Applied Superconductivity最新文献

Near-dc ionic currents arise after injury of nerves. The detection of their concomitant magnetic fields requires new techniques, to be applied in the growing field of biomagnetic measurement technology. The mechanical modulation of the source-to-detector distance enables monitoring of such slowly varying magnetic fields. In this report we describe technical details of the modulation device, including signal processing and system performance. The functionality of the device is demonstrated with an investigation on an in vitro human nerve specimen and with an in vivo investigation, performed with a patient undergoing a diagnostically indicated muscle biopsy.

We report our studies on light-induced changes in electrical properties of YBa₂Cu₃O_x (YBCO) grain-boundary Josephson weak links. We discuss the erasable process (photodoping), in which light-induced enhancement of junctions properties at low temperatures relaxes back above 250 K. Our studies are focused on the junction critical current versus time, light intensity, and magnetic field measurements. In tested few-μm-wide step-edge junctions, photodoping manifested itself as above 50% increase of the critical current and the decrease of the junction magnetic penetration depth, leading to reduction of the effective junction width. Contrary to the photodoping effect, the high-intensity laser modification of junctions led to permanent modification of their electrical characteristics that remained unchanged after subsequent room-temperature/helium thermal cycling of the sample. The laser-modified junctions exhibited up to 25% increase of the critical current times normal-state resistance product.

Complex high temperature superconductor digital RSFQ circuits require a multilayer technology that is compatible with freely positionable Josephson junctions. For RSFQ circuits, especially small inductances are required. This can be achieved by using superconducting striplines on a YBCO groundplane. We have produced and examined all basic elements like, e.g., superconducting cross-overs, vias, pinhole free insulation layers and on-chip resistors that are necessary to design and fabricate HTS RSFQ circuits. All structures have been successfully tested and integrated with ramp type junctions with PBCO barriers. For the superconducting lines over the groundplane an inductance of 0.8 pH/square was measured which shows the advantage of a superconducting groundplane. Based on this technology we fabricated a simple multilayer circuit consisting of two magnetically coupled d.c.-SQUIDs. Here we used bicrystal junction stacks due to the smaller spread of junction parameters. The circuit allowed essential operations like generating, erasing and detecting a single flux quantum. One d.c.-SQUID served as storage loop while the other was kept permanently in the voltage state and was used to determine the internal flux state of the storage loop.

We report on the effects of electromigration of basal plane oxygen vacancies on SNS ramp edge Josephson junctions where the N-layer is YBa₂Cu_2.79Co_0.21O_7−δ, a doped version of the YBCO electrodes. Through the application of a 4–10 mA (∼2–5 MA/cm²) current bias at room temperature, the basal plane oxygen order and content in the N and S layers were improved. This is demonstrated by an increase in I_cR_n from <5 μV, to as much as 205 μV. The implications of these results on SNS junction fabrication, and the nature of tunneling in such devices are discussed.

Ring-shaped rapid single flux quantum (RSFQ) circuits composed of segments of Josephson transmission lines (JTLs) and other RSFQ circuits enable permanent SFQ pulse circulation. New ring structures of different designs have been realized which comprise T-flipflop (TFF) and multiplier (MULT) circuits. Reliability in circuit operation has been proven experimentally by a bit error rate BER≅10⁻¹⁶. The fabrication process has been optimized by using PTB-4 μm Nb/Al₂O₃–Al/Nb trilayer technology with externally shunted tunnel junctions of critical current densities of j_c=≅1 kA/cm². Characteristic voltage is V_c=250 μV and Steward–McCumber parameter β_c≤1. A linear dependence of pulse circulation frequency on JTL bias currents has been measured within a bias current interval of 20%.

We have performed tunneling spectroscopy experiments on different kinds of high-temperature superconductors (HTS) and different types of junctions. Firstly, spectra were taken on surfaces of YBa₂Cu₃O_7-δ (YBCO) thin films and Nd_1.85Ce_0.15CuO_4-y (NCCO) single crystals by low-temperature scanning tunneling microscopy/spectroscopy (LTSTM/S) in a superconductor-insulator-normal metal (SIN) configuration. Second, the tunneling spectra of grain-boundary junctions (GBJs) fabricated on bicrystal substrates were measured for the materials YBCO, La_1.85Sr_0.15CuO_4-δ (LSCO), Bi₂Sr₂CaCu₂O_8+δ (BSCCO) and NCCO in a SIS configuration. The characteristics of the spectra in both systems could be related to the symmetry of the order parameter in the superconducting electrode(s). In particular, a zero bias conductance peak (ZBCP) was observed only in systems with a d-wave symmetry, but was completely absent in the s-wave system NCCO.

RSFQ-toggle-flipflops with a SFQ-trigger circuit a Josephson transmission line at the input and a SFQ/dc-circuit at the output of each stage are implemented in the Nb–Al₂O₃–Nb Josephson junction technology on a single chip having coplanar wave guides at input and output. The counter is tested successfully at 4.2 K via coplanar/coaxial transitions using a bit pattern generator and a digital oscilloscope at room temperature up to f_I≈2 GHz pulse repetition frequency at the input. The highest test frequency f_I is limited by the available pattern generator.

Experiments like the test of the weak equivalence principle at the Bremen drop-tower, Germany, and STEP (Satellite Test of the Equivalence Principle) require a position detector with an extremely high resolution to measure tiny displacements of free falling test bodies.

In order to develop a SQUID position detector numerous configurations of test bodies and pick-up coils with different geometries were tested experimentally. As a function of the position of the test body the inductance of the pick-up coil was measured with a commercial LCR meter as well as with a LTS DC SQUID system. This SQUID system, which is developed and manufactured at the Jena University, provides high sensitivity and extremely low intrinsic noise, especially at low frequencies.

This contribution will also discuss some recent results in measuring the motion of one body during its free fall over 109 m at the Bremen drop-tower.

Shunted tunnel junctions with a small parasitic inductance have been developed for improving the operating frequency of Josephson array oscillators. The inductance was minimized by reducing the inductive length to 1 μm and was estimated to be about 40 fH. The analysis of resonant properties for the shunted junctions gave a high resonant frequency up to 1.4 THz. Josephson array oscillators were designed and fabricated to operate at near Nb gap frequency (700 GHz) using 11 shunted Nb/AlOx/Nb tunnel junctions with Nb microstrip resonators. Shapiro steps induced by Josephson oscillation were clearly observed above the Nb gap frequency (up to 830 GHz). By fitting the step height to the simulation result using the RLCSJ model, the output power of the Josephson oscillator to the load resistor was estimated to be about 0.1 μW at 680 GHz.

Nuclear magnetic resonance (NMR) experiments at ultralow temperatures place high demands on superconducting quantum interference device (SQUID) amplifiers. We have developed a suitable direct-coupled readout SQUID electronics system, which utilizes additional positive feedback (APF), and have achieved a system bandwidth of 5 MHz, maximum slew rate of 1.3×10⁷ Φ₀/s at 14 kHz and a white noise level of 3.4 μΦ₀/$\text{Hz}$. We have also investigated improving the signal-to-noise ratio by using a low noise series array of dc SQUIDs and by placing the SQUID amplifier closer to the ultralow temperature experiment. We will discuss how we have configured our SQUID amplifiers for operation at T≪1 K.