Applied Superconductivity最新文献

We have measured the low frequency (5 mHz<f<30 Hz) noise in current biased aluminium single electron tunneling (SET) transistors. A refined high frequency (HF) shielding allows us to maintain and study a given background charge configuration for many hours at T<100 mK. At frequencies below 10 Hz the noise is mainly due to charge traps, and the noise pattern superimposed on the V(V_g)-curve strongly depends on the particular background charge configuration resulting from the cooling sequence and the applied RF irradiation, including thermal radiation from the 4.2 K environment. The noise spectra, which show both 1/f and 1/f^1/2 dependencies and saturate at f<100 mHz can be fitted by two-level fluctuators (TLF) with Debye–Lorentz spectra and relaxation times on the order of seconds.

The noninvasive measurement of magnetic heart activity based on high-T_C DC SQUIDs, could be a tool for investigating cardiac electrophysiological properties, if such a system is able to work in unshielded environments. For the realization of such a system we use thin film planar SQUID gradiometers with bicrystal or step-edge Josephson junctions. Even for a small baseline of approximately 4 mm (limited by substrate dimensions), the field gradent resolution is in the range required for clinical analysis. We use a portable system which consists of a glass fiber cryostat with a measuring unit where we can place up to four sensors, different read-out electronics, and signal filtering methods. Firstly, measurements inside a magnetically shielded room were used to show the capability of our magnetocardiography (MCG) system in medical applcations. The system works in an unshielded environment also, without additional field compensation. We demonstrate a measurement of a magnetocardiogram in a real clinical environment, and discuss the possibilties as well as the limitations of this system in magnetocardiography.

The unshunted (un) SQUID (superconducting quantum interference device) is a novel device with a frequency-dependent damping on the Josephson junctions. The un SQUID circuit contains four adjustable parameters, including equivalents of the β_c and β_l in dc SQUIDs. We have studied numerically the effect of the parameters to the un SQUID characteristics. To solve the Langevin equations, explicit Euler integration is used. We have also mapped the device noise as a function of the parameters. The energy resolution of the un SQUID appears to be better than that of the dc SQUID.

We investigated the main dependencies of the gradient resolution of planar galvanometer superconducting quantum interference device (SQUID) gradiometers made of YBa₂Cu₃O_7-x (YBCO). We focussed especially on the influence of antenna layout and the parameters of the galvanometer SQUID such as effective and parasitic areas on the performance of the gradiometer and the behavior in an unshielded environment. The efficiency (that is, the quotient of effective area to inductance) of different geometries of antennas will be compared. Some aspects of the layout of the galvanometer SQUIDs are discussed in terms of parasitic area and best current resolution. Special problems due to the use of bicrystal Josephson junctions in gradiometers for operation in highly disturbed environment are shown. Step-edge Josephson junctions can offer alternative concepts. Reached gradient sensitivity values in the white noise region are 0.46 pT/(cm/$\text{H}$z) in the case of bicrystal junctions and 0.69 pT/(cm/$\text{H}$z) for step-edge junctions.

Random telegraph voltage noise signals characterized by strongly nonlinear and nonmonotonic dependence of the telegraph amplitude on current flow has been observed in good quality epitaxial BiSrCaCuO thin film strips biased with d.c. current flow and immersed in an external magnetic field. The shape of the amplitude dependence on current flow closely resembled that observed previously in granular films, where intergranular Josephson junctions were involved in noise generation. The dependence of noise amplitude on bias current and magnetic field in epitaxial films is discussed in the context of the possible Josephson flux-to-voltage conversion mechanism.

Two-dimensional arrays of shunted Nb/AlOx/Nb Josephson junctions have been investigated. Shapiro steps in the current–voltage (I–V) curve of an on-chip detector junction can be used to sense mutual phase locking of the array junctions. By scanning the array with a low-power electron beam, the phase locked junctions remain locked, whereas the unlocked junctions generate a beam-induced additional voltage drop along the array. In regions of the array’s bias current where pronounced Shapiro steps occur in the detector’s I–V curve, practically no array junction generates a beam-induced voltage signal, thus indicating complete mutual locking. For other bias currents of the array with less than maximum detected power, some of the junctions generate beam-induced signals, and can be identified as being non-locked. Our investigations allow a detailed and spatially resolved analysis of mutual phase locking in arrays of Josephson junctions. Moreover, the results obtained with autonomous arrays are compared to experiments performed with injection locked arrays.

We present some numerical simulation results for a non-latching Josephson gate which is referred as a coupled-superconducting quantum interference device (SQUID) (C-SQUID) gate. TheC-SQUID gate can perform several logic functions by tuning the bias current to the single-junction-SQUID input-stage. The encoder and the decoder composed of C-SQUID BUFFER and NOR gates are successfully simulated. A modified C-SQUID gate which utilizes kinetic inductance of Josephson junctions is also presented.

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.

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.

We report recent progress on an all-electronic, on-chip, 100 mK refrigerator. This refrigerator utilizes the unique thermal transport properties of a normal-insulator-superconductor (NIS) tunnel junction to preferentially remove electrons whose energy is higher than the Fermi energy from the normal electrode, and thus, to lower the temperature of electrons in the normal electrode. We present the first measurements demonstrating electron cooling in a device having a substantially larger area (∼10⁴ μm²) than typical submicron-scale devices demonstrated previously. The performance of this refrigerator is analyzed by a simple thermal model that includes energy transport through the junction, thermal loading from the environment, recombination of quasiparticles in the superconductor electrode, and non-ideal junction characteristics.