电阻并联约瑟夫森结阵列中的混沌行为

采用数值方法,对存在并联电阻的约瑟夫森结串联阵列进行了研究。首次发现当阵列中结的个数为3时,其中存在明显的混沌行为,并给出了存在混沌行为的参数范围。根据A.A.Chernikov和G.Schmidt的理论,只有当约瑟夫森结的个数大于或者等于4时才能观察到混沌行为;这一发现说明了其理论存在一定的局限性。     

并联电阻对串联约瑟夫森结阵列相位锁定的影响研究

利用约瑟夫森结阵列制作毫米/亚毫米微波源的一个重要问题是阵列中约瑟夫森结的相位锁定.本文采用仿真的方法研究了约瑟夫森结RCSJ模型存在并联电阻时的相位锁定问题.研究结果包括约瑟夫森结临界电流的不一致性、McCumber参数和结的个数对能够实现相位锁定的参数范围的影响.结果对于约瑟夫森结阵列作为太赫兹产生和检测应用具有重要意义.     

Design optimization for microwave coupling structures of a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions

A new coupling scheme of the external microwave signal for a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions has been proposed. It is suggested to couple the meander-like array inductively to a parallel microwave transmission line. Within the resistively shunted junction model different types of microwave coupling structures were analyzed with the goal to reduce the mutual phase locking and to achieve high output power and phase-locking stability. A harmonic balance technique was used to derive an analytic expression connecting the amplitudes of microwave voltages and currents in the array and the frequency of the external signal. Numerical simulations were performed to determine the boundaries of stable locking as a function of circuit parameters.     

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.     

二维约瑟夫森结阵列中的相变及噪声频谱研究

采用电阻分路(RSJ)模型，运用数值计算方法研究二维约瑟夫森结阵列的涡旋度,涡旋密度涨落随温度的变化，表明了超导阵列中存在KTB型相变.还研究了相变点附近的涡旋噪声频谱随温度、驱动电流变化的特性.计算结果与最近实验报导定性一致，并能用涡旋的运动图像来解释噪声频谱的变化规律. 关键词： 高温超导 噪声频谱 约瑟夫森结阵列 KTB相变     

Gate-tuned high frequency response of carbon nanotube Josephson junctions

Carbon nanotube Josephson junctions in the open quantum dot limit are fabricated using Pd/Al bilayer electrodes, and exhibit gate-controlled superconducting switching currents. Shapiro voltage steps can be observed under radio frequency current excitations, with a damping of the phase dynamics that strongly depends on the gate voltage. These measurements are described by a standard resistively and capacitively shunted junction model showing that the switching currents from the superconducting to the normal state are close to the critical current of the junction. The effective dynamical capacitance of the nanotube junction is found to be strongly gate dependent, suggesting a diffusive contact of the nanotube.     

Dissipation-driven phase transition in two-dimensional Josephson arrays

We analyze the interplay of dissipative and quantum effects in the proximity of a quantum phase transition. The prototypical system is a resistively shunted two-dimensional Josephson junction array, studied by means of an advanced Fourier path-integral Monte Carlo algorithm. The reentrant superconducting-to-normal phase transition driven by quantum fluctuations, recently discovered in the limit of infinite shunt resistance, persists for moderate dissipation strength but disappears in the limit of small resistance. For large quantum coupling our numerical results show that, beyond a critical dissipation strength, the superconducting phase is always stabilized at sufficiently low temperature. Our phase diagram explains recent experimental findings.     

Efficient simulation of resistively shunted Josephson junctions

We present a cluster algorithm for resistively shunted Josephson junctions or similar physical systems, which dramatically improves sampling efficiency, and apply it to the superconductor-to-metal transition in a single junction. Measuring the temperature dependence of the zero bias resistance, we confirm that the critical point does not depend on the strength of the Josephson coupling. However, we find that the correlation exponents vary continuously along the phase boundary, indicating that the Schmid-Bulgadaev transition is a line of fixed points.     

Ac Josephson effect in Corbino-geometry Josephson junctions constructed on Bi2Te3 surface

Recently, a Corbino-geometry type of Josephson junction constructed on the surface of topological insulators has been proposed for hosting and braiding Majorana zero modes. As a first step to test this proposal, we successfully fabricated Corbino-geometry Josephson junctions (JJs) on the surface of Bi₂Te₃ flakes. Ac Josephson effect with fractional Shapiro steps was observed in the Corbino-geometry JJs while the flux in the junction area was quantized. By analyzing the experimental data using the resistively shunted Josephson junction model, we found that the Corbino-geometry JJs exhibit a skewed current—phase relation due to its high transparency. The results suggest that Corbino-geometry JJs constructed on the surface of topological insulators may provide a promising platform for studying Majorana-related physics.     

Simulation and measurement of millimeter-wave radiation from Josephson junction array

We report the circuit simulations and experiments of millimeter-wave radiation from a high temperature superconducting(HTS) bicrystal Josephson junction(BJJ) array. To study the effects of junction characteristic parameters on radiation properties, new radiation circuit models are proposed in this paper. The series resistively and capacitively shunted junction(RCSJ) models are packaged into a Josephson junction array(JJA) model in the simulation. The current-voltage characteristics(IVCs) curve and radiation peaks are simulated and analyzed by circuit models, which are also observed from the experiment at liquid nitrogen temperature. The experimental radiation linewidth and power are in good agreement with simulated results. The presented circuit models clearly demonstrate that the inconsistency of the JJA will cause a broad linewidth and a low detected power. The junction radiation properties are also investigated at the optimal situation by circuit simulation. The results further confirm that the consistent JJA characteristic parameters can successfully narrow the radiation linewidth and increase the power of junction radiation.     

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。     

无序二维约瑟夫森结阵列中磁场引起的涡旋玻璃态相变

采用电阻阻错结的无序二维约瑟夫森结阵列模型,数值研究超导薄膜中垂直磁场引起的涡旋运动.通过分析磁场激发产生的涡旋度Ne及低频电压噪声S₀的变化特性,得到如下结论：在无序超导体中固定温度不变,随着磁场的减弱涡旋液态经过准有序的布拉格相,涡旋玻璃相重新进入到低磁场下的钉扎稀磁液相. 由于在涡旋玻璃相中,电流驱动下的噪声值表现出一个峰,表明系统处于无序与有序相互竞争的亚稳态,并且临界电流应有峰值效应. 计算得到噪声值的变化与Okuma等得到的无序超导Mo_xSi_1-x膜实验现象一致,并能解释磁场降低引起的重新进入钉扎的稀磁液相行为. 关键词： 约瑟夫森结阵列 磁通玻璃 重新进入 峰值效应     

Analytical theory of DC SQUID with a resistively shunted inductance driven by thermal noises

An analytical expression for the stationary probability distribution of the DC superconducting quantum interference device (SQUID) with a resistively shunted inductance driven by thermal noise is derived from the two-dimensional Fokker—Planck equation. The effects on the SQUID characteristics subject to a large thermal fluctuation with a noise parameter Γ>0.20 are discussed by taking into account the thermal noise in the accuracy of numerical simulation. This theory is valid for a reduced inductance β≤1. The analytical formulae for the SQUID characteristics, e.g. the circulating current, the average voltage and the voltage modulation, are obtained and discussed. The theory shows that the voltage modulation increases with the shunted inductance more efficiently for a large inductance parameter β and small fluctuation parameter Γ.     

Observation of negative absolute resistance in a Josephson junction

We experimentally demonstrate the occurrence of negative absolute resistance (NAR) up to about -1 Omega in response to an externally applied dc current for a shunted Nb-Al/AlO_{x}-Nb Josephson junction, exposed to a microwave current at frequencies in the GHz range. The realization (or not) of NAR depends crucially on the amplitude of the applied microwave current. Theoretically, the system is described by means of the resistively and capacitively shunted junction model in terms of a moderately damped, classical Brownian particle dynamics in a one-dimensional potential. We find excellent agreement of the experimental results with numerical simulations of the model.     

Investigation of step-edge Josephson device of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">d</Emphasis></Subscript> formed on MgO by a pulsed laser deposition method

We fabricated step-edge Josephson junctions of YBa₂Cu₃O_7−d on MgO substrates by a pulsed laser deposition method for high frequency applications. On the basis of temperature dependence of critical current, noticeable deterioration has not been observed in a step-edge area. The dynamic resistance was between 2 and 3Ω under the superconducting critical temperature. A deviation from a resistively shunted junction model was observed, which implies the excess current not flowing through Josephson junctions.     

利用一种改进模型研究谐振器耦合时的约瑟夫森结的响应(英文)

提出了一种改进的谐振器耦合时的约瑟夫森结模型,模型考虑了结与谐振器之间的耦合强度。利用这种模型,研究了谐振器耦合时的约瑟夫森结的响应,包括单结的电压自锁定和结阵列的相位互锁定现象。研究了耦合强度、谐振器的品质因数和谐振频率对电压自锁定的影响,并给出了合理的解释。此外,还对结阵列的相位互锁定强度对锁定电压和结数量的依赖关系进行了研究,并与理论值进行了比较。所得结果对谐振器耦合下的约瑟夫森结的应用具有指导意义。     

高温超导GdBa2Cu3O7-δ薄膜双晶晶界结特性研究

在晶界夹角为24°的YSZ人工双晶基片上,采用原位直流磁控溅射法制备了高温超导双晶GdBa₂Cu₃O_7-δ超导薄膜。采用光刻技术在晶界上刻出了尺寸5×5μm²的双晶晶界结。在77K下观测了结的直流I-V特性和在10GHz微波辐射下结的Shapiro台阶。这表明我们的人工双晶晶界结具有约瑟夫森弱连接行为。对结的特性进行了初步的理论分析和在实验基础上的数值模拟,模拟结果与RSJ理论符合较好,表明我们的双晶晶界结基本符合RSJ模型。     

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。     

Absolute negative mobility induced by thermal equilibrium fluctuations

A novel transport phenomenon is identified that is induced by inertial Brownian particles which move in simple one-dimensional, symmetric periodic potentials under the influence of both a time periodic and a constant, biasing driving force. Within tailored parameter regimes, thermal equilibrium fluctuations induce the phenomenon of absolute negative mobility (ANM), which means that the particle noisily moves backwards against a small constant bias. When no thermal fluctuations act, the transport vanishes identically in these tailored regimes. ANM can also occur in the absence of fluctuations on grounds which are rooted solely in the complex, inertial deterministic dynamics. The experimental verification of this new transport scheme is elucidated for the archetype symmetric physical system: a convenient setup consisting of a resistively and capacitively shunted Josephson junction device.