Superconducting electronics

During the last decades superconducting electronics has been the most prominent area of research for small scale applications of superconductivity. It has experienced quite a stormy development, from individual low frequency devices to devices with high integration density and pico second switching time. Nowadays it offers small losses, high speed and the potential for large scale integration and is superior to semiconducting devices in many ways ¿ apart from the need for cooling by liquid helium for devices based on classical superconductors like niobium, or cooling by liquid nitrogen or cryocoolers (40K to 77K) for high-Tc superconductors like YBa2Cu3O7. This article gives a short overview over the current state of the art on typical devices out of the main application areas of superconducting electronics

Noise in (double) relaxation oscillation SQUIDs

We have modelled the effect of two intrinsic noise sources on the flux noise spectral density of (Double) Relaxation Oscillation SQUIDs ((D)ROSs) based on hysteretic Josephson tunnel junctions. An important noise source is the spread in the critical current of the SQUID due to thermal fluctuations. Critical current noise mainly determines the noise on the average output voltage of DROSs with high flux to voltage transfer. A second noise source is the spread in the relaxation frequency due to the random interaction between the Josephson oscillations and the relaxation oscillations during switching to the zero-voltage state. This effect can dominate the voltage noise of a ROS

Scanning Auger microscopy as applied to the analysis of highly textured YBaCu3Ox thin films

Scanning Auger electron spectroscopy and scanning electron microscopy have been used to investigate the local composition and structure of highly textured axis oriented YBaCuO films with thicknesses in the range 0.4–1 μm. The cuprate films were sputtered on MgO and sapphire (100)-oriented single-crystal substrates at room temperature followed by several anneal stages below or at 920°C in pure oxygen. The YBaCuO/sapphire sample was examined again after an additional 750°C air anneal for 24 h. By applying Auger line profiling on a freshly prepared cross-sectional surface of a thin cuprate film deposited on a sapphire substrate we have been able to show that barium aluminate segregation at grain boundaries is the main cause of the higher electrical resistance usually observed for cuprate films on Al2O3. The (drastic) reduction in Tc can be attributed to the substitution of aluminium in the cuprate at copper sites. Severe interdiffusion has been observed for the epitaxial c axis oriented YBaCu oxide films grown on an MgO substrate, which leads to a deterioration in the superconductivity. The main reason for reduced Tc and quality of cuprate films on MgO is the copper loss into the substrate, the depth of penetration of copper extending more than 400 nm below the YBaCuO---MgO interface. From our experimental results it is evident that Auger line profiling is an important tool in the analysis of high Tc superconducting thin films

Design of a Fast Digital Double Relaxation Oscillation SQUID

A fast digital Double Relaxation Oscillation SQUID (DROS) with a relaxation oscillation frequency of 100 MHz has been developed. The digital DROS incorporates a DROS and a superconducting up-down counter that supplies the feedback flux. The major advantage of a DROS is that the relaxation oscillations generate an on-chip clock signal and therefore, no external clock is required. In order to maximize the slew rate without compromising the sensitivity, the quantization unit of the feedback flux was adapted to the flux noise of the DROS. This resulted in a designed flux slew rate of 5·106 ¿0/s. We will discuss the design optimization, numerical simulations, the layout and some experimental results of the digital DRO

A high-Tc 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 demonstrate

Transistor performance of high-Tc three terminal devices based on carrier concentration modulation

Electric field effect devices and quasiparticle injection effect devices are good candidates for the realization of three terminal devices from high-T/sub c/ materials, since they take explicit advantage of the low carrier concentration in these compounds. We describe the fabrication and operation of both types of devices, and discuss their performance as transistor-like element

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 increase