Bicrystal Junctions Research Articles

Andreev bound states in high temperature superconductors

Andreev bound states (ABS) at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of ABS in HTS employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBCO, BSCCO, and LSCO show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS NCCO no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.

Direct-Coupled High Tc DC Superconducting Quantum Interference Device Magnetometers on SrTiO3 Substrate: Theoretical Description and Comparison with Experiment

A comprehensive quantitative comparison between the measured performance of direct-coupled high Tc superconducting quantum interference device (SQUID) magnetometers with 30° bicrystal junctions and a numerical simulation is presented. It is shown that the characteristics of the SQUID magnetometer are considerably affected by resonances due to the large dielectric constant of the SrTiO3 substrate. In the realized magnetometer layout, the strip line resonance occurring in the SQUID inductance increases the voltage modulation depth and the output voltage noise of the SQUID, while the flux noise of the SQUID is nearly unchanged. It is also shown that the distortion of the voltage versus flux characteristic is caused by the LC resonance in the pickup loop of the magnetometer in combination with a capacitive feedback. Good agreement between experiment and simulation has been obtained.

Superconducting and normal-state properties of YBa2Cu3O7−δ -bicrystal grain boundary junctions in thin films

A model, based on universal properties of high-Tc superconductors only, is presented for the transport characteristics of grain boundaries in these materials. The model accounts for the superconducting as well as for the normal-state transport properties observed in a comprehensive study of YBa2Cu3O7−δ [001]-tilt bicrystalline grain boundaries with varying misorientation angles and of two different symmetries.

Large critical currents and current steps in shunted bicrystal Josephson junctions at liquid nitrogen temperatures

The amplitudes of the critical current and constant-voltage steps in shunted bicrystal Josephson junctions were explored as a function of the grain boundary type (symmetrical or asymmetrical), misorientation angle and thickness of films. Two new results were demonstrated. First, we found that for the given angles of to the junctions on symmetrical grain boundaries generated the highest critical current density . Second, the increase of superconducting film thickness up to allowed us to measure the first current step height of 1 mA and 2 mA in high- junctions operated at 80 K and 50 K respectively. Simulations with a wide-junction model have shown that the maximal current step amplitude in large critical current bicrystal junctions is controlled by a self-field effect.

Development of planar thin film HTSC–SQUID gradiometers for different applications

Abstract We investigated the main dependencies of the gradient resolution of planar galvanometer superconducting quantum interference device (SQUID) gradiometers made of YBa 2 Cu 3 O 7-x (YBCO). We focussed especially on the influence of antenna layout and the parameters of the galvanometer SQUID such as effective and parasitic areas on the performance of the gradiometer and the behavior in an unshielded environment. The efficiency (that is, the quotient of effective area to inductance) of different geometries of antennas will be compared. Some aspects of the layout of the galvanometer SQUIDs are discussed in terms of parasitic area and best current resolution. Special problems due to the use of bicrystal Josephson junctions in gradiometers for operation in highly disturbed environment are shown. Step-edge Josephson junctions can offer alternative concepts. Reached gradient sensitivity values in the white noise region are 0.46 pT/(cm/ H z) in the case of bicrystal junctions and 0.69 pT/(cm/ H z) for step-edge junctions.

Effect of proton irradiation on the resistance in Tl 2Ba 2CaCu 2O 8 Josephson junctions and thin films

The effect of 2 MeV proton irradiation on the bulk onset transition temperature T c, the bulk resistivity and the resistance of bicrystal Josephson junctions fabricated from Tl 2Ca 2BaCu 2O 8 is reported. Each of the measured parameters are found to vary nearly linearly with proton fluence. This is the first time a correlation between junction resistance and bulk T c has been reported in the cuprate superconductors.

Magnetocardiography with high-TC Gradiometers Working in Unshielded Environments

Abstract The noninvasive measurement of magnetic heart activity based on high- T C DC SQUIDs, could be a tool for investigating cardiac electrophysiological properties, if such a system is able to work in unshielded environments. For the realization of such a system we use thin film planar SQUID gradiometers with bicrystal or step-edge Josephson junctions. Even for a small baseline of approximately 4 mm (limited by substrate dimensions), the field gradent resolution is in the range required for clinical analysis. We use a portable system which consists of a glass fiber cryostat with a measuring unit where we can place up to four sensors, different read-out electronics, and signal filtering methods. Firstly, measurements inside a magnetically shielded room were used to show the capability of our magnetocardiography (MCG) system in medical applcations. The system works in an unshielded environment also, without additional field compensation. We demonstrate a measurement of a magnetocardiogram in a real clinical environment, and discuss the possibilties as well as the limitations of this system in magnetocardiography.

Superconducting Nd1.85Ce0.15CuO4−y bicrystal grain boundary Josephson junctions

We have studied the electric transport properties of symmetrical [001] tilt Nd1.85Ce0.15CuO4−y (NCCO) bicrystal grain boundary Josephson junctions (GBJs) fabricated on SrTiO3 bicrystal substrates with misorientation angles of 24° and 36.8°. The superconducting properties of the NCCO GBJs are similar to those of GBJs fabricated from the hole doped high temperature superconductors (HTS). The critical current density Jc decreases strongly with increasing misorientation angle. The products of the critical current Ic and the normal resistance Rn (∼100 μV at 4.2 K) are small compared to the gap voltage and fit well to the universal scaling law IcRn∝Jc found for GBJs fabricated from the hole doped HTS. This suggests that the symmetry of the order parameter, which most likely is different for the electron and the hole doped HTS, has little influence on the characteristic properties of symmetrical [001] tilt GBJs.

Application of silicon substrates for high-Tc Josephson junctions and SQUIDs

We investigated the realization of YBCO Josephson junctions and SQUIDs on 10 x 10 mm 2 silicon bicrystal substrates and on 2 inch single crystalline silicon wafers. All buffer layers, YBCO films, passivation and metallization layers were deposited with the laser ablation technique. Bicrystal junctions with RSJ-like I-V characteristics, I C R N products of 150 μV at 77 K and microwave response up to the THz range were realized. DC-SQUIDs consisting of these junctions have working temperatures up to 80 K and transfer functions up to 30 μV/Φ 0 at 77 K. Further we enlarge our standard preparation technique of Josephson step-edge junctions on 5x10 mm 2 to 2 inch silicon wafers. YBCO films of this size have a critical temperature T C 87 K and critical current densities j C (77K) >3.10 6 A/cm 2 over the whole wafer area.

Measurement of the static error rate of a storage cell for single magnetic flux quanta, fabricated from high-Tc multilayer bicrystal Josephson junctions

We measured the static error rate of a high-Tc superconductor dc superconducting quantum interference device (SQUID), which, in the form as a storage loop for single flux quanta, is a basic element of rapid single flux quantum circuits. Using high-Tc multilayer bicrystal technology, we fabricated a stacked dc SQUID pair, one SQUID serving as the storage loop, the other one as the readout device. The escape rate of a stored flux quantum was measured as a function of the bias current at a temperature of 28 K. The measured error rates were in good agreement with a model calculation based on thermally activated barrier crossing.

Low-noise YBa2Cu3O7−x single layer dc superconducting quantum interference device (SQUID) magnetometer based on bicrystal junctions with 30° misorientation angle

We have fabricated and characterized a low-noise direct-coupled magnetometer based on a 100 pH YBa2Cu3O7−x dc superconducting quantum interference device (SQUID) on a 10 mm×10 mm SrTiO3 bicrystal substrate with 30° misorientation angle. The thin films were deposited by hollow cathode discharge sputtering and patterned using conventional photolithography and Ar ion beam etching. The SQUID magnetometer was operated using direct-coupled flux-locked-loop electronics with bias reversal. The sensor had a usable voltage swing of 39 μV and a white magnetic field noise of 32 fTHz−1/2 with a 1/f corner at 2 Hz, including electronics and environmental noise. The voltage versus flux (V−Φ) characteristic showed a pronounced distortion on the negative slope. Numerical simulations were performed to explain the distorted V−Φ characteristic. Measurements of magnetocardiograms demonstrate the suitability of this sensor for biomagnetic applications.

A new preparation technique for YBa 2Cu 3O 7 bicrystal junctions

We have developed an Airion-beam-assisted technique for preparing YBa 2Cu 3O 7 (YBCO) bicrystal junctions with the apropos critical current, high normal resistance and high characteristic voltage. The dependence of the critical current and normal resisrance of YBCO bicrystal junctions on the ion beam energy and annealing temperature has been investigated. By using this technique, junctions with normal resistance R n up to3–8 Ω and characteristic voltage I c R n of 200–720 μV have been obtained. In addition, the scaling relation of the characteristic voltage with the critical current has been examined, which is in good agreement with the results obtained by other researchers. An infrared detector was fabricated by the YBCO bicrystal junction with high normal resistance of 6 Ω and characteristic voltage of 720 μV. The obtained maximum responsibility of the detector was up to 220 V/W at 77 K. Its noise equivalent power (NEP) at 78 K was of 9.0 × 10 −13W/√Hz.

Critical-current measurements in planar Josephson junctions

The properties of planar high-temperature superconducting Josephson junctions are not entirely consistent with low Tc models. Some of these disparities may be intrinsic, related to the unusual superconducting properties of the electrodes, while others are due to the nonstandard planar geometry. By comparing experimental measurements of a 24° bicrystal substrate Josephson junction with a simple model of nonuniform current flow in a planar geometry, we show that several unusual properties can be explained as extrinsic. We show that nonuniform current feed in a planar geometry leads to a reduced critical current, an unchanged Meissner field Hco, and a power-law dependence of the critical current on Hco different from that expected in an overlap geometry.

Electromagnetic radiation induced current steps in biepitaxial Josephson junctions

Ordinary Shapiro steps were observed in the I - V curves of biepitaxial Josephson junctions at when microwave radiation at f = 11 GHz was applied to the junction. However, pronounced current steps were observed at twice the period, when the d.c. Josephson current was suppressed by high power microwave radiation. The extension of the voltage region, within which the double period steps were seen, increased with microwave power. Modified biepitaxial heterostructures of , with as a seed layer and as an intermediate buffer layer were grown by laser ablation on . The I - V characteristics for the biepitaxial Josephson junctions agreed well with the RSJ model for T > 50 K but showed hysteresis at T < 30 K. No large excess current was observed. The critical current density, about at 4 K, was an order of magnitude higher than for bicrystal junctions and prevalent biepitaxial junctions with a misorientation between the crystal planes on either side of the grain boundary. An irregular magnetic field dependence of the critical current, , indicated an uneven current distribution within the junction. The products for the junctions were about 1.5 mV at 4.2 K and 20 - 60 at 77 K. Pronounced current steps in the I - V dependencies due to a.c. Josephson current coupling to self-induced electromagnetic modes at the biepitaxial boundary were used to characterize the junctions.

Properties of normal metal shunted YBCO bi-crystal junctions

We present experimental results on YBCO thin film bi-crystal junctions in which additional metallic shunt resistors are added by in-situ thin film evaporation of gold or silver. The current-voltage characteristics have been measured, as well as the broad band noise in the system between 1 Hz and 100 kHz, using a HTS dc SQUID as a low-noise preamplifier. The influence of the shunt resistors on the excess noise contributions is discussed and the measured shunt resistance results are compared with a resistive network model.

Infrared detector fabricated by YBCO Josephson junctions in series

The infrared detectors were fabricated by YBCO bicrystal and step-edge junctions in series. The detectors exhibited an high responsivity of 780 V/W at 77K and low NEP of 9.0 × 10−13 W/√Hz. The optical response of the detectors below Tc is mainly of nonequilibrium origin.

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.

TEM study of microstructure of YBa2Cu3Oy bicrystal grain boundary junctions grown on Y2O3 stabilized ZrO2 bicrystal

The microstructure details of bicrystal boundaries of both the YBa2Cu3Oy thin film and the ZrO2 bicrystals substrate were studied by means of transmission electron microscopy (TEM).

Shunted YBCO bi-crystal junctions for noise thermometry

Abstract We present experimental results on YBCO thin film bi-crystal junctions in which additional metallic shunt resistors are added by in-situ evaporation of gold or silver. These junctions are intended for use in the development of an HTS Josephson noise thermometer for primary temperature scale use. The temperature dependence of the critical current and effective shunt resistance are reported and the noise arising from the shunted junctions when biased into the finite voltage regime has been assessed as a function of frequency. This is compared with the predictions of a simple model and the implications for the potential accuracy of the proposed thermometer are considered.

YBa2Cu3O7−x dc squid magnetometers with bicrystal junctions for biomagnetic multichannel applications

Abstract We have fabricated magnetometers based on direct-coupled YBa2Cu3O7−x superconducting quantum interference devices (SQUID) with SQUID inductances of 50 pH and 100 pH on 10×10 mm2 24° SrTiO3 bicrystal substrates. The thin films were deposited by hollow cathode discharge sputtering and patterned using conventional photolithography and Ar ion beam etching. The optimization of the fabrication process was performed utilizing design of experiment methodology. The SQUID magnetometers were operated with an improved direct-coupled flux-locked loop electronics with a preamplifier voltage noise of 0.44 nV Hz−1/2 and bias reversal (fb=0.1–500 kHz). The best sensor had a white noise of 41 fT Hz−1/2 and a 1/f corner at 2 Hz, including electronics and environmental noise. High-quality magnetocardiograms recorded in the Berlin magnetically shielded room (BMSR) illustrate that the sensors are suitable for biomagnetic multichannel systems.