Fundamental characteristics of three-terminal thin-film microbridges

Current-voltage characteristics of a new type of superconducting weak link are investigated in which three bulk superconductors are coupled via a common weak-order-parameter region, in contrast to the usual weak links, where the coupling is limited to two superconductors. The indium three-terminal weak links studied in this experiment were made by using an electron beam lithography technique. The weak link length is typically shorter than the Ginzburg-Landau coherence length at the bath temperature, allowing strong interactions between phase-slip centers to occur inside the weak link. Pronounced structure in theI–V curves can be understood in terms of quasiparticle-exchange interactions between the phase-slip centers and order parameter variation due to current depairing. An aluminum three-terminal weak link was also investigated, showing behavior somewhat different from that of the indium weak links.     

A sinusoidal current-phase relation for superconducting thin-film microbridges

Superconducting systems separated by a very short weak link are treated using the one-dimensional Ginzburg—Landau theory. It is shown that in the limit of extremely narrow and/or dirty links I sin . In the dirty limit we obtain the well-known expression for the Josephson tunnel current. However, the normal tunnel resistance has been replaced by the resistance of the link in the normal state. The theoretical results are compared with our experimental investigations on thin-film microbridges of the Dayem type. Deviations from a sinusoidal current—phase relation are discussed in connection with microwave-induced subharmonic steps sometimes observed in the current—voltage characteristics.     

Comparison between the resistively shunted Josephson junction model incorporating a voltage-dependent current-phase relation and experiments on superconducting thin-film microbridges

A voltage-dependent current-phase relation for short microbridges, % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% GaaeysaWGaae4CaOGaaiikaiaabAfacaGGSaGaaeiiaiabgwGiglaa% cMcacaqGGaGaeyypa0JaaeiiaiaabMeamiaaicdakiaacIcacaqGwb% GaaiykaiaabccaciGGZbGaaiyAaiaac6gacaqGGaGaeyybIySaaeii% aiabgUcaRiaabccacaqGjbadcaaIXaGccaGGOaGaaeOvaiaabccaca% GGPaGaaeiiaiGacogacaGGVbGaai4CaiaabccacqGHfiIXcaqGGaGa% ey4kaSIaaeiiaiaabMeamiaaikdakiaacIcacaqGwbGaaiykaaaa!6017!\[{\rm{Is}}({\rm{V}},{\rm{ }}\emptyset ){\rm{ }} = {\rm{ I}}0({\rm{V}}){\rm{ }}\sin {\rm{ }}\emptyset {\rm{ }} + {\rm{ I}}1({\rm{V }}){\rm{ }}\cos {\rm{ }}\emptyset {\rm{ }} + {\rm{ I}}2({\rm{V}})\] is calculated on the basis of time-dependent Ginzburg—Landau theory. Used in the current-controlled resistively shunted Josephson junction model such a current-phase relation enables us to explain a number of features in the I–V characteristics of thin-film microbridges. Experimentally we can estimate the upper frequency limit of the Josephson effect in thin-film microbridges.     

Josephson effects in Nb3Sn microbridges

We have studied Josephson effects in long narrow Nb3Sn microbridges at temperatures up to 17 K. These microbridges are formed by photolithographic techniques and subsequently subjected to controlled electrical discharges to modify the intrinsic Tcof the bridge region. The bridges exhibit 10 GHz micro wave steps in their I-V characteristics whose amplitudes are in excellent agreement with the Resistively Shunted Junction (RSJ) model. I-V characteristics (with and without microwaves) can be fit assuming an effective temperature approximately 15 K above the bath temperature. We have also investigated in detail structures in the I-V characteristics in the absence of microwaves. We show experimentally that phase-slip centers are induced at weak superconducting positions along the bridge when the S-N boundary of an expanding hot spot reaches within a thermal healing distance. The critical current of the phase-slip center thus formed exhibits a temperature dependence (1-T/Tc)^1/2instead of the usual mean field result (1-T/Tc)^3/2.     

Analog simulation of the nonideal behavior of superconducting Josephson microbridges

The nonideal behavior of superconducting Josephson microbridges due to current-induced pair breaking and self-heating is investigated in an analog simulation experiment. The investigation is based upon the resistively shunted-junction model first treated by Aslamasov and Larkin, which has been extended phenomenologically. The new features in the behavior of the model and their dependence upon the strength and relaxation time of the additional pair-breaking and heating effects are demonstrated by means of their effect on the current-phase relation and the current-voltage characteristic of the Josephson element.     

Superheating of YBa2Cu3O7−x thin-film microbridges

It is found that the hysteresis that develops with decreasing temperature in the current-voltage characteristics of thin-film YBa₂Cu₃O_7−x microbridges is due to superheating of the bridges by the transport current. The heat transport coefficient is determined to be α=5500–7500 V/(cm²·K) and the thermal resistance of the YBa₂Cu₃O_7−x /MgO film-substrate interface is R _if=(1.3–1.8)×10₋₄ (cm²·K)/W. Pis’ma Zh. Tekh. Fiz. 23, 56–62 (June 26, 1997)     

Study of Josephson effects in microbridges of Bi-Sr-Ca-Cu-O films made by spray pyrolysis

Superconducting Bi-Sr-Ca-Cu-O (2212) films were prepared by spraying stoichiometric aqueous solutions of nitrates of bismuth, strontium, calcium and copper on heated MgO (100) substrates and subsequent annealing in air. TheR-T curves of the films show metallic behaviour above the superconducting transition temperature.T _c (R=0) is observed between 80 and 85 K. Annealing temperature has a profound effect onT _c (R=0) and on the orientation of the film. Annealing in air in near-melting region yields highly oriented films withc-axis perpendicular to the substrate. These films show a sharp superconducting transition with zero resistance at 85 K. Microbridges of the dimensions of 50 μm × 50 μm have been patterned photolithographically followed with chemical etching. The 1 V characteristics of the microbridges show Josephson effects due to the presence of grain boundary weaklinks. The temperature dependence of the critical current for these microbridges suggest formation of superconductor-normal-superconductor type weaklinks.     

Ginzburg-Landau theory for three-dimensional Josephson junction arrays

We have derived a mean-field theory and a Ginzburg-Landau (GL) theory for classical three-dimensional Josephson junction arrays. We show that the full GL theory for arrays is formally equivalent to the GL theory for a bulk superconductor, demonstrating that for low fields and close to the critical temperature, the two systems are electrodynamically equivalent. We derive expressions for the penetration depth and coherence length, the effective superfluid density, critical fields, and the current-phase relationship for arrays. Using the Ginzburg criterion, we show that the critical region is very wide and that three-dimensional critical fluctuations should be experimentally accessible. Although the wide critical region casts doubt on the strict validity of the GL theory, suitable modifications of the coefficients should allow it to be used at lower temperatures, as is done for bulk superconductors.     

Coherent state of a chain of Josephson tunnel junctions

The regions of stability of coherent Josephson oscillation in a chain of Josephson tunnel junctions with nonlocal electrodynamic coupling were studied by means of a numerical simulation of the dynamics of this structure using the Wertheimer macroscopic theory of Josephson junctions. The possibility of using these systems to develop generators of narrow-band electromagnetic radiation at frequencies of 1 THz is discussed. Pis’ma Zh. Tekh. Fiz. 24, 1–7 (August 12, 1998)     

Phase transition of frustrated two-dimensional Josephson junction arrays

We have investigated the resistive behavior of frustrated Josephson tunnel junction arrays. The transitions nearf=1/2, atf=1 and at a nonspecial value off=0.38 are studied in detail with linear and nonlinear resistance measurements. The parameterf is the frustration index, the applied flux per unit cell of the array normalized to the flux quantum ₀=h/2e. The transition atf=1/2 looks similar to the zero-field Kosterlitz-Thouless transition, including a universal jump in the nonlinear resistance exponent. Compared tof=0, the transition is shifted to much lower temperatures. Nearf=1/2, below the transition temperature, single vortex crossings dominate the resistance. The transition atf=1 is qualitively the same as thef=0 Kosterlitz-Thouless transition, but small deviations are found. Forf=0.38, there is no experimental evidence for a special phase transition; over the whole temperature range, the resistance decreases exponentially with an energy barrier close to two in units of the Josephson coupling energy.     

Specific features of a model with Josephson frustrated arrays

Using the model for layered superstructures with the Josephson bond and additional boundary conditions (frustration), the authors consider the dependence of the critical temperatureT _c ^(p) on the number P of evaporated layers. The characteristics defined through precise solutions of the Josephson functions functional distinctions on the fluxon latticeT _c ^(p) are presented.     

Microstrip coupling techniques applied to thin-film Josephson junctions at microwave frequencies

Three different schemes for coupling to low impedance Josephson devices have been investigated. They all employ superconducting thin-film microstrip circuit techniques. The schemes are: (i) a quarterwave stepped impedance transformer, (ii) a microstrip resonator, (iii) an adjustable impedance transformer in inverted microstrip. Using single microbridges to probe the performance we found that the most primising scheme in terms of coupling efficiency and useful bandwidth was the adjustable inverted microstrip transformer.     

Fabrication of thin-film micropolarizer arrays for visible imaging polarimetry

We describe a microfabrication process for fabricating micropolarizer devices with polarization thin film. The polarization film is less than a 0.5 mum thick and can have a polarization extinction ratio of ~330 in the visible wavelength range. A single-state micropolarizer array with polarizing pixels as small as 5 mum x 5 mum has been fabricated. A multilayer spatially multiplexed three-state micropolarizer line array with a 14.4-mum resolution has also been fabricated for visible imaging polarimetry application.     

Application of Josephson series arrays to a DC quantum voltmeter

Josephson series arrays with a binary sequence of nonhysteretic superconductor-insulator-normal metal-insulator-superconductor (SINIS) junctions have been realized. The arrays of up to 8192 junctions (14 bits) are operated at microwave frequencies of about 70 GHz with low microwave power. An electronic system was built to drive an automated dc quantum voltmeter. Two electrically isolated quantum voltmeters (QVMs) in the same cryoprobe and driven by one frequency have been used for calibrating resistive dividers with the intention of demonstrating some advantages of these instruments     

Microwave wideband tunable oscillators using coherent arrays of Josephson junctions

Coherent microwave radiation from arrays containing up to 100 indium microbridges has been directly detected. As expected the power scales as N²and the linewidth as 1/N for an array containing N junctions. Design criteria and fabrication methods for these arrays are discussed.     

Reduction of the average access time with thin-film head arrays

The average access time, ta, of a moving head disc file can be drastically reduced by the application of multi-track Thin-Film Head Arrays. It is shown analytically that ta= ts+ 16/15(Sm/a)^1/2, where tsis the settling time, Sm, the maximum seek length and a, the constant acceleration and deceleration. When eight arrays each containing 25 thin-film heads are used to cover a 50 mm span of disc recording surface with a track density of about 40 track/mm, the average access time on the order of 2 to 6 m sec. can be achieved depending on the settling time, with present batch fabricated vertical inductive head design and constant acceleration and deceleration of 10,000 to 20,000 cm/sec².     

Microwave-induced steps in Josephson junctions, DC-SQUIDs and parallel arrays

We have studied the characteristics of Shapiro steps in DC-SQUIDs and parallel Josephson junction arrays by means of numerical circuit simulation within the RCSJ model. Particular interest was dedicated to the occurence of fractional (subharmonic) steps, looking for possible relations to recent observations of this effect in high-T_C junctions. Besides this, some new, pecliar effects in the dependence of step height vs. RF amplitude have been found, that correspond to characteristics measured on high-T_C junctions and SQUIDs. The results can be useful for characterizing experimental Josephson junctions and circuits by studying dependencies of Shapiro steps.