Bicrystal Junctions Research Articles

Characterization of grain boundaries in YBa/sub 2/Cu/sub 3/O/sub y/ bicrystal junctions [SQUIDs

I-V, V-/spl Phi/ characteristics and atomic force images of grain boundary Josephson junctions fabricated on SrTiO/sub 3/ bicrystal substrates were investigated. The half integer Shapiro steps and I/sub c/ versus B curves show evidence of the inhomogeneous current distribution along the grain boundary junction. The dc SQUID formed on the grain boundary shows the expected V-/spl Phi/ curve. The AFM images, R-T curves and I/sub c/R/sub n/ product reveal sequential destructive deterioration of the junction originating from the underlying grooved substrate. The results are discussed.

High-performance high-T/sub c/ SQUID sensors for multichannel systems in magnetically disturbed environment

We have fabricated and characterized high-T/sub c/ SQUID sensors that are suited for operation in multichannel systems in magnetically disturbed environment. Utilizing the superior properties of our 30/spl deg/ SrTiO/sub 3/ bicrystal junctions, we are able to fabricate high-performance single-layer direct-coupled SQUID magnetometers. System noise levels down to 24 fT Hz/sup -1/2/ and typical 1/f corners of 4 Hz/sup -1/2/ were obtained for devices having a 9 mm/spl times/9 mm pickup loop and a nominal SQUID inductance of 100 pH. To protect the SQUIDs from moisture, they are hermetically encapsulated in a ceramic housing. A thick film resistor integrated on the chip carrier allows one to heat the SQUID device above T/sub c/ in order to release trapped magnetic flux. For the operation of our low-noise SQUIDs, a compact direct-coupled read out electronics with a preamplifier voltage noise of 0.4 nV Hz/sup -1/2/ and 100 kHz bias reversal was developed. Sensor dynamics and linearity were investigated with regard to the operation in magnetically disturbed environment. The maximum system bandwidth and slew rate of our bias reversal electronics were 1 MHz and about 5/spl times/10/sup 5/ /spl Phi//sub 0//s, respectively. For signal frequencies /spl les/50 Hz/sup -1/2/, a minimum total harmonic distortion of -118 dB was measured, dominated by nonlinearities in the read-out electronics. A new magnetometer design with improved noise performance in magnetically disturbed environment is presented.

Vortices in HTS junctions and SQUIDs

Three different problems of a vortex penetrating a superconducting thin film have been addressed. It has been shown that the flux coupled from a vortex into a pickup loop or SQUID washer can be obtained from knowledge of the current distributions which flow in the absence of a vortex, a much easier calculation. Results are given for the case of the SQUID washer. The current distribution for a vortex near a circular hole in a film has been found in the strong shielding limit, and the force on the vortex deduced. The current distribution of a Josephson vortex penetrating a planar junction, such as a bicrystal junction, has been calculated. The form of the Josephson vortex is different from that usually assumed, with a long range tail. The size of the Josephson vortex is typically a factor of two larger than that obtained by taking the conventional expression in terms of the Josephson penetration depth.

Measurement of the error rate of single flux quantum circuits with high temperature superconductors

The determination of bit error rates in single flux quantum logic circuits operating at temperatures well above the temperature of liquid helium is essential since the question of stability against thermal noise arises. We determined experimentally the static and for the first time the dynamic error rate at temperatures of about 40 K with the help of simple test circuits. The static error rate has been investigated using two different circuits with bicrystal junctions in either single-layer or multilayer configuration. In both cases the internal state of a storage loop was observed by a dc-SQUID. A ring oscillator based on a Josephson transmission line allowed us to measure the dynamic error rate of a Josephson comparator at high frequencies. This circuit has been fabricated using junctions by focused-electron-beam irradiation. It is specially suited for the detection of seldom occurring switching errors.

Very large YBa/sub 2/Cu/sub 3/O/sub 7/-Josephson-junction-arrays

Arrays with YBa/sub 2/Cu/sub 3/O/sub 7/-Josephson junction ramp-edge geometry and on bicrystal substrates were investigated. The ramp-edge Josephson junctions were prepared in arrays of three, five and seven junctions in series and in a mixed array with a very large number of up to two million on one chip. First measurements are presented. On bicrystal substrates Josephson junctions were prepared up to 105 Josephson junctions in series. They show scaled current-voltage characteristics and Shapiro-steps under influence of microwave radiation.

Bicrystal submicrometer Josephson junctions and dc SQUIDs

Submicrometer Josephson junctions and dc SQUIDs with such junctions have been prepared on symmetrical 24/spl deg/ bicrystal substrates and were electrically characterized. The submicrometer structures are patterned using e-beam lithography and a C/Ti/e-beam resist mask system. Junctions with dimensions down to about 350 nm show no degradation of their superconducting properties at 77 K, when patterned at low temperatures. Series connections of directly coupled SQUIDs with large (110-160 pH) coupling-inductance, prepared with 0.5 /spl mu/m wide junctions show flux-to-voltage transfer function values up to 100 /spl mu/V//spl Phi//sub 0/. For these SQUIDs, typical white noise levels of 10 /spl mu//spl Phi//sub 0///spl radic/Hz are measured at 77 K.

Fabrication of QFP devices from Au/SrTiO/sub 3//YBa/sub 2/Cu/sub 3/O/sub 7/ structures

Quantum Flux Parametron (QFP) devices were fabricated from Au/SrTiO/sub 3//YBa/sub 2/Cu/sub 3/O/sub 7/ structures based on bicrystal grain boundary junctions. The junction connected to a load inductor in series was utilized as a sampling gate which reads out the load current. External fluxes were applied to two closed loops separately in the QFP by using two independent activation lines in order to reduce the load inductance. A multiplexer consisting of a couple of the QFPs and the sampling junction was fabricated and was tested at 6.5 K.

Nd/sub 1.85/Ce/sub 0.15/CuO/sub 4-y/ bicrystal grain boundary Josephson junctions

The study of the detailed influence of the order parameter (OP) symmetry on the properties of high temperature superconducting (HTS) Josephson junctions still is a key issue. Whereas the hole doped HTS such as YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO), Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/ (BSCCO), or La/sub 1.85/Sr/sub 0.15/CuO/sub 4-/spl delta// (LSCO) are known to have a dominating d-wave component of the OP, there is significant evidence that the electron doped material Nd/sub 1.85/Ce/sub 0.15/CuO/sub 4-y/ (NCCO) has an s-wave symmetry of the OP. Therefore, we have studied the electrical transport properties of [001] tilt NCCO bicrystal grain boundary Josephson junctions (GBJs) with misorientation angles between 7/spl deg/ and 36.8/spl deg/ and compared them to those of the hole doped HTS. For the NCCO-GBJs an exponential decay of the critical current density J/sub c/ with increasing misorientation angle as well as a scaling of the characteristic junction voltage V/sub c//spl prop/J/sub c//sup p/ was found very similar to what is observed for the hole doped HTS. This strongly suggests that the OP symmetry is not the key parameter controlling the characteristic properties of HTS-GBJs. In contrast, they are most likely related to the presence of a disorder induced, insulating grain boundary barrier which is similar for both the d- and s-wave HTS.

Josephson junctions with hysteretic current voltage characteristics at high temperatures

The properties of bicrystal Josephson junctions with external capacitors are reported. It was found that the hysteresis in the current voltage (IV) characteristic was very sensitive to the wavelength of the Josephson oscillation in the dielectric and thermal noise suppression of the critical current. A McCumber parameter /spl beta//sub c/ of 1.2 at 65 K has been achieved. In addition intrinsic grain boundary capacitance has been found to give a large /spl beta//sub c/ value of 9.6 at 4.2 K.

A discrete HTS Josephson flux-flow structure with gain at 77 K

We have developed flux-flow devices based on parallel arrays of HTS bi-crystal grain-boundary Josephson junctions (GBJs) with current gains well above unity (1.6-1.8) at 77 K. The symmetric control characteristics are similar to those of the LTS SuperCIT with the critical current maximum at zero control current and decreasing linearly with increasing control currents. Our design featured a control line that is defined by a parallel array of long slots in the base electrode. The slots confined the control current to within 2 /spl mu/m of the grain-boundary junctions resulting in tight coupling of the control fields to the junction array. Furthermore, the slots allowed the bias current to remain uniform as it crossed the control line into the base electrode. This combination was necessary to produce SuperCIT-like control characteristics as well as useful current gains in single-layer device structures. Device transresistance and usable output voltage range (typically 0.2-0.3 /spl Omega/ and 0.1 mV, respectively, for YBCO devices at 77 K) were limited by the low intrinsic shunt resistance and I/sub c/R/sub n/ product of the bi-crystal GBJs. Better device characteristics can be expected with improved junction qualities.

Interactions between bicrystal Josephson junctions in a multilayer structure

We have fabricate and studied a variety of devices based on stacked YBa/sub 2/Cu/sub 3/O/sub x/ bicrystal Josephson junctions in a multilayer structure. The proximity of the junctions in the two layers produces a large number of effects based on interactions between the junctions. Voltage locking and current locking were observed in the stacked junctions. The voltage locking is due to the ac path provided by a SrTiO/sub 3/ layer between the stacked junctions. The current locking is likely the result of a Josephson vortex interaction.

New microwave circuits for programmable voltage standards using high-temperature Josephson junction arrays

New microwave circuits for voltage standard application containing 256 shunted bicrystal junctions were designed and investigated. The series array follows a meander line, which prevents the uniform distribution of microwave current along the array. To overcome this drawback, we are placing the meander parallel to a coplanar waveguide line. This provides parallel feeding of microwave power to series connected junctions. A circuit divided in 8 sub-arrays represents a seven-bit digital-to-analog converter. At a frequency f=27332 MHz and for a constant power each of the segments of the array demonstrated the first current step at 78 K.

High temperature superconductor transimpedance amplifiers using serially connected bi-crystal junction SQUID arrays

This article discusses the design and measurement of high temperature superconductor transimpedance amplifiers using bi-crystal junction SQUID arrays. The peak to peak modulation voltages of the arrays as a function of their SQUID inductance were measured. The measured transresistance, dV/sub o//dI/sub i/, of a 30-element array of 5-pH SQUIDs was 4 V/A. The voltage modulation depth of the 30-element array was 2 mV and the white noise of the array was 1.0 nV/Hz/sup 1/2/ at 100 kHz. The dynamic range of the array was 120 dB in a one hertz bandwidth. This device has the potential to be used as a broadband amplifier for low impedance sources.

High T/sub c/ dc SQUID utilizing bicrystal junctions with 30 degree misorientation angle

Performances of high T/sub c/ dc SQUID utilizing bicrystal junctions with 30/spl deg/ misorientation angle have been studied. Junction resistance R/sub s/=10 /spl Omega/ and critical current I/sub 0/=10-20 /spl mu/A can be obtained rather easily at T=77 K with this technology. As a result, voltage modulation depth as high as 50 /spl mu/V, and flux noise as low as 5 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ obtained for the 100 pH-SQUID. The measured values of the voltage modulation depth agree reasonably well with the numerical simulation, while the measured values of the flux noise are about 2 or 3 times larger than the simulation. The reason for the high noise level is discussed by taking into account imperfection of junctions.

Digital and analog measurements of HTS SFQ RS flip-flops and shift register circuits

We fabricated reset-set (RS) flip-flops and shift registers by using YBCO bicrystal junctions and tested their operations by using a computer-controlled digital measurement set-up and an analog measurement set-up. The RS flip-flop circuits operated successfully at temperatures up to 71 K. The RS flip-flop circuit was observed over 600 computer-generated clock cycles with nearly no errors. The circuits were reset or set during each clock cycle. By using an analog measurement technique, we operated the RS flip-flop circuits at frequencies of up to 500 Hz. Since our probe was not designed for high speed operation, the test speed was limited to this frequency. Although we could not operate the shift register completely, we observed flux propagation in the shift register.

Design and Optimization of HTC DC Squids for Magnetometric Applications

In order to optimize the magnetic field sensitivity [Formula: see text] we have followed two different approaches. In the former we have analyzed the influence of the quality of YBCO films on the superconducting properties and noise characteristics of bicrystal grain boundary junctions and dc-SQUIDs. For this purpose we have studied and compared three different YBCO deposition processes. In the latter we have reported the different criteria for the design of SQUID magnetometers usable for applicalions in shielded and unshielded environment.

On the nature of the electric-field effect on YBa2Cu3O7−δ grain boundary junctions employing epitaxial SrTiO3 gate insulators

We have fabricated Josephson field-effect transistors based on YBa2Cu3O7−δ bicrystal grain-boundary junctions (GBJs) and epitaxial SrTiO3 films as gate insulators. The SrTiO3 gate insulator shows high products of the breakdown field Ebd and the dielectric constant εr up to Ebdεr=1.3×1010 V/m allowing measurements over a wide range of applied gate electric-field Eg. The critical current Ic of the GBJs is found to depend highly nonlinear on Eg. Remarkably, the measured Ic(Eg) are very similar to the εr(Eg) curves. This strongly suggests that the observed electric-field effect is not due to a field-induced change in carrier concentration but is related to the dielectric properties of the SrTiO3 gate insulator.

YBCO thin film rf-SQUID using bicrystal junction

A YBCO thin film rf-SQUID has been fabricated using bicrystal junction. The YBCO film is deposited on SrTiO 3 36.8° bicrystal substrate. The rf-SQUID structure has been fabricated by laser patterning technique. The geometry consists of one hole of dimension 50×10 μm 2 and two asymmetric bicrystal junctions of dimensions 5 and 20 μm. The rf-SQUID operates in hysteretic mode and its peak to peak amplitude of V– Φ modulations is 60 μV at 77 K. The flux noise density of the bicrystal junction rf-SQUID is 2×10 −4 Φ 0/ Hz at 2 Hz and 77 K.

YBCO bicrystal junctions on sapphire: d-wave impact and possible applications

The current transport and Josephson effect have been investigated in YBCO junctions, fabricated on the sapphire bicrystal substrates. The YBCO film with thickness t≅150 nm was deposited by dc sputtering on the epitaxial CeO 2 buffer layer made by rf magnetron sputtering. The junctions were characterized at dc and at mm waves. 5 μm wide junctions have moderately high normal state resistance R N = 5÷30 Ω with critical currents I c =50÷200 μA which give I c R N product of order of 0.5÷2 mV. The tolerance of characteristic interface resistance (R N S) was around 30% for the junctions on a chip. Experimental data, discussed in terms of d-wave symmetry, demonstrate possibility of design of small scale integrated Josephson microwave circuits.

Observation of bound surface states in grain-boundary junctions of high-temperature superconductors

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.