Superconductor-normal Conductor-superconductor Research Articles

Standard measurement method for normal state resistance and critical current of resistively shunted Josephson junctions

An important parameter of Josephson junctions (JJs) is the product of normal state resistance (R n) and critical current (I c) for designing superconductor analogue devices or digital circuits. Determination of R n and I c from voltage–current (U–I) characteristic curves often faces difficulties; in particular I c is considerably reduced by intrinsic thermal or extrinsic electrical noises. Here, we propose a standard measurement method of R n and intrinsic critical current (I ci) for high-T c superconductor (HTS) grain boundary JJs operated in liquid nitrogen and low-T c superconductor (LTS) multilayer superconductor/normal-conductor/superconductor (SNS) JJs in liquid helium. The applicable condition of this method is that both HTS and LTS JJs have U–I curves compatible with resistively-shunted junction (RSJ) model. Both R n and I ci values are extracted by combining a geometric mean criterion to select a data set and a least-squares fitting method with the RSJ model, eliminating two distortion effects on U–I curves: noise-rounding and self-heating. The combined method ensures relative standard uncertainty values of 1.9% for R n and 8% for I ci or better, when the users follow the standard protocol. It is demonstrated that the combined method is valid for d-wave HTS JJs near 77 K, regardless of peculiarities such as 0–π junction transition in d-wave superconductors at lower temperatures, and s-wave LTS SNS JJs with a wide range of junction parameters. This work is the first step to facilitate standardization for superconductor electronics with JJs.

Design and Fabrication of Coupled NanoSQUIDs and NEMS

Two significant recent developments in measurement science involve nano-electromechanical systems (NEMS) resonators and nanoSQUIDs. Both show remarkable properties with important implications for the future of metrology and sensing at the nanoscale. In this paper we describe early work in bringing these two developments together into a combined system which will further enable novel and ultrasensitive measurements. The ability to operate SQUIDs at the nanoscale without compromising the near quantum-limited performance demonstrated at larger scale is shown through two different SQUID technologies. The first is based on superconductor-normal conductor-superconductor (SNS) Josephson junctions while the second uses nanoscale Dayem bridges. The noise performance of both is similar. We also present results on fabrication of NEMS resonators using focused ion beam (FIB) milling and show how such a resonator can be assembled in close coupling with a nanoSQUID.

Investigation of TiOx barriers for their use in hybrid Josephson and tunneling junctions based on pnictide thin films

We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high IcRn-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes, such as lead and niobium, respectively. By tuning the barrier thickness, we were able to change the junction's behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe2As2 thin films prepared by pulsed laser deposition as base electrode and evaporated Pb as counter electrode. For barriers with a thickness of 1.5 nm, we observe clear Josephson effects with IcRn≈90 μV at 4.2 K. These junctions behave SNS'-like (SNS: superconductor-normal conductor-superconductor) and are dominated by Andreev reflection transport mechanism. For junctions with barrier thickness of 2.0 nm and higher, no Josephson but SIS'- (SIS: superconductor-insulator-superconductor) or SINS'-like (SINS: superconductor-normal conductor-insulator-superconductor) behavior with a tunnel-like conductance spectrum was observed.

Precision Comparison of Sine Waveforms With Pulse-Driven Josephson Arrays

The equivalence of two sinusoidal waveforms synthesized by pulse-driven Josephson arrays using two Josephson arbitrary waveform synthesizers (JAWS) has been investigated by a direct, high-precision comparison. The arrays are based on double-stacked SNS-type Josephson junctions (superconductor-normal conductor-superconductor) with Nb <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> as barrier material. Two arrays, consisting of 4000 junctions each, are arranged on one silicon chip and are connected in opposite voltage direction directly at the chip at 4.2 K. Careful phase tuning between the two JAWS systems allowed the minimization of the residual ac voltage, which was characterized by a fast digitizer and a lock-in amplifier. For an output voltage of 8.8 mV, a difference voltage of 0.14 nV with 0.25 nV uncertainty was measured, i.e., (1.6 ±2.9) × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> in relative units. This result demonstrates the excellent agreement of both voltages and therefore the quantization of the JAWS itself.

Vertical SNS weak-link Josephson junction fabricated from only boron-doped diamond

A vertical superconductor/normal conductor/superconductor (SNS) weak-link Josephson junction has been successfully fabricated from only homoepitaxial diamond films. Superconducting diamond, normal-state diamond, and superconducting diamond are homoepitaxially grown in a sequence. The temperature dependence of the critical superconducting current was fitted using Likharev's theory of SNS weak-link junctions. When the normal-state-diamond thickness $L$ is much larger than the coherent length in the normal-state-diamond ${\ensuremath{\xi}}_{n}$, an exponential temperature dependence of normalized critical current was observed, indicating a proximity effect. Shapiro steps in the current-voltage characteristics under microwave irradiation were observed in the junction with $L\ensuremath{\gg}{\ensuremath{\xi}}_{n}$.

Perspectives of Overdamped Josephson Junctions in Voltage Standard Applications

We discuss the main characteristics that must be satisfied by Josephson junctions for metrological applications and, specifically, in the development of voltage standards of new generation. Data reported in the literature for the most established technologies, like SNS (Superconductor-Normal conductor-Superconductor), SINIS (Superconductor- Insulator-Normal conductor-Insulator-Superconductor) and others, are analyzed and compared. The properties of Nb/Al- AlOx/Nb overdamped SNIS (Superconductor-Normal conductor-Insulator-Superconductor) Josephson junctions developed at INRiM are also presented, since these junctions can optimize crucial parameters such as the current density and the characteristic voltage. Operation at temperatures above 4.2 K is also discussed, in view of using cryocooler setups to replace liquid helium refrigeration systems.

Stacked SNS Josephson junction of all boron doped diamond

It is important for a new superconducting device application to demonstrate Josephson effect by homoepitaxial all diamond junction. Using only CVD diamond, we have fabricated stacked SNS (superconductor–normal conductor superconductor) Josephson junction where normal state diamond film is sandwiched by superconducting diamond films. The current–voltage (I–V) characteristics of the junction showed clear Josephson properties at 2K.

Self-consistent solution for proximity effect and Josephson current in ballistic graphene SNS Josephson junctions

We use a tight-binding Bogoliubov--de Gennes (BdG) formalism to self-consistently calculate the proximity effect, Josephson current, and local density of states in ballistic graphene superconductor-normal conductor-superconductor (SNS) Josephson junctions. Both short and long junctions, with respect to the superconducting coherence length, are considered, as well as different doping levels of the graphene. We show that self-consistency does not notably change the current-phase relationship derived earlier for short junctions using the non-self-consistent Dirac--BdG formalism [M. Titov and C. W. J. Beenakker, Phys. Rev. B 74, 041401(R) (2006)] but predict a significantly increased critical current with a stronger junction-length dependence. In addition, we show that in junctions with no Fermi-level mismatch between the N and S regions, superconductivity persists even in the longest junctions we can investigate, indicating a diverging Ginzburg-Landau superconducting coherence length in the normal region.

Improved design of superconductor/normal conductor/superconductor Josephson junctionseries arrays for an ac Josephson voltage standard

Superconductor/normal conductor/superconductor (SNS) Josephson junction series arraysare the subject of research for the realization of a Josephson arbitrary waveform synthesizer(JAWS) based on pulse-driven Josephson junctions. Lumped arrays of SNS Josephsonjunctions are investigated for this application at the Physikalisch-Technische Bundesanstalt(PTB). The series arrays consisting of Nb/HfTi/Nb SNS junctions are embedded in thecenter line of a coplanar waveguide transmission line. To increase the ac outputvoltage, the Josephson junction series array is designed as a meander-like structure.A broadband response of the arrays is required for the complex pulse operation of theJAWS. The arrays are investigated by dc measurements and for test purposes undercontinuous microwave irradiation at different frequencies. Flat Shapiro steps weremeasured for meander-like series arrays with 21 parallel lines containing up to10 752 junctions. A sine wave with a frequency of 2.5 kHz and a peak voltage of214 mVPeak was synthesized under pulse mode operation.

SNS Josephson Junction Series Arrays for the Josephson Arbitrary Waveform Synthesizer

The Josephson arbitrary waveform synthesizer (JAWS) allows AC voltages to be generated with fundamental accuracy. Lumped arrays of superconductor/normal conductor/superconductor (SNS) Josephson junctions were developed and investigated for this application. Combining electron-beam lithography and chemical-mechanical polishing, series arrays of sub-micrometer SNS junctions consisting of Nb/HfTi/Nb were fabricated. The junction size varied from 0.04 mum2 to 4 mum2 in series arrays up to 2560 junctions in different microwave designs. Stable Shapiro steps are generated in a frequency range from 5 GHz to 20 GHz using continuous microwave irradiation. Using a simple pulse pattern at different pulse repetition frequencies, the arrays show current-voltage characteristics with flat Shapiro steps. Sinusoidal AC voltages were synthesized using a commercial pulse pattern generator. Peak-to-zero voltages of some mV for frequencies below 1 kHz were achieved. Higher harmonics were measured to be more than 80 dB below the fundamental tone. The behavior of the arrays was investigated in more detail by measuring the amplitude-frequency spectra.

Interface dependence of the Josephson-current fluctuations in short mesoscopic superconductor/normal-conductor/superconductor junctions

We discuss the dependence of the Josephson current correlations in mesoscopic superconductor/normal-conductor/superconductor (SNS) devices on the transparency of the superconductor/normal-conductor (SN) interfaces. Focusing on short junctions we apply the supersymmetry method to construct an effective field theory for mesoscopic SNS devices which is evaluated in the limit of highly and weakly transparent interfaces. We show that the two-point Josephson-current correlator differs by an universal factor 2 in these two cases.

Internally shunted Josephson junctions with barriers tuned near the metal–insulatortransition for RSFQ logic applications

The fabrication of self-shunted SNS (superconductor/normal conductor/superconductor)Josephson junctions for rapid single flux quantum (RSFQ) logic couldpotentially facilitate increased circuit density, as well as reduced parasiticcapacitance and inductance over the currently used externally shunted SIS(superconductor/insulator/superconductor) trilayer junction process. We report thedeposition, fabrication, and device characterization of Josephson junctions prepared withNb1−yTiyN electrodesand TaxN barriers tuned near the metal–insulator transition, deposited on practical large-area oxide-bufferedsilicon wafers. When scaled to practical device dimensions, this type of junction is found to have anIcRn product of over 0.5 mV and a critical current(Ic) and normalresistance (Rn) of magnitudes suitable for single flux quantum digital circuits. A longer than expected normal-metal coherencelength (ξn) of 5.8 nm is inferred from the thickness dependence ofJc at 4.2 K for junctions fabricated using a barrier resistivity of13 mΩ cm. Althoughnot well understood and not quantitatively predicted by conventional theories, this results in a sufficientlyhigh Ic and IcRn to make the junctions suitable for practical applications. Similar observationsof unexpectedly large Josephson coupling currents in SNS junctions have beendocumented in other systems, particularly in cases when the barrier is near theM–I transition, and have become known as the giant proximity effect. The temperature dependence ofξn,IcRn,and Jc are also reported. For this technology to be used in practical applications, significantimprovements in our fabrication process are needed as we observe large variations inIc and Rn values across a 100 mm wafer, presumably as a result of variationsin the Ta:N stoichiometry and the resulting changes in theTaxN barrier resistivity.

Supercurrent-phase relationship of aNb∕InAs(2DES)∕NbJosephson junction in overlapping geometry

Superconductor/normal conductor/superconductor (SNS) Josephson junctions with highly transparent interfaces are predicted to show significant deviations from sinusoidal supercurrent-phase relationships (CPR) at low temperatures. We investigate experimentally the CPR of a ballistic Nb/InAs(2DES)/Nb junction in the temperature range from 1.3 K to 9 K using a modified Rifkin-Deaver method. The CPR is obtained from the inductance of the phase-biased junction. Transport measurements complement the investigation. At low temperatures, substantial deviations of the CPR from conventional tunnel-junction behavior have been observed. A theoretical model yielding good agreement to the data is presented.

Optimized technology for freely positionable Josephson junctions in YBa/sub 2/Cu/sub 3/O/sub 7-x/ fabricated by local ion damaging

Freely positionable Josephson junctions in high-temperature superconductor thin films are fabricated by a combination of photo- and electron-beam lithography. In selected regions of YBCO thin films point defects are induced by oxygen implantation. These weak links exhibit resistively shunted junction (RSJ)-like voltage-current characteristics close to the junction transition-temperature. Below this temperature, a transition from RSJ to flux-flow behavior is observed. Fraunhofer patterns appear in magnetic modulation measurements at different temperatures. The temperature dependence of the critical current reveals superconductor/normal conductor/superconductor (SNS) features. The dependence of Shapiro steps on microwave power demonstrates Josephson contributions over the entire temperature range.

Impedance control of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub x/ Josephson junctions on MgO substrates for millimeter wave sensing

SIS-type (Superconductor-Insulator-Superconductor) intrinsic stacked Josephson junctions (ISJJs) and SNS-type (Superconductor-Normal conductor-Superconductor) step-edge Josephson junctions (SEJJs) have been successfully fabricated using anisotropic Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub x/ thin films on MgO substrates. In order to apply them to millimeter wave receivers, their normal resistance (Rn) was controlled, which determines the impedance matching for millimeter wave signals. For electrical transport along the c-axis, the Rn of a single unit ISJJ with a 5 /spl mu/m-square mesa was estimated to be 580 /spl Omega/. For electrical transport parallel to the ab-plane, the R/sub n/ of series connected six SEJJs (6-SEJJs) with the gentle slope of 28/spl deg/ was 2.0 to 3.8 /spl Omega/. Since their responsivities as millimeter wave detectors were two to three orders of magnitude larger than those of single SEJJs, efficient IF (intermediate frequency) output of fundamental mixing with LO (local oscillation) power of a few microwatts was observed up to 95 K.

Fluctuations of the DC Josephson Current in a Superconductor-Normal Conductor-Superconductor Junction

We study mesoscopic fluctuations of the dc Josephson effect in a superconductor-dirty normal conductor-superconductor junction using the Green function method. Our theoretical approach is valid if the pair potential is considerably higher than the Thouless correlation energy. The effects of temperature and sample geometry are taken into account. The calculated result is in fairly good agreement with that of the recent experiment by Takayanagi et al .

YBa&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;7-x&lt;/sub&gt;超電導薄膜の抵抗と臨界電流密度の周波数依存性

YBa2Cu3O7-x superconducting thin films were prepared using the Excimer laser ablation technique. The current-voltage characteristics of these films was measured under various ramp current waves with rise rates of 50A/s-10kA/s. It was shown that the resistance of the flux flow increased with current rise rate. Critical current density decreased with increasing the current rise rate. The critical current density-temperature characteristics of the YBa2Cu3O7-x superconducting thin films were consistent with a proximity effect junction behavior. The frequency dependence of the critical current density was found to be attributed to the electon diffusion coefficient (electron mobility) in the normal conductor layer of the superconductor-normal conductorsuperconductor (SNS) junction.

In-situ fabrication of SNS junction consisting of YBaCuO/PrBaCuO/YBaCuO structure

All high-T/sub C/ superconductor-normal conductor-superconductor (SNS) junctions have been fabricated using in-situ deposition of YBaCuO/PrBaCuO/YBaCuO trilayer films. The trilayer films of YBaCuO(200 nm)/PrBaCuO(50 nm)/YBaCuO(100 nm) with c-axis orientation were fabricated on the

High-T/sub c/ Josephson junctions and DC SQUIDs

Step edge Josephson junctions and DC superconducting quantum interference devices (SQUIDs) made of laser ablated and sputtered YBaCuO (YBCO) films and grain boundary junctions in polycrystalline TlBaCaCuO (TBCCO) films have been investigated. Bridges on the micrometer scale have been structured by laser or ion beam etching as well as by the inhibit-layer technique. The Josephson junctions are characterized comparing I-V measurements under microwave irradiation to calculations within the resistively shunted junction (RSJ) model with finite capacitance and white noise. The dependence of the I/sub C/R/sub N/ product on temperature can be fitted to the microscopic theory of superconductor-normal insulator-normal superconductor (SNINS) or superconductor-normal conductor-superconductor (SNS) junctions with additional temperature-independent pair breaking. Flux modulation and noise properties of the DC SQUIDs at 77 K are shown. >

Electrical characteristics of HoBa 2Cu 3O 7 − x-PrBa 2Cu 3O 7 − y-HoBa 2Cu 3O 7 − x junctions with planar-type structures

All-oxide superconductor-normal conductor-superconductor (S-N-S) junctions with a planar-type structure are fabricated using HoBa 2 Cu 3 O 7 − x and PrBa 2 Cu 3 O 7 − y as the superconductor and normal conductor, respectively. Their electrical characteristics are measured. The junction's normal resistance is almost comparable to that of a single PrBa 2 Cu 3 O 7 − y film. However, it deviates from the T − 1/3 ( T is temperature) law and saturates at low temperatures for junction lengths smaller than 1.0 μm. Superconducting characteristics obtained for this type of junction may be accompanied by this saturation of normal resistance. Nonlinear current-voltage characteristics that may reflect correlation energy in the PrBa 2 Cu 3 O 7 − y layer are also detected.