Applied Superconductivity最新文献

The structural and superconducting properties of (Gd_1−yCa_y)Ba₂(Cu_1−xMo_x)₃O_z samples are investigated using X-ray diffraction, A.C. susceptibility, electrical resistivity and oxygen content measurements. The effect of increasing the Mo concentration in GdBa₂(Cu_1−xMo_x)₃O_z changes the structure from orthorhombic to tetragonal, accompanied by an increase in the normal state resistivity and decreases both the oxygen content and T_c, which is attributed to hole filling by Mo. This suppression in T_c has been compensated for by appropriate hole doping with Ca, for instance in the oxide (Gd_1−yCa_y)Ba₂(Cu_0.92Mo_0.08)₃O_z, the T_c increases from 27 K (y=0.0) to 86.5 K (y=0.24, compensated oxide), closer to the value of 91 K for pure GdBa₂Cu₃O_7−δ due to the balance between the hole filling by Mo and hole doping by Ca.

The lack of high density memories at 4 K has severely constrained the applications of digital Josephson electronics. Superconductor–semiconductor hybrid technology can take advantage of the high speed of a superconductor processor and the high density of a semiconductor memory and make superconducting electronics applicable. Currently we are developing a hybrid memory system to achieve low power (135 mW) and high speed (128 Gb/s) data access between a 16 GHz 8-bit superconducting rapid single flux quantum (RSFQ) vector processor and a 512 kbit complimentary metal-oxide silicon (CMOS) memory system. In this paper, we give a detailed description of both the high-level system organization and low-level circuit design, as well as simulation and test results for some circuit components of this hybrid RSFQ–CMOS memory-processor interface.

A resistive d.c. SQUID (superconducting quantum interference device), d.c. RSQUID for short, is a superconducting loop that contains two shunted Josephson junctions and a resistor. Our d.c. RSQUIDs are intended for noise thermometry below 4.2 K. They are fully integrated thin film devices fabricated in standard Nb technology. The resistor is made from an evaporated silver layer. It is specially designed in order to suppress the proximity effect, and thus the resistor remains normally conductive down to mK temperatures.

For read-out, modified d.c. SQUID electronics are used. It is directly coupled to the d.c. RSQUID and represents a preamplifier with a voltage noise of 0.8 nV/Hz^1/2, a gain of 4000, and a bandwidth of 4 MHz.

Due to this large bandwidth, the d.c. RSQUID can be operated at a high frequency enabling a short averaging time for the temperature measurement. At a frequency of 2 MHz and a temperature of 4.2 K we measured the temperature with a statistical uncertainty of 0.8% for 500 s averaging time.

A Josephson 256 word×16 bit RAM that includes a power circuit has been designed to enable high-frequency clock operation. This RAM consists of a 4×4 matrix array of 256 RAM blocks, impedance matching lines, and input signal amplifiers. A power-supply circuit, composed of a transformer and a Josephson regulator, is included in each 256 RAM block. Fail bit maps for the 256 RAM block were measured, and perfect operation with a 100% bit yield was obtained. The 256 RAM block functioned properly at a high clock frequency of 1 GHz with less than 3 mW of power dissipation from an external power source.

Integrated magnetometers based on superconducting quantum interference devices (SQUIDs) are employed for the very sensitive measurement of magnetic flux and magnetic flux density. Today, most superconducting magnetometers made from high-temperature superconductors (HTS) are based on YB_a2Cu₃O₇ and are optimized for a temperature of 77 K. Depending on the application, the size of the magnetometers is restricted. Many applications in the non-destructive evaluation (NDE) of materials allow the use of large chips with up to 5 cm diameter. The need for integrated, multilayer magnetometers arises, when high sensitivity has to be combined with high spatial resolution with areas below 1 cm². This applies to the sensors in multichannel systems for biomagnetism which have to be adapted to the size of the sources of magnetic signals in the human heart or brain. Here, a survey on the current status of integrated, thin-film YB_a2Cu₃O₇ magnetometers is given.

We have measured the switching current distributions out of the zero-voltage state of Josephson junctions in nonstationary conditions, obtained by means of fast sweeping of the current bias (≌100 kHz) resulting in a dI/dt up to 25 A/s. In this way we have analysed the effects of the levels quantization on the escape rate out of the zero voltage state in conditions where the occupancy probability of the energy levels is far from being in equilibrium.

We have developed a novel coplanar resonator serving as a tank circuit for rf washer supercenductry quantum interference device (SQUID)s. Two coplanar lines surround flux concentrator washers. The SQUID, 2.5 or 3.5 mm in diameter, is coupled to the concentrator in the flip-chip configuration. In these layouts, the adjustable resonant frequency is up to the GHz-range. With SQUID loops of 10×500 μm² (SQUID inductance L_s=260 pH), we measured at 77 K white flux noise levels S_φ^1/2 of 8.5 μφ₀/√Hz. This corresponded to an energy resolution ϵ of 850 h and a field resolution of about 16 fT/√Hz for a concentrator diameter of 13.4 mm.

An all-NbN quasi-optical superconductor–insulator–superconductor (SIS) mixer for the terahertz band has been designed and tested. This mixer consists of a MgO hyperhemispherical lens with anti-reflection cap, an NbN twin-slot antenna, and NbN tuning circuits. The size of the NbN/AlN/NbN junction was about 0.5 μm in diameter, and the current density was about 30 kA/cm². The junctions showed good d.c. I–V characteristics, with a high gap voltage of about 5.4 mV and a small sub-gap leakage current. The double side band (DSB) receiver noise temperature, measured by the standard Y-factor method, was about 2700 K at 761 GHz. This value is much higher than the theoretical sensitivity based on the experimental I–V curve calculated using Tucker’s quantum theory of mixing. The difference between experimental and theoretical mixer performance may result from large RF losses in the tuning circuit of NbN fabricated on SiO.

Josephson junctions based on thin films of the Bi₂Sr₂Ca₂Cu₃O_10+δ compound show I_cR_n products compatible with YBa₂Cu₃O_7−δ samples. Using quasiparticle tunneling experiments we found evidence for a superconductor–insulator–superconductor tunneling process via localized states in the barrier. The Bi₂Sr₂CaCu₂O_8+δ compound is investigated regarding possible applications in superconducting field effect devices. We present thin films of four unit cells thickness that are superconducting at 58 K. An inverted metal–insulator–superconductor structure was prepared. From the modulation of the normal state resistance we estimate a carrier density of 7×10¹⁹ cm⁻³ for a superconducting sample. The shift of the transition temperature was 1 K/V. The modulation of the I–V characteristics was demonstrated.

We have studied the electrical transport properties of the grain boundaries (GBs) formed at the top and at the bottom edges of YBa₂Cu₃O_7-δ step-edge Josephson junctions, for different values of the step angle α. The step-edge junctions were fabricated on (100) LaAlO₃ steps using a tilted Ar ion milling. Due to the shadowing effect of the step, the middle and the bottom part of the step edge junction were contacted by thin stripes. We found that for α≈60° the top GB is responsible for the weak link behaviour of our step-edge junctions. On less steep steps, α≈45°, a series of GBs with weak link properties were found to nucleate along the step profile. We also correlated these results with the different microstructural properties of the GBs formed on steps with different angles.