[en] A dc getter- and bias-sputtering apparatus is constructed. Good-quality superconducting thin Nb films are successfully prepared using the apparatus in a conventional vacuum system. Thin-film microbridges are fabricated out of the Nb films by employing photoresist and anodization techniques. Microwave-induced current steps are observed at 10 GHz in variable tickness type bridges, typically 0.5μ x 0.5μ and 250 Angstroem thick with two wide films 400 Angstroem thick adjacent. The current steps are observable up to 360μV and down to 4.2 K in a low critical current bridge (Ic (T=0.9Tc)=0.05 mA) with a normal resistance of 2.5Ω and a critical temperature of 7.6 K. The data of microwave response imply that heating is limiting the high voltage operation. The I-V characteristic, temperature variation of the critical current and hysteresis are also discussed. The relatively high resistance, low capacitance and temperature sensitivity of the critical current are promising for millimeter wave detector application. (orig.)

[en] We have fabricated Nb₃Ge/amorphous-silicon/Pb Josephson tunnel junctions using high T/sub c/(> or =20 K) and stoichiometric (25 +- 1 at. %Ge) Nb₃Ge films. Good I-V characteristic with large Josephson current I/sub c/ and high gap voltage is obtained when the amorphous-silicon barrier is sputter deposited at low rf voltage and high Ar pressure to reduce the damage to the Nb₃Ge by the sputtered particles and to obtain good coverage over the Nb₃Ge surface. Peculiar I-V characteristics showing the inverse ac Josephson effect and chaotic transition betwen current steps are observed in the response to microwave. The temperature dependence of I/sub c/ agrees well with the theoretical calculation when the Ge enrichment at the top surface of the Nb₃Ge electrode is reduced

[en] Nb₃Ge thin-film microbridges are fabricated on a cylindrical substrate to form rf SQUIDs. High temperature operations well above 10 K (up to 17.5 K) are obtained. An intrinsic flux noise of the order of 10^-4 Phi₀/(Hz)/sup 1/2/ is estimated from the step rise parameter. Comparison with a theoretical noise indicates a nonsinusoidal current phase relation of the microbridge

[en] We have observed a telegraph-like noise in dc superconducting quantum interference devices (SQUID's) made of low-temperature synthesized superconducting Y-Ba-Cu oxide thin films. The voltage of the SQUID jumps among two to four discrete levels randomly with time, and exhibits a Lorentzian type frequency dependence in the power spectrum. Periodic modulations of the voltage with magnetic flux are observed in a SQUID having narrow bridges of submicrometer width. The flux noise of the SQUID is dominated by the telegraph-like noise at low frequencies

[en] We have studied the surface structure of rf-sputtered Nb₃Ge films by means of reflection electron diffraction and Auger electron spectroscopy. It is found that a tetragonal Nb₅Ge₃ phase exists, being partly disordered, at the surface of high-T/sub c/ films, whereas A15 phase is predominant in the body of the film as proved by x-ray diffraction. The tetragonal surface phase is replaced by an amorphous phase in thin films of <1000 A, where reduced T/sub c/'s as compared with those of thick films are found. Single A15 phase occurs at the surface of Ge-deficient films whch have low T/sub c/'s. Auger analysis has shown a pronounced Ge-rich layer with a depth of 60-100 A, depending on film composition and thickness. At the top of the surface layer, Ge is enriched beyond a composition corresponding to Nb₅Ge₃. It is inferred that the Ge-rich layer is responsible for formation of the tetragonal phase

[en] The authors have synthesized superconducting Y-Ba-Cu oxide thin films by rf sputtering at a relatively low substrate temperature. The films of 300-700 nm thickness have Tc of 60-72K without post-annealing after the deposition. A preferential orientation with a c-axis perpendicular to the film surface was observed. DC-SQUIDs having two parallel microbridges with constriction of submicrometer to 40μm width were fabricated from the oxide films by chemical etching. The microbridges having more than 2μm width did not operate as Josephson devices. The characteristics of these SQUIDs suggested vortex-flow. Periodic modulation of the voltage with magnetic flux was observed in a SQUID having submicrometer microbridges. Telegraph-like noise of the voltage which exhibits switching between two or more discrete levels was observed in all of the SQUIDs irrespective of the width of the microbridges. This telegraph-like noise was also observed in a single microbridge, and its effective switching-rate increased exponentially with the increase of temperature above 15K. A bump structure of Lorentzian due to the telegraph-like noise was dominant in the power spectral density of the flux noise of the SQUID

[en] In the characteristics of short (l < or = 0.2 μm) variable-thickness bridges made of Nb films we have found a transition into a regime of vortex flow when the thickness of the weak-coupling region is varied. The vortex flow is favored below a thickness of the order of the size of Nb grains. It is confirmed that the weak-coupling region exceeds the size of the vortex core because of the decrease in coherence length. A linear variation of the critical current with temperature and a linear or quadratic voltage increase in the I-V characteristic are observed in the vortex flow regime in agreement with theory. However, some disagreement is found in the microwave response

[en] We have fabricated Nb₃Ge microbridges, including those with variable-thickness geometry, with submicrometer lengths, by means of plasma etching combined with lithographic techniques. The measured I-V characteristic and the temperature variation of the critical current are consistent with that expected from the vortex concept. However, a deviation is observed in the microwave steps: the magnitude of the critical current oscillates with microwave power in a manner which agrees with an analog simulation based on the RSJ model. We have found a strong correlation between the bridge behavior and the film parameters T_c and rho_n (normal resisitivity). The critical current is reduced rapidly with a decrease in T_c and an increase in rho_n, while in high-T_c and low-rho_n bridges a linear I-V characteristic appears at low voltages. Taking into account the influence of pinning, we discuss the results in terms of vortex motion

No abstract available

[en] A planar gradiometer composed of three concentric superconductive loops is analyzed. The gradiometer performs the second derivative with a rotational symmetry in a form of partial²B/sub z//partialr², where r² = x²+y². In response to the biomagnetic field generated by a current dipole, an isoflux line distribution which resembles well the magnetic field distribution is obtained. The location and the strength of the current-dipole source can readily be estimated from the isoflux pattern. Reduction of the magnetic field noise from distant sources with respect to the signal of a near source is calculated to be comparable with that of conventional axial gradiometers