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。     

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.     

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     

Subharmonic gap structure in superconducting scanning tunneling microscope junctions

We observe a subharmonic gap structure (SGS) and the Josephson effect in superconducting scanning tunneling microscope junctions with resistances below 100 kΩ. The magnitude of the n=2 SGS is shown to scale with the square of the junction normal state conductance, in agreement with theory. We show by analyzing the Josephson effect in these junctions that the superconducting phase dynamics are strongly affected by thermal fluctuations. We estimate the linewidth of the Josephson oscillations due to phase fluctuations, a quantity that may be important in modern theories of the subgap structure. While phase fluctuations may smear the SGS current onsets, we conclude that the sharpness of these onsets in our data is not limited by fluctuations.     

Staircase Structure of Shapiro Steps

We investigate IV-characteristics of coupled Josephson junctions which model the intrinsic Josephson junctions in high temperature superconductors under external electromagnetic radiation. A staircase structure of Shapiro steps is found in the branching region. Its origin is related to the coupling between junctions and their switching from rotating to oscillating states. This conclusion is tested by detailed analysis of the IV-characteristics as for total stack and for each junction in the stack. IV-curves of junctions in the stack are compared with the average of time derivative of phase difference. Experimental observation of this staircase structure would give us a proof of coupling between junctions and a way for precise measurement of its value. Such investigations would be also useful for a diagnostic of Josephson junctions in the stack.     

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.     

Superconducting-insulator transition in disordered Josephson junctions networks

The superconducting-insulator transition is simulated in disordered networks of Josephson junctions with thermally activated Arrhenius-like resistive shunt. By solving the conductance matrix of the network, the transition is reproduced in different experimental conditions by tuning thickness, charge density and disorder degree. In particular, on increasing fluctuations of the parameters entering the Josephson coupling and the Coulomb energy of the junctions, the transition occurs for decreasing values of the critical temperature T _c and increasing values of the activation temperature T _o . The results of the simulation compare well with recent experiments where the mesoscopic fluctuations of the phase have been suggested as the mechanism underlying the phenomenon of emergent granularityin otherwise homogeneous films. The proposed approach is compared with the results obtained on TiN films and nanopatterned arrays of weak-links, where the superconductor-insulator transition is directly stimulated.     

New model for systems of mesoscopic Josephson junctions

Quantum fluctuations of the phases of the order parameter in two-dimensional arrays of mesoscopic Josephson junctions and their effect on the destruction of superconductivity in the system are investigated by means of a quantum-cosine model that is free of the incorrect application of the phase operator. The proposed model employs trigonometric phase operators and makes it possible to study arrays of small superconducting granules, pores containing superfluid helium, or Josephson junctions in which the average number of particles n ₀ (effective bosons, He atoms, and so on) is small, and the standard approach employing the phase operator and the particle number operator as conjugate operators is inapplicable. There is a large difference in the phase diagrams between arrays of macroscopic and mesoscopic objects for n ₀<5 and U<J (U is the characteristic interaction energy of the particles per granule and J is the Josephson coupling constant). Re-entrant superconductivity phenomena are discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 649–654 (25 November 1997)     

Quantum dynamics of superconducting point contacts: chiral anomaly,Landau–Zener transitions,and all that

Quantum effects in the dynamics of the Josephson phase difference in Josephson junctions with large electron transparency D are studied in the adiabatic regime, when the characteristic charging energyE_C of the junction is much smaller than the superconducting energy gap Δ. In isolated junctions, quantum phase fluctuations are large and manifest themselves as Coulomb blockade of Cooper pair tunneling. The amplitude of the Coulomb blockade oscillations is calculated for single-mode junctions with arbitrary D. In particular, it is shown that the chiral anomaly completely suppresses Coulomb blockade in ballistic junctions with D =  1, and the suppression process at D →  1 can be described as the Landau–Zener transition in imaginary time. In the regime when quantum phase fluctuations are small, they lead to quantum decay of supercurrent states due to macroscopic quantum tunneling of phase through the Josephson potential barrier. The decay rate is found in the nearly-ballistic junctions.     

Effects of disorder on voltage coherence in a modified linear Josephson array

We report numerical simulations on the effect of disorder in parallel-biased series of arrays of overdamped Josephson junctions. Our arrays feature additional parallel-conected ‘shunt’ junctions which provide strong local coupling between pairs of junctions along the series-connected ‘backbone’. We find that series arrays of 20 junctions have phase-locked and phase-coherent dynamical modes which are tolerant of more than 50% spread in critical currents,I _C , and plaquette area, S. The arrays, however, show moderate tolerance to (10% spread) disorder in bias currents,I _B . This tolerance to disorder in array parameters implies that practical RF sources might be possible using these arrays.     

Quantum fluctuations and the universal kinetic inductance of granular superconducting films

The same mean-field theory for an array of Josephson junctions that describes the onset of superconductivity at normal state resistance R_N^(c) yields the kinetic inductance, L, for R_N<R_N^(c). The predicted universal behavior of L, as a function of R_N/R_N^(c), provides a further test of the theory of quantum fluctuations.     

Self-radiation from arrays of niobium Josephson junctions embedded in the open resonator

This paper focuses on self-radiation from arrays of Josephson junctions embedded in a quasi-optical resonator. The mechanism of coupling this radiation to the principal mode of the open resonator is illustrated using experiments and simulations with CST microwave studio software. Comparing the microstrips and dielectric resonators used as the antennas of the series arrays of discrete Josephson junctions, we demonstrate that the dielectric resonator antennas are more effective than microstrips.     

Renormalization and quantum effects in a two-dimensional planar model

We examine the influence of the charging energy on the vortex unbinding transition in two-dimensional arrays of Josephson junctions. The fluctuations of the phase about the time-dependent vortex solutions are treated within the self-consistent harmonic approximation. A new relation between the renormalized vortex unbinding temperature T_c and the charging energy U is derived. We predict a continuous drop of T_c at a critical value of U well below the spurious first order transition of the rigidity constant. The results are interpreted as a quantum assisted dissociation of vortex pairs.     

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.     

Synchronisationserscheinungen in Anordnungen aus zwei Josephson-Verbindungen

Synchronization Phenomena in Arrays of Two Josephson Junctions Within the frame of the RSJ model we investigate the synchronization of the oscillations of two Josephson junctions biased separately and shunted by a resonator. Using an adiabatic approximation to the first order of the mismatch the basic equations of the slowly varying phases are derived. These equations allow us to calculate the explicite shape of the current-voltage characteristic and the radiation properties in the locking region of the array. It is also possible to determine the phase locking range in the case of an additionally applied alternating current. The modification of the internal coupling caused by the external force is discussed by concidering the stability of the solutions.     

Bi-SQUID arrays and parallel SQIF structures for active electrically small antennas

The basic physics of the development of broadband active electrically small superconductive antennas for the subgigahertz and gigahertz frequency ranges, based on series arrays with cells characterized by a highly linear voltage response to the magnetic component B of an electromagnetic signal, are under consideration. As such cells, bi-SQUIDs and cells based on two parallel SQIF structures that are differentially connected are proposed. Series arrays of cells with linear voltage response, including an antenna prototype, are fabricated using standard niobium technology with a critical current density of Josephson junctions of 4.5 kA/cm² and are studied experimentally. The data obtained allow estimation of the achievable dV/dB conversion factor and the sensitivity ??B for an antenna integrated with a magnetic flux converter, placed on an available area of 3.3 × 3.3 mm of a chip 5 × 5 mm in size. These values are found to be 10 ??V/nT and 20 fT/Hz^1/2, respectively. The conversion factor increases in proportion with the area a ² occupied by the antenna with a square flux converter, and the sensitivity is improved as a ^?3/2.     

Spectral linewidths of Josephson oscillators

We show that the linewidth of a Josephson flux-flow oscillator has the same functional dependence on temperature, static, and dynamic resistances as the ones of Josephson single-fluxon oscillators and small Josephson junctions. This suggests a universal formula for the linewidth of Josephson oscillators.     

Quantum effects on capacitively coupled intrinsic Josephson junctions

We numerically study quantum effects in intrinsic Josephson junctions of layered high-T_c superconductors in order to explain recent experimental observations on the switching rate enhancement in the low temperature quantum regime. We pay attention to the capacitive coupling between neighboring junctions and perform simulations for the Schrödinger equation derived from the Hamiltonian describing the capacitive coupling. The simulation results reveal that the phase dynamics show synchronous behaviors when entering the quantum regime. This is qualitatively consistent with the experimental result.     

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.     

Chaotic behavior of a stack of intrinsic Josephson junctions at the transition to branching for overcritical currents

Phase dynamics of a stack of coupled intrinsic Josephson junctions was investigated in the framework of capacitively coupled Josephson junctions with diffusion current model. We study the transition from the current-voltage characteristic specific to Josephson junctions arrays with small dissipation and weak coupling between the junctions to the arrays with strong coupling between the junctions and high dissipation. Low dissipative arrays of Josephson junctions are characterized by the absence of branching for overcritical currents which appears for highly dissipative arrays. Described branching appears due to charging on the superconducting layers and charge traveling waves generation. Arrays of Josephson junctions with intermediate values of coupling and dissipation parameters are characterized by the chaotic behavior, confirmed by positive Lyapunov exponent, and branching on the current voltage characteristic for both sub- and overcritical currents.