Co-doped BaFe_(2)As_(2) Josephson junction fabricated with a focused helium ion beam

Josephson junction plays a key role not only in studying the basic physics of unconventional iron-based superconductors but also in realizing practical application of thin-film based devices,therefore the preparation of high-quality iron pnictide Josephson junctions is of great importance.In this work,we have successfully fabricated Josephson junctions from Co-doped BaFe_(2)As_(2)thin films using a direct junction fabrication technique which utilizes high energy focused helium ion beam(FHIB).The electrical transport properties were investigated for junctions fabricated with various He^(+)irradiation doses.The junctions show sharp superconducting transition around 24 K with a narrow transition width of 2.5 K,and a dose correlated foot-structure resistance which corresponds to the effective tuning of junction properties by He^(+)irradiation.Significant J_c suppression by more than two orders of magnitude can be achieved by increasing the He^(+)irradiation dose,which is advantageous for the realization of low noise ion pnictide thin film devices.Clear Shapiro steps are observed under 10 GHz microwave irradiation.The above results demonstrate the successful fabrication of high quality and controllable Co-doped BaFe_(2)As_(2)Josephson junction with high reproducibility using the FHIB technique,laying the foundation for future investigating the mechanism of iron-based superconductors,and also the further implementation in various superconducting electronic devices.

Harmonic balance simulation of the influence of component uniformity and reliability on the performance of a Josephson traveling wave parametric amplifier

A Josephson traveling wave parametric amplifier(JTWPA),which is a quantum-limited amplifier with high gain and large bandwidth,is the core device of large-scale measurement and control systems for quantum computing.A typical JTWPA consists of thousands of Josephson junctions connected in series to form a transmission line and hundreds of shunt LC resonators periodically loaded along the line for phase matching.Because the variation of these capacitors and inductors can be detrimental to their high-frequency characteristics,the fabrication of a JTWPA typically necessitates precise processing equipment.To guide the fabrication process and further improve the design for manufacturability,it is necessary to understand how each electronic component affects the amplifier.In this paper,we use the harmonic balance method to conduct a comprehensive study on the impact of nonuniformity and fabrication yield of the electronic components on the performance of a JTWPA.The results provide insightful and scientific guidance for device design and fabrication processes.

Half-integer Shapiro steps in MgB_(2) focused He ion beam Josephson junctions

Half-integer microwave induced steps(Shapiro steps)have been observed in many different Josephson junction systems,which have attracted a lot of attention because they signify the deviation of current phase relation(CPR)and uncover many unconventional physical properties.In this article,we first report the discovery of half-integer Shapiro steps in MgB_(2)focused He ion beam(He-FIB)Josephson junctions.The half-integer steps'dependence on microwave frequency,temperature,microwave power,and magnetic field is also analyzed.We find that the existence of half-integer steps can be controlled by the magnetic field periodically,which is similar to that of high temperature superconductor(HTS)grain boundary junctions,and the similarity of the microstructures between gain boundary junctions and He-FIB junctions is discussed.As a consequence,we mainly attribute the physical origin of half-integer steps in MgB_(2)He-FIB junctions to the model that a He-FIB junction is analogous to a parallel junctions'array.Our results show that He-FIB technology is a promising platform for researching CPR in junctions made of different superconductors.

Anomalous spin Josephson effect in spin superconductors

The spin superconductor state is the spin-polarized triplet exciton condensate,which can be viewed as a counterpart of the charge superconductor state.As an analogy of the charge Josephson effect,the spin Josephson effect can be generated in the spin superconductor/normal metal/spin superconductor junctions.Here we study the spin supercurrent in the Josephson junctions consisting of two spin superconductors with noncollinear spin polarizations.For the Josephson junctions with out-of-plane spin polarizations,the possibleπ-state spin supercurrent appears due to the Fermi momentum-splitting Andreev-like reflections at the normal metal/spin superconductor interfaces.For the Josephson junctions with in-plane spin polarizations,the anomalous spin supercurrent appears and is driven by the misorientation angle of the in-plane polarizations.The symmetry analysis shows that the appearance of the anomalous spin Josephson current is possible when the combined symmetry of the spin rotation and the time reversal is broken.

Light-modulated graphene-based φ_(0) Josephson junction and -φ_(0) to φ_(0) transition

We investigate the chiral edge states-induced Josephson current–phase relation in a graphene-based Josephson junction modulated by the off-resonant circularly polarized light and the staggered sublattice potential.By solving the Bogoliubov–de Gennes equation,a φ_(0) Josephson junction is induced in the coaction of the off-resonant circularly polarized light and the staggered sublattice potential,which arises from the fact that the center of-mass wave vector of Cooper pair becomes finite and the opposite center of-mass wave vector to compensate is lacking in the nonsuperconducting region.Interestingly,when the direction of polarization of light is changed,-φ_(0) to φ_(0) transition generates,which generalizes the concept of traditional 0–πtransition.Our findings provide a purely optical way to manipulate a phase-controllable Josephson device and guidelines for future experiments to confirm the presence of graphene-based φ_(0)Josephson junction.

自旋混合库珀对引起的Josephson电流

通过求解Bogoliubov-de Gennes方程研究一维S/FL-F-FR/S结中Josephson电流的输运特性,其中S和F分别是超导体和铁磁体,FL,R是左右两侧具有非共线磁矩的界面层.研究发现,FL和FR界面能够产生自旋混合和自旋翻转效应,该效应能够将S内一部分自旋单重对转化为F内自旋相同的三重对.对于短S/FL-F-FR/S结,F层内同时存在自旋单重对和自旋相同的三重对.于是,随着铁磁交换场和界面磁矩偏转角度差的增加,临界电流在一个基准面上振荡.若F转变为半金属,则F内仅剩自旋相同的三重对,临界电流的振荡特征消失.此外,FL和FR界面还能起到普通势垒的作用.这将导致临界电流随着铁磁厚度的增加展现出双重振荡行为,其中长波振荡源于铁磁体内自旋单重对的位相变化,短波振荡是由自旋单重对和自旋相同的三重对经过两个界面势垒时的共振隧穿效应引起的.

Anomalous Josephson effect between d-wave superconductors through a two-dimensional electron gas with both Rashba spin-orbit coupling and Zeeman splitting

Based on the Bogoliubov-de Gennes equation and the extended McMillan’s Green’s function formalism,we study theoretically the Josephson effect between two d-wave superconductors bridged by a ballistic two-dimensional electron gas with both Rashba spin-orbit coupling and Zeeman splitting.We show that due to the interplay of Rashba spin-orbit coupling and Zeeman splitting and d-wave pairing,the current-phase relation in such a heterostructure may exhibit a series of novel features and can change significantly as some relevant parameters are tuned.In particular,anomalous Josephson current may occur at zero phase bias under various different situations if both time reversal symmetry and inversion symmetry of the system are simultaneously broken,which can be realized by tuning some relevant parameters of the system,including the relative orientations and the strengths of the Zeeman field and the spin-orbit field in the bridge region,the relative orientations of the a axes in two superconductor leads,or the relative orientations between the Zeeman field in the bridge region and the a axes in the superconductor leads.We show that both the magnitude and the direction of the anomalous Josephson current may depend sensitively on these relevant parameters.

Enhanced topological superconductivity in an asymmetrical planar Josephson junction

As a platform for holding Majorana zero models(MZMs),the two-dimensional planar topological Josephson junction that can be used as carriers for topological quantum computing faces some challenges.One is a combination of mirror and time-reversal symmetries may make the system hold multiple pairs of MZMs.The other is that a soft gap dominated by a large momentum occurs in a clean system.To solve these problems,asymmetric junction can be introduced.Breaking this symmetry changes the symmetry class from class BDI to class D,and only a single pair of MZMs can be left at the boundary of the system.We numerically study four cases that create an asymmetric system and find out different superconducting pairing potential,different coupling coefficients between two-dimensional electron gases(2DEGs)and two superconducting bulks,different widths of two superconducting bulks make the gap of the system decrease at the optimal value,but make the gap at the minimum value increases.And the zigzag-shape quasi-one-dimensional junction eliminates the large momentum parallel to the junction and enhances the gap at the large momentum.However,the zigzag-shape junction cannot increase the gap at the region of multiple pairs of MZMs in a symmetric system.We show that by combining zigzag-shape junction with different coupling coefficients,the system can maintain a large gap(≈0.2△)in a wide region of the parameter space.

Research Progress of Reverse Monte Carlo and Its Application in Josephson Junction Barrier Layer

As indispensable components of superconducting circuit-based quantum computers,Josephson junctions determine how well superconducting qubits perform.Reverse Monte Carlo(RMC)can be used to recreate Josephson junction’s atomic structure based on experimental data,and the impact of the structure on junctions’properties can be investigated by combining different analysis techniques.In order to build a physical model of the atomic structure and then analyze the factors that affect its performance,this paper briefly reviews the development and evolution of the RMC algorithm.It also summarizes the modeling process and structural feature analysis of the Josephson junction in combination with different feature extraction techniques for electrical characterization devices.Additionally,the obstacles and potential directions of Josephson junction modeling,which serves as the theoretical foundation for the production of superconducting quantum devices at the atomic level,are discussed.

Algorithms for automatic measurement of SIS-type hysteretic underdamped Josephson junction’s parameters by current-voltage characteristics

Some electrical parameters of the SIS-type hysteretic underdamped Josephson junction(JJ)can be measured by its current-voltage characteristics(IVCs).Currents and voltages at JJ are commensurate with the intrinsic noise level of measuring instruments.This leads to the need for multiple measurements with subsequent statistical processing.In this paper,the digital algorithms are proposed for the automatic measurement of the JJ parameters by IVC.These algorithms make it possible to implement multiple measurements and check these JJ parameters in an automatic mode with the required accuracy.The complete sufficient statistics are used to minimize the root-mean-square error of parameter measurement.A sequence of current pulses with slow rising and falling edges is used to drive JJ,and synchronous current and voltage readings at JJ are used to realize measurement algorithms.The algorithm performance is estimated through computer simulations.The significant advantage of the proposed algorithms is the independence from current source noise and intrinsic noise of current and voltage meters,as well as the simple implementation in automatic digital measuring systems.The proposed algorithms can be used to control JJ parameters during mass production of superconducting integrated circuits,which will improve the production efficiency and product quality.

SSeS超导结的Josephson效应

以推广了的Jacobson方法 ,由Ginzburg Landau方程出发 ,推导出了SSeS超导结的直流Josephson效应 .从而进一步求得了SSeS超导结的临界电流与温度之间的关系 ,以及对结区厚度的依赖性 .

外场中超导体/半导体/超导体结的直流Josephson电流

当外加磁场时,超导体/半导体/超导体结的临界电流受外加磁场的影响.超导结的临界电流与磁场的曲线非常类似于单色光在单狭缝衍射的夫琅和费图样.在未考虑外加磁场的超导体/半导体/超导体结的临界电流的基础上,进一步对外加磁场的情形进行了研究.

直流和交流Josephson效应的研究

基于Feynman模型得到的Josephson方程,在求解过程中采取了ρ1=ρ2的假设,但这个假设在数学处理上并不严格。为了得到严格的结果,未进行上述假设,通过解微分方程得到了严格的Josephson方程,结果显示在一定条件下才能得到通常所说的Josephson方程。

介观LC回路数-相量子化和Josephson结的Cooper对数-相量子化

传统的介观LC回路的量子化是将电量q和电感与电流的乘积L×I分别作为量子力学中的坐标算符Q和动量算符P来处理;本文采取另外一种量子化的观点,即将电量q(q=en)中的n作为荷数算符,并建立电流和相算符θ之间对应关系,就能实现介观LC回路的数-相范畴的量子化,并得到以数-相算符表示的Hamiltonian;通过引进纠缠态表象,对超导Josephson结也可以实现Cooper对数-相量子化,并给出了相应的物理解释。

超导Josephson结直流I-V特性的数值计算

为了研究超导Josephson结的性质,选用四阶龙格-库塔法对RSJ模型下单个Josephson结及其阵列以及微波辐照下Josephson结的直流I-V特性进行了数值计算,并将模拟结果与实验结果进行了比较.

Josephson结的I-V曲线的理论分析

本文从理论上分析了Josephson结的I-V曲线的混沌、台阶和滞后等一系列现象,并给出了描述上述现象的解析关系式,所得结果与实验结果比较符合。

用Josephson结电子模拟器测量磁通量子2e/h

介绍用 Josephson结电子模拟器在正常温度下 ,模拟测量磁通量子 2 e/ h,用模拟器来研究 Josephson结的特性 .该实验可作为普通物理实验中课题设计实验的一个内容 .

Josephson环耦合量子细胞神经网络混沌动力学分析

量子细胞神经网络(QCNN)在大规模信号处理上是一种崭新的结构,将是未来细胞神经网络(CNN)一个新的发展方向。以Josephson环的幅值和相位作为状态变量,研究了两个Josephson环耦合的量子细胞神经网络的非线性动力学行为。通过理论研究和计算机仿真,发现系统具有丰富的动力学行为,如周期、拟周期、混沌和超混沌状态。分析了细胞内隧穿矩阵元比例系数和相邻细胞互感系数对系统分岔与混沌特性的影响,发现系统经拟周期分岔进入混沌,在不对称细胞耦合的量子细胞神经网络中,系统能产生混沌的参数范围较大,且在混沌区域没有周期窗口,是一种鲁棒混沌;在对称细胞耦合的量子细胞神经网络中,系统能产生混沌的的参数范围相对较小,且在产生混沌振荡的区域内有周期、拟周期窗口。

Schrdinger Cat States in a Mesoscopic Josephson Junction

To study the nonclassical effects of the mesoscopic Josephson junction in the presence of a nonclassical microwave, the mesoscopic Josephson junction and the field were both treated quantum mechanically, and the external field approximation was used. It is shown that if the external field is in the coherent state and the state of the junction is initially prepared in the vacuum state, the state of the junction can evolve into a quantum superposition of two coherent states. The Schrdinger cat states can be produced in a mesoscopic Josephson junction.

全铌超导Josephson结的EWB仿真模型

通过超低温实验直接测量了制备的全铌Josephson结的电压源驱动RCSJ模型的I-V特性曲线。建立了Josephson结在电路仿真软件EWB中的模型,极大的方便了Josephson结及相关电路的仿真研究。利用这一新模型对Josephson结的基本特性进行了较为系统的研究,包括RSJ模型和RCSJ模型的直流驱动I-V特性曲线。