigahertz.

Engineering of YBa2Cu3O7–δ grain boundary Josephson junctions by Au nanocrystals

AbstractWe prepared and investigated grain boundary Josephson junctions based on SrTiO3 bicrystal substrates. During the deposition of YBa2Cu3O7–δ (YBCO) gold nanocrystals forming from an intermediate gold layer can modify the crystalline structure and thus the properties of the YBCO grain boundaries. The variation of the film thickness of the Au seed layer changes the growth conditions of the YBCO film and the Au nanocrystals. The values of the characteristic ICRN product do not change whereas the values of the critical current IC decrease.magnified image(© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Transport properties of variable-angleYBa2Cu3O7 − δ step-edgejunctions in the a–b plane

The superconductor–barrier–superconductor interface in a range ofYBa2Cu3O7 − δ (YBCO) step-edge Josephson junctions was engineered to lie at anglesθ other thannormal to the a–b axes of the YBCO crystal. This systematic study enabled theeffect of the d-wave anisotropy to be probed over the rangeθ = 0° ([100]direction) to θ = 45° ([110] direction) in a way not possible with junction technologies such as bicrystals.Anomalous temperature dependence of the critical current and a peak in the differentialconductance at low voltage were reminiscent of features attributed elsewhere tozero-energy states in a range of [001]-tilt bicrystal and ramp-edge junctions. Moreover,IcRN products decreasedwith increasing θ, and conductance minima or gap-related structures were measured for largeθ. The data reported here for variable-angle junctions are compared with results fromstudies of other grain boundary junctions (such as bicrystal and biepitaxial junctions) andprovide support for theories developed by Shirai et al (2003 J. Phys. Soc. Japan 722299–307) in particular.

Aluminum hard mask technique for the fabrication of high quality submicronNb/Al–AlOx/Nb Josephson junctions

We have developed a combined photolithography and electron-beam lithography fabrication process forsub-µm to µm-size Nb/Al–AlOx/Nb Josephson junctions. In order to define the junction size and protect itstop electrode during anodic oxidation, we developed and used the newconcept of an aluminum hard mask. Josephson junctions of sizes down to 0.5 µm2 have been fabricated and thoroughly characterized. We foundthat they have a very high quality, which is witnessed by theIV curves with qualityparameters Vm > 50 mV and Vgap = 2.8 mV at4.2 K, as well as IcRN products of 1.75–1.93 mV obtained at lower temperatures. In order to test the usability of our fabrication processfor superconducting quantum bits, we have also designed, fabricated and experimentallyinvestigated phase qubits made of these junctions. We found a relaxation time ofT1 = 26 ns and a dephasingtime of T2 = 21 ns.

The I C R N Value in Intrinsic Josephson Tunnel Junctions in Bi2Sr2CaCu2O8+δ (Bi2212) Mesas

The c-axis current-voltage I(V) characteristics have been obtained on a set of mesas of varying height sculpted on Bi2Sr2CaCu2O8+δ (Bi2212) crystals intercalated with HgB2. The intercalation, along with the small number of junctions in the mesa, N=6–30, minimizes the degree of self-heating, leading to a consistent Josephson critical current, IC, among junctions in the mesa. The Bi2212 crystals with a bulk TC=74 K are overdoped and display negligible pseudogap effects allowing an accurate measure of the normal state resistance, RN. These properties make the mesas nearly ideal for the determination of the Josephson ICRN product. We find ICRN values consistently ∼30% of the quasiparticle gap parameter, Δ/e, which was measured independently using a mechanical contact, break junction technique. The latter was necessitated by higher bias heating effects in the mesas which prevented direct measurements of the superconducting gap. These values are among the highest reported and may represent the maximum intrinsic value for ICRN. The results indicate that the c-axis transport is a mixture of coherent and incoherent tunneling.

BaFe1.8Co0.2As2 thin film hybrid Josephson junctions

Josephson junctions with iron pnictides open the way for fundamental experiments on superconductivity in these materials and their application in superconducting devices. Here, we present hybrid Josephson junctions with a BaFe1.8Co0.2As2 thin film electrode, an Au barrier, and a PbIn counter electrode. The junction shows resistively shunted junction-like current-voltage characteristics up to the critical temperature of the counter electrode of about 7.2 K. The temperature dependence of the critical current shows nearly linear behavior near TC. Well-pronounced Shapiro steps are observed at microwave frequencies of 10–18 GHz. Assuming an excess current of 200 μA at 4.2 K the effective ICRN product calculates to 7.9 μV.

Electron transport and microwave dynamics of hybrid Nb/Au/CaSrCuO/YBaCuO planar Josephson junctions

We report on measurements of dc electron transport and microwave dynamics of thin film hybrid Josephson junctions with an oxide antiferromagnetic interlayer. The base superconducting electrode YBaCuO and the antiferromagnetic (AF) interlayer CaSrCuO (with thickness d = 20 – 70 nm) were grown by laser ablation on NdGaO3 substrates. I-V curves were well fitted to RSJ model and had no excess current, ICRN products were of order 0.2 mV at T=4.2 K. We did not observe any noticeable reduction of ICRN with increasing d. Such "tunnellike" behaviour also resulted in appearance of singularities on I-V curve when magnetic field was applied. Oscillating with microwave power integer and half-integer Shapiro steps were registered along with sub-harmonic detector response. Moreover, for some of junctions a "devil" staircase structure was observed on I-V curves and giant noise-like signals were measured in 1-2 GHz band at the certain levels of microwave power. Observed features, noise performance and the impact of the second harmonic in current-phase relation on junction dynamics are discussed taking into account data for structures without AF interlayer.

C -axis critical current of a PrFeAsO0.7 single crystal

The c-axis transport properties of a high-pressure synthesized PrFeAsO0.7 single crystal are studied using s-shaped junctions. Resistivity anisotropy of about 120 detected at 50 K shows the presence of strong anisotropy in the electronic states. The obtained critical current density for the c-axis of 2.9×105 A/cm2 is two orders of magnitude larger than that in Bi2Sr1.6La0.4CuO6+δ. The appearance of a hysteresis in the current-voltage curve below Tc is the manifestation of the intrinsic Josephson effect similar to that in cuprate superconductors. The suppression of the critical current-normal resistance (IcRn) product is explained by an inspecular transport in s±-wave pair potential.

Niobium-Silicide Junction Technology for Superconducting Digital Electronics

We present a technology based on Nb/NbxSi1-x/Nb junctions, with barriers near the metal-insulator transition, for applications in superconducting electronics (SCE) as an alternative to Nb/AlOx/Nb tunnel junctions. Josephson junctions with co-sputtered amorphous Nb-Si barriers can be made with a wide variety of electrical properties: critical current density (Jc), capacitance (C), and normal resistance (Rn) can be reliably selected within wide ranges by choosing both the barrier thickness and Nb concentration. Nonhysteretic Nb/NbxSi1-x/Nb junctions with IcRn products greater than 1mV, where Ic is the critical current, and Jc values near 100kA/cm2 have been fabricated and are promising for superconductive digital electronics. These barriers have thicknesses of several nanometers; this improves fabrication reproducibility and junction uniformity, both of which are necessary for complex digital circuits. Recent improvements to our deposition system have allowed us to obtain better uniformity across the wafer.

Overdamped characteristics of multilayered MgB2/AlN/Al/MgB2 Josephson junction

MgB 2 / Al / AlN / MgB 2 multilayered Josephson junctions were fabricated on c-plane sapphire substrates. The measured current-voltage characteristics were well fitted with a resistively and capacitively shunted junction model. For a junction with 0.56-nm-thick AlN and 10-nm-thick Al layers, the current density was 740 A/cm2 and the ICRN product was 210 μV. The Josephson currents were found to be ideally modulated in accordance with theoretical calculations by an external magnetic field. Clear Shapiro steps were observed under irradiation at 95.622 GHz, and fourth step was obtained. Shapiro step heights were consistent with the resistively and capacitively shunted junction model.

High-Tc superconducting quantum interference devices: Status and perspectives

In this paper, an overview of the current status of high-Tc superconducting quantum interference devices (SQUIDs), from device engineering to biomagnetic applications, is given. The authors offer a description of the current status of SQUID sensors, challenges encountered, and the solution of fabricating SQUID sensors with low flux noises. The current challenge that we face is to fabricate high-Tc SQUIDs that are not only more reproducible than the current technology but also capable of providing a high IcRn product and fabricating SQUID with high yield. Improvement of flux noises and fabrication yield in the integrated multichoices directly coupled SQUID magnetometer or gradiometer with series SQUID array are presented. High-Tc SQUID magnetometers exhibiting magnetic field sensitivity of ∼30–50fT∕Hz1∕2 or better at 100Hz was demonstrated by incorporating serial SQUID into the pickup loop of the magnetometers. New technologies currently being developed and applications for high-Tc SQUIDs are addressed.

Improving the IcRn product and the reproducibility of high Tc Josephson junctions made by ion irradiation

A simple model has been proposed to explain the spread in the characteristics of high Tc Josephson junctions made by ion irradiation, assuming that the source of dispersion is the slit’s size variation. Accordingly, increasing ion energy should lead to a significant reduction of inhomogeneities. Test samples have been fabricated using two different beam energies. As predicted, the spread in critical current decreases upon increasing energy. Moreover, since the actual width of the barrier is reduced in this case, the IcRn product increases significantly. These results seem promising for future technological applications.

Improving ion irradiated high Tc Josephson junctions by annealing: The role of vacancy-interstitial annihilation

The authors have studied the annealing effect in the transport properties of high Tc Josephson junctions (JJs) made by ion irradiation. Low temperature annealing (80°C) increases the JJ coupling temperature (TJ) and the IcRn product, where Ic is the critical current and Rn the normal resistance. They have found that the spread in JJ characteristics can be reduced by sufficient long annealing times, increasing the reproducibility of ion irradiated Josephson junctions. The characteristic annealing time and the evolution of the spread in the JJ characteristics can be explained by a vacancy-interstitial annihilation process rather than by an oxygen diffusion one.

0-π oscillations in nanostructured Nb/Fe/Nb Josephson junctions

The physics of the π phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from “0” to “π” states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of 1.98 ×105 m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the ICRN product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.

Observation of SFQ pulse using HTS sampler

We have succeeded in observing a pico-second current pulse accompanying a single flux quantum (SFQ) using a high-Tc superconducting (HTS) sampler. The sampler circuit used in this study, an on-chip sampler, has two pulsers connected to each other at the comparator in one chip. The waveform of an SFQ current pulse from one pulser is measured by using another SFQ pulse from the other pulser as a sampling pulse. We observed a waveform exhibiting two peaks with 5.8 and 1.6ps widths. Based on these values, we estimated the sampling-pulse width as 2.8ps, suggesting the potential bandwidth for the HTS sampler of 125GHz. This value coincides well with the bandwidth expected from the critical current-normal resistance (IcRn) product of the Josephson junctions of 1mV.

Fabrication of Inductively-Coupled Double-Josephson Junctions Using High-Temperature Superconductors

We fabricated double interface-treated Josephson junctions with vertically-stacked geometry based on the high temperature superconductor YBCO. The junctions have a superconductor-insulator-superconductor-insulator-superconductor structure. If the intermediate layer is thin enough, the two junctions are coupled inductively through the intermediate layer, and the two junctions switch simultaneously. The double junctions with 30 nm thick intermediate layer exhibited resistively-shunted-junction characteristics and had no kinks in their I - V curves. In a typical chip, an average of the critical current density was 1.65 kA/cm2 with 1 sigma spread of 14.6%. The nominal IcRn product for a double junction was 2.43 mV with 1 sigma spread of 5.72%. This IcRn product is 1.5-2 times larger than that of the single vertically-stacked junction having similar critical current density. These experimental results suggest the simultaneous switch of the two junctions. We expect that higher nominal IcRn products would be obtained after choosing appropriate process parameters, because the IcRn products of our previous single junctions have reached 2 mV.