约瑟夫森结非线性特性的计算机模拟

通过计算机对约瑟夫森结方程求解，并模拟约瑟夫森结在直流和交流条件下V-I特性曲线．通过约瑟夫森结的相图与庞加莱截面图直观的表现了倍周期分岔以及混沌现象，并与用约瑟夫森结电子模拟器所做的实验进行比较．     

双模非经典光场辐照下介观约瑟夫森结中超流的动力学行为

研究了直流偏置电压下介观约瑟夫森结在SU（1，1）双模相干态光场作用下超流的动力学行为、流压台阶和直流分量等特性．研究发现超流能够呈现明显的崩塌与复苏现象，对于经典电磁场和非耦合的双模相干态光场作用下的情况，超流不出现此现象，且其流压台阶结构与后两种情况也不相同． 关键词：     

高温超导双晶结混频器噪声系数测量

噪声系数(Noise Figure,NF)是超导混频器的一个重要参数,是衡量超导混频器内部噪声、影响混频器灵敏度的一个重要指标.实验基于小型脉冲管制冷系统,搭建高温超导约瑟夫森双晶结混频器噪声系数自动测量系统,工作温度为60K.Ar离子刻蚀工艺技术制备高温超导YBa2Cu3O7-δ(YBCO)/MgO约瑟夫森双晶结,通过Y-因子噪声测量法,测量YBCO/MgO约瑟夫森双晶结混频器的噪声系数.实验成功制备了特征电压约1mV的YBCO/MgO约瑟夫森双晶结,在650GHz信号辐照下,有明显的夏皮罗(Shapiro)台阶.在210GHz信号辐照下,进行30次谐波混频,约瑟夫森双晶结在不同偏置电压点中频输出信噪比变化范围约为30dB.在77K与300K不同输入噪声功率下,测得双晶结混频器噪声系数最小值为10dB,偏置电压在300μV.     

热噪声对约瑟夫森结I-V特性及微波感应台阶的影响研究

采用数值仿真的方法研究了热噪声对约瑟夫森结I-V特性及微波感应台阶的影响。研究表明,热噪声导致约瑟夫森结的I-V特性曲线呈现"圆拱化",也使得微波感应台阶高度减小。得出了取不同约瑟夫森结临界电流和结电阻时,热噪声对台阶的影响规律     

约瑟夫森结的混沌行为及其在保密通信中的应用(英文)

约瑟夫森结的混沌行为在研究和应用方面都具有重要的价值。电阻-电容-电感并联约瑟夫森结在直流电流的驱动下会出现混沌行为,采用数值计算的方法,对混沌行为进行了研究。通过电压波形、频谱、对初始条件的敏感依赖以及奇怪吸引子,证实了混沌的存在,在此基础上首次提出了基于约瑟夫森结混沌行为的保密通信方案。     

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.     

超导约瑟夫森结在频谱信息检测中的应用

开展了利用超导约瑟夫森效应进行频谱检测的研究。利用小型制冷机得到液氮温区环境,并使探测系统小型化、智能化。利用DSP系统进行数据采集、信号处理及对整个测量系统的控制。改进后的系统检测了约100GHz微波辐照的响应,展示了超导约瑟夫森结在频谱信息检测中的优越性。     

SNS与SINIS结阵制作工艺的比较

国家电压基准是基于约瑟夫森量子化效应的。传统的SIS结阵具有不能快速选择特定的台阶,相位锁定时间短的问题。I-V曲线无回滞的约瑟夫森结阵,其I-V曲线单值,电压台阶宽,解决了SIS结阵的问题。美国标准技术研究院的Hamilton提出并制作高度阻尼的可编程SNS约瑟夫森结阵电压标准;德国物理技术研究院则选择SINIS结来制作可编程约瑟夫森结阵电压基准。从结构、性能和制作方法等方面对SNS和SINIS可编程约瑟夫森结阵进行了比较。     

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.     

SrTiO3基片上Tl-2212双晶约瑟夫森结的动态特性及噪声影响研究

在SrTiO₃（STO）基片上制作了Tl-2212双晶约瑟夫森结,并对其进行了微波辐照下的I-V特性测试,观察到了夏皮罗台阶,符合约瑟夫森电压-频率关系.利用数值仿真对约瑟夫森结建立了RCSJ模型,仿真结果与实验数据符合较好,利用此模型深入研究了噪声对结动态特性的影响,解释了噪声影响下结的微波感应台阶幅度减小和极小值展宽现象,提出了有效噪声温度为工作温度和外部噪声的等效温度之和. 关键词： RCSJ模型 噪声 Tl-2212双晶约瑟夫森结     

Subharmonic shapiro steps and assisted tunneling in superconducting point contacts

We analyze the current in a superconducting point contact of arbitrary transmission in the presence of a microwave radiation. The interplay between the ac Josephson current and the microwave signal gives rise to Shapiro steps at voltages V = (m/n)h(omega)(r)/2e, where n,m are integer numbers and omega(r) is the radiation frequency. The subharmonic steps (n not equal 1) are a consequence of multiple Andreev reflections (MAR) and provide a signature of the peculiar ac Josephson effect at high transmission. Moreover, the dc current exhibits a rich subgap structure due to photon-assisted MARs.     

On the cryocooler-based cooling of Josephson microchips fabricated from cuprate superconductors for use in voltage standards

The features of the cooling of arrays of Josephson junctions fabricated from cuprate superconductors, as well as their interaction with microwave radiation during treatment in a closed-cycle cryocooler, are investigated. Estimation of microchip heating sources is carried out by measuring the current–voltage characteristics (IVCs) of the Josephson junctions. On the developed the developed experimental setup, Shapiro steps with a voltage of about 25 mV are obtained on the IVCs of the structure exposed to microwave radiation at a frequency of 75 GHz.     

An experimental study of Josephson effect in bulk YBaCUO

We have observed Josephson effect in bulk YBaCuO. The size of bulk is 7.5×2.2×0.3 mm and the microwave frequency is 9.82 GHz in our experiment. Several microwave induced steps can easily be observed. When an external magnetic field is applied to the bulk, the critical supercurrent at zero voltage is suppressed significantly. It has been demonstrated that YBaCuO bulk can be seen as a network of Josephson junctions. An experimental study of Josephson effect in bulk YBaCuO at millimeter wave frquency is in progress.     

约瑟夫森隧道结微波感应效应的实验和理论研究

在实验中观察到半波长约瑟夫森隧道结微波感应阶梯高度随微波电压振幅近似地按贝塞耳函数的二次方变化,并且奇数号码阶梯有时不出现,对于上述现象本文做了理论解释。     

Multiple Andreev reflections in ballistic Nb-InGaAs/InP-Nb junctions

The current–voltage characteristics of ballistic Nb-InGaAs/InP-Nb Josephson junctions have been investigated. At temperatures below 1 K a negative differential conductance, which usually leads to a hysteresis in the current–voltage characteristics, was resolved by connecting an additional external shunt resistor to the junction. The negative differential conductance is explained by heating and conductance enhancement due to multiple Andreev reflections. The structures observed in the differential resistance measurements as a function of the bias voltage are explained by self-detection of Josephson radiation at low bias voltages and subharmonic gap structures at higher bias voltages.     

Developing Josephson junction array chips for microvolt applications

Josephson junction array chips for microvolt applications have been designed and fabricated. A voltage step as small as 1 μV has been observed for a single junction in the array when it is driven by 483.59 MHz microwave. By selecting different parts of the array, it can output a voltage from 1 μV to 256 μV. The flat region of the voltage steps is over 200 μA.This kind of array is useful for potential microvolt applications.     

The Influence of the McCumber Parameter on Parametric Amplification of High-Frequency Radiation by Josephson Junctions

The influence of the internal capacity of a Josephson junction on the parametric amplification of external electromagnetic radiation was studied in terms of a resistively and capacitively shunted junction model. The influence of the McCumber parameter on parametric amplification was clarified. It is shown that the additional regions of amplification can occur near subharmonic Shapiro steps in the case of Josephson junctions with internal capacity.     

Ac-Josephson effect in YBa2Cu3O7/PbSn point contact junctions

Adjustable YBa₂Cu₃O₇/PbSn point contact Josephson junctions are exposed to a microwave field. The current voltage characteristics of the junctions show microwave-induced constant voltage steps. The power dependence of the current width of the steps is well described by the Bessel function behaviour. This clearly demonstrates the presence of macroscopic quantum effects in the new highT _c material.     

Subharmonic Shapiro steps and noise in high-T c superconductor Josephson junctions

An experimental investigation is made of the subharmonic Shapiro steps observed on the I-V curves of high-T _c superconductor Josephson junctions and on the bias-voltage dependences of the rf noise and detector response when the junctions are subjected to external submillimeter radiation. Structures of this type are ordinarily described by a nonsinusoidal current-phase relation, which is why subharmonic steps appear. Numerical modeling of the processes occurring in a Josephson junction by means of a simple current-phase relation, as in the case of an SNS junction, gives good agreement with experiment. The width of the characteristic Josephson generation line of the junction was estimated on the basis of the noise dependences and the selective detector response. The width can be explained by taking into account the shot noise of the tunneling component of the conductivity. A model of the conductivity of a high-T _c superconductor Josephson junction, consisting of a tunnel junction with microshorts possessing metallic conductivity, is discussed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 426–430 (10 September 1998)     

High-efficiency deterministic Josephson vortex ratchet

We investigate experimentally a Josephson vortex ratchet--a fluxon in an asymmetric periodic potential driven by a deterministic force with zero time average. The highly asymmetric periodic potential is created in an underdamped annular long Josephson junction by means of a current injector providing an efficiency of the device up to 91%. We measured the ratchet effect for driving forces with different spectral content. For monochromatic high-frequency drive the rectified voltage becomes quantized. At high driving frequencies we also observe chaos, subharmonic dynamics, and voltage reversal due to the inertial mass of a fluxon.