Bicrystal Junctions Research Articles

Bicrystal pi-interferometers

Experimental results for DC interferometers of asymmetric topology with high- T c bicrystal Josephson junctions are reported. Model of the tilted bicrystal junctions is discussed. As a model, parallel array of alternating `0' and ` π' contacts was studied numerically. It has been shown that the array can act as either 0- or π-junction. Both the array characteristics and the critical current dependence on applied magnetic field for the array and DC interferometer are discussed.

HTS dc SQUID based rf amplifier: development concept

We present a concept of a rf amplifier based on a directly coupled dc SQUID with bicrystal junctions, which have high saturation power and can be used with SIS mixers or possibly for satellite and cellular phone communications. A novel input resonant circuit is proposed using single layer of HTS. Estimated parameters are (per stage): central frequency≈11 GHz, bandwidth≈400 MHz, noise temperature≈10 K, gain≈10 dB and input saturation≈1000 K GHz.

Experimental study of noise and Josephson oscillation linewidths in bicrystal YBCO junctions

The intensities of the noise in a bicrystal high-TC (HTS) Josephson junction have been precision-measured at 1–2 GHz frequency band at bias voltages up to 50 mV at T = 4.2 K. At large bias voltages, V > 30 mV, the dependence of current noise density was found exactly coinciding with the Schottky shot noise asymptote 2eI. At relatively low voltages, V < 4 mV, a noticeable noise rise has been registered. The broadening of Josephson oscillation linewidths ΔfJ over the values ΔfRSJ predicted by the RSJ model has been experimentally studied at different frequencies in the mm and submm wave range up to voltages V = 2 mV in connection with low-voltage noise rise. Both the features observed, the linewidth broadening and the excess noise over the noise level of thermal fluctuations, are discussed in terms of multiple Andreev reflection, giving rise to a nonequilibrium shot noise—the case which may take place in the d-wave superconducting junctions. Experimental results on noise performance are also compared with the qualitatively similar dependences of the current noise, known for the s-superconducting ballistic point-like or diffusive-type SNS junctions, where the excess low-voltage noise is manifested due to multiple Andreev reflections. Increasing the operating temperature, the thermal (equilibrium) fluctuations were found to predominate, resulting in a decrease of ratio ΔfJ/ΔfRSJ. The characteristics of the ac Josephson effect in HTS junctions measured at submm wave frequencies at temperatures close to the transition temperature TC are also discussed.

Direct flux noise measurement of YBCO grain boundary junctions

The flux generated by vortices in a field-cooled ( 10 μT ) wide bicrystal grain boundary junction (GBJ) in a YBa 2Cu 3 O 7− δ thin film was measured at 77 K, using a superconducting thin-film coil and Nb-based superconducting quantum interference device. The sample film was filled with slots, forming a mesh structure. When the field applied to the film was changed above a threshold value, either in positive or negative direction, random switching noise appeared. The switching heights implied long-distance movement of vortices over 20– 500 μm within the GBJ, driven by the shielding current. Almost symmetric threshold fields showed the effectiveness of the mesh structure in reducing demagnetization effect in thin-film geometry. The same slot structure made along the GBJ was also effective to suppress long-distance vortex motion in the GBJ to lengths of less than 1 μm .

Properties of (Hg,Re)-1212 bicrystal junctions with different misorientation angles

The relationship between the properties of bicrystal junctions with the Hg-based superconductor and the misorientation angle has been investigated. Smooth and homogeneous (Hg,Re)Ba 2CaCu 2O y ((Hg,Re)-1212) thin films exhibiting T C( zero)=114–116 K were fabricated on SrTiO 3 bicrystal substrates with various misorientation angles θ of 5–45°. In the low angle region up to 10°, the junctions exhibited flux-flow (FF) type current–voltage ( I–V) characteristics. For the θ⩾20°, they exhibited resistively shunted junction (RSJ) like I–V characteristics. The θ=15° junctions had both FF and RSJ like characteristics. The critical current density ( J C) value decreased monotonically with an increase in θ in the range form 0° to 24°. On the other hand, the J C of the junctions with θ⩾24° remained unchanged. A large I C R n product of 706 μV was recorded at 77 K in the θ=20° junctions.

Properties of a Flux Dam Inserted in the Pickup Coil of a High-Tc Superconducting Quantum Interference Device Magnetometer

The properties of a flux dam inserted in a pickup coil of a high-Tc superconducting quantum interference device (SQUID) are studied. First, the effect of thermal noise at T=77 K on the magnetic flux entry into in the pickup coil through the flux dam is discussed. It is shown that the thermal noise causes the flux entry even when the circulating current in the pickup coil is less than the critical current of the flux dam. The behavior of this thermally activated flux entry is analyzed using the so-called potential-barrier model. It is shown that the flux entry into the coil proceeds logarithmically with time, indicating the necessity of a long waiting time in the use of the flux dam. It is also shown that the critical current of the flux dam must be large enough to obtain a quasistable state, i.e., a state where no thermally activated flux entry occurs during the measurement time. The critical current should be larger than 50 µA in the magnetic field under operation. Next, in order to satisfy this requirement, the magnetic field dependence of the critical current of the flux dam is studied when the flux dam is made of a wide bicrystal junction. It is shown that the magnetic field dependence of the wide junction can be explained reasonably well if we take into account the so-called overlap-type-junction model and the flux-focusing effect of the bicrystal junction. The obtained results will be useful in the design of the flux dam.

Symmetrical high-T c superconducting bicrystal Josephson junctions: Dependence of the electrical properties on the misorientation angle

The electrical properties of junctions at symmetrical bicrystal boundaries in high-T c superconducting films are studied as functions of the misorientation angle in the range 8°–45°. The junctions are prepared by growing YBa2Cu3O7 (YBCO) epitaxial films on Y-ZrO2 (YSZ) bicrystal substrates. The proportional relationship between the characteristic voltages and the normal conductivities of junctions is derived from the dependences of the critical current and the normal resistance on the misorientation angle. The results are interpreted within the model of a superconductor-dielectric with defect levels in the band gap of the superconductor. The deviations from the proportional relationship are explained by the junction inhomogeneity. The thickness of the effective dielectric layer in the bicrystal junction and the Bohr radius of electrons on the defects are estimated.

Terahertz spectral analysis by ac Josephson effect in high-T/sub c/ bicrystal junctions

A prototype of a terahertz Hilbert-transform spectrum analyzer based on a high-T/sub c/ Josephson junction integrated into a Stirling cooler has been developed. The detector response of YBa/sub 2/Cu/sub 3/O/sub 7-x/ Josephson grain-boundary junctions to monochromatic radiation with the frequency f in the range from 60 GHz to 5 THz has been studied. Odd-symmetric resonances near the voltages V=hf/2e in the responses /spl Delta/I(V) of these junctions to radiation with different frequencies f have been observed in a decade of spectral range for any operating temperature between 30 to 85 K. Decreasing the junction temperature from 85 to 30 K by a Stirling cooler, the spectral analysis could be made in two decades of spectral range. A resolving power /spl delta/f/f/spl sim/10/sup -3/ has been shown in the terahertz spectral analysis with the low-resistive Josephson junctions. As an example of application of this analyzer, an optimization of the single-line operation of a far-infrared optically-pumped CH/sub 3/OH laser has been demonstrated.

Step-edge Josephson junctions and their use in HTS single-layer gradiometers

We describe reproducible technology for fabricating YBa/sub 2/Cu/sub 3/O/sub 7/ step edge Josephson junctions on SrTiO/sub 3/ substrates. We report on the electrical characteristics of the junctions at 77 K and the dependence on the ratio of film thickness to step height. Single-layer gradiometers incorporating step-edge junction dc SQUIDs have been fabricated. We compare the performance of identical SQUID gradiometers based on our step-edge junctions and on 24/spl deg/ SrTiO/sub 3/ bicrystal junctions. Gradiometric SQUIDs (G-SQUIDs) incorporating step-edges have also been fabricated. We measured the effective area of several G-SQUIDs to be in the range of 1-2 /spl mu/m/sup 2/, approximately two orders of magnitude lower than for conventional SQUIDs of similar inductance. We demonstrate that when incorporated into a gradiometer the G-SQUID leads to an improved performance in an unshielded environment.

Spread of critical currents in thin-film YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions

A spread of the critical currents in a series array of up to 100 YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions has been studied by Laser Scanning Microscopy. The values of the critical current I/sub c/ of individual junctions in the array have been obtained by focusing a laser beam on each junction and measuring the current at which the maximum laser-induced voltage response /spl Lambda/V on the array has appeared. The distribution of critical currents in logarithmic scale was close to a Gaussian one. The I/sub c/-spread has been found to increase with the increase of misorientation angle of bicrystal substrate and the decrease of the width of the junctions in the array.

Arrays of Josephson junctions coupled by distributed circuits

Both the linewidth reduction and the gain in output power have been analyzed for Josephson-junction arrays with distributed coupling circuits. Different ways of Josephson junction connection to the distributed circuits, as well as both the nonlinear interaction effect and the crucial role of high frequency losses have been studied. It has been found that the contradictory requirements to the junctions as nonlinear active elements on (i) low wave reflection and (ii) high total power produced can be fulfilled by an increase in number of Josephson junctions with a proper impedance value. The first condition is necessary to provide the most reduction in the linewidth, and the second one is needed to compensate for the power output. An array of biased in parallel bicrystal Josephson HTSC junctions coupled by a microstrip circuit has been fabricated and preliminary tested.

Transport properties of bicrystal Josephson junctions in Y-Ba-Cu-O films grown by liquid phase epitaxy

We studied charge transport across 24/spl deg/, 36/spl deg/, 45/spl deg/ symmetrical and 45/spl deg/ asymmetrical grain boundaries (GB's) in YBCO bicrystals grown by liquid phase epitaxy (LPE). We show that transport properties of symmetrical GB's are similar to those in bicrystals formed by various physical vapour deposition (PVD) methods while for 45/spl deg/ asymmetrical GB's, a difference between transport properties of LPE and PVD bicrystals has been found. In particular, in contrast to PVD bicrystals, selected LPE samples demonstrate regular magnetic interference patterns. In an agreement with transmission electron microscopy data, this result suggests better quality of 45/spl deg/ asymmetrical GB's in LPE films.

YBa/sub 2/Cu/sub 3/O/sub 7-δ/ bicrystal grain boundary T/sub c/ microstructure

We use an S/sub /spl alpha//S/sub /spl beta//IS/spl beta//sub S/spl alpha// tunneling/proximity-effect model with measured YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) grain boundary function (GBJ) normalized critical current temperature dependence /spl eta/(T)=I/sub c/(T)I/sub c/(O) to infer microstructural transition temperature profiles T/sub c/(x) for bicrystal GBJs with various misorientation angles /spl theta/. We discuss the rationale for selecting an optimum value for a numerical analysis parameter /spl epsi/ (associated with the S/sub /spl alpha//S/sub /spl beta// interface T/sub c/ discontinuity). We determine the extent to which inferred T/sub c/(x) profiles vary with /spl plusmn/20% rms deviations from nominal values for the fixed input parameters whose a priori values are not precisely known. This sensitivity analysis shows that specified T/sub c/ locations can be determined with /spl les//spl plusmn/1 nm precision for T/sub c/<60 K. Finally, we present new inferred T/sub c/(x) profiles for bicrystal junctions with /spl theta/=24/spl deg/ and 32/spl deg/ and compare these profiles with those of bicrystal JJs with /spl theta/=19/spl deg/ and 36.8/spl deg/ which we previously reported. Our inferred T/sub c/(x) profiles for these functions exhibit a clear correlation with /spl theta/: as /spl theta/ increases so does the extent of the grain boundary region with suppressed T/sub c/. This expected result gives us confidence that our approach should prove useful as a tool for elucidating the structure and properties of high temperature superconductors.

Transport and noise characteristics of submicron high-temperature superconductor grain-boundary junctions

We have investigated the transport and noise properties of submicron YBa2Cu3O7−δ bicrystal grain-boundary junctions prepared using electron beam lithography. The junctions show an increased conductance for low voltages reminiscent of Josephson junctions having a barrier with high transmissivity. The voltage noise spectra are dominated by a few Lorentzian components. At low temperatures clear two-level random telegraph switching signals are observable in the voltage versus time traces. We have investigated the temperature and voltage dependence of individual fluctuators both from statistical analysis of voltage versus time traces and from fits to noise spectra. A transition from tunneling to thermally activated behavior of individual fluctuators was clearly observed. The experimental results support the model of charge carrier traps in the barrier region.

Fabrication of low noise high-T c bicrystal junction rf SQUID

We fabricate rf superconducting quantum interference devices (SQUIDs) using a YBCO bicrystal junction, which is made on a chip consisting of a 10×12mm 2 YSZ bicrystal substrate with a 24° angle of in-plane misorientation. The magnetic field sensitivity is 84 fT/Hz 1 2 . The tank frequency at which the rf SQUID operated is near 800MHz. According to our result, the boundary across the superconducting loop does not show significant effects on the 1/f noise level.

Dynamics of order parameter and microwave emission for a YBa 2Cu 3O 7− δ bicrystal junction

It is shown that the experimental result and the numerical result by using time dependent Ginzburg–Landau (TDGL) equations for the current–voltage characteristics of a YBa 2Cu 3O 7− δ bicrystal junction with a finite excess current agree well. The YBa 2Cu 3O 7− δ bicrystal Josephson junction was fabricated on a MgO(1 0 0) 24° bicrystal substrate and it was integrated with a bow-tie antenna structure for broadband high-frequency coupling. The critical current of the bicrystal junction was 505 μA and there was a finite excess current I ex on the current–voltage characteristics. The microwave emission power per bandwidth of Δf=0.9 GHz at 141 GHz from the same junction was also measured and the total emission power was of the order of 10 −9 W. A schematic model of the order parameter in a weak link, which is dominated by the normalized one-dimensional TDGL equations, is presented. Comparing the experimental result with the numerical result for current–voltage characteristics, the values of ξλ 2=1.88×10 −19 m 3 and σλ 2=7.32×10 −7 Ω −1 m are obtained. By using these values, the microwave emission power of the order of 10 −9 W is estimated. This value is consistent with the experimental result.

Theory of c-axis twist Bi2212 Josephson junctions: Strong evidence for incoherent tunneling and s-wave superconductivity

We studied the Bi 2Sr 2CaCu 2O 8+δ bicrystal c-axis twist junctions of Li et al. In one model, two coherent tight-binding layered superconductors are coupled across the twist via coherent c-axis tunneling. In the other model, the c-axis tunneling across the twist is a Gaussian in the momentum change across the junction. We conclude that the c-axis tunneling is strongly incoherent, and contains an isotropic s-wave component for T ≤ T c .

Bicrystal grain boundary Josephson junctions with epitaxial Tl 2Ba 2CaCu 2O x thin films

Epitaxial Tl 2Ba 2CaCu 2O x (Tl-2212) thin film grain boundary Josephson junctions have been fabricated on 24° SrTiO 3(STO) bicrystal substates by magnetron sputtering and post-annealing process. The junction characteristics have RSJ-like behavior with V c = I c R n of 95 μV at 95 K. The critical current of the junctions near T c (105K) has a temperature dependence similar to that of SNS type ones. The junctions have been used to form a dc-SQUID, which can operate at temperatures up to 99 K.

HTc dc SQUID Magnetometers

High sensitivity HTc dc-SQUID magnetometers, of bicrystal junction type, have been designed, fabricated and characterized. The magnetometers are characterized by a direct-coupling planar design and their parameters and geometrical dimensions have been optimized in order to achieve the lowest possible noise level. The IV characteristics are RSJ-like and show no excess current. The measured SQUID normalized inductances and magnetic field modulation periods are in good agreement with the expected values. The magnetic field noise spectral densities, measured with a flux locked loop electronic setup, exhibit typical white noise levels of about 150 fT/Hz 1/2 at frequencies above 2 KHz. At lower frequencies a 1/f noise is observed. By employing a bias-reversal electronic readout scheme, the 1/f corner frequency can be reduced to values less than 100 Hz.

Electron transport in high- Tc superconducting grain boundary junctions

The current state and the prospects of the application of high- T c superconducting grain boundary Josephson junctions in microwave electronics devices are given. It is approached by sketching the typical fabrication technique of the junction. Josephson bicrystal junctions on sapphire substrate are considered in detail. The results of dc microwave and magnetic measurements of YBCO bicrystal junctions on r-cut sapphire are presented. The junctions with high resistance 10–20 Ω and I c R N=1–2 mV and tolerance of R N S around 30% on the chip allow to create microwave circuits with low integration (up to 10 junctions on chip). The microwave dynamics of the junction with superconducting current-phase relation fits with sin ϕ relation better than 5%, that clearly indicates the tunnel conductivity between two YBCO electrodes. It was found that critical current density depends on the square root of interface transparency in accordance with the prediction of superconducting current transport via Andreev's bound surface states. The specific properties of current transport in high- T c grain boundary junctions with taking into account d-wave type of gap order in high- T c superconductor are discussed.