Ramp-type Josephson Junctions Research Articles

Characterization of ramp-type Josephson junctions with a Co-doped PrBaCuO barrier

Current transport mechanism in Go-doped PrBaCuO barriers was investigated for ramp-type junctions. The junction characteristics were extremely sensitive to a slight variation in the substrate temperature and the oxygen atmosphere during the heating process for the subsequent barrier layer deposition. Such sensitivity was related to the thermodynamic stability of YBaCuO. The conductance of junctions fabricated under the optimized conditions exhibited an exponential dependence on the barrier layer thickness at low temperatures, and the decay length was estimated to be around 1 nm. Characteristic power law dependence of junction conductance on temperature was confirmed for these junctions, indicating that resonant tunneling and hopping conduction via a small number of localized states were predominant. Clear Josephson characteristics were observed for junctions with a barrier thinner than 11 nm. We found that the experimental I/sub c/ versus barrier thickness relation was also explained well by resonant tunneling models.

High-T c Ramp-Type Josephson Junctions for Rapid Single Flux Quantum Circuits

Ramp-type Josephson junctions with YBCO superconducting electrodes and PrBa 2 Cu 2.6 Ga 0.4 O 6+δ (PBGCO) barriers were fabricated and characterized. The YBCO ramp junctions with 25 nm-thick barriers of PBGCO showed typical resistively- shunted-junction (RSJ)-like I-V behavior and 1σ spread of less than 13% in critical current density. Differential conductance-voltage characteristics registered using a lock-in technique indicate that the resonant tunneling via localized states in the barriers plays a dominant role in the conduction process of the ramp junctions.

Evidence for coherent structures within tokamak plasma turbulence

The microstructures of YBa2Cu3O7-delta/PrBa2Cu3O7-delta/YBa2Cu3O7-delta ramp type Josephson junctions with different ramp slopes fabricated on (100) SrTiO3 substrates were studied by means of transmission electron microscopy. The results show that for the ramp slope angles of 15 degrees-40 degrees, 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. For a steep ramp junction, a dimple was created in the substrate, which caused the bending in the film and the nucleation of misoriented grains, 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. (C) 1998 Elsevier Science B.V.

Microstructural study of ramp type multilayer Josephson junctions with different ramp slopes

The microstructures of YBa 2Cu 3O 7− δ /PrBa 2Cu 3O 7− δ /YBa 2Cu 3O 7− δ ramp type Josephson junctions with different ramp slopes fabricated on (100) SrTiO 3 substrates were studied by means of transmission electron microscopy. The results show that for the ramp slope angles of 15°–40°, 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. For a steep ramp junction, a dimple was created in the substrate, which caused the bending in the film and the nucleation of misoriented grains, 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.

High-Tc ramp-type Josephson junctions on MgO substrates for terahertz applications

The authors successfully fabricated high-Tc ramp-type junctions with PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// (PBCGO: x=0.1, 0.4) barriers on MgO substrates. The junctions showed resistively shunted Josephson junction (RSJ)-like I-V curves with thermally and voltage activated conductivity. The I/sub c/R/sub n/ products for these junctions scaled very well with the Ga-doping. Maximum response of the junctions for 100-GHz millimeter-wave irradiation could be observed up to 12 mV corresponding to 6 THz. Using far infrared laser radiation, we confirmed a terahertz (THz) response of these junctions. These results show promise for THz-wave applications of ramp-type Josephson junctions.

The effect of ion milling on the morphology of ramp-type Josephson junctions

Artificial barriers in Josephson junctions make it possible to change the height and width of the barrier independently. This technique can be realized in high-Tc Josephson junctions using the ramp technique. In this article the fabrication of ramp-type junctions is discussed and the importance of the morphology of the ramp is pointed out. Detailed investigations are described which address the surface roughness and the damage due to ion-beam structuring of ramps. It is shown that hard masks can significantly improve the ramp quality by reducing the ion impact angle. Furthermore, annealing of the so structured ramps leads to unit cell steps enforcing a step-flow growth mode.

A new concept for integrated YBa/sub 2/Cu/sub 3/O/sub 7/ magnetometers

We present an integrated magnetometer concept that combines inductance matching by a flux transformer with the low inductance of a multiloop pick-up coil. Using ramp-type Josephson junctions with PrBa/sub 2/Cu/sub 3/O/sub 7/ barriers, a 90 pH dc SQUID together with a five turn flux transformer is coupled to a multiloop pick-up coil. The dependence of the effective area of the magnetometer on the temperature is investigated by successively disconnecting the pick-up loops. The 1/f flux noise of the integrated magnetometer is suppressed by operation in a flux-locked loop mode with bias current modulation. This results in a flux density noise of /spl radic/(S/sub B/)(1 Hz,77 K)=100 fT//spl radic/(Hz). Using this magnetometer, different biomagnetic measurements have been performed.

Electrical and noise properties of YBa/sub 2/Cu/sub 3/O/sub 7/ ramp-type Josephson junctions

Josephson junctions with PrBa/sub 2/Cu/sub 3/O/sub 7/ barriers are used in integrated magnetometers, where for biomagnetic applications the noise at low frequencies should be as low as possible. The 1/f-noise contribution close to the critical current is governed by critical current fluctuations. We measure the electrical and noise properties of our ramp-type Josephson junctions and interpret them in a model of resonant tunneling through the PrBa/sub 2/Cu/sub 3/O/sub 7/ barrier. We find that the critical current fluctuations /spl delta/I/sub c//I/sub c/ scale with the temperature dependent critical current. The consequences for the noise properties of integrated magnetometers are discussed.

A high-T/sub c/ 4-bit periodic threshold analog-to-digital converter

Using ramp-type Josephson junctions a 4-bit periodic threshold ADC has been designed, fabricated and tested. Practical design constraints will be discussed in terms of noise immunity, flux flow, available technology, switching speed etc. In a period of four years we fabricated about 100 chips in order to bring the technology to an acceptable level and to test various designs and circuit layouts. This resulted in a basic comparator that is rather insensitive to the stray field generated by the analog input signal or variations in mask alignment during fabrication. The input signal is fed into the comparators using a resistive divider network. Full functionality at low frequencies has been demonstrated.

Noise properties and epitaxial quality in low-noise integrated YBa2Cu3O7 magnetometers

Temperature dependent flux-locked loop noise measurements have been performed on low-noise integrated multiloop pickup coil magnetometers using ramp-type Josephson junctions with PrBa2Cu3O7 barriers. We show that the noise properties of such multilayer devices depend on the epitaxy of the YBa2Cu3O7 top film as analyzed by micro-Raman spectroscopy. At high epitaxial film quality the low-frequency noise is dominated solely by fluctuations of the critical currents of the Josephson junctions and is suppressible by additional bias-current modulation. The noise and the epitaxial properties of different samples are correlated to details in the laser-deposition process of the insulating SrTiO3 layer.

Ga segregation in DyBa2Cu3O7−δ/PrBa2Cu3−xGaxO7−δ/DyBa2Cu3O7−δ ramp-type Josephson junctions

Ramp-type Josephson junctions with highly doped PrBa2Cu3−xGaxO7−δ barrier layers (x=0.7, 1.0) have been investigated by high-resolution electron microscopy. A Ga-rich intergrowth and Ga diffusion in the ion-milled SrTiO3 substrate are observed. The Ga segregation is responsible for the deviating electrical behavior as expected from extrapolation of low doping levels (x=0.1–0.4). The Ga diffusion in the ion-milled substrate, and possibly in the base electrode, may hamper Josephson junction fabrication using substituted REBa2Cu3O7−δ materials (RE=rare earth).

Ramp-type YBa2Cu3O7−δ Josephson junctions with high characteristic voltage, fabricated by a new, completely in situ, growth technique

We have fabricated YBa2Cu3O7−δ/PrBa2Cu3O7−δ/YBa2Cu3O7−δ ramp-type Josephson junctions with high characteristic voltage Vc=IcRn at a temperature of 77 K. A microshadow mask technique was used to grow completely in situ a ramp-type multilayer structure. Junctions with barrier thicknesses of about 13 and 45 nm were characterized. Junctions with a thicker barrier exhibited supercurrent up to 80 K and showed a resistively shunted-junction-like current voltage characteristics with a Vc as high as ∼1.5 mV at 77 K and 16 mV at 15 K. Junctions with a thinner barrier worked up to 90 K and exhibited the ac Josephson effect at 77 K under 8.9 GHz microwave radiation.

Low- or high-angle Ar ion-beam etching to create ramp-type Josephson junctions

The dependence of the ramp geometry on high- or low-angle ion-beam etching, used to structure ramp-type Josephson junctions based on high- superconductors, is investigated by cross-section transmission electron microscopy. The surface quality, interfaces and crystal defects are analysed by high-resolution electron microscopy. Technical difficulties to reproducibly obtain the desired slope angle and shape make high-angle ion-beam etching less interesting although the surface quality is comparable and no systematic differences in electrical properties are observed.

Biomagnetic measurements with an integrated YBa2Cu3O7 magnetometer in a hand-held cryostat

With low-noise magnetometers made from high-temperature superconductors, mobile applications of superconducting quantum interference device (SQUID) magnetometry become possible. Due to the high volume heat of evaporation of liquid nitrogen, a SQUID magnetometer can be operated in a small size cryostat for some hours. For the first time biomagnetic measurements are presented using an integrated YBa2Cu3O7 magnetometer mounted in a hand-held cryostat with a content of 100 cm3 of liquid nitrogen. These measurements and the noise properties are compared to those made in a conventional cryostat. The low-noise magnetometer consists of a multiloop pick-up coil coupled inductively to a dc SQUID based on ramp-type Josephson junctions with PrBa2Cu3O7 barriers.

Integrated YBa2Cu3O7 magnetometer with flux transformer and multiloop pick-up coil

We report on a novel integrated magnetometer concept that combines inductance matching by a flux transformer with the low inductance of a multiloop pick-up coil. A 90 pH dc superconducting quantum interference device together with a five turn flux transformer is coupled to a multiloop pick-up coil resulting in an effective magnetometer area of 1.4 mm2 on a 10×10 mm2 substrate. Using ramp-type Josephson junctions with PrBa2Cu3O7 barriers, a white noise level of √SB(1 kHz) = 44 fT/√Hz is measured. The 1/f-flux noise of the integrated magnetometer is suppressed successfully by operation in a flux-locked loop mode with bias current modulation. At 77 K we find a flux density noise of √SB(1 Hz) = 100 fT/√Hz.

Magnetic-field dependence of microwave properties of ramp-type Josephson junctions with PrBa 2Cu 3O 7 barriers

Josephson junctions in ramp-type geometry with artificial PrBa 2 Cu 3 O 7 barriers are prepared in a multilayer process using laser-deposited films and patterning with conventional photolithography and argon-ion etching. The Josephson junctions have been prepared with a barrier thickness of 53 nm to obtain a small critical current at low temperature and allow measurements to be made without the thermal noise rounding occurring at high temperature. The current-voltage characteristics of these junctions at 8.8 K follow the resistively shunted junction model with excess current. We study the microwave properties of the Josephson junctions and the influence of an external magnetic field. By applying microwaves we observe Shapiro steps and in the presence of an additional static magnetic field also half-integral constant-voltage steps are observed. The dependence of the widths of the Shapiro steps and of the half-integral constant-voltage steps on induced microwave current and magnetic field is investigated.

Ramp type HTS Josephson junctions with PrBaCuGaO barriers

Ramp type Josephson junctions have been fabricated using DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as electrode material and PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// with x=0, 0.10 and 0.40 as junction barriers. Barrier thickness lie between 6-30 nm. Several junctions without barrier were made in order to find ways to minimize the damage of the ramp interface. In total about 40 chips were fabricated each containing several junctions and their I-V characteristics measured for various temperatures down to 4.2 K. Only those junctions showing clear RSJ-like curves were selected to be analyzed. In some cases we also measured I/sub c/ as a function of a small applied field and obtained a clear Fraunhofer pattern, but there is a tendency to flux trapping as evidenced by LTSEM. It was found at 4.2 K that the critical current density J/sub c/ scales with the specific resistance R/sub n/A as J/sub c/=C/sub bar/(R/sub n/A)/sup -m/ (m=1.8/spl plusmn/0.5). The barrier material dependent constant C/sub bar/ increases with x, whereas, for a given d, J/sub c/ is constant and R/sub n/A increases.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Preparation of ramp junctions and multilayers for superconducting comparators

YBa 2Cu 3O/Y 0.3Pr 0.7Ba 2Cu 3O/YBa 2Cu 3O ramp type Josephson junctions have been prepared using Ar +-ion beam milling. For low barrier layer thicknesses the junctions show AC and DC Josephson effect up to 82 K. The Shapiro step height of junctions in a microwave field depends on the microwave power as predicted by the RSJ-model. The properties of junctions located on different sides of a bridge have been made equal within 30% by changing the angle between the Ar +-ion beam and the substrate during the preparation of the ramp. The critical current density of the YBa 2Cu 3O (YBCO) counter electrode over a ramp consisting of a Y 0.3Pr 0.7BCO/YBCO/CeO 2-bilayer is larger than 1 · 10 6A/cm 2 at 40 K.

Resonant tunneling in Y(Dy)Ba 2Cu 3O 7−δ/PrBa 2Cu 3−xGa xO 7−δ/Y(Dy)Ba 2Cu 3O 7−δ ramp-type Josephson junctions

We have investigated both experimentally and theoretically the normal state resistance and Josephson critical current of ramp-type Josephson junctions having YBCO (DyBCO) electrodes and 8–30 nm thick Ga-doped barriers PrBa 2Cu 3−xGa xO 7−δ with Ga content x = 0, 0.05 and 0.1. Analysis of the data shows that the behavior of the junctions can be well described by the model assuming transport through a finite number of localized states in the barrier.

Response of YBCO/PBCO/YBCO ramp type Josephson junctions to near mm wave irradiation

A high T c Josephson device for high frequency detection applications is being developed, consisting of an YBCO/PBCO/YBCO ramp type junction and a broad band log-periodic antenna. In this contribution we present the response of such a device to (near) mm wave irradiation. Shapiro steps have been observed up to very high voltage values - nearly 4 mV at 10 K, at the maximum of the radiation power. The modulation of the step amplitudes shows very good resemblence with the predictions from the Resistively Shunted Junction model.