Bicrystal Grain Boundary Junctions Research Articles

The Effect of Magnetic Flux Focusing on the Current–Voltage Characteristics of YBa2Cu3O7- δ Grain Boundary Josephson Junctions

In order to obtain experimental evidence for the influence of flux focusing effect on the characteristics of planar grain boundary (GB) Josephson junctions, we measured the current–voltage characteristics and magnetic-field-dependent current–voltage steps of integrated YBa2Cu3O7- δ bicrystal GB Junctions with different junction widths. Under the self-magnetic field, the critical current of the GB junction is proportional to the square root of the junction width, which is different from the traditional linear relationship between the critical current and the junction width. When a vertical magnetic field is applied, the flux flow current–voltage step could be observed, and the step voltage increases monotonously with an increase in the junction width. When a magnetic field rotating in the bc plane is applied, the voltage value of the flux flow step decreases slowly as the magnetic field inclined angle increases when the field inclined angle is relatively large. Based on these results, we demonstrate the presence of the magnetic flux focusing effect on YBa2Cu3O7- δ GB Josephson junctions. The size of flux focusing in the GB is closely associated with the width of the junctions and the orientation of the magnetic field.

Effects of electron-beam irradiation on the transition properties of YBa2Cu3O7−d grain boundary junctions

We have studied the effects of high-energy electron-beam irradiation on the transition properties of superconducting YBa2Cu3O7−d (YBCO) grain boundary junctions, bicrystal junctions and step-edge junctions, on SrTiO3 substrates. A uniform 1-MeV electron beam irradiated all over the samples. The irradiation doses were 0, 4.7 × 1014, 4.7 × 1015, and 4.7 × 1016 e/cm2. For each junction type, we used at least two samples for each dose level and compared the transition parameters before and after irradiation. For comparison, we also studied the same irradiation effects for YBCO microbridges. We measured the resistive transition temperature, the current-voltage characteristics, the normal-state resistance, and the critical current. The effect of irradiation was the most significant for the bicrystal grain-boundary junction and the least significant for the microbridges. The critical current data for the YBCO bicrystal grain-boundary junction showed a maximum at (0.47 ∼ 0.9) × 1015 e/cm2, and those for the microbridges showed a monotonic decrease with increasing dose. The normal-state resistance increased monotonically with increasing dose for all samples by up to ∼40% for the microbridges and ∼20% for the grain-boundary junctions at 4.7 × 1015 e/cm2. The change in the superconducting temperature (Tc) was negligible except for the bicrystal junction at 4.7 × 1016 e/cm2, which was not superconducting at 77 K. These results show that grain-boundary junctions are more susceptive to irradiation, indicating that their critical currents are controllable by using high-energy electron-beam irradiation.

The effect of bias current asymmetry on the flux-flow steps in the grain boundary YBaCuO long Josephson junctions

The current-voltage characteristics and critical current versus magnetic field dependence of long 24°[001]-tilt YBa2Cu3O7−δ bicrystal grain-boundary junctions are studied both experimentally and theoretically. For the opposite magnetic field directions, the flux-flow steps with significantly different height and slope are observed. It is demonstrated that the most probable reason of this discrepancy is recently predicted asymmetry of spatial bias current distribution due to crystallographic anisotropy of bicrystal substrates [Kupriyanov et al., JETP Lett. 95, 289 (2012)].

Effective-barrier-thickness fluctuation and critical-current spread of grain-boundary Josephson junctions

The phenomenological formulas for the critical-current spread of the bicrystal grain-boundary Josephson junctions are proposed. By considering the intrinsically shunted-junction model and the gamma distribution, the fluctuation of effective-barrier thickness from 0.01to0.1nm corresponds to the 1−σ critical-current spread from 14% to 106%. With a fixed scale parameter, the critical-current spread is inversely proportional to the square root of the bridge width. When the bridge width is fixed, the spread is inversely proportional to the square root of the mean critical-current density. These predictions are qualitatively in agreement with the experimental results reported to date.

Influence of bicrystal microstructural defects on high-transition-temperature direct-current superconducting quantum interference device

Using atomic force microscopy and scanning electron microscopy (SEM), we investigate the correlations between the microstructural defects and the electrical characteristics of the bicrystal grain-boundary Josephson junctions and dc superconducting quantum inference devices (SQUIDs). The structural defects are shown to correlate qualitatively with the characteristics of grain-boundary Josephson junctions patterned on the YBa2Cu3O7−x film. SEM images show that these defects grown on the grain boundary were a few submicron depth of the groove. The low flux noise characteristics were observed when the groove depth was smaller than 18nm in the junctions of the SQUID. The existence of these defects is expected to affect the supercurrent and the motion of the magnetic flux in the films, which dominate the excess noise in the SQUID with bicrystal junctions.

Evidence of midgap-state-mediated transport in 45° symmetric [001] tiltYBa2Cu3O7−xbicrystal grain-boundary junctions

We have investigated the effect of midgap bound states on the Josephson properties of symmetric 45\ifmmode^\circ\else\textdegree\fi{} [001] tilt bicrystal grain-boundary junctions (GBJ's) characterized by different junction widths. GBJ's, $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wide, show a monotonic temperature dependence of the Josephson current with a slight upwards curvature, the result of the competition between midgap states and the opposite-sign continuous current. In some junctions, an increasing contribution of the midgap-state-mediated current is observed at low temperatures, leading to an anomalous nonmonotonic temperature dependence of the critical current. This behavior is more evident for junctions smaller than $500\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, where, in some cases, a 0- to $\ensuremath{\pi}$-junction transition is clearly observed. Experimental results agree qualitatively with theoretical models taking into account midgap states in strongly nonuniform barrier interfaces.

Magnetoresistance in bicrystal grain-boundary junctions analyzed within the Stoner–Wohlfarth model

Experimental data of bicrystal grain-boundary magnetoresistance hysteresis shows that the resistance at remanence after saturation depends on whether the magnetic field was applied parallel or perpendicular to the grain boundary. In the latter case the behavior can be described by the Stoner–Wohlfarth magnetization reversal model for coherent rotation of the magnetization, which leads to an alignment along the easy magnetization directions of the bicrystal in zero field. In the parallel configuration the magnetization directions of the two grains remain parallel in zero field. We have used this information to deduce an effective spin polarization of P≈40% of the grain boundary in a bicrystal of La0.7Sr0.3MnO3.

Advances in high-Tc grain-boundary junctions

We review results on two novel types of grain-boundary Josephson junctions in the high-critical-temperature cuprate superconductors. The quality of YBa2Cu3O7−δ biepitaxial grain-boundary Josephson junctions has been improved using a new growth geometry to the point that “tunnel-like” characteristics are reproducibly obtained. The consequent low barrier transmission probabilities are apparently favorable for isolating intrinsic d-wave induced effects. We also focus on peculiar aspects of spontaneous currents in HTS Josephson junctions, which are unique and reflect the intimate relation between vortex matter and the Josephson effect. Bicrystal grain-boundary Josephson junctions have also been fabricated using artificial superlattice films with as few as six superconducting copper–oxygen planes. Extrinsic and intrinsic d-wave effects in these junctions are discussed within the framework of novel designs for π-circuitry and qubits.

Characterization of YBCO bicrystal grain-boundary Josephson junctions on NdGaO 3 substrate in terms of conventional superconductivity models

We have fabricated high quality grain-boundary bicrystal YBCO Josephson junctions on the NdGaO 3 substrates. At the temperature T=77 K, the I– V characteristic can be explained by means of the conventional overdamped resistively shunted junction model comprising thermal noise. The deviations of the magnetic dependence of the Josephson critical current I c from the usual Fraunhofer diffraction pattern can be described by a combination of a peaked-at-the-edges distribution of the critical current density and of the rounding of the I– V characteristic due to thermal fluctuations. The same measurements were done also at T=4.2 K, at low thermal noise conditions. The experimental magnetic field dependence I c vs H was described by using a peaked-at-the-edges asymmetric profile, where the asymmetry depends on the applied magnetic field. The relation between our phenomenological model and advanced microscopical theory of grain-boundary bicrystal YBCO junction based on alternating `0' and ` π' contacts is discussed.

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.

Observation of individual Josephson vortices inYBa2Cu3O7−δbicrystal grain-boundary junctions

The response of YBCO bicrystal grain-boundary junctions to small dc magnetic fields (0 - 10 Oe) has been probed with a low-power microwave (rf) signal of 4.4 GHz in a microwave-resonator setup. Peaks in the microwave loss at certain dc magnetic fields are observed that result from individual Josephson vortices penetrating into the grain-boundary junctions under study. The system is modeled as a long Josephson junction described by the sine-Gordon equation with the appropriate boundary conditions. Excellent quantitative agreement between the experimental data and the model has been obtained. Hysteresis effect of dc magnetic field is also studied and the results of measurement and calculation are compared.

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.

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.

Properties of doped-YBCO bicrystal grain-boundary junctions for Josephson field effect transistor

We report the properties of hole-overdoped and underdoped YBa2Cu3O7−x (YBCO) grain-boundary junctions (GBJ) on the 24° tilt SrTiO3 (100) bicrystals as a function of temperature and junction width. Epitaxial thin films of YBCO, Y0.7Ca0.3Ba2Cu3O7−y (Co-YBCO) were deposited by pulsed laser deposition, and the GBJs were fabricated by standard photolithography with argon ion-milling. We measured the IcRn products of ∼ 1.2 mV for Ca-YBCO and ∼ 0.3 mV for Co-YBCO GBJ at 27 K. Critical current densities(Jc) of the junctions were 1.4×103 and 3.0x104 A/cm2 at 27 K for the Ca- and Co-YBCO GBJs, respectively. This is explained in terms of a weak-link behavior at the grain-boundaries in comparison with the bulk properties of the doped YBCO films.

High-T/sub c/ superconductor oversampled delta modulator for analog-to-digital converters

Based on a single-level YBCO thin film topology and seven bicrystal grain-boundary junctions, a high temperature superconductor delta modulator for analog-to-digital converters has been designed, fabricated, and tested. A single bit quantizer based on a quantum flux parametron (QFP) comparator is used for the modulator. A large superconducting inductor is used both for the signal input and for the feedback. A small resistor is chosen for connecting the comparator to the read-out section. The oversampling provides large input signal bandwidth with high dynamic range. Circuit simulations prove the concept of the design. The effective LSB and MSB of the modulator are discussed in terms of input inductance, junction critical current, and QFP noise current. The density of latched output pulses is proportional to the change of input amplitude. Au wires were employed to make crossover of bias lines. Although this modulator did not function as expected for low-speed measurements, it shows the proper latching behaviour of the output section at 4.2 K and 35 K.

YBa/sub 2/Cu/sub 3/O/sub 7-x/ Josephson junctions on bicrystal Al/sub 2/O/sub 3/ and SrTiO/sub 3/ substrates

Bicrystal grain-boundary junctions (bi-GBJs) have been reproducibly fabricated on SrTiO/sub 3/ (STO) and r-plane Al/sub 2/O/sub 3/ (sapphire) bicrystal substrates. Sapphire bicrystals are candidates for high-frequency applications due to their low dielectric constant and loss tangent. The sapphire bi-GBJs demonstrated resistively shunted junction (RSJ)-like current voltage characteristics, with junction parameters comparable to the STO bi-GBJs and critical current densities Jc/spl sim/10/sup 5/ A/cm/sup 2/. Independent control of junction resistance (R/sub N/) was demonstrated with the use of Au shunt layers. In addition, overlayers such as Au or STO may act to passivate the GBJs and improve long term stability.

Design and implementation of a dual-control active device using YBCO grain-boundary junctions

We propose a dual-control active device based on overdamped long junctions. In analogy to the semiconductor dual-gate field effect transistor which can be considered a cascode (output terminals in series) of two single-gate FETs, the dual-control device consists of two single devices in parallel at the outputs. The transresistance of one device is shown to be a linear function of the second control current over a sizable range. This unique feature makes the dual-control device highly desirable for applications such as gain control and mixing, Active devices have been fabricated using arrays of YBCO bi-crystal grain-boundary junctions. Tight coupling of the control fields to the array was achieved by injecting the control current into an ear structure at one end of the array. The large-signal current gain, however, is less than 1 due to the asymmetric bias and end injection. Improved current gain with tight coupling to the entire array is necessary for a practical dual-control device.

Josephson coupling behavior of YBa 2Cu 3O 7− x bicrystal grain-boundary junctions

The resistive transition tail often observed for superconducting YBa 2Cu 3O 7− x grain-boundary junctions has been attributed to the thermally activated phase slippage [R. Gross et al., Phys. Rev. Lett. 64 (1990) 228] through the junction. The model predicts the normalized broadening of the transition tail, ΔT T c , to scale linearly with the sqaure root of the junction's normal-state resistance, R n 1 2 . In this experiment, by successfully varying junctions parameters in a broader range, we provide further evidence for this interpretation and explicitly verified this relationship. Combining data from two different grain-boundary angles, 24° and 36°, we have shown that universality is found when R n 1 2 T c /Δ i (0) is plotted versus ΔT T c , where Δ i(0) is the zero-temperature reduced energy gap at the junction interface. Furthermore, we have studied the effect of oxygen deficiency on grain-boundary junctions and have found a linear dependence of Δ i(0) versus (T cmax − T c ) 1 2 where T cmax is the maximum critical temperature of the fully oxygenated thin film.

1/f noise in Bi2Sr2CaCu2O8+x bicrystal grain-boundary Josephson junctions.

We have measured the low-frequency 1/f voltage noise of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$ bicrystal grain-boundary Josephson junctions (GBJ's). The origin of the 1/f noise are fluctuations \ensuremath{\delta}${\mathit{I}}_{\mathit{c}}$ and \ensuremath{\delta}${\mathit{R}}_{\mathit{n}}$ of their critical current and normal resistance, respectively. The ratio p of the normalized fluctuations \ensuremath{\Vert}\ensuremath{\delta}${\mathit{I}}_{\mathit{c}}$/${\mathit{I}}_{\mathit{c}}$\ensuremath{\Vert}/\ensuremath{\Vert}\ensuremath{\delta}${\mathit{R}}_{\mathit{n}}$/${\mathit{R}}_{\mathit{n}}$\ensuremath{\Vert} is found to be p=1.9\ifmmode\pm\else\textpm\fi{}0.25 independent of temperature. The analysis of the noise data indicates that the critical current and resistance fluctuations are correlated. The measured noise properties can be consistently explained by the intrinsically shunted junction model based on an insulating layer at the grain boundary containing a high density of localized defect states.

High-quality YBCO d.c. SQUIDs based on bicrystal substrates

An analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of ac-axis-oriented YBCO film on a SrTiO3 bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle θ=20° are reported. The SQUID shows a critical temperatureT c∼89 K and a high critical current densityJ c∼3·106 A/cm2 atT=4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependenceJ c(T) shows a linear behaviour at small reduced temperatures and a depressedJ c value close toT c. High-quality flux-voltage curves have been found upT=87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of θ make YBCO bicrystal GBJs very useful for applications in electronics.