含有两个约瑟夫森结的射频SQUID的本征磁通噪声

我们研究了双结射频超导量子干涉器件（rf SQUID）的含时特性，给出了总磁通的动力学方程，并分析了在回滞模式和非回滞模式下的本征磁通噪声。结果显示：两个结的临界电流差别越大则越有助于获得小的本征噪声，在这种情况下，双结rf SQUID的本征磁通噪声会比单结rf SQUID的噪声稍大，但不超过一个数量级。     

Peculiarities of rf SQUID response in finite magnetic fields

Single-layer washer-type high-T_c YBa₂Cu₃O_7−x rf SQUIDs with grain-boundary Josephson junctions, as well as low-T_c Nb rf SQUIDs with Nb–Al₂O₃–Nb tunnel junctions, have been investigated in finite magnetic fields. It was shown experimentally that the suppression of the critical current of the Josephson junction due to the magnetic field leads to a modulation of the amplitude of the SQUID output signal. The role of the “unwanted” junction in high-T_c rf SQUIDs, which is formed by the grain boundary running through the washer of the SQUIDs on bicrystal substrates, has also been clarified. The drop of the SQUID signal at a finite magnetic field is originated by the penetration of the magnetic field into the unwanted junction. Based on these results, a direct radio-frequency method for the determination of the first critical field H_c1 for long Josephson junctions has been developed.     

在一个磁通量子内dc Josephson电流的阶梯效应(Ⅰ)——双结SQUID

如果将由两个相同的Josephson结组成的双结SQUID放置于Q值足够高的谐振腔内,当其Josephson频率ω=2eV₀/h与谐振腔的本征频率ω_r发生谐振时,腔内就被激起一个驻波电磁场,这个场对两个结的反馈作用,将导致双结SQUID的dc Josephson电流在一个磁通量子内随磁场产生多次阶梯效应。理论给出两个结的Josephson电流产生一系列新的干涉作用:如果两个结分别位于反馈场的波峰,则阶跃电流加强;如果其中一个位于波谷,则使SQUID的干涉图形改变π/2位相,且两个结电流产生相干性减小;如果一个在波峰一个在波节,则SQUID退化到单结;如果两个结都在波节,则不出现n≠0的阶梯。 关键词：     

MgO衬底上YBa2Cu3O7-δ台阶边沿型约瑟夫森结的制备及特性

MgO衬底上的YBa2Cu3O7-δ(YBCO)台阶边沿型约瑟夫森结(台阶结)在高灵敏度高温超导量子干涉器(superconducting quantum interference device,SQUID)等超导器件研制方面具有重要的应用价值和前景.本文对此类YBCO台阶结的制备和特性进行了研究.首先利用离子束刻蚀技术和两步刻蚀法在MgO(100)衬底上制备陡度合适、边沿整齐的台阶,然后利用脉冲激光沉积法在衬底上生长YBCO超导薄膜,进而利用紫外光刻制备出YBCO台阶结.在结样品的电阻-温度转变曲线中,观测到低于超导转变温度时的电阻拖尾现象,与约瑟夫森结的热激活相位滑移理论一致.伏安特性曲线测量表明结的行为符合电阻分路结模型,在超导转变温度TC附近结的约瑟夫森临界电流密度TC随温度T呈现出(TC-T)^2的变化规律,77 K时JC值为1.4×10^5 A/cm^2.利用制备的台阶结,初步制备了YBCO射频高温超导SQUID,器件测试观察到良好的三角波电压调制曲线,温度77 K、频率1 kHz时的磁通噪声为250μΦ0/Hz^1/2.本文结果为进一步利用MgO衬底YBCO台阶结研制高性能的高温超导SQUID等超导器件奠定了基础.     

Magnetic field rectifiers using semicircular Josephson junctions

A novel method for rectifying alternating magnetic fields is demonstrated using fluxons in semicircular Josephson junctions. An external magnetic field applied parallel to the dielectric barrier of the semicircular junction has opposite polarities at the ends of the junction and supports penetration of opposite polarity fluxons into the junction in the presence of a constant dc bias. When the direction of the field is reversed, flux penetration is not possible and a flux-free state exists in the junction. Thus, effective rectification of an alternating magnetic field can be achieved in semicircular Josephson junctions. This unique phenomenon is specific to this geometry and can be employed in rf SQUID magnetometers.     

Aharonov-Casher-effect suppression of macroscopic tunneling of magnetic flux

We suggest a system in which the amplitude of macroscopic flux tunneling can be modulated via the Aharonov-Casher effect. The system is an rf SQUID with the Josephson junction replaced by a Bloch transistor--two junctions separated by a small superconducting island on which the charge can be induced by an external gate voltage. When the Josephson coupling energies of the junctions are equal and the induced charge is q = e, destructive interference between tunneling paths brings the flux tunneling rate to zero. The device may also be useful as a qubit for quantum computation.     

Josephson effect on YBCO break junctions

We have investigated dc and ac Josephson effects on YBa₂Cu₃O_7–x ceramic samples using the break junction technique. The critical current was measured as a function of external magnetic field and temperature. The field dependence shows superposition effects of junction and SQUID behavior. The critical current depends linearly on temperature like for SNS junctions. We observed Shapiro steps with microwave radiation with a frequency of 35 GHz at 4.2 K and 77 K, respectively. Deviations of the step height from Bessel behavior at low microwave power are explained within the Stewart-McCumber model using small capacitances of the junction in simulation calculations.     

Mesoscopic SNS junctions and their application to SQUIDs and millimeter-wave detectors

Mesoscopic SNS junctions have been studied both in the ballistic and diffusive regimes. SNS junctions in the ballistic regime behave as an ideal Fermion oscillator which is to be compared with the Boson oscillator or the Planck theory of blackbody radiation. The current of mesoscopic SNS junctions in the diffusive regime has the same phase dependence as that of dirty-limit short weak links derived by a transport equation. Recent theories of mesoscopic SNS junctions have successfully unified the theories of the tunnel Josephson junction, the clean-limit short weak link and the dirty-limit short weak link which look very different conceptionally. We can even observe transitions among the three types of junctions when we change the transmission coefficients of the barriers between the superconducting electrodes experimentally. We looked experimentally for the optimum transmission coefficient which gives the minimum low-frequency telegraph noise in order to make a low-noise SQUID magnetometor for brain science. We have observed signals of 5 fT from human brains with a good signal-to-noise ratio using the SQUID magnetometor of the SNS junctions. The 64-channel SQUID magnetometer of SNS junctions has confirmed that mesoscopic SNS junctions are important not only theoretically but also practically. These data could encourage people studying SNS junctions of high-T_c superconductors.     

Equivalent parameters of a Josephson junction in a microwave SQUID structure

The equivalent parameters of a Josephson junction in a microwave SQUID structure are calculated on the basis of relations obtained as a result of an analysis of the operation of an rf SQUID. This analysis is based on the sawtooth variation of the voltage on the resonator as a function of the constant flux bias. The quantitative characteristics permit regarding the Josephson junction as a linear impedance in the rf or microwave circuit, whose real and imaginary parts are controlled by the constant magnetic flux passing through the SQUID loop. Zh. Tekh. Fiz. 69, 106–112 (November 1999)     

在一个磁通量子内dc Josephson电流阶梯或振荡效应的物理本质

本文分析了放置于高Q谐振腔中的Josephson结、双结SQUID和单结超导环,当其Josephson频率ω=2eV₀/h和腔的本征频率相等时,Josephson电流在一个磁通量子内可以产生多次阶梯或振荡的物理本质。指出对于Josephson结或双结SQUID,当ω=ω_r时,被激发起的腔的本征振荡将反馈辐照结或SQUID,反馈场的频率作用使Josephson电流产生一般地感应台阶;而反馈场的振幅是受到外磁场调制的,当这种调制作用使反馈场振幅的空间部分与辐射场的空间部分匹配时,则引起电流在一个磁通量子内产生一系列的新的阶跃。对单结超导环,当nω=ω_r时,被激发起的腔的振荡反馈辐照作用将迫使其超流电流产生周期小于φ₀的振荡。 关键词：     

Modulation depth of series SQUIDs modified by Josephson junction area

The superconducting quantum interference device(SQUID) amplifier is widely used in the field of weak signal detection for its low input impedance, low noise, and low power consumption. In this paper, the SQUIDs with identical junctions and the series SQUIDs with different junctions were successfully fabricated. The Nb/Al-AlO_x/Nb trilayer and input Nb coils were prepared by asputtering equipment. The SQUID devices were prepared by a sputtering and the lift-off method.Investigations by AFM, OM and SEM revealed the morphology and roughness of the Nb films and Nb/Al-AlO_x/Nb trilayer.In addition, the current–voltage characteristics of the SQUID devices with identical junction and different junction areas were measured at 2.5 K in the He~3 refrigerator. The results show that the SQUID modulation depth is obviously affected by the junction area. The modulation depth obviously increases with the increase of the junction area in a certain range. It is found that the series SQUID with identical junction area has a transimpedance gain of 58 ? approximately.     

Coulomb blockade and coherent single-cooper-pair tunneling in single Josephson junctions

We have measured the current-voltage characteristics of small-capacitance single Josephson junctions at low temperatures ( T< or =0.04 K), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc SQUIDs. We have clearly observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, when the zero-bias resistance of the SQUID arrays is much higher than the quantum resistance h/e(2) approximately 26 kOmega. The negative differential resistance is evidence of coherent single-Cooper-pair tunneling in the single Josephson junction.     

Submm heterodyne mixing using NbCN/Nb sis tunnel junctions

We describe heterodyne mixing experiments with NbCN/Nb quasi-particle tunnel junctions at submillimeter wavelengths. In this wavelength range junctions with niobium nitride as superconducting material are promising because of the high gap voltage, about 5.7 mV, as compared to 3 mV for the more commonly used niobium. As a first step towards all-NbN junctions we investigate the development of junctions with a NbN base electrode and a Nb upper electrode. We present results from samples with two different insulating barriers: aluminum oxide and magnesium oxide. Measured noise temperatures range from 500 K at 345 GHz and 1600 K at 482 GHz to 8500 K at 644 GHz. These results are about one order of magnitude worse than the best results obtained with all-Nb junctions. The difference can be attributed partly to the relatively high radio-frequency (rf) losses in NbN films as compared to Nb. Also sensitivity is reduced because of the relatively high leakage current in the sub-gap region of the I-V curve of the junction.     

Development of 1.3GHz high-T-c rf SQUID

A new high-T_c (HT_c) rf SQUID working at around 1.3GHz has been developed to avoid electromagnetic interference such as growing mobile communication jamming. This new system works in a frequency range from 1.23 to 1.42GHz (centred at 1.3GHz), which is not occupied by commercial communication. The sensor used in the 1.3GHz rf SQUID is made of a HT_c coplanar superconducting resonator and a large-area HT_c superconducting film concentrator. We have achieved in the 1.3GHz HT_c rf SQUID system a minimal flux noise of 2.5×10^{-5}Φ_0/\sqrt{Hz} and a magnetic field sensitivity of 38fT/\sqrt{Hz} in white noise range, respectively. The effective area of the concentrator fabricated on a 15×15mm^2 substrate is 1.35mm^2. It is shown that the 1.3GHz rf SQUID system has a high field sensitivity. Design and implementation of 1.3GHz HT_c rf SQUID offers a promising direction of rf SQUID development for higher working frequency ranges.     

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.     

Development of 1.3GHz high-T-c rf SQUID

A new high-T_c (HT_c) rf SQUID working at around 1.3GHz has been developed to avoid electromagnetic interference such as growing mobile communication jamming. This new system works in a frequency range from 1.23 to 1.42GHz (centred at 1.3GHz), which is not occupied by commercial communication. The sensor used in the 1.3GHz rf SQUID is made of a HT_c coplanar superconducting resonator and a large-area HT_c superconducting film concentrator. We have achieved in the 1.3GHz HT_c rf SQUID system a minimal flux noise of 2.5×10^{-5}Φ_0/\sqrt{Hz} and a magnetic field sensitivity of 38fT/\sqrt{Hz} in white noise range, respectively. The effective area of the concentrator fabricated on a 15×15mm^2 substrate is 1.35mm^2. It is shown that the 1.3GHz rf SQUID system has a high field sensitivity. Design and implementation of 1.3GHz HT_c rf SQUID offers a promising direction of rf SQUID development for higher working frequency ranges.     

Macroscopic quantum interference effects in superconducting multiterminal microstructures

We review the macroscopic quantum phenomena in superconducting microstructures based on multiterminal junctions. The multiterminal Josephson junction presents a system in which the weak coupling takes place between several massive superconducting banks (terminals). Compared with the conventional (two-terminal) junctions such systems have additional degrees of freedom and a corresponding set of control parameters, preset transport currents and (or) applied magnetic fluxes. The general phenomenological theory of multiterminal Josephson junctions is presented. The specific multichannel interference effects (studied theoretically and experimentally) are described for two microstructures: the four-terminal SQUID and a system consisting of two weakly coupled superconducting rings.     

具有Washer型输入线圈的超导量子干涉放大器的制备与表征

超导量子干涉仪(SQUID)放大器具有低输入阻抗、低噪声、低功耗等优点, 目前被广泛用于微弱信号的检测领域. 与其他工艺相比, Nb/Al-AlO_x/Nb结构的约瑟夫森结具有相对较高的转变温度(T_c)、高的磁通电压调制系数以及良好的热循环能力、较宽的临界电流范围, 因此是制备SQUID放大器的很好选择. 设计并制作了欠阻尼、过阻尼约瑟夫森结以及具有Washer型输入线圈的单SQUID放大器, 通过在He³制冷机3 K温区下对器件电流-电压特性进行测量, 得到良好的结I-V特性曲线、SQUID调制特性, 初步实现利用SQUID进行放大作用, 并计算了SQUID的电流分辨率. 此项工作对于超导转变边沿传感器读出电路的实现具有重要的意义.     

手持移动型高温超导SQUID在无损检测中的应用

在SQUID的无损检测应用中,如果被测样品非常巨大,不能移动样品,必须来移动SQUID进行扫描。而移动SQUID最方便最直接的方式是用手移动或者用机械臂移动,这就要求SQUID能在无屏蔽环境下非常稳定地工作,地磁场的影响要补偿掉,在移动中手的抖动产生的噪声也要消除。成功地解决了这些问题,用手持移动型高温超导SQUID在无屏蔽环境下检测到了6 mm铝板下的孔缺陷信号。报道了在这方面的初步研究结果。     

Simulation of dielectric resonator for high-Tc radio frequency superconducting quantum interference device

Nowadays, the high-critical-temperature radio frequency superconducting quantum interference device (high-$T_{\rm c }$ rf SQUID) is usually coupled to a dielectric resonator that is a standard $10\times 10\times 1$~mm³ SrTiO$₃ (STO) substrate with a YBa₂Cu$₃O$_{7 - \delta }$ (YBCO) thin-film flux focuser deposited on it. Recently, we have simulated a dielectric resonator for the high-$T_{\rm c }$ rf SQUID by using the ANSOFT High Frequency Structure Simulator (ANSOFT HFSS). We simulate the resonant frequency and the quality factor of our dielectric resonator when it is unloaded or matches a 50-$\Omega$ impedance. The simulation results are quite close to the practical measurements. Our study shows that ANSOFT HFSS is quite suitable for simulating the dielectric resonator used for the high-$T_c rf SQUID. Therefore, we think the ANSOFT HFSS can be very helpful for investigating the characteristics of dielectric resonators for high-$T_c rf SQUIDs.