Current-voltage characteristics of large Josephson tunnel junctions

An analytic expression of dc supercurrents through large Josephson tunnel junctions in the presence of both a dc voltage and a dc magnetic field is obtained. The magnetic field dependence of the calculated dc supercurrents is reasonably consistent with experiments.     

dc Josephson Effect in s-Wave Superconductor/Ferromagnet Insulator/p-Wave Superconductor Junctions

The Josephson currents in s-wave superconductor/ferromagnet insulator/p-wave superconductor（s/FI/p） junctions are calculated as a function of temperature and the phase taking into account the roughness scattering effect at interface. The phase dependence of the Josephson current I （φ） between s-wave and px-wave superconductor is predicted to be sin（2φ）. The ferromagnet scattering effect, the barrier strength, and the roughness strength at interface suppress the dc currents in s/FI/p junction.     

Dynamics of spin transport in voltage-biased Josephson junctions

We investigate spin transport in voltage-biased spin-active Josephson junctions. The interplay of spin filtering, spin mixing, and multiple Andreev reflection leads to nonlinear voltage dependence of the dc and ac spin current. We compute the voltage characteristics of the spin current (IS) for superconductor-ferromagnet-superconductor Josephson junctions. The subharmonic gap structure of IS(V) is shown to be sensitive to the degree of spin mixing generated by the ferromagnetic interface, and exhibits a pronounced even-odd effect associated with spin transport during multiple Andreev reflection processes. For strong spin mixing both the magnitude and the direction of the dc spin current can be sensitively controlled by the bias voltage.     

s波超导体绝缘层dx2-y2波超导体结的直流Josephson电流

在s波超导体绝缘层dx2-y2波超导体结(sId)中,考虑到结界面粗糙散射,运用BogoliubovdeGennes(BdG)方程和FurusakiTsukada(FT)电流公式,计算超导结中的准粒子传输系数和直流Josephson电流.结果表明:sId超导结的直流Josephson电流随温度以及结两侧的相位差变化的关系曲线强烈地依赖于d波超导体的晶轴方位;结界面的粗糙散射对Josephson电流有抑制作用 关键词： s/I/d超导结 dx2-y2波超导体 直流Josephson电流     

Observation of Shapiro steps in the charge-density-wave state of NbSe3

We report the observation of ac-induced Shapiro-like steps in the dc I-V characteristics of NbSe₃. Analysis of the steps in terms of a classical model of charge-density-wave (CDW) transport, mathematically equivalent to the Stewart-McCumber model of Josephson tunnel junctions, indicates a highly coherent sample response, and that coupling phenomena well known in the Josephson literature also occur for CDW transport.     

Resonant effects in ballistic Josephson junctions

We study the Josephson effect in ballistic double-barrier SINIS planar junctions, consisting of bulk superconductors (S), a clean normal metal or semiconductor (N), and insulating interfaces (I) modeled as a δ-function potential-energy barriers. We solve the scattering problem based on the Bogoliubov-de Gennes equations and derive a general expression for the dc Josephson current, valid for arbitrary interfacial transparency, the Fermi wave vectors mismatch, and for different effective band masses. The effect of transmission resonances on the Josephson current and on the normal conductance is analyzed for short junctions. Curvature of the temperature dependence of the critical Josephson current is related to the presence of resonances at the Fermi level and to the interfacial transparency. For thin semiconductor layers with negative effective masses of the carriers, finite interfacial transparency and large Fermi wave vectors mismatch we find that an unusual and significant enhancement of both the normal conductance and the critical Josephson current occurs at low temperatures due to the presence of an evanescent mode localized at interfaces.     

Evidence for two-band superconductivity from break-junction tunneling on MgB2

Superconductor-insulator-superconductor tunnel junctions have been fabricated on MgB2 that display Josephson and quasiparticle currents. These junctions exhibit a gap magnitude, Delta approximately 2.5 meV, that is considerably smaller than the BCS value, but which clearly and reproducibly closes near the bulk T(c). In conjunction with fits of the conductance spectra, these results are interpreted as direct evidence of two-band superconductivity.     

Josephson current carried by Andreev levels in superconducting quantum point contacts

The dc Josephson effect in a superconducting quantum point contact, where supercurrent flows through a small number of channels, is reviewed. The central role of Andreev levels is emphasized which carry the whole supercurrent in short symmetric Josephson junctions including tunnel junctions. A simple intuitive view of the dc Josephson effect in a quantum point contact is given in terms of multiple Andreev reflections. The quantization of the critical current in superconducting quantum point contacts is briefly discussed.     

硅衬底MgB_2-B-MgB_2超导SNS约瑟夫森结的制备与特性

利用两步原位电子束蒸发技术,在Si(111)衬底上制备了MgB2-B-MgB2超导SNS约瑟夫森夹心结.夹心硼(B)层厚度从10nm到80nm范围内MgB2-B-MgB2/Si(111)薄膜表现出明显的SNS约瑟夫森结特性,而在5nm和100nm处薄膜分别是整体超导和正常金属特性.在同一温度下,随着硼层厚度的增加,临界结电流减小,对同一厚度下,临界结电流随着温度的增加而减小.同时,实验指出,夹心硼SNS超导MgB2约瑟夫森结的电流-电压(I-V)曲线具有回滞现象,符合SM模型.     

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：     

Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, current injection and electromagnetic radiation. In particular we will consider the ‘giant’ Josephson critical current in ‘long’ SIS tunnel junctions and the regular and anomalous nonequilibrium Josepson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).     

Resonant breather states in Josephson coupled systems

A review of diverse resonant effects appearing in weakly dissipative Josephson coupled systems in the presence of inhomogeneous dynamic localized state (discrete breather) is given. As particular examples I discuss the resonant interaction of breather states with linear electromagnetic excitations (EEs) in dc driven Josephson junction ladders and a single plaquette containing three Josephson junctions. Such resonant interaction manifests itself by resonant steps and various sharp switchings (voltage jumps) in the current-voltage characteristics. Moreover, the resonant interaction leads to an increase of breather dynamical complexity, e.g., enlargement of the breather core, low symmetry or quasiperiodic breather states. I show that the application of an external magnetic field allows to tune the resonant interaction, and correspondingly to increase (or decrease) the height of the resonant steps, to change the stability of the breather states.     

s/FI/d超导结的直流Josephson电流

在 s波超导体 /铁磁绝缘层 / d波超导体 Josephson结 (s/ FI/ d)中 ,考虑结界面铁磁绝缘层的磁散射和粗糙散射情况下 ,运用 Bd G方程和 FT的电流公式计算准粒子的输运系数及 s/ FI/ d结的直流 Josephson电流与温度 T、结两侧的相位差之间的关系。研究表明 :结界面的磁散射和粗造散射均抑制结中准粒子的 Andreev反射 ,降低了流过 s/ FI/ d结的直流 Josephson电流 ,直流Josephson电流 I随温度 T、相位差φ的变化曲线强烈地依赖于 d波超导体的晶轴方位     

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.     

Terahertz responses of intrinsic Josephson junctions in high T(c) superconductors

High frequency responses of intrinsic Josephson junctions up to 2.5 THz, including the observation of Shapiro steps under various conditions, are reported and discussed in this Letter. The sample was an array of intrinsic Josephson junctions singled out from inside a high T(C) superconducting Bi2Sr2CaCu2O(8+x) single crystal, with a bow-tie antenna integrated to it. The number of junctions in the array was controllable, the junctions were homogeneous, the distribution of applied irradiation among the junctions was even, and the junctions could synchronously respond to high frequency irradiation.     

Coherence effects in S/I/N/I/FS tunnel junctions

The dc Josephson effect in superconductor / insulator / normal metal / insulator/ferromagnetic superconductor junctions has been studied. We calculate the de Josephson current based on the Bogoliubov de Gennes equation. The Josephson current is derived as a function of exchange field in ferromagnetic superconductor, normal metal thickness and insulating barrier strength. It is found that there exists an oscillation relation between the critical Josephson current and the normal metal thickness. The oscillation amplitude decreases as the thickness of the normal metal increases or the exchange field augments.[第一段]     

Rectifying effect in a Josephson junction ratchet array

We have studied numerically a rectifying effect in an underdamped Josephson junction ratchet array driven by dc and ac current. The array consists of both alternating potential barriers and alternating inter-capacitances along the direction of vortex flow. The guide banks of high critical currents are assigned for all the longitudinal junctions to prevent the percolative pattern of vortex motion. In some junction parameters, we see a rectifying effect which indicates a finite value of the time-averaged voltage at zero dc bias. The directional dependence of the vortex motion becomes fairly large when the junction parameters lie in an optimal range which gives rise to a Shapiro step at zero dc bias. Such a rectifying effect survives for small thermal fluctuation, but eventually disappear beyond a certain critical temperature.     

Josephson effect in supercondutor/ferromagnetic semiconductor/superconductor junctions

Using a general expression for dc Josephson current, we study the Josephson effect in ballistic superconductor (SC)/ferromagnetic semiconductor (FS)/SC junctions, in which the mismatches of the effective mass and Fermi velocity between the FS and SC, spin polarization P in the FS, as well as strengths of potential scattering Z at the interfaces are included. It is shown that in the coherent regime, the oscillatory dependences of the maximum Josephson current on the FS layer thickness L and Josephson current on the macroscopic phase difference φ for the heavy and light holes, resulting from the spin splitting energy gained or lost by a quasiparticle Andreev-reflected at the FS/SC interface, are much different due to the different mismatches in the effective mass and Fermi velocity between the FS and the SC, which is related to the crossovers between positive (0) and negative (π) couplings or equivalently 0 and π junctions. Also, we find that, for the same reason, Z and P are required not to surpass different critical values for the Josephson currents of the heavy and light holes. Furthermore, it is found that, for the dependence of the Josephson current on φ, regardless of how L,Z, and P change, the Josephson junctions do not transit between 0 and π junctions for the light hole.     

Broadly tunable subterahertz emission from internal branches of the current-voltage characteristics of superconducting Bi2Sr2CaCu2O(8+δ) single crystals

Continuous, coherent subterahertz radiation arises when a dc voltage is applied across a stack of the many intrinsic Josephson junctions in a Bi2Sr2CaCu2O(8+δ) single crystal. The active junctions produce an equal number of I-V characteristic branches. Each branch radiates at a slightly tunable frequency obeying the Josephson relation. The overall output is broadly tunable and nearly independent of heating effects and internal cavity frequencies. Amplification by a surrounding external cavity to allow for the development of a useful high-power source is proposed.