Quasiperiodically driven Josephson junctions: strange nonchaotic attractors, symmetries and transport

We consider the dynamics of the overdamped Josephson junction under the influence of an external quasiperiodic driving field. In dependence on parameter values either a quasiperiodic motion or a strange nochaotic attractor (SNA) can be observed. The latter corresponds to a resistive state in the current-voltage characteristics while for quasiperiodic motion a finite superconducting current exists for zero voltage. It is shown that in the case of SNA a nonzero mean voltage across the junction can appear due to symmetry breakings. Based on this observation a detailed symmetry consideration of the generalized equation of motion is performed and symmetry conditions ensuring zero mean voltage across the junction are found. Received 16 August 2001 and Received in final form 22 January 2002     

Cooper-pair number-phase quantization analysis in double-Josephson-junction mesoscopic circuit coupled by a capacitor

This paper examines the quantization of mesoscopic circuit including Josephson junctions. Following Feynman＇s assumption, via the Hamilton dynamic approach and by virtue of the entangled state representation, it constructs Hamiltonian operator for the double-Josephson-junction mesoscopic circuit coupled by a capacitor. Then it uses the Heisenberg equation of motion to derive the induction voltage across each Josephson junction. The result manifestly shows how the voltage is affected by the capacitance coupling.     

Emission and absorption asymmetry in the quantum noise of a Josephson junction

We measure current fluctuations of mesoscopic devices in the quantum regime, when the frequency is of the order of or higher than the applied voltage or temperature. Detection is designed to probe separately the absorption and emission contributions of current fluctuations, i.e. the positive and negative frequencies of the Fourier transformed nonsymmetrized noise correlator. It relies on measuring the quasiparticles photon assisted tunneling current across a superconductor-insulator-superconductor junction (the detector junction) caused by the excess current fluctuations generated by quasiparticles tunneling across a Josephson junction (the source junction). We demonstrate unambiguously that the negative and positive frequency parts of the nonsymmetrized noise correlator are separately detected and that the excess current fluctuations of a voltage biased Josephson junction present a strong asymmetry between emission and absorption.     

Modified Josephson Equation for Mesoscopic Parallel LC Circuit Including a Josephson Junction

By introducing the entangled state representation, parallel LC circuit including a 3osephson junction equation associated with the modification of the motion equation. the Cooper-pair number-phase quantization of the mesoscopic is realized. In the Heisenberg picture, the modified Josephson Faraday equation about the inductance is deduced from the     

一个带阻尼项Josephson振动的半经典模型

在双势阱模型的基础上考虑了Josephson流与凝聚体的相互作用而引起的阻尼效应,得出相对粒子数Z(t)的表达式,并用一个简单的碰撞模型得出Josephson振动的振幅和能量及随时间呈指数衰减,以及凝聚体的质量越大,衰减越慢。     

Tunneling spectroscopy of two-level systems inside a Josephson junction

We consider a two-level system (TLS) with energy level separation plankvOmega0 inside a Josephson junction. The junction is shunted by a resistor R and is voltage V biased. If the TLS modulates the Josephson energy and/or is optically active, it is Rabi driven by the Josephson oscillations in the running phase regime near the resonance 2eV=plankvOmega0. The Rabi oscillations, in turn, translate into oscillations of current and voltage that can be detected in noise measurements. This effect provides an option to fully characterize the TLS inside Josephson junction and to find the TLS's contribution to the decoherence when the junction is used as a qubit.     

SQUID ratchets: basics and experiments

Superconducting quantum interference devices (SQUIDs) are very well suited for experimental investigations of ratchet effects. This is due to the periodicity of the Josephson coupling energy with respect to the phase difference δ of the superconducting macroscopic wave function across a Josephson junction. We show first that, within the resistively and capacitively shunted junction model, the equation of motion for δ is equivalent to the motion of a particle in the so-called tilted washboard potential, and we derive the conditions which have to be satisfied to build a ratchet potential based on asymmetric dc SQUIDs. We then present results from numerical simulations and experimental investigations of dc SQUID ratchets with critical-current asymmetry under harmonic excitation (periodically rocking ratchets). We discuss the impact of important properties like damping or thermal noise on the operation of SQUID ratchets in various regimes, such as adiabatically slow or fast nonadiabatic excitation. Received: 22 November 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002     

Electrodynamic properties of a Josephson junction biased with a sequence of delta-function pulses

An analytic method is developed for studying the electrodynamic properties of Josephson junctions biased with a sequence of delta-function pulses. Such investigations are of interest in connection with developing, on the basis of the Josephson effect, analog-to-digital converters and synthesizers producing signals of a preset form with a fundamental precision. Analytic expressions are obtained that determine phase lock conditions, the mean voltage across the junction, and the boundaries of Shapiro’s steps. The results obtained are presented in a graphical form for the cases of unipolar and bipolar bias of a junction. The results are discussed and compared with those known from previous investigations.     

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。     

ac Josephson effect and resonant Cooper pair tunneling emission of a single Cooper pair transistor

We measure the high-frequency emission of a single Cooper pair transistor (SCPT) in the regime where transport is only due to tunneling of Cooper pairs. This is achieved by coupling on chip the SCPT to a superconductor-insulator-superconductor junction and by measuring the photon assisted tunneling current of quasiparticles across the junction. This technique allows a direct detection of the ac Josephson effect of the SCPT and provides evidence of Landau-Zener transitions for proper gate voltage. The emission in the regime of resonant Cooper pair tunneling is also investigated. It is interpreted in terms of transitions between charge states coupled by the Josephson effect.     

Very high frequency spectroscopy and tuning of a single-cooper-pair transistor with an on-chip generator

A single-Cooper-pair transistor (SCPT) is coupled capacitively to a voltage biased Josephson junction, used as a high-frequency generator. Thanks to the high energy of photons generated by the Josephson junction, transitions between energy levels, not limited to the first two levels, were induced and the effect of this irradiation on the dc Josephson current of the SCPT was measured. The phase and gate bias dependence of energy levels of the SCPT at high energy is probed. Because the energies of photons can be higher than the superconducting gap we can induce not only transfer of Cooper pairs but also transfer of quasiparticles through the island of the SCPT, thus controlling the poisoning of the SCPT. This can both decrease and increase the average Josephson energy of the SCPT: its supercurrent is then controlled by high-frequency irradiation.     

Superconducting transport in an LED with Nb electrodes

Superconducting transport is measured between two superconducting electrodes at the n-type semiconductor side of a superconductor-based LED where a Josephson junction is formed. The characteristics of the Josephson junction are found to be modulated by applying voltage to the normal electrode at the p-type semiconductor side. The Josephson junction characteristics show an extraordinary sensitivity to the radiative recombination process, which we estimate as the recombination efficiency.     

Phase-charge duality of a josephson junction in a fluctuating electromagnetic environment

We have measured the current-voltage characteristics of a single Josephson junction placed in a high impedance environment. The transfer of Cooper pairs through the junction is governed by overdamped quasicharge dynamics, leading to Coulomb blockade and Bloch oscillations. Exact duality exists to the standard overdamped phase dynamics of a Josephson junction, resulting in a dual shape of the current-voltage characteristic, with current and voltage changing roles. We demonstrate this duality with experiments which allow for a quantitative comparison with a theory that includes the effect of fluctuations due to the finite temperature of the electromagnetic environment.     

圆对称环域Josephson结中的零场台阶

对计及干涉项的描述圆对称环域Josephson结扰动sine-Gordon方程数值研究表明:在无外磁场时,对于内外半径分别为7.5λ_j和12.5λ_j的结,在它的Ⅰ-V特性曲线上呈现两支零场台阶,它们分别对应于单孤子和三孤子激发,双孤子的激发是不稳定的,它仅存在于暂态过程之中,三孤子激发相当于一维结中三孤子的对称模式,且伴随着包括不同振荡周期等的运动不规则性。 关键词：     

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。     

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.     

Quantum fluctuations in a distributed Josephson junction and the spectral properties of Josephson oscillators

Within the framework of a tunneling Hamiltonian, we obtain an equation for the reduced density matrix to describe quasiclassical dynamics and fluctuation effects in distributed Josephson junctions for voltages comparable with the superconducting gap. For quasiclassical dynamics, we derive the Langevin equation describing in a self-consistent way the resistive state and fluctuations due to both the tunneling current and the electromagnetic field. Current-voltage characteristics of a Josephson oscillator are calculated in the high magnetic field approximation. The intensity and shape of the spectral line of radiation due to vortices moving in a distributed Josephson junction are found.     

嵌入到Fabry-Perot谐振腔的双晶约瑟夫森结阵列的阻抗匹配和相位锁定研究

在He等人所做的嵌入到Fabry-Perot谐振腔中约瑟夫森结阵列的微波辐照研究基础上,提出了同时实现约瑟夫森结阵列阻抗匹配和相位锁定的方法,进行了相关的电磁仿真和数值计算.双晶约瑟夫森结阵列被制作在YSZ双晶基片上,同时被嵌入到Fabry-Perot谐振腔内.通过在基片上制作与结阵列集成的串联馈电半波偶极天线阵,并对其结构进行优化实现了结与天线的匹配,数值计算表明结的辐射效率达到94%;利用天线阵辐射场的特征和对模型合理的设计,使Fabry-Perot谐振腔和基片同时谐振在合适的模式下,从而使结阵列与谐 关键词： 约瑟夫森结阵列 阻抗匹配 相位锁定 Fabry-Perot谐振腔     

Superposition of Coherent States in a Mesoscopic Josephson Junction with Dissipation

A mesoscopic Josephson junction with dissipation is considered. Usually the dissipation in the system is described as a consequence of its coupling to a reservoir. By solving the master equation we show that the state of the junction can evolve in a quantum superposition of two coherent states even when the dissipation is present.``     

Driving of a single vortex in a long Josephson junction

The driving of a single vortex by a bias current in a long Josephson junction with a finite width has been studied with the two-dimensional Josephson equation. The applicable range of the equivalent one-dimensional equation is shown.