Superconducting Josephson structures with high linearity of transformation of magnetic signal into voltage

Possible approaches to the creation of Josephson superconducting circuits with a high linearity of transformation of a magnetic signal into voltage and a wide dynamic range have been described. Realization of such circuit by the creation of a special spatial inhomogeneity in chains of Josephson junctions and connection of these chains into a differential circuit has been considered. A new method for linearizing the transfer function of a single two-junction superconducting quantum interference device, which is an elementary cell of the chain, by connecting to it an additional loop playing the role of a nonlinear transformer of the magnetic flux has been described. For both approaches, the basic analytical expressions describing the synthesized circuits are presented. The data on numerical calculation and experimental measurements of samples produced on the basis of niobium technology with the critical current density of Josephson junctions of 4.5 kA/cm² are reported. The approaches developed in the work can serve as a basis for the creation of highly efficient gigahertz superconducting amplifiers for cellular and satellite communications and can be applied for improving the characteristics of SQUID-based devices.     

基于高温超导双晶结的约瑟夫森效应观测实验

基于YBa2Cu3O7-δ高温超导双晶结,组建了约瑟夫森效应测试装置.利用此装置观察了直流和交流约瑟夫森效应,并测量了约瑟夫森结的临界电流和夏皮罗台阶.     

利用超导结的直流约瑟夫逊效应探测β粒子的初步实验结果

利用超导结的直流约瑟夫逊效应探测粒子,具有能量分辨率高和时间响应快的优点。我们首次进行了用Sr⁹⁰-Y⁹⁰β源辐照Nb-NbO_x-Pb超导结的实验。结果表明:在β粒子的辐照下,不仅超导结的临界电流和能隙电压都变小,而且在超导结的两端产生电压脉冲讯号。本文报道了利用直流约瑟夫逊效应探测β粒子所观察到的一些现象。 关键词：     

Surface Acoustoelectric Waves in a Piezoelectric–High-Temperature Granular Superconductor Layered Structure

Interactions of surface acoustoelectric waves with a granular high-temperature superconducting medium are studied. Dispersion equations describing the characteristics of surface acoustoelectric waves are derived for piezoelectrics of 4 and 6mm and 3m symmetry. It is shown that at a temperature above the critical point an attenuation jump and a sudden change in the phase velocity of surface acoustoelectric waves are observed. This effect increases with increase in the electromechanical coupling coefficient and decrease in the thickness of the high-temperature superconducting film. The results obtained can be used in designing frequency selectors and transient photodetectors.     

High-Temperature Superconducting Microstrip Transmission Lines

A model calculation of a two-wire Josephson transmission line with a quasi-transverse electromagnetic wave is performed. The dispersion characteristics of the wave are estimated. The group velocity is shown to be (4.1–2.5)·10⁷ m/s for a temperature of 76–85.9 K, a critical current of 10⁹ A/m², a dielectric plate thickness of 10 m, and a relative permittivity of 40. The wave attenuation in this temperature range is 2 db/dm at a frequency of 10 GHz. The rough estimates suggest the feasibility of designing microstrip transmission lines based on granular high-temperature superconducting films. These lines will provide delays up to 1 s and picosecond pulse transmission.     

THE ELECTRON-BEAM IRRADIATION ON THE HIGH TEMPERATURE SUPERCONDUCTING THIN FILMS: A NUMERICAL SIMULATION OF THE LOW TEMPERATURE SCANNING ELECTRON MICROSCOPY

Low-temperature scanning electron microscopy (LTSEM) is a promising measuring technique for probing the spatial distribution of the superconducting properties of the high-temperature superconducting (HTS) thin films. A theoretical analysis of the electron-beam irradiation on the HTS thin films in the LTSEM has been carried out. An inhomogeneously distributed grain array model has been applied in the analysis, and some numerical sim-ulations have been carried out on the electron-beam induced voltage (EIV) signals in the LTSEM experiments. The comparisons of our numerical results with the LTSEM experi-mental data indicate that it is quite reasonable to use a two-dimensional Josephson junction array for stimulating the inhomogeneous HTS thin film sample. Our numerical results also show that the EIV signals are influenced by the electron-beam power used in the LTSEM, and a reduction of the electron-beam power is suggested in order to eliminate the errors in estimating the local values of critical temperature T_c and critical current I_c by the sample temperature and the bias current at which the first EIV signal occurs.     

Mechanisms of dissipation in a Josephson medium based on a high-temperature superconductor in a magnetic field

This paper reports on the results of an investigation into the influence of magnetic fields (0–60 kOe) on the temperature dependences of the electrical resistance R(T) of the Y_3/4Lu_1/4Ba₂Cu₃O₇ + CuO composites. The structure of these composites is considered to be a network of tunnel-type Josephson junctions in which a nonsuperconducting component (CuO) forms boundaries (barriers) between high-temperature superconducting crystallites. The temperature dependence R(T) of the composites has two steps characteristic of granular superconductors: (i) an abrupt change in the electrical resistance at the critical temperature of high-temperature superconducting crystallites and (ii) a smooth transition to the superconducting state under the influence of the boundaries between the crystallites. The experimental dependences R(T) are analyzed within the Ambegaokar-Halperin model of thermal fluctuations in Josephson junctions and the flux creep model. An increase in the magnetic field leads to a crossover from the Ambegaokar-Halperin mechanism to the flux creep mechanism. The temperature dependences R(T) in the range of weak magnetic fields (from 0 to 10² Oe) are adequately described by the relationship following from the Ambegaokar-Halperin model. In the range of strong magnetic fields (from 10³ to 6 × 10⁴ Oe), the dissipation obeys the Arrhenius law R ～ exp(?U(H)/T)], which is characteristic of the flux creep model with a temperature-independent pinning energy U(H). The effective Josephson coupling energies and the pinning energies corresponding to the Ambegaokar-Halperin and flux creep mechanisms are determined.     

A new model analysis of the third harmonic voltage in inductive measurement for critical current density of superconducting films

The critical current density J c is one of the most important parameters of high temperature superconducting films in superconducting applications,such as superconducting filter and superconducting Josephson devices.This paper presents a new model to describe inhomogeneous current distribution throughout the thickness of superconducting films applying magnetic field by solving the differential equation derived from Maxwell equation and the second London equation.Using this model,it accurately calculates the inductive third-harmonic voltage when the film applying magnetic field with the inductive measurement for J c.The theoretic curve is consistent with the experimental results about measuring superconducting film,especially when the third-harmonic voltage just exceeds zero.The J c value of superconducting films determined by the inductive method is also compared with results measured by four-probe transport method.The agreements between inductive method and transport method are very good.     

Dominant influence of the compression effect of a magnetic flux in the intergranular medium of a granular high-temperature superconductor on dissipation processes in an external magnetic field

Experiments have been presented that demonstrate the effect of the compression of a magnetic flux in grain boundaries of a granular high-temperature superconductor in an external magnetic field on the dissipation processes. The compression of the magnetic flux is associated with the diamagnetic behavior of superconducting grains and the existence of a Josephson medium in grain boundaries. Under these conditions, grain boundaries are in an effective magnetic field that depends on the magnetic state (magnetization) of the superconducting grains. Based on the analysis of experimental data (dependences of the electrical resistance R and magnetization on the magnetic field H and temperature T, as well as current-voltage characteristics), the conclusion has been drawn that it is the temperature evolution of the effective magnetic field in the intergranular medium which primarily determines the behavior of the dependences R(T) in weak external magnetic fields of no more than ～10³ Oe. This should be taken into account in the interpretation of experiments on the magnetoresistance effect in granular high-temperature superconductors in terms of different theories. The conclusion drawn here also implies a significant correction of the previously obtained results.     

高温超导薄膜临界电流密度的无损测量研究

临界电流密度Jc是超导薄膜的一个重要参量,它可以衡量超导薄膜的功率承载能力。大面积高温超导薄膜制成后,其Jc需要被无损精确测量。文中提出了一种新的交流磁场下的高温超导薄膜临界态模型:(1)基于此模型对薄膜的临界电流密度进行了精确无损测量;(2)并将实验测量的三次谐波电压曲线进行拟合研究。首先,根据麦克斯韦方程和伦敦方程,计算外加直流磁场超导薄膜Meissner态下电流和磁场在薄膜内的分布;然后分析薄膜进入临界态后内部电流的变化,在考虑顶扎力作用的情况下,提出了临界态电流和磁场非均匀分布模型;最后根据其模型,推导出三次谐波电压的表达式。为了验证该理论,分别对四片超导薄膜在不同频率下进行了三次谐波和临界电流密度测量。实验结果表明:三次谐波电压的理论与实验曲线一致;与四点传输法的测量结果相比较,该方法测量超导薄膜临界电流密度的误差在5%左右,具有高精度、无损伤、方便快捷等优点。     

Dynamical properties of superconducting nanowires

The dynamical properties of thin superconducting wires (nanowires) are studied using numerical simulations based on a one-dimensional time-dependent Ginzburg-Landau equation, which is modified by introducing an order parameter u characterizing the “purity” of the superconductor material. It is established that relatively long nanowires (with lengths much greater than the coherence length) made of a “pure” superconductor (u > 1) are characterized by two critical current density values: j _c1 and j _c2. For j < j _c1, the total current is entirely superconducting, whereas for j > j _c2, the current is purely normal. In the intermediate region of current densities, j _c1 < j < j _c2, the total current contains both superconducting and normal components (mixed state) and the nanowire exhibits the generation of high-frequency electromagnetic waves. The current-voltage characteristics are constructed and the radiation spectrum is obtained. The properties of short superconducting nanowires (with lengths on the order of the coherence length) coincide with those of the Josephson junction. In the case of an “impure” superconductor (u < 1), the nanowire is characterized by a single critical current density.     

Dependence of the maximal superconducting current on the resonance frequency in a shunted Josephson junction

We have analyzed the phase dynamics and current–voltage characteristics of a Josephson junction shunted by an LC circuit. When the Josephson frequency ω _J becomes equal to the natural frequency ω_rc of the formed resonance circuit, the I–V curve acquires additional branches. We have studied the features of the rc branch and the superconducting circuit for different values of the resonance frequency. It is shown that the maximal superconducting current through the Josephson junction on the rc-branch depends on the resonance frequency and is determined by the closeness of the end point of the rc branch to the critical current. We have determined the dependence of the maximal superconducting current on the resonance frequency for different values of the dissipation parameters. The limiting value of the maximal superconducting current is independent (to within 1%) of the parameters of the system.     

Magnetically dependent superconducting transport in oxide heterostructures with an antiferromagnetic layer

The superconducting current in hybrid superconducting structures Nb/Au/Ca_1?x Sr _x CuO₂/YBa₂Cu₃O_7?δ with an antiferromagnetic layer is experimentally shown to have a Josephson nature, and the deviation from the sinusoidal dependence of the superconducting current on the phase difference between superconducting electrodes is about 20% of the second harmonic. These heterostructures are found to have sensitivity to an applied magnetic field that is much higher than that of conventional Josephson junctions. The experimental shape of the magnetic-field dependence of the critical current in the heterostructures differs from the usual Fraunhofer shape by oscillation with a significantly smaller period along a magnetic field.     

A model for layered high-temperature superconductors with two CuO2 layers per unit cell

We extend a model for layered high-temperature superconductors to systems with two CuO₂ layers per unit cell and two interlayer spacings with different physical properties. The carriers are assumed to occupy Fermi liquid states, forming narrow tight-binding bands. The layers are coupled by weak interlayer-hopping matrix elements between adjacent sheets, as well as by an attractive interaction between carriers in neighboring layers in addition to an on-site intralayer coupling. We solve the Gorkov equations for this model to obtain the critical temperature and the density of states of the oneparticle excitations from the superconducting condensate, and discuss various parameter regimes concerning the coupling between the two layers. We compare our results with current experimental findings for high-temperature superconductors. The presence of two CuO₂ layers leads to multi-peak features in the superconducting density of states, as has been observed in recent tunneling measurements.     

Critical temperature and critical current density of superconducting V-Ga-Al alloys with A 15 structure

V-Ga-Al alloys containing 68–82 at.-% vanadium can be easily worked mechanically in the A2 phase and transformed into the superconducting A15 phase by subsequent heat treatment. The preparation, critical temperature and critical current density of the A15 phase are described. The critical current density, which at 5 Tesla is in the 10⁴ to 10⁵ A/cm² range, is compared with values of technically used superconductors. Ternary V-Ga-Al alloys are suitable for superconducting shielding devices for high fields.     

Magnetoresistance of superconducting grains in ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics $E(j)_{H_{treat} } = const$ of ceramic (granular) YBa₂Cu₃O_6.95 samples preliminarily magnetized in different transverse magnetic fields H _treat have been measured in a zero field (H _ext = 0) at T = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0 ≤ H _trap ≤ H _c2J (where H _trap is the magnetic field trapped as a result of the pretreatment in H _treat and H _c2J is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance ρ_A(H _treat) _{j = const} of superconducting grains. It has been established that the magnetoresistance ρ_A of the superconducting grains is significantly lower than the magnetoresistance ρ_J for the Josephson medium. The dependence of ρ_A on H _treat and on the transport current density j has been investigated. It has been shown that the dependences ρ_A(H _treat) _{j = const} exhibit a clearly pronounced tendency to saturation, ρ_satur, and the value of ρ_satur increases with increasing j. It has been found that the lower critical field H _c1A of the superconducting grains strongly depends on the transport current density.     

On the use of STM superconducting tips at very low temperatures

We report on high quality local tunnel spectroscopy measurements in superconductors using in-situ fabricated superconducting tips as counterelectrode. The experiments were made at very low temperatures using a dilution refrigerator and a ³He cryostat. Spectra obtained with superconducting tip and sample of Al show that the spectroscopic resolution of our set-up is of 15 eV. Following the observation of Josephson current in tunnelling regime (with tips of Pb and of Al), we discuss the feasibility of Scanning Josephson Spectroscopy with atomic size resolution. Experiments showing new applications of these superconducting tips under applied external magnetic fields are also reported.Received: 12 May 2004, Published online: 7 September 2004PACS: 73.63.Rt Nanoscale contacts - 74.25.Fy Transport properties (electric and thermal conductivity, thermoelectric effects, etc.) - 74.25.Ha Magnetic properties - 74.50. + r Tunnelling phenomena; point contacts, weak links, Josephson effects - 74.78.Na Mesoscopic and nanoscale systems - 74.80.Fp Point contacts; SN and SNS junctions     

Josephson effects in long superconducting bridges with variable thickness

We have studied the properties of long (L≈20μm) superconducting threads (Pb or Sn) formed by pulse electrical breakdown in monocrystalline silicon. These long bridges with variable thickness have well defined d.c. and a.c. Josephson effects in large temperature range (T/T_c?0.2 and 0.4 for Pb and Sn, respectively) and high magnetic fields (up to 0.14T for Pb at 4.2 K). In the whole temperature range, the critical current has linear temperature dependence, as for “ideal” Josephson junctions. The results are interpreted by coherent motion of vortices. Some interesting results about satellites of quasiparticle gap structures and the asymmetry of these structures with respect to the current direction are presented.