Modulation instability of electromagnetic excitations in nonlocal Josephson electrodynamics of thin films of nonmagnetic and magnetic (two-and three-dimensional) superconductors

Modulation instability of nonlinear electromagnetic excitations (oscillating with the Josephson frequency) of finite amplitude is investigated in a Josephson junction in a film of a nonmagnetic, as well as of a magnetic (two-or three-dimensional), superconductor. The instability is accompanied by a nonlinear shift in frequency. Dispersion relations are derived for the time increment of small perturbations of the amplitude. It is shown that, for this type of excitations in a Josephson junction in a thin film of nonmagnetic superconductor, modulation instability develops only in a certain finite range of wave vectors, whereas in a thin film of a two-or three-dimensional magnetic superconductor it develops for any wave vector.     

嵌入Fabry-Perot谐振腔的高温超导双晶约瑟夫森结的毫米波辐照特性研究

采用嵌入Fabry-Perot谐振腔的方式,研究了Tl2Ba2CaCu2O8(Tl-2212)高温超导薄膜双晶约瑟夫森结的毫米波辐照特性.重点研究了双晶约瑟夫森结与Fabry-Perot谐振腔的耦合特性,发现放置在谐振腔中的结的位置、角度、结的图形以及Fabry-Perot谐振腔中两镜面间的距离等对耦合都有很大的影响.通过精细调节这些参数,可使双晶约瑟夫森结与外加毫米波达到最佳耦合.在最佳耦合情况下,能观察到9级明显的夏皮罗台阶.利用Fabry-Perot谐振腔技术,解决了约瑟夫森结与毫米波的耦合问题,为高温超导约瑟夫森结的毫米波及太赫兹波的辐射和检测研究奠定了基础.     

硼膜制备工艺、微观结构及其在硼化镁超导约瑟夫森结中的应用

MgB_2材料具备临界转变温度较高、相干长度大、临界电流和临界磁场高等优点,被认为有替代Nb基超导材料的潜力.研究了不同温度下以化学气相沉积法制备的硼(B)薄膜的微观结构.实验结果表明:较低温度沉积的B先驱薄膜为无定形B膜,可以与Mg蒸气反应生成MgB_2超导薄膜;当沉积温度高于550?C时,所得硼薄膜为晶型薄膜;以晶型硼薄膜为先驱膜在镁蒸气中退火,不能生成硼化镁超导薄膜.利用晶型B膜的这一特点,成功制备了以晶型硼薄膜为介质层的硼化镁超导约瑟夫森结.     

Nonlinear response of a thin metamaterial film containing Josephson junctions

An interaction of electromagnetic field with metamaterial thin film containing split-ring resonators with Josephson junctions is considered. It is shown that dynamical self-inductance in a split-rings results in reduction of magnetic flux through a ring and this reduction is proportional to a time derivative of split-ring magnetization. Evolution of thin film magnetization taking into account dynamical self-inductance is studied. New mechanism for excitation of waves in one dimensional array of split-ring resonators with Josephson junctions is proposed. Nonlinear magnetic susceptibility of such thin films is obtained in the weak amplitude approximation.     

Observation of latent coherent effects in randomized systems

Coherent interference effects of the following three types are experimentally discovered in disordered (randomized) systems: (i) Josephson behavior of the HTSC polycrystal BaKBiO in the phase-separated state; (ii) oscillations of bismuth film resistance, which are periodic in “direct” magnetic field; and (iii) mesoscopic oscillations of the resistance in the course of film growth. In the first case, the method for detecting the “latent” nonstationary Josephson effect is substantiated by the frequency modulation method for microwave radiation, while in the other two cases, simple models are proposed to explain the nature of coherent oscillations of the resistance. The analogy between the observed oscillations and the Josephson effect in randomized systems is discussed.     

YBCO/Nb bulk junctions: Observation of the Josephson effect

Summary The fabrication procedure of reliable YBCO-based bulk junctions with a Nb film as a counterelectrode is described. Typical currentvoltage characteristics are presented, clearly showing the presence of the Josephson effect.     

Experimental evidence for the Coulomb blockade of Cooper pair tunneling and Bloch oscillations in single Josephson junctions

We report on measurements of the current-voltage characteristics of ultrasmall Josephson junctions. The junctions were made of either Al or Pb alloy, and the leads connecting the junctions to the outside world were high resistance thin film microstrips fabricated on the chip very close to the junction. The high frequency impedance of these leads was sufficiently large to enable the observation in a single Josephson junction of the Coulomb blockade of Cooper pair tunneling, as well as evidence for the time correlation of Cooper pair tunneling (Bloch Oscillations).     

Fluctuation dominated Josephson tunneling with a scanning tunneling microscope

We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 k Omega. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltage. In the presence of microwave fields ( f = 15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory.     

用于电压基准的Nb/NbxSi1-x/Nb约瑟夫森单结的研制

现代可编程约瑟夫森电压基准的核心器件是约瑟夫森结阵.目前最具有优势的约瑟夫森结阵是 Nb/Nb_xSi_1-x/Nb材料的结阵. Nb/Nb_xSi_1-x/Nb材料的约瑟夫森结 具有三层薄膜的制作过程简便, Nb和Nb_xSi_1-x刻蚀工艺相同以及Nb_xSi_1-x 势垒层成分可调等优点.中国计量科学研究院设计制作了Nb/Nb_xSi_1-x/Nb约瑟夫森单结. 通过在4.2 K低温下对所做单结进行直流电流-电压特性测量,观测到了清晰的超导隧穿电流和 从零电压态向电压态的跳变,最后就测量结果进行了分析讨论.此项工作属于国内首个开展 Nb/Nb_xSi_1-x/Nb材料约瑟夫森单结研究的工作.     

Intrinsic Josephson tunneling for basic studies of high-temperature superconductors

High-temperature superconductors (HTS) are highly anisotropic with layers of CuO₂ separated by layers of charge reservoirs. The superconducting CuO₂ planes are weakly coupled by Josephson tunneling of Cooper pairs in the c-axis direction. It may be possible to use the intrinsic tunneling to realize a stack of intrinsic Josephson junctions at the surface of a single crystal (or epitaxial film) of HTS. These will form excellent tools in the study of superconducting properties of HTS.     

纳米金属掩膜和离子辐照技术制备高温超导约瑟夫森结

研究了一种利用纳米金属掩膜和离子辐照技术在高温超导YBCO薄膜上制备Josephson结的方法.首先用在YBCO薄膜甩上一层800nm左右的光刻胶(PMMA),继而在光刻胶上用直流磁控溅射的方法镀上一层大约300nm左右的Cr膜,利用紫外曝光和离子刻蚀的方法在YBCO薄膜上形成覆盖有Cr膜的微桥,然后,利用聚焦离子束系统(FIB)在微桥上刻出一个50nm左右的狭缝,最后利用120keV的H2 对狭缝内的材料进行辐照,从而使狭缝部分的材料超导电性减弱,形成类似SNS型的Josephson结.     

高温超导约瑟夫森结阵列的相位锁定

根据交流约瑟夫森效应,研究了嵌入Fabry-Perot谐振腔里的约瑟夫森结阵列在微波辐照下的相位锁定问题。结阵列是由生长在双晶钇稳定氧化锆(YSZ)基片上的YB2Cu3O7(YBCO)超导薄膜光刻成微桥得到的。通过优化设计,在温度为79.2K辐照频率为77.465GHz的条件下,包含620个串联约瑟夫森双晶结的结阵列在外加微波辐照下实现了相位锁定,得到了陡峭的夏皮罗台阶。其第一级夏皮罗台阶的电压约为0.1V,台阶高度约为0.17mA。试验结果表明,这种结阵列结构在电压基准的应用上有极大的优势,而且这种准光学耦合方法在太赫兹信号发生和检测方面有很好的应用前景。     

Nonlocal Josephson electrodynamics of a magnetic (two-or three-dimensional) superconductive thin film

For the Josephson junction in a two-or three-dimensional magnetic superconductive thin film, an integro-differential equation of phase-difference dynamics, including time and space nonlocality, is deduced. It is shown that the magnetic subsystem induces the substantial renormalization of the spectrum of small-amplitude electromagnetic excitations and causes them to decay.     

Enhancement of the Josephson current by an exchange field in superconductor-ferromagnet structures

We calculate the dc Josephson current for two superconductor/ferromagnet (S/F) bilayers separated by a thin insulating film. It is demonstrated that the critical Josephson current I(c) in the junction strongly depends on the relative orientation of the effective exchange field h of the bilayers. We found that in the case of an antiparallel orientation I(c) increases at low temperatures with increasing h and at zero temperature has a singularity when h equals the superconducting gap Delta. This striking behavior contrasts with the suppression of the critical current by the magnetic moments aligned in parallel and is an interesting new effect of the interplay between superconductors and ferromagnets.     

Resistively shunted Josephson tunnel junctions

SIS tunnel junctions have externally been shunted by a thin metallic film. It is shown that such devices exhibit radiation induced steps as point contacts or microbridges. This result agrees qualitatively with the commonly used lumped circuit model of Josephson junctions.     

Nb/AlOx-Al/Nb超导隧道结制备氧化工艺与特性研究

提出一种氧的等离子氧化的方法改善结区边缘绝缘性能,降低超导隧道结的漏电流。对Al膜进行等离子氧化能够有效的改善氧化膜的绝缘性能,AES分析表明:氧化绝缘层均匀,界面清晰;应用此方法成功制备出较好性能的Nb隧道结。     

Nonlocal Josephson electrodynamics of electromagnetic excitations in a superconducting magnetic (three-dimensional) thin film

The integro-differential equation for the dynamics of the phase difference with temporal and spatial nonlocality for a Josephson junction in a superconducting three-dimensional magnetic thin film is derived. The equation is valid for any type of magnetic ordering. The magnetic subsystem is shown to significantly renormalize the spectrum of small-amplitude electromagnetic excitations, which, as a result, are damped.     

Preliminary results on tunnel junctions on YBCO bulk with an artificial barrier

YBCO-bulk based tunnel junctions employing a cadmium sulphide artificial barrier have been fabricated. Preliminary results on the role played by the CdS film thickness and different YBCO surface processing are discussed. dc Josephson effect has been detected in samples with a low tunneling resistance. The dependence of the critical current on the external magnetic field is reported.     

The Electrodynamics of Granular High-Temperature Superconducting Media

The electrodynamic properties of granular high-temperature superconducting media are studied. Relations for the surface resistance and impedance of a high-temperature superconducting medium are derived. The temperature and frequency dependence of the phase velocity and group velocity and depth of penetration of the electromagnetic field into the Josephson medium are calculated. The possibility of using high-temperature superconducting films as high-efficiency shields is shown. The shielding properties are improved appreciably with increase in the critical current density of the high-temperature superconducting film. The shield thickness is several microns or several fractions of a micron for a critical current density of >10⁷ A/m². The results obtained can be used in designing superhigh-frequency shields, microstrip lines, and other devices based on high-temperature superconducting Josephson media.     

Energy gap suppression and excess current in Tl_2Ba_2CaCu_2O_8 intrinsic Josephson junctions

Intrinsic Josephson junctions in misaligned Tl2Ba2CaCu2O8 thin film were fabricated on LaAlO3 substrate. The temperature dependence of the critical current is investigated around liquid nitrogen temperature. In the current voltage characteristic, large voltage jump and lack of resistive branch are observed, which shows good consistency with the intrinsic Josephson junctions. By analyzing the large gap voltage in the curve, great suppression of the energy gap is found. Through discussing the temperature dependence of the gap voltage in liquid nitrogen temperature, it is shown that this phenomenon can be caused by the non-equilibrium quasiparticle injection. The temperature influence on the excess current also confirms the non-equilibrium effect.