Properties Of Weak Links Research Articles

Reduced-Scale Transition-Edge Sensor Detectors for Solar and X-Ray Astrophysics

We have developed large-format, close-packed X-ray microcalorimeter arrays fabricated on solid substrates, designed to achieve high energy resolution with count rates up to a few hundred counts per second per pixel for X-ray photon energies up to 8 keV. Our most recent arrays feature 31-micron absorbers on a 35-micron pitch, reducing the size of pixels by about a factor of two. This change will enable an instrument with significantly higher angular resolution. In order to wire out large format arrays with an increased density of smaller pixels, we have reduced the lateral size of both the microstrip wiring and the Mo/Au transition-edge sensors (TES). We report on the key physical properties of these small TESs and the fine Nb leads attached, including the critical currents and weak-link properties associated with the longitudinal proximity effect.

Alternating-current properties of short Josephson weak links

The admittance of two types of Josephson weak links is calculated, i.e., of a one-dimensional superconducting wire with a local suppression of the order parameter, and the second is a short S-c-S structure, where S denotes a superconductor and c---a constriction. The systems of the first type are analyzed on the basis of time-dependent Ginzburg-Landau equations. We show that the impedance $Z(\Omega)$ has a maximum as a function of the frequency $\Omega$, and the electric field $E_{\Omega}$ is determined by two gauge-invariant quantities---the condensate momentum $Q_{\Omega}$ and the potential $\mu$ related to charge imbalance. The structures of the second type are studied on the basis of microscopic equations for quasiclassical Green's functions in the Keldysh technique. For short S-c-S contacts (the Thouless energy ${E_{\text{Th}} = D/L^{2} \gg \Delta}$) we present a formula for admittance $Y$ valid at frequencies $\Omega$ and temperatures $T$ less than the Thouless energy but arbitrary with respect to the energy gap $\Delta$. It is shown that, at low temperatures, the absorption is absent [${\mathrm{Re}(Y) = 0}$] if the frequency does not exceed the energy gap in the center of the constriction (${\Omega < \Delta \cos \varphi_{0}}$, where $2 \varphi_{0}$ is the phase difference between the S reservoirs). The absorption gradually increases with increasing the difference ${(\Omega - \Delta \cos \varphi_{0})}$ if $2 \varphi_{0}$ is less than the phase difference $2 \varphi_{\text{c}}$ corresponding to the critical Josephson current. In the interval ${2 \varphi_{\text{c}} < 2 \varphi_{0} < \pi}$, the absorption has a maximum. This interval of the phase difference is achievable in phase-biased Josephson junctions. Close to $T_{\text{c}}$ the admittance has a maximum at low $\Omega$ which is described by an analytical formula.

A GENERAL ORNSTEIN–UHLENBECK STOCHASTIC VOLATILITY MODEL WITH LÉVY JUMPS

We present a general class of stochastic volatility models with jumps where the stochastic variance process follows a Lévy-driven Ornstein–Uhlenbeck (OU) process and the jumps in the log-price process follow a Lévy process. This financial market model is a true extension of the Barndorff-Nielsen–Shephard (BNS) model class and can establish a weak link between log-price jumps and volatility jumps. Furthermore, we investigate the weak-link [Formula: see text]-OU-BNS model as a special case, where we calculate the characteristic function of the logarithmic price in closed form. The classical [Formula: see text]-OU-BNS model can be obtained as a limit of weak-link [Formula: see text]-OU-BNS models in the Skorokhod topology. We highlight that the weak-link property may be a useful model extension in the case of pricing barrier options.

Critical current characteristics and history dependence in superconducting SmFeAsOF bulk

The superconducting SmFeAsO1−xFx (x=0.2) polycrystalline bulks were prepared by the powder-in-tube (PIT) method. The magnetic field and temperature dependences of critical current densities in the samples were investigated by resistive and ac inductive (Campbell's) methods. It was found that a fairly large shielding current density over 109 A/m2, which is considered to correspond to the local critical current density, flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low transport current density of about 105 A/m2 corresponding to the global critical current density flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller value in the increasing-field process than that in the decreasing-field process. The history dependence of global critical current characteristic in our case can be ascribed to the existence of the weak-link property between the grains in SmFeAsO1−xFx bulk.

Magnetoresistance hysteresis and critical current density in granularRuSr2Gd2−xCexCu2O10−δ

We report resistivity data on ceramic RuSr 2 Gd 2 - x Ce x Cu 2 O 1 0 - δ which is the first Cu-O based system in which superconductivity (SC) in the CuO 2 planes and weak-ferromagnetism (W-FM) in the Ru sublattice are considered to coexist. Due to the granular nature of the materials the critical current density at 5 K is extremely small as compared to other high-T C superconducting materials. Below the superconducting transition (T C ), the magnetoresistance ΔR(H) = R(H)-R(0) is positive and unexpected hysteresis loops are observed. The hysteresis loop is completely different than that observed in conventional homogeneous superconductors. ΔR(H) on decreasing the applied field (H e x t ) is much smaller than the ΔR(H) for increasing (H e x t ). The width of the loops depends strongly on the weak-link properties. Similar hysteresis loops are observed in the reference nonmagnetic NbSr 2 Gd 1 . 5 Ce 0 . 5 Cu 2 O 1 0 which is SC at T C =28 K, thus the possibility that the hysteresis phenomenom is caused by the coexistence of both SC and magnetic states is excluded.

Structure and properties of an intragranular weak link in Bi 2Sr 2CaCu 2O 8+x single crystals

Coupled magneto-optic imaging, four-point probe electrical characterization and transmission electron microscopy (TEM) of Bi 2Sr 2CaCu 2O 8+ x (2212) single crystals grown by directional solidification has revealed the presence of extensive, lamellar defects that lie parallel to the ( 0 1 0 ) planes of the structure. These defects block the supercurrent over areas that can reach hundreds of micrometers squared. Diffraction-contrast and high-resolution TEM revealed sheet-shaped defects that lie parallel to the ( 0 1 0 ) plane and in which the wavelength of the incommensurate modulation is perturbed. These lamellae are a few nanometers wide and, extends to tens to hundreds of micrometers on the ( 0 1 0 ) plane. The approximate doubling of the wavelength of the incommensurate modulation within these lamellae indicates severe oxygen deficiency. High-spatial-resolution X-ray spectroscopy indicates that the defects are also Ca deficient. These macroscopic defects which are common in directionally-solidified 2212 single crystals, must be recognized in the interpretation of single and synthetic bicrystal data, and may play a role in determining the percolative current path in bulk-scale 2212 materials.

Electromagnetic properties andHe+irradiation effects onYBa2Cu3O7−xgrain-boundary Josephson junctions

We have analyzed the electromagnetic properties of small and long YBa2Cu3O7-x grain-boundary Josephson junctions. The Swihart velocity increases with junction width (rv) while the ratio of the relative dielectric constant to the barrier thickness (epsilon/t) decreases. We found that the product wX epsilon/t is approximately constant. These results have been explained in the framework of the filamentary model, where the barrier can be regarded as a disordered dielectric medium with a high density of superconducting filaments. Experiments demonstrate that a controllable variation of these parameters can be achieved by helium irradiation at 80 keV. We give examples of an enhancement of weak-link properties of junctions for doses in the range of 10(13) cm(-2) Raising the dose we can sweep the modification of the weak-link properties from an increase of the junction critical current of about 10% to a severe degradation of the coupling energy of the barrier, although the superconducting properties of the electrodes always worsen gradually.

Transport properties of Nb/InAs(2DEG)/Nb Josephson field-effect transistors

We investigate transport properties of mesoscopic semiconductor-superconductor weak links. The superconducting Nb electrodes of our junctions are coupled by the two-dimensional electron gas of an InAs heterostructure grown on a GaAs substrate. We report on the properties of Josephson field-effect transistors utilizing these junctions.

The microwave power handling of an FIB-generated weak link in a YBCO film

We have measured the power-dependent microwave properties of a weak link in a YBa2Cu3O7- thin film formed by writing a line of damage using a focused ion beam. The measurement was made using a parallel plate resonator at 5.5 GHz with the weak link written across the width of one of the plates. The ion-induced damage was characterized using a TRIM computer simulation and the dc properties of similar weak links were measured. Using a 200 eV Si ion dose of 2 × 1013 cm-2, Tc of the damaged region was reduced by 5.5 K and the normal resistivity was doubled. Surprisingly, the microwave measurements did not show any Josephson junction characteristics. Rather, the ion-damaged region exhibited a greatly increased microwave resistivity that was constant as a function of microwave power up to rf fields of 20 mT at 21 K.

The influence of the top and the bottom grain boundaries on the current transport in YBa2Cu3O7-δ step-edge Josephson junction

Abstract We have studied the electrical transport properties of the grain boundaries (GBs) formed at the top and at the bottom edges of YBa2Cu3O7-δ step-edge Josephson junctions, for different values of the step angle α. The step-edge junctions were fabricated on (100) LaAlO3 steps using a tilted Ar ion milling. Due to the shadowing effect of the step, the middle and the bottom part of the step edge junction were contacted by thin stripes. We found that for α≈60° the top GB is responsible for the weak link behaviour of our step-edge junctions. On less steep steps, α≈45°, a series of GBs with weak link properties were found to nucleate along the step profile. We also correlated these results with the different microstructural properties of the GBs formed on steps with different angles.

Difference in mechanical properties of weak and strong links in Bi-2223/Ag superconducting tapes

Transport critical current ( I c) was measured as a function of magnetic field ( H) at 77 K, for Bi-2223/Ag tapes which were progressively bent. The field was perpendicular to the tape plane. Proportion of critical current through strong links in total I c ( I c0 s/ I c0) was used as a measure of grain connectivity by strong links. It was found that I c0 s/ I c0 changes with bending, going through a peak as the tapes progressively bend. A change of flux pinning with bending was also observed. However, the effect of the change of flux pinning on I c0 s/ I c0 was only a minor one. Therefore, change of I c0 s/ I c0 with bending is defined by grain connectivity and it reflects qualitatively different electromagnetic and mechanical properties of weak and strong links. This is in agreement with previous studies of electromagnetic properties and formation of weak and strong links.

Limitations To The Performance Of PIT Tl(Bi)-1223 Tapes

We have investigated the possibility of improving the weak link properties of Tl-1223 tape by melt processing, and also studied the technical limitations of coil fabrication using multimeter lengths of PIT tape. We conclude that melt processing does offer some potential for grain growth, but that grain alignment is only achieved in some places at the Ag/1223 interface. Until we can establish more control of the local grain alignment by altering the geometry of the samples, the properties of coils in high fields remains rather poor.

Fabrication and characterization of YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary Josephson junctions on [110] NdGaO/sub 3/ bicrystal and single-twin substrates

To meet the requirements of high-frequency applications, we have fabricated YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary junctions on 2/spl times/12/spl deg/ [110] NdGaO/sub 3/ bicrystal substrates. Compared to reference junctions made on (100) SrTiO/sub 3/ bicrystals, the specific resistances R/sub n/A of the junctions on NdGaO/sub 3/ were usually several times higher. Furthermore, they showed RSJ-like behavior in a more limited temperature range, which might be due to weak links in the YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin film formed on twin boundaries of the NdGaO/sub 3/ bicrystal substrate. The electrical properties of these additional weak links have been studied by preparing YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin-film bridges crossing a single twin boundary of a NdGaO/sub 3/ substrate. The critical current densities j/sub c/ were found to be on the order of 1/spl times/10/sup 6/ A/cm/sup 2/. Therefore, even for the smallest twin boundary junctions, a pronounced flux-flow-behavior is observed.

Josephson transport in Tl-cuprate bicrystal weak links

Superconducting epitaxial Tl2Ba2CaCu2O8 (Tl-2212) and Tl2Ba2CuO6 (Tl-22O1) thin films are synthesised on single crystal and bicrystal LaAlO3 substrates by an ex-situ process. Tl-2212 films were optically smooth, with T-c's from 105 to 108K and Delta T-c <2 K, I-c reaching 2.10(6) A/cm(2) at 77 K. Tl-2201 films exhibited tetragonal or orthorhombic symmetry depending on the thallium content. The T-c, of the as-synthesised Tl-2201 films was in the range 40-55 K. By oxygen depletion, T-c was increased by 15 - 20 K. Properties of weak links in Tl-2212 and Tl-2201 thin films are reported.

STM observation of dislocation chains suitable for flux pinning in YBa 2Cu 3O 7− x films

Systematic variations of the film-growth conditions during pulsed-laser deposition (PLD) with regard to substrate temperature, oxygen partial pressure and substrate quality, allow us to establish correlations between the as-grown defect structure and the current transport properties of c-axis oriented YBa 2Cu 3O 7− x films. By investigating the film surface with scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) the nature and the density of various defects as well as the general film-growth mode were determined. SEM pictures show the overall surface roughness and the presence of other phases (outgrowths), while higher-resolution STM pictures reveal small-scale defects down to the nm range. Transport current measurements on patterned parts of the same films show differences in the magnitude of J c as well as in its field and angle dependence. Influences of the defect structure on intrinsic pinning, weak-link properties and special features in the angular dependence of J c are discussed. Dislocation chains extending over several layers along the c-axis within the terraces of growth spirals are of special interest with regard to flux pinning. The existence of these dislocation chains can be directly implied from the present data. A higher substrate roughness and a faster growth speed of the terraces along the ab plane seem to favor the appearance of the dislocation chains.

Properties of YBCO weak links prepared by local oxygen-ion induced modification

High- T c weak links consisting of two superconducting thin film YBCO electrodes coupled by a short (∼ 300 nm) bridge modified by oxygen-ion irradiation of the same YBCO film are investigated in this work. To fabricate these devices we have developed a technology, which is compatible with conventional integrated circuit fabrication techniques and enables controllable and reproducible preparation of the weak links and SQUID's operating at liquid-nitrogen temperature. We describe in detail the electrical behavior of the bridges, which show RSJ-like (resistively shunted junction model) properties for a small critical current (∼ 10 μA), with resistances of up to 1 Ω. The transport mechanism providing the Josephson coupling across such barriers is still not unambiguously identified. We believe that our devices are SNS type in the operation range of the SQUIDs and SS'S at lower temperatures.

Transport properties of submicron YBaCuO low angle grain boundary weak links

Submicron weak links in YBaCuO thin films fabricated on low angle grain boundary bi-crystals were studied. Their transport properties may be explained in terms of an S-S'-S weak link model.

Transport superconducting properties of grain boundaries in Tl1Ba2Ca2Cu3Ox thin films

In order to investigate weak link properties in the Tl1Ba2Ca2Cu3Ox[Tl-(1223)] system, we measured the transport properties of artificially grown grain boundaries in Tl-(1223) films. Epitaxial thin films were grown on SrTiO3 bicrystal substrates with five tilt angles of 5°, 10°, 15°, 24°, and 36.8°. We found that the grain boundaries Jc (Jcg.b.) of large tilt angles (θ≥15°) at 77 and 5 K were substantially less than intragrain Jc(Jcg) and limited by a weak link. However, the Jcg.b./Jcg values for low tilt angle grain boundaries (θ≤10°) were almost unity. Moreover, magnetic-field history dependent Jc values for low tilt angle grain boundaries were not observed in a low magnetic field (&amp;lt;0.1 T). These data indicate that the low tilt angle grain boundaries do not work as a weak link and retain high Jc.

Modulated microwave surface impedance of superconducting films

We present details of an experimental method to measure the field modulated surface resistance (dRs/dH) of superconducting films accurately, which enables both field and temperature sweep experiments and is particularly useful for low-field or near Tc measurements. The complex surface impedance Zs(H,T)=Rs(H,T)+iXs(H,T) can also be simultaneously measured. This technique utilizes specially built circuitry to lock the microwave source to the cavity and keep the input power of the cavity constant, even as the resonant frequency or Q value of the cavity changes with changing temperature or applied field. With the above circuitry, we were able to measure (dRs/dH) directly rather than (dP/dH) as in some modulated microwave absorption measurements. With the high accuracy offered by the modulation technique and the high Q of the cavity, important information about the magnetic-field response of superconducting films, such as dynamics of vortices and low-field properties of weak links can be obtained. The technique can also be employed to measure any possible spontaneously induced internal fields in two-dimensional high-Tc superconductors.

Observation of Jc oscillations in bitextured YBa2Cu3O7- delta films.

A series of in-plane bitextured ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films were obtained on yttria-stabilized ${\mathrm{ZrO}}_{2}$ substrates with various ratios of [100] and [110] grains. Their critical-current-density properties were measured in an applied magnetic field of up to 4 T. A diffraction pattern in ${\mathit{J}}_{\mathit{c}}$(H)\ensuremath{\sim}H is observed at high magnetic fields in thin-film samples. This diffraction pattern is due to the weak-link character of 45\ifmmode^\circ\else\textdegree\fi{} grain boundaries. The volume-pinning-force densities were calculated, with two separate mechanisms governing thin-film ${\mathit{J}}_{\mathit{c}}$ behavior in the magnetic fields identified. In the range of H0.5 T, weak-link properties of 45\ifmmode^\circ\else\textdegree\fi{} grain boundary dominated. At fields above 1 T, flux flow along strongly coupled grains was determined to be the main mechanism of dissipation.