Applied Superconductivity最新文献

We have developed a process for fabricating YBa₂Cu₃O₇ thin-film, ramp-type edge junctions in which no deposited barrier is employed. These devices display excellent RSJ-type current–voltage (I–V) characteristics with values of I_c and R_n tunable over a useful range for operation of digital circuits. Initial junction reproducibility and uniformity are very encouraging.

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 superconductive rapid single flux quantum (RSFQ) demultiplexer was designed, implemented and tested as an interface between high speed RSFQ circuits and low speed semiconductor circuits. We employed a data-driven self-timed (DDST) architecture to eliminate the timing constraint in the synchronous clocking. In this asynchronous architecture, a clock signal is localized within 2-bit basic demux modules, and complementary data signals are used between the modules to transmit timing information. A larger size of demux can be simply designed by connecting the 2-bit modules in a binary tree structure without any timing consideration. Successful operation has been observed in 4-bit and 8-bit systems at low frequency.

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.

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.

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.

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.

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.

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.

The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm² microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity $\text{R}$=3.9 V/W.