Ramp-type Josephson Junctions Research Articles

Imaging of Order Parameter InducedπPhase Shifts in Cuprate Superconductors by Low-Temperature Scanning Electron Microscopy

Low-temperature scanning electron microscopy (LTSEM) has been used to image the supercurrent distribution in ramp-type Josephson junctions between Nb and either the electron-doped cuprate Nd_{2-x}Ce_{x}CuO_{4-y} or the hole-doped cuprate YBa_{2}Cu_{3}O_{7}. For zigzag-shaped devices in the short junction limit the critical current is strongly suppressed at zero applied magnetic field. The LTSEM images show that this is due to the Josephson current counterflow in neighboring 0 and pi facets, which is induced by the d_{x;{2}-y;{2}} order parameter in the cuprates. Thus, LTSEM provides imaging of the sign change of the superconducting order parameter, which can also be applied to other types of Josephson junctions.

Transport and interface properties in ramp-type Josephson junctions with a graded Y 1− xPr xBa 2Cu 3O y barrier

High T c ramp-type Josephson junctions have been successfully fabricated using a continually graded barrier Y 1− x Pr x Ba 2Cu 3O y . In these junctions the Josephson coupling occurs at the naturally formed S/N (superconducting/normal) interfaces within the Y 1− x Pr x Ba 2Cu 3O y layer. Such a barrier can lead to a highly transparent S/N boundary hence to greatly enhance the performance of junction. In contrast to most high- T c junctions, the properties of our junctions would be dominated by the barrier material rather than the boundary as such a boundary is absent in the weak link region. A unique feature of this novel barrier structure is that the effective thickness of the barrier can be varied even after fabrication, depending on the measuring temperature and the Pr-concentration gradient. It could make the transport process and the Josephson coupling differ from that of the conventional junctions. The temperature dependences of the barrier thickness and Josephson properties were investigated and compared with those of junctions with a conventional single barrier. Low values of proximity effect parameters found in our junctions show an advantage of this barrier structure.

Preparation of ramp-type Josephson junctions using an Nd2CuO4 barrier

Ramp-type Josephson junctions with a Nd2CuO4 (NCO) barrier have been fabricated on (001)-SrTiO3 using RF magnetron sputtering. Such an NCO barrier has a highly stable T′-214 structure and is chemically compatible with the 123-superconductors. The lattice constant and thermal expansion coefficient of YBa2Cu3Oy and NCO being very similar ensures a high transparency at the superconductor/barrier interface, thus significantly increasing the reproducibility. Its resistivity is also remarkably higher than that of many currently used barrier materials. These junctions showed a typical resistively shunted junction-like current–voltage characterization and Shapiro steps under irradiation using a microwave field. The current uniformity of these junctions has also been enhanced significantly. The temperature dependence of the junction parameters and the scaling behaviour are discussed and compared with other high temperature Josephson junctions.

Characterization of high-Tc ramp-type Josephson junctions constructed with a Nd2CuO4 barrier

High-Tc ramp-type Josephson junctions were fabricated using a Nd2CuO4 barrier. Such a Nd2CuO4 material has excellent structural, thermal and chemical compatibility with YBa2Cu3O7−δ. Its very stable 214-T′ structure, superior electric properties and closed lattice matching make it desirable to construct multilayer junctions. The growth and nature of the heteroepitaxial Nd2CuO4/YBa2Cu3O7−δ structures were investigated by using x-ray diffraction, small angle x-ray reflection, rocking curve, electron microscopy and surface scan measurements. Junctions with a Nd2CuO4 barrier display typical resistively shunted junction-like I–V characteristics with reasonable IcRn products. The large normal resistance, absence of the potential barrier at the SN boundary and a high transparency for the quasi-particles show the advantages of the Nd2CuO4 barrier. The temperature dependence of Josephson parameters and the transport process were studied and discussed.

Enhanced transparency ramp-type Josephson contacts through interlayer deposition

A thin interlayer is incorporated in ramp-type Josephson junctions to obtain an increased transparency. The interlayer restores the surface damaged by ion milling and has the advantage of an all in situ barrier deposition between two superconductors, leading to clean and well-defined interfaces. The method has been applied to Josephson junctions between high (YBa2Cu3O7−δ) and low temperature (Nb) superconductors, separated by a Au barrier. Transmission electron microscopy images of these junctions reveal crystalline YBa2Cu3O7−δ up to the interface with the Au barrier. The junctions have improved critical current density values exceeding 20 kA/cm2, normal state resistances of 3×10−8 Ω cm2 and IcRn products of 0.7 mV at 4.2 K. Furthermore, the junction properties can be controlled by varying the Au barrier thickness.

Low-Tcramp-type Josephson junctions for SQUIDS

The Josephson tunnel junction is the basic element of a superconducting quantum interference device (SQUID). Amongst other parameters, the junction capacitance determines the characteristics of a (digital) SQUID. In a conventional dc SQUID, reducing the junction capacitance decreases the flux noise of the sensor, whereas in digital SQUIDs, the operating frequency can be increased when reducing the junction capacitance. For digital SQUIDs, this means that not only the flux noise decreases, but also the flux slew rate increases. Slew rates up to l0(8) @ds can be achieved by reducing the junction size to the sub-pm2 level. Using a ramp-type structure allows sub-pm2 Josephson junctions sizes using standard lithography. In this paper we present the first results on low-T, ramp-type Josephson junctions and dc SQUIDs based on these junctions. The first junctions and SQUIDs showed nonhysteretic behavior at 4.2 K caused by the A1 bottom layer in the design.

Submicron YBa2Cu3Ox ramp Josephson junctions

Submicron YBa2Cu3Ox/PrBa2Cu2.6Ga0.4Ox/YBa2Cu3Ox ramp-type Josephson junctions were fabricated and tested. The submicron bridges in the top electrode were patterned by e-beam lithography and Ar ion milling through an amorphous carbon (a-C) mask. Junctions with width ranging from 0.2 to 8 μm and oriented along different crystal directions of YBa2Cu3Ox have been produced. Current–voltage characteristics show a behavior consistent with the resistively shunted junction model with small excess current. Junction critical current densities of about 10 kA/cm2 and characteristic voltages up to 6 mV were measured at 4.2 K for the submicron junctions. Junctions along different crystal orientations showed different characteristics suggesting an influence from the d-wave order parameter.

D-Wave-induced Josephson current counterflow in YBa2Cu3O7/Nb zigzag junctions.

Well-defined zigzag-shaped ramp-type Josephson junctions between YBa2Cu3O7 and Nb have been studied. The magnetic field dependencies of the critical currents provide evidence for d-wave--induced alternations in the direction of the Josephson current between neighboring sides of the zigzag structure. The arrays present controllable model systems to study the influences of pi facets in high-angle high- T(c) grain boundaries. From the characteristics, we estimate a possible imaginary s-wave admixture to the order parameter of the YBa2Cu3O7 to be below 1%.

Results on a fast digital DROS

A digital double relaxation oscillation (DROS) superconductingquantum interference device (SQUID) has the potential for a very large dynamicrange and a slew rate that is orders of magnitude larger than that ofconventional SQUID systems. This is, for example, advantageous when using theSQUID in an unshielded environment. An important characteristic of the digitalDROS is that the feedback flux is quantized since this flux is supplied by asuperconducting up-down counter. Together with the clock frequency, thequantization unit of the feedback flux determines the maximum slew rate. In anoptimized design, the quantization unit is adapted to the broadband flux noiseof the DROS, so that the slew rate is maximized without compromising thesensitivity. Simulations on our optimized digital DROS showed proper operationfor slew rates up to 5×106 Φ0 s-1. The relaxationoscillations generated at a frequency of 100 MHz deliver an on-chip clocksignal, so that no external clock is required. Slew rates up to108 Φ0 s-1 can be achieved by increasing the relaxationoscillation frequency to a few GHz. This means that sub-µm2 Josephsonjunctions with high critical current densities are required. The firstexperiments using low-Tc ramp-type Josephson junctions are presented inthis paper.

High-Tc ramp-type Josephson junctions with a continually graded Y1−xPrxBa2Cu3Oy barrier

High-Tc Josephson junctions with a graded barrier have been prepared by using a composite target. Such a barrier is synthesized by utilizing Y1−xPrxBa2Cu3Oy with a continually graded concentration of Pr, in which no lattice mismatch and other incompatible problems take place. The structural interfaces are absent in the weak link region and Josephson coupling occurs at the naturally formed superconducting/normal interfaces within the Y1−xPrxBa2Cu3Oy layer. Thus, it can significantly enhance the reproducibilty and performance of these junctions. The temperature dependences of the barrier thickness and Josephson were also studied.

Effect of the magnetic-field orientation on the modulation period of the critical current of ramp-type Josephson junctions

We have investigated the dependence of the critical current IC on the value and orientation of an externally applied magnetic field H for interface-engineered YBa2Cu3O7−x ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The IC versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the IC(H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material. © 2001 American Institute of Physics.

The stationary properties of YBCO/PrBCO/YBCO ramp junctions with a graded barrier layer

We studied the stationary properties of ramp type Josephson junctions with a graded barrier layer formed by varying the doping level of Pr in a Y 1− x Pr x Ba 2Cu 3O y barrier. The temperature dependence of the I c R n product is nearly linear for the junctions with a barrier thickness ranged from 20 nm to over 50 nm. A theoretical model, which assumes a linear order parameter structure in the graded barrier layer, was used to simulate the stationary properties of these junctions. Numerical result based on Usadel diffusive equation for this model is in good agreement with the experimental results.

Development of sub-micron SNS ramp-type Josephson junctions

At PTB, a fabrication technology for sub-micron superconductor-normal metal-superconductor (SNS) ramp-type Josephson junctions has been developed which allows these junctions to be used as active elements in highly integrated circuits. Test circuits of series arrays containing up to 10000 junctions with contact areas below 0.4 /spl mu/m/sup 2/ and of single junctions with contact areas reduced down to 0.03 /spl mu/m/sup 2/ have been successfully realized and measured. To achieve high values of the characteristic voltage V/sub c/, different N-layer materials, i.e. Al, PdAu and HfTi with thicknesses down to d=15 nm and different layer sequences have been investigated. Typical parameters of SNS junctions with a thickness of the HfTi N-layer of d=20nm are about j/sub c/=470 kA/cm/sup 2/ and V/sub c/=100 /spl mu/V. The junctions realized allow for application in superconducting circuits.

Investigation of ramp-type Josephson junctions with surface-modified barriers

We have investigated the properties of YBa/sub 2/Cu/sub 3/O/sub 7-x/ ramp-edge Josephson junctions with surface-modified barriers produced by Ar-ion irradiation followed by oxygen annealing. The fabricated junctions displayed RSJ-like I-V characteristics and excellent uniformity. The stray capacitance of the junctions was estimated from the ramp-edge structure. The junction capacitance was obtained by subtracting the stray capacitance from the shunting capacitance. We estimated the barrier thickness from the junction capacitance, and found that the critical current density of the junction increased exponentially with decreasing barrier thickness. The relative dielectric constant of the barriers ranged from 13 to 18.

Realization and properties of ramp-type YBa 2Cu 3O 7− δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa 2Cu 3O 7− δ and Nb have been fabricated. This procedure allows connections between high- T c and low- T c superconductors at different crystal sides of the high- T c superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa 2Cu 3O 7− δ to the Nb. Critical current densities up to 600 A/cm 2 are obtained at T=4.2 K, with typical R n A values of ∼0.8 μΩ cm 2. The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa 2Cu 3O 7− δ crystal axes is explained by considering a predominant d x 2− y 2 order parameter symmetry of the YBa 2Cu 3O 7− δ . The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high- T c superconductor.

Characteristics of YBa 2Cu 3O 7−y Josephson junctions prepared with smooth ramp-edge surfaces

YBa 2Cu 3O 7−y ramp-type Josephson junctions with PrBa 2Cu 3O 7−y and SrTiO 3 barriers have been fabricated on SrTiO 3 (001) substrates. The surface morphology of the ramp-edge was examined using the atomic force microscope (AFM). The conditions of the photlithography process and the ion beam incident angle were optimized to improve the surface roughness at the ramp-edge. Typically, smooth surfaces at the ramp-edge with a ramp edge of about 25° and an average roughness of about 3 nm was achieved. Results indicate the junction properties are improved with a smooth ramp-edge surface. The details are discussed.

THE TRANSPORT BEHAVIOR NEAR THE JUNCTION INTERFACE IN RAMP JOSEPHSON JUNCTIONS WITH A GRADED BARRIER

The stationary properties of high- T c ramp type Josephson junctions with a graded Y 1-x Pr x Ba 2 Cu 3 O y barrier have been studied. In such a structure the composition of the Y 1-x Pr x Ba 2 Cu 3 O y barrier is continuely changed as x increasing from 0 to 1 at one side of the PrBCO barrier and vice verse at the other side. The critical current of these junctions showed a nearly linear dependence on the temperature. The experimental results were discussed in comparing with theoretical models.

Interfacial microstructures in ramp type multilayer Josephson junctions studied by TEM

The microstructures of high T/sub c/ ramp type Josephson junctions were studied by using transmission electron microscopy. The work was emphasized at the interfacial defects and the influence of the ramp slope. The results show that for the ramp slope angles of 15/spl deg//spl sim/40/spl deg/, the epitaxy was still remained through all layers at the ramp region without the formation of big grain boundaries. No amorphous layers and secondary phases were observed at the barrier interfaces. For a gentle ramp junction, small misoriented grains appeared in some portions of the barrier. The substrate ramp formed during the ion etching process had little influence on the growth of the upper layers. In junctions with a steep ramp, defects increased near the interface, although the epitaxy of the barrier was of good quality. The results demonstrate that the slope angle of the junction ramp is an important factor that influences the performance of the Josephson junctions.

BaTbO/sub 3/ as a new material for insulation and junction barriers in High-T/sub c/ devices

It is shown that BaTbO/sub 3/ has a high degree of chemical and structural compatibility with YBa/sub 2/Cu/sub 3/O/sub 7/ and that this new material has many properties beneficial for different device applications. The growth morphology of several YBa/sub 2/Cu/sub 3/O/sub 7//BaTbO/sub 3//YBa/sub 2/Cu/sub 3/O/sub 7/ multilayer films was investigated by high-resolution transmission electron microscopy (HRTEM). Depending on the deposition temperature two types of interface between YBa/sub 2/Cu/sub 3/O/sub 7/ and BaTbO/sub 3/ were found. At relatively high deposition temperatures the misfit strain is mainly restricted to a narrow (smaller 1 nm) interface layer, whereas at lower temperature a semi coherent interface with well-localized misfit dislocations was observed. By /sup 18/O//sup 16/O tracer experiments it was shown that significant oxygen diffusion in a BaTbO/sub 3/ layer is possible at temperatures below 500/spl deg/C. The diffusion rate is much higher than in conventionally used insulators like e.g. SrTiO/sub 3/ and even higher than in YBa/sub 2/Cu/sub 3/O/sub 7/. Insulating properties of BaTbO/sub 3/ were successfully tested for crossovers in multilayer devices. Furthermore field effect devices and Josephson junctions using BaTbO/sub 3/ as dielectric or barrier material have been investigated. The ramp-type Josephson junctions included chemically as well as ion beam etched ramp-edges. In both cases the normal resistance decreases with decreasing temperature for a barrier thickness of 10 nm. For chemically etched junctions well defined Shapiro steps and characteristic voltage of about 0.2 mV at 77 K were observed.

MM and sub-MM properties of ramp-type Josephson junctions on MgO with STO buffer layers

We have successfully fabricated ramp-type junctions on MgO substrates using an SrTiO/sub 3/ (STO) buffer layer. The observed I/sub c/R/sub n/ product for the junctions on MgO with STO buffer layer were about 2 mV at 4.2 K and 0.1 mV at 60 K. The junctions clearly showed Shapiro steps under irradiation of MM-waves and sub-MM-waves. We observed Josephson emission at 50 GHz from a junction on a MgO substrate with STO buffer layer at 17 K. We have also confirmed mixing in the self-oscillating mode using MM-wave and sub-MM-wave signals.