high-Tc Josephson Junctions Research Articles

Quantum interference in a high-transition-temperature superconductor based on nanoslits on SrTiO3 substrate

The Josephson junction, a key component of superconducting quantum circuits, is much less mature, especially for the high-transition (high-Tc) temperature cuprate superconductors. Herein, we report on the realization of high-Tc Josephson junction based on nanoslits on a SrTiO3 substrate. We demonstrate the ability to manipulate the current–voltage characteristics of YBa2Cu3O7−x bridges continuously from superconducting current-dominated to Josephson junction behavior by changing the irradiation parameters of the focused Ga+ ion beam on a single-crystal SrTiO3 substrate. The periodic critical current that depends on the magnetic flux coupled into the superconducting quantum interference devices was observed, which exactly reflects the effects of quantum tunneling and flux quantization. To some extent, this weak link within the framework of the semiconductor manufacturing process shows the potential to provide a cost-effective, highly efficient, and reliable pathway for scaling up quantum mechanical superconducting circuits, which is promising for the fabrication process.

Examples of superconducting technology application: Sensing and interfacing

Technological processes for the fabrication of low- and high-Tc Josephson junctions, aimed for certain applications, are described. On the one hand, the integration of low-Tc superconductor digital electronics with superconducting sensor arrays enables input signal processing with quantum limited resolution at millikelvin temperatures. We describe this mixed signal superconductor technology for analogue sensor readout and signal multiplexing for operating temperatures down to 300 mK. On the other hand, by making use of modern high-Tc Josephson junction technology, sensitive magnetometers, which require a modest cooling power, can be developed. Examples of the application of the mentioned processes are shown.

High-Tc Step-Edge Josephson Junction Arrays: Comparison of Simulated and Experimental Voltage Responses

With the recent development of multijunction superconducting quantum interference device (SQUID) arrays and superconducting quantum interference filters (SQIFs) using high-temperature superconducting (HTS) materials, it has become increasingly desirable to accurately model HTS devices to enable the optimization of the design for enhanced performance before fabrication. Initially, to develop our simulation capability for HTS devices, we have been working with a commercial software package to fit the data obtained from the characterization of two HTS devices. The models were iteratively adjusted by increasing the number of parameters taken into account, including the aspect ratio of the SQUID loop and the bias injection point. These models were developed for devices fabricated using YBCO step-edge junctions formed on MgO substrates. It was found that the biasing mechanism affects the SQIF voltage response to applied magnetic fields. This has implications for the design of larger arrays.

Optimization of electrical and structural parameters of YBa2Cu3O7−x thin-film bicrystal Josephson junctions with chemical and thermal treatments of substrates

The [100]-tilt high-Tc Josephson junctions are characterized by high characteristic voltages IcRn but their current range is limited by low critical currents Icf of the thin-film electrodes with tilted c-axes. The impact of chemical and thermal treatments of NdGaO3 substrates on morphology and IV curves of the YBa2Cu3O7−x thin films and the bicrystal thin-film junctions have been studied. It was found that IV curves of the films above a critical current Icf are described by a power-law dependence V = V0(I/Icf – 1)n, where Icf, V0 and n are dependent on the c-axis tilt of the film and treatment of the substrate. Critical current densities jc(78K) up to 1.4107 A/cm2 have been reached in 1.7°-tilt YBa2Cu3O7−x films perpendicular to the tilt. Bicrystal junctions have been fabricated with optimized YBa2Cu3O7−x thin films, and RSJ-like IV-curves in the current ranges from Ic up to 5 Ic with IcRn (78K)-values of 1 mV have been achieved.

YBCO Josephson Junctions and Striplines for RSFQ Circuits Made by Ion Irradiation

We have investigated the electrodynamic properties of high-Tc Josephson junctions (JJs) and striplines made by ion irradiation in order to evaluate the potential of such a technology for low-power rapid single flux quantum superconductor digital electronics. The process is briefly presented, together with the JJ characteristics in the context of high-speed electronics. Line inductances of different geometries and at different temperatures have been measured through superconducting quantum interference device (SQUID) modulation. A detailed comparison with 3-D numerical calculations has been made, and a quantitative agreement that shows typical values of 4-8 pH per square without a ground plane, depending on linewidth and the gap to the ground line, has been obtained.

High-Tc Josephson junctions fabricated by focused ion beam direct milling

High-Tc Josephson junctions were successfully fabricated by focused ion beam (FIB) direct milling. The characteristics of the junction barrier were carefully controlled by tuning the thickness of the link region. The optimal remaining thickness for the YBCO is about 70–80 nm even though the YBCO thicknesses are different. The temperature-dependence of the critical current provides good evidence of the superconductor–normal–superconductor weak link of the junctions. Although the flux-flow behavior increasingly dominates at lower temperatures, the voltage–current shows resistively shunted junction-like characteristics at temperatures close to Tc. The Shapiro steps of a single junction irradiated with microwaves were measured, revealing strong Josephson effects in ion-damaged junctions. This method will be of great benefit to nanoSQUIDs for the detection of spin systems.

Structural and electrical characterization of ultra-thin SrTiO3 tunnel barriers grown over YBa2Cu3O7 electrodes for the development of high Tc Josephson junctions

The transport properties of ultra-thin SrTiO3 (STO) layers grown over YBa2Cu3O7 electrodes were studied by conductive atomic force microscopy at the nano-scale. A very good control of the barrier thickness was achieved during the deposition process. A phenomenological approach was used to obtain critical parameters regarding the structural and electrical properties of the system. The STO layers present an energy barrier of 0.9 eV and an attenuation length of 0.23 nm, indicating very good insulating properties for the development of high-quality Josephson junctions.

Towards “Ideal” High-Tc Josephson Junction: [100]-tilt YBa2Cu3O7-x Bicrystal Boundary

Abstract Based on structural, electrical and THz characterization of [100]-tilt YBa2Cu3O7-x bicrystal junctions, criteria have been formulated for the “ideal” high- Tc Josephson junction: a single crystallographic facet for each high- Tc electrodes at the bicrystal boundary, optimal oxygen content inside the boundary, IcRn -values up to 10 mV, phonon-induced structures on the I-V curves at the voltages from 4 to 12 mV at T = 4.2 K and RSJ-like I-V curves at T ≥ 50 K within an accuracy of 1%. When compared with the “ideal” low- Tc junctions, the “ideal” high- Tc junctions are characterized by the IcRn -values of around 4-5 times below Ambegaokar-Baratoff value πΔ(0)/2 and strong interaction between Josephson oscillations and optical phonons of electrodes.

Hilbert spectroscopy based on the ac Josephson effect for liquid identification

Fast and reliable identification of liquids is of great importance in developing new security measures at public places. A concept of liquid identification is presented, based on our Hilbert spectroscopy and high-Tc Josephson junctions, that can operate at the frequency range of main dispersions of liquids under concern, i.e. at the intermediate range from microwaves to terahertz frequencies. Several demonstration setups, consisting of synthesized polychromatic radiation sources and compact Hilbert spectrometers integrated in Stirling coolers, have been developed and characterized. Reflection polychromatic spectra of various bottled liquids have been measured at the spectral range of 15 – 400 GHz with total scanning time down to 0.5 second and the possibility of reliable identification of liquids has been demonstrated.

Liquid identification by Hilbert spectroscopy

Fast and reliable identification of liquids is of great importance in, for example, security,biology and the beverage industry. An unambiguous identification of liquids can be madeby electromagnetic measurements of their dielectric functions in the frequencyrange of their main dispersions, but this frequency range, from a few GHz to afew THz, is not covered by any conventional spectroscopy. We have developed aconcept of liquid identification based on our new Hilbert spectroscopy and high-Tc Josephson junctions, which can operate at the intermediate range from microwaves to THzfrequencies. A demonstration setup has been developed consisting of a polychromaticradiation source and a compact Hilbert spectrometer integrated in a Stirling cryocooler.Reflection polychromatic spectra of various bottled liquids have been measured at thespectral range of 15–300 GHz with total scanning time down to 0.2 s and identification ofliquids has been demonstrated.

Characteristics of High-$T_{\rm c}$ Josephson Junction Fabricated by Focused Ion Beam and Ion Damage

A high-Tc Josephson junction and superconducting quantum interference devices (SQUIDs) were fabricated by focused ion beam (FIB) milling and 150 keV oxygen ion implantation. A single layer gold mask with a small aperture of 28-73 nm defined by direct milling with FIB, was used. The voltage versus current characteristics of high-Tc YBa2Cu3O7-x Josephson junctions were measured under microwaves. Shapiro steps were observed in the single junction. The voltage versus current and voltage versus magnetic field characteristics of a SQUID were measured.

Quantum phase dynamics of high-Tc Josephson junctions

We study quantum phase fluctuations in planar junctions between high-Tc superconductors for a range of relative orientations of the a-b crystal axes. Specifically, attention is directed towards the fermionic subspace of low energy Andreev bound states, midgap states (MGS). This subspace, being responsible for most of the Josephson current, strongly couples to the phase degree of freedom. Low frequency phase fluctuations may introduce intrinsic dynamics in this subspace, which would lead to a strong dynamical modulation of the Josephson current, and thus affect the phase quantum dynamics. In this article it is found that transitions between pairs of MGS induced by the phase fluctuations are forbidden due to specific symmetry properties of the MGS, and the macroscopic quantum Hamiltonian remains essentially adiabatic within the assumption of a specular interface.

Annealing of ion irradiated high TC Josephson junctions studied by numerical simulations

Recently, annealing of ion irradiated high Tc Josephson iunctions (JJs) has been studied experimentally in the perspective of improving their reproducibility. Here we present numerical simulations based on random walk and Monte Carlo calculations of the evolution of JJ characteristics such as the transition temperature Tc′ and its spread ΔTc′, and compare them with experimental results on junctions irradiated with 100 and 150 keV oxygen ions, and annealed at low temperatures (below 80 °C). We have successfully used a vacancy-interstitial annihilation mechanism to describe the evolution of the Tc′ and the homogeneity of a JJ array, analyzing the evolution of the defects density mean value and its distribution width. The annealing first increases the spread in Tc′ for short annealing times due to the stochastic nature of the process, but then tends to reduce it for longer times, which is interesting for technological applications.

High-$T_{c}$ Josephson Square-Law Detectors and Hilbert Spectroscopy for Security Applications

Among various discussed ways of explosive detection, the techniques using electromagnetic radiation are considered as having great potential and research activities are recommended in this field. To identify new threats, like liquid explosives, with low rate of false alarms, fast spectral measurements are required in a broad frequency range from microwave to terahertz. We attract attention to a great potential of high-Tc Josephson technology in security applications and present our results in developing high-Tc Josephson junctions for Hilbert spectroscopy and detector arrays.

Annealing effect on the reproducibility of Josephson Junctions made by ion irradiation

We have studied the annealing effects on the transport properties of High Tc Josephson Junctions (JJ) made by ion irradiation. Several JJ were measured for different annealing times and the experimental data were compared to numerical simulations. We have successfully used a vacancy-interstitial annihilation mechanism to describe the evolution of the JJ coupling temperature (TJ) and the homogeneity of a JJ array, related to the evolution of the defects density mean value and its distribution width. For sufficient long annealing times (t > 600 min), ΔTJ was significatively reduced. This result appears to be very encouraging for future applications where the spread in JJ characteristics has to be as low as possible.

Optimization of the coupling of mm wave power to arrays of high-Tc Josephson junctions

We explored high-temperature superconductor Josephson junction arrays embedded in a hemispherical Fabry-Perot resonator. We compared the characteristics of three designs of arrays to achieve steps at higher voltage with a better coupling to the millimeter wave irradiation power. With an optimal design, we achieved a maximum Josephson voltage of about 0.1V for an array of 620 bicrystal junctions at a temperature of 79.2K and a frequency of 77.465 GHz. Also steps from 0.01 V up to 0.1 V were observed. Our results showed that such circuits are challenging for applications in quantum voltage metrology. It is important to note that our quasioptical coupling method can be extended up to terahertz frequencies.

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.

Using ion irradiation to make high-Tc Josephson junctions

In this article we describe the effect of ion irradiation on high-Tc superconductor thin film and its interest for the fabrication of Josephson junctions. In particular, we show that these alternative techniques allow to go beyond most of the limitations encountered in standard junction fabrication methods, both in the case of fundamental and technological purposes. Two different geometries are presented: a planar one using a single high-Tc film and a mesa one defined in a trilayer structure.

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.

Study and optimization of ion-irradiated high Tc Josephson junctions by Monte Carlo simulations

High Tc Josephson junctions (HTc JJ) made by irradiation have remarkable properties for technological applications. However, the spread in their electrical characteristics increases with the ion dose. We present a simple model to explain the JJ inhomogeneities, which accounts quantitatively for experimental data. The spread in the slit’s width of the irradiation mask is the limiting factor. Monte Carlo simulations have been performed using different irradiation conditions to study their influence on the spread of the JJ characteristics. A “universal” behavior has been evidenced, which allows us to propose new strategies to optimize JJ reproducibility.