Step-edge Junctions Research Articles

Step-edge Josephson junctions and their use in HTS single-layer gradiometers

We describe reproducible technology for fabricating YBa/sub 2/Cu/sub 3/O/sub 7/ step edge Josephson junctions on SrTiO/sub 3/ substrates. We report on the electrical characteristics of the junctions at 77 K and the dependence on the ratio of film thickness to step height. Single-layer gradiometers incorporating step-edge junction dc SQUIDs have been fabricated. We compare the performance of identical SQUID gradiometers based on our step-edge junctions and on 24/spl deg/ SrTiO/sub 3/ bicrystal junctions. Gradiometric SQUIDs (G-SQUIDs) incorporating step-edges have also been fabricated. We measured the effective area of several G-SQUIDs to be in the range of 1-2 /spl mu/m/sup 2/, approximately two orders of magnitude lower than for conventional SQUIDs of similar inductance. We demonstrate that when incorporated into a gradiometer the G-SQUID leads to an improved performance in an unshielded environment.

Noise properties of YBa/sub 2/Cu/sub 3/O/sub 7-x/ step-edge junction dc SQUID magnetometers prepared on sapphire substrates

Step-edge YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) dc SQUID magnetometers on sapphire substrates have been fabricated. CeO/sub 2/ buffer layers and YBCO films were deposited in situ on the low angle (/spl sim/35/spl deg/) steps formed on the sapphire substrates with various thickness ratio of YBCO thin film to step height. Noise properties of the magnetometer exhibit quite different behavior according to the thickness ratio. The field noise of the dc SQUID magnetometer was measured to be 100/spl sim/300 fT//spl radic/Hz at 100 Hz and about 1.5 pT//spl radic/Hz at 1 Hz with a dc bias method, and 450 fT//spl radic/Hz at 1 Hz with an ac bias method.

Flux flow effects induced by a control current in a four terminal Josephson device

We report on the characterisation of a new Josephson device, based on a YBa 2Cu 3O 7 (YBCO) step edge junction and a control line. The control current is injected into the region close to the GB region through two Au/YBCO contacts. Feeding the device with a suitable control current, linear branches are observed in the current-voltage characteristic of the step edge junction, in the range 0.5–2.5 mV. No similar effects appear when an external magnetic field is applied. The stability of the linear branches and their wide-range voltage tunability are such that applications can potentially be developed in the field off submillimiter-wave generation and detection.

Voltage modulation of high-T c SQUIDs with step-edge junctions

We studied the V-I characteristics and the voltage modulation of washer-type step-edge YBa 2C 3uO y dc SQUIDs. It was found that the banks of the SQUID were composed of two junctions in series. The V-I curve can be fitted to the RSJ model with a sinusoidal current phase relation for the first grain-boundary junction while the second junction show a non-sinusoidal current phase relation. The V-Φ curves of SQUIDs can be characterized by the parameters of the first junctions if the SQUID is biased at I ∼ I c1 where I c1 is the critical current of the first junction. The implications of the data are discussed.

YBa 2Cu 3O 7 step-edge Josephson junction dc SQUID magnetometers prepared on sapphire substrates

YBa 2Cu 3O 7 direct coupled dc SQUID magnetometers based on the step-edge junctions on sapphire substrates have been fabricated. 15-nm-thick CeO 2 buffer layer and 120-nm-thick YBa 2Cu 3O 7 film were in situ deposited on the 35° steps formed on the sapphire substrates by Ar ion milling. Large I C R N product of 250 μV with junction resistance R N ∼ 5 ω and critical current I C ∼ 50 μA at 77 K could be obatined reproducibly with 5 μm microbridges. The SQUID magnetometers fabricated on the 1×1 cm 2 sapphire substrates based on the step-edge junctions exhibit a voltage modulation depth of 16μV and magnetic field noise of 170 fT/√Hz at 100 Hz for the 70 pH-SQUID.

Effects of d-wave symmetry in high T c step-edge Josephson junctions

We have studied critical currents of YBa 2Cu 3O 7 step-edge junctions with different step-line angles with respect to the major axes of SrTiO 3 (100) substrates. Measurements of the junction critical current indicated a periodic modulation with the step-line angle: maxima near 0° and 90° degree and minima near 45° and 135°. These results are in qualitative agreement with d-wave coupling superconductivity at the step-edge grain-boundary of the junctions.

Step angle effects in YBa 2Cu 3O 7 step-edge Josephson junctions prepared on sapphire substrates

YBa 2Cu 3O 7 (YBCO) step edge Josephson junctions were fabricated on sapphire substrates. The steps were formed on r-plane sapphire substrates by using Ar-ion milling with PR masks. The step angle was controlled in the wide range from 20° to 70° by adjusting both the Ar-ion incident angle and the PR mask rotation angle relative to the incident Ar-ion beam. CeO 2 buffer layer and in situ YBCO thin films were deposited on the stepped r-plane sapphire substrates by pulsed laser deposition. The YBCO film thickness was varied to obtain the ratio of film thickness to the step height in the range from 0.5 to 1. The step-edge junction exhibited RSJ-like behavior with I C R N product of 100∼300 μV, critical current density of 10 3∼10 5 A/cm 2 at 77 K.

Improving on the yield and performance of high Tc rf SQUIDs

A method to control and determined the critical currents I c of step-edge junctions has been developed. Using this method, the critical currents of step-edge junctions are easily adjusted to within a range useful for obtaining high performance rf SQUID magnetometers or gradiometers.

Superconducting properties of YBa2Cu3O7 step-edge Josephsonjunctions prepared on CeO2-buffered sapphire substrates

Step-edge Josephson junctions in the c-axis oriented YBa2Cu3O7 films were fabricated on the CeO2-bufferedsapphire substrates. The step angles were controlled in the wide range of20-75° by an Ar ion milling technique. The I-V curves ofjunctions fabricated on the 35° step with the film thickness to thestep height ratio of about 0.8 exhibited a resistively shunted junction-likebehaviour with an ICRN product of about 250 µV and a criticalcurrent density of about 2×104 A cm-2 at 77 K. Thecritical currents of the step-edge junctions increased approximately linearlywith decreasing temperature but the normal resistance remained almostconstant. All the samples of the step-edge Josephson junctions satisfied ascaling behaviour of ICRN∝(JC)0.5, and a dc SQUID made out ofthose junctions exhibited a typical voltage modulation in an applied magneticfield.

Low-frequency noise reduction in YBa2Cu3O7−δ superconducting quantum interference devices by antidots

It is demonstrated that the low-frequency noise due to vortex motion in high-temperature superconducting quantum interference devices (SQUIDs) in ambient magnetic fields can strongly be reduced by a simple arrangement of antidots patterned into the SQUID. Sputter-deposited YBa2Cu3O7−δ radio-frequency SQUIDs (rf-SQUIDs) with step edge junctions are characterized before and after patterning of antidots in the vicinity of the Josephson junction. No deterioration of the rf-SQUIDs due to the introduction of the antidots can be detected. In contrary, the onset of the increase of the low-frequency noise in an applied magnetic field is shifted from 10 μT for the bare SQUID to 40 (field cooled) and 18 μT (zero-field cooled) for the rf-SQUIDs with antidots. The reduction of low-frequency noise in ambient field is explained by trapping of vortices by the antidots. The comparison of zero-field and field-cooled experiments demonstrates that flux penetrating the washer does not affect the low-frequency noise as long as the vortices are homogeneously distributed and the flux is properly pinned in the vicinity of the junction.

Gap symmetry and critical current of YBa2Cu3O7 step-edge Josephson junctions

Effects of d-wave symmetry on the critical current of high Tc step-edge Josephson junctions were studied theoretically by using the concept of grain-boundary junctions, and the theory was compared with experimental results of YBa2Cu3O7 step-edge junctions on SrTiO3 (100) substrates. A step-edge junction contains two grain boundaries, and each grain boundary has both in-plane and out-of-plane misorientation angles at the same time. The critical current of the step-edge junction as a function of the in-plane misorientation angle has a periodic modulation with maxima when the step line is parallel to one of the major axes of the SrTiO3 (100) substrate. Experimental results of c-oriented YBa2Cu3O7 step-edge junctions with various in-plane misorientation angles were qualitatively in good agreement with the theory. The out-of-plane misorientation angle is formed between two grains usually with the c axes perpendicular to each other and is normally not controllable.

Effects of sputtered SiO2 passivation layers on YBCO microbridges and step-edge junctions

High-temperature superconducting YBa2Cu3O7- microbridges and step-edge junctions (SEJs) were coated with SiO2 thin film by an rf magnetron sputtering technique. The effect of the coating on critical temperature Tc and critical current Ic was studied. It was found that Tc and Ic of microbridges increased as a result of the coating, by up to 3 K for Tc and up to 23% for Ic. We believe that this is due to the oxygen in the plasma penetrating into YBa2Cu3O7- resulting in an increase in oxygen content and restoration of the degraded microbridge edges. Ic of the SEJ, however, was found to decrease by 20-30% after deposition of SiO2 films despite no degradation in Tc. The results of ageing tests showed that SiO2 film forms a good passivation layer, protecting YBa2Cu3O7- film from reacting with water.

Characteristics of the planar second-order high-Tc SQUID gradiometer

We have studied the planar-type single-layer second-order high-Tc SQUID gradiometer that detects the transverse field gradient, d2Bz/dx2. The gradiometer consists of three parallel-connected rectangular pickup loops that are directly coupled to a SQUID. The pickup loops are designed in such a way that the SQUID measures the difference in the differential screening currents of neighbouring pickup loops. The SQUID has either step-edge or bicrystal Josephson junctions. The gradiometer was made from a single layer of YBa2Cu3O7 film on a 1 cm × 1 cm chip. The response of the gradiometer was tested in various flux distributions generated by three identical multiturn film coils patterned on the same chip in parallel with each pickup loop. The balanced gradiometer responded sensitively only to the second-order field gradient. Reduction of the inductive coupling between loops improved the intrinsic balance.

Noise and mixing properties of high-Tc Josephson junctions at W-band frequencies

Abstract We report on the noise and Josephson mixing properties of high-Tc superconductor (HTS) Josephson junctions. Direct radiation measurements and heterodyne mixing experiments in the frequency range 45–141 GHz have been performed by using YBa2Cu3O7−x (YBCO) step-edge junctions (SEJ) on LaAlO3 and MgO and bicrystal junctions (BCJ) on MgO substrates. Junctions with current voltage characteristics (CVC) close to predictions of the resistivity shunted junction (RSJ) model were mounted into a high sensitive radiometer system. From linewidth measurements we calculated an effective noise temperature of our junctions. In heterodyne mixing experiments we obtained conversion efficiencies around −14 dB in the 11 GHz intermediate frequency (IF) band under the radiation of two monochromatic signals. In the fundamental mixing regime we observed response at IF at working temperatures up to 72 K. The measured receiver and mixer noise temperature of the Josephson mixer at 94 GHz local oscillator (LO) frequency, an IF of 1.4 GHz and at a working temperature of 10 K was 4700 and 3400 K, respectively.

Shorted two-dimensional high-Tc Josephson arrays for oscillator applications

Abstract We present results of our experimental investigations of the dynamic properties of two types of shorted two-dimensional (2D) arrays. The first type consisted of 4×4 YBCO step edge junctions integrated into a circuit, which allowed the simultaneous detection of all single row voltages. Thus, using only dc-measurements, the interaction and synchronization between the rows could be observed. In the second investigated array type the edge junctions of the rows were closed into superconducting loops in the form of coplanar resonators. Resonant steps were observed on the current–voltage characteristic due to interaction of the junctions with the resonators. The circuits were integrated into a stripline geometry and coupled to another (detector) Josephson junction. A clear detector response, i.e. Shapiro steps, was measured up to 460 GHz. Steps up to 4th harmonic were observed in the frequency range 150–200 GHz.

Optically-induced effects in Y–ba–cu–O Josephson junctions

Abstract We report our studies on light-induced changes in electrical properties of YBa 2 Cu 3 O x (YBCO) grain-boundary Josephson weak links. We discuss the erasable process (photodoping), in which light-induced enhancement of junctions properties at low temperatures relaxes back above 250 K. Our studies are focused on the junction critical current versus time, light intensity, and magnetic field measurements. In tested few- μ m-wide step-edge junctions, photodoping manifested itself as above 50% increase of the critical current and the decrease of the junction magnetic penetration depth, leading to reduction of the effective junction width. Contrary to the photodoping effect, the high-intensity laser modification of junctions led to permanent modification of their electrical characteristics that remained unchanged after subsequent room-temperature/helium thermal cycling of the sample. The laser-modified junctions exhibited up to 25% increase of the critical current times normal-state resistance product.

Ion-beam etching of substrate steps: SNS compared to step-edge grain boundary Josephson-junctions

Abstract For “SNS step and gap”, and for “step-edge grain boundary”, Josephson-junctions steep steps in the substrate are needed to get either a break in the YBa2Cu3O7-film (SNS) or to induce grain boundaries in the growth of the YBa2Cu3O7-film across the step (step-edge). We report on the influence of the profiles of ion-beam etched substrate steps in MgO and SrTiO3 on the growth of YBa2Cu3O7-films at the step-edges and on the electrical properties of the different Josephson-junction types. On MgO we have fabricated Josephson-junctions of the SNS-type (using Ag as “N”), while for the step-edge junctions SrTiO3 was used. Both types of junctions exhibit Josephson-effects at 77 K. For the steps we use photoresist AZ 5214 E with steep edges as the mask. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis on differently fabricated steps are presented and compared to the electrical properties of patterned YBa2Cu3O7-films grown on them. Especially the role of material redeposited at the edge of a step during ion-beam etching is discussed. The reproducible step edge profiles are reflected in reproducible electrical properties of the Josephson-junctions and dc-SQUIDs (on chip and chip to chip).

Vortex penetration and hysteretic behaviour of narrow planar Josephson junctions in a magnetic field

We have studied narrow YBa 2Cu 3O 7 (YBCO) step edge junctions (width w=0.8–18 μm) in applied magnetic fields up to B a∼35 mT. At B a=0, their I– V characteristics show good agreement with other YBCO grain boundary junctions. We use the magnetic field dependence of the junction critical current, I c( B a), to study the penetration of vortices. At 77 K, the junctions exhibit jumps in I c( B a) due to the creation, annihilation and motion of a small number of vortices close to the junction. The fields at which the jumps occur and the number of jumps measured for a particular B a show a dependence on w. These data suggest that field enhancement affects the penetration of vortices and support a field enhancement factor which is proportional to w. The data also reveal information about the hysteretic trapping of vortices near the junction. The entire I c( B a) pattern demonstrates a shift along the B a axis depending on the size and direction of the maximum applied field, B max. The magnitude of this hysteretic behaviour increases both with w and with B max until showing signs of saturation for B max greater than ∼15 mT.

Fabrication and characterisation of YBCO single grain boundary step edge junctions

We use ion beam etching techniques to fabricate YBCO step edge junctions (SEJ) on MgO substrates. Argon ion-beam etching (IBE) of the substrate at angles other than at normal incidence is used to define the step height and angle. Thin (/spl sim/300 nm) magnetron sputtered YBCO films are deposited over the step and patterned using microlithography and cold substrate ion-beam etching techniques. The critical current, I/sub c/ of these SEJs can be controlled by varying the angle of the step etched into the substrate. Fabrication techniques are described which produce one grain boundary at the top of the step and include a smooth return path thereby avoiding a second grain boundary at the bottom of the step. At 77 K, the current-voltage (I-V) characteristics show resistively shunted junction behaviour. These junctions routinely demonstrate reasonably large I/sub C/R/sub N/ products (0.1-0.6 mV), making them suitable for applications in high temperature SQUID devices.

Flux trapping and magnetic field dependence of critical currents in narrow YBCO step edge junctions

We have studied narrow (0.8-18 /spl mu/m wide) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// step edge junctions in moderate applied magnetic fields as high as B/sub a//spl sim/25 mT. The magnetic field dependence of the junction critical current, I/sub c/, resembles a Fraunhofer diffraction pattern for all the junctions studied, although the field values of the pattern minima are modified due to the planar geometry. At 77 K these narrow junctions exhibit abrupt discontinuities in the I/sub c/(B/sub a/) pattern due to jumps in I/sub c/. These jumps are most likely due to sudden changes in the position of an individual vortex in close proximity to the junction. The I/sub c/(B/sub a/) patterns show hysteretic behaviour for applied fields B/sub a/>1-2 mT at 77 K, and the magnitude of this hysteretic behaviour appears to plateau for B/sub a/>/spl sim/12 mT, depending on junction width.