high-Tc Superconductor Josephson Junctions Research Articles

Standard measurement method for normal state resistance and critical current of resistively shunted Josephson junctions

An important parameter of Josephson junctions (JJs) is the product of normal state resistance (R n) and critical current (I c) for designing superconductor analogue devices or digital circuits. Determination of R n and I c from voltage–current (U–I) characteristic curves often faces difficulties; in particular I c is considerably reduced by intrinsic thermal or extrinsic electrical noises. Here, we propose a standard measurement method of R n and intrinsic critical current (I ci) for high-T c superconductor (HTS) grain boundary JJs operated in liquid nitrogen and low-T c superconductor (LTS) multilayer superconductor/normal-conductor/superconductor (SNS) JJs in liquid helium. The applicable condition of this method is that both HTS and LTS JJs have U–I curves compatible with resistively-shunted junction (RSJ) model. Both R n and I ci values are extracted by combining a geometric mean criterion to select a data set and a least-squares fitting method with the RSJ model, eliminating two distortion effects on U–I curves: noise-rounding and self-heating. The combined method ensures relative standard uncertainty values of 1.9% for R n and 8% for I ci or better, when the users follow the standard protocol. It is demonstrated that the combined method is valid for d-wave HTS JJs near 77 K, regardless of peculiarities such as 0–π junction transition in d-wave superconductors at lower temperatures, and s-wave LTS SNS JJs with a wide range of junction parameters. This work is the first step to facilitate standardization for superconductor electronics with JJs.

2D Bifurcation diagram of capacitively coupled Josephson junctions in high-Tc superconductors

We study the nonlinear dynamics in an array capacitively coupled Josephson junctions with N=3 junctions. We consider the superconductor/ insulator/ superconductor, SIS, type junctions as a model for high temperature superconductors with layered structure. In this study, we introduce the 2D bifurcation diagrams for the Josephson junctions. First we show a current voltage characteristics (CVC) of Intrinsic Josephson Junctions (IJJs) with N=3 Junctions, then we show the breakpoint region,BPR (a region in which a net electric charge appear on superconducting layers, (S-layers), of junctions), in the CVC of the system, then we try to investigate the chaos in BPR region by the 2D bifurcation diagram of the system. We show the 2D bifurcation diagrams in two cases in which either the coupling factor, α or the β parameter which is related to dissipation, be variable. In the later case we showed there is a range of β parameter in which the chaotic region of BPR is disappeared, also we show that there is another range of β in which the BPR structure disappeared.

Detailed investigation of the bifurcation diagram of capacitively coupled Josephson junctions in high-Tc superconductors and its self similarity

We study an array of coupled Josephson junction of superconductor/insulator/superconductor type (SIS junction) as a model for high temperature superconductors with layered structure. In the current-voltage characteristics of this system there is a breakpoint region in which a net electric charge appear on superconducting layers, S-layers, of junctions which motivate us to study the charge dynamics in this region. In this paper first of all we show a current voltage characteristics (CVC) of Intrinsic Josephson Junctions (IJJs) with N=3 Junctions, then we show the breakpoint region in that CVC, then we try to investigate the chaos in this region. We will see that at the end of the breakpoint region, behavior of the system is chaotic and Lyapunov exponent become positive. We also study the route by which the system become chaotic and will see this route is bifurcation. Next goal of this paper is to show the self similarity in the bifurcation diagram of the system and detailed analysis of bifurcation diagram.

Modeling of LC-shunted intrinsic Josephson junctions in high-Tc superconductors

Resonance phenomena in a model of intrinsic Josephson junctions shunted by LC-elements (L-inductance, C-capacitance) are studied. The phase dynamics and IV-characteristics are investigated in detail when the Josephson frequency approaches the frequency of the resonance circuit. A realization of parametric resonance through the excitation of a longitudinal plasma wave, within the bias current interval corresponding to the resonance circuit branch, is demonstrated. It is found that the temporal dependence of the total voltage of the stack, and the voltage measured across the shunt capacitor, reflect the charging of superconducting layers, a phenomenon which might be useful as a means of detecting such charging experimentally. Thus, based on the voltage dynamics, a novel method for the determination of charging in the superconducting layers of coupled Josephson junctions is proposed. A demonstration and discussion of the influence of external electromagnetic radiation on the IV-characteristics and charge-time dependence is given. Over certain parameter ranges the radiation causes an interesting new type of temporal splitting in the charge-time oscillations within the superconducting layers.

Imaging of local temperature distributions in mesas of high-Tc superconducting terahertz sources

Stacks of intrinsic Josephson junctions in high-Tc superconductors are a promising source of intense, continuous, and monochromatic terahertz waves. In this paer, we establish a fluorescence-based temperature imaging system to directly image the surface temperature on a Bi2Sr2CaCu2O8+δ mesa sample. Intense terahertz emissions are observed in both high- and low-bias regimes, where the mesa voltage satisfies the cavity resonance condition. In the high- bias regime, the temperature distributions are shown to be inhomogeneous with a considerable temperature rise. In contrast, in the low-bias regime, the distributions are rather uniform and the local temperature is close to the bath temperature over the entire sample.

Terahertz wave emission from intrinsic Josephson junctions in high-Tc superconductors

Recently, we experimentally demonstrated that rectangular mesa structures of intrinsic Josephson junctions(IJJ) in Bi2Sr2CaCu2O8+d (Bi2212) can be used as a compact solid-state generator of continuous, coherent and polarizedterahertz (THz) radiation. In the present work, we will exhibit tall mesas (over 600 junctions)which were fabricated using UV lithography, e-beam lithography with photoresist and e-beamlithography with a Ti selective etching technique. We will present measurements of thec-axis resistance as a function of temperature and of current–voltagecharacteristics of THz emitting mesas with lateral sizes ranging from30 × 300 to100 × 300 µm2. Furthermore, we will discuss the dependence of the characteristics of the mesa structureson the oxygen doping level of the Bi2212 crystals. We will also experimentally show thatthe voltage–frequency relation of the ac Josephson effect has to match the cavity resonancefor successful emission.

Macroscopic Quantum Tunneling and Resonant Activation of Current Biased Intrinsic Josephson Junctions in Bi-2212

The utilization of a high-Tc superconductor for implementing a superconducting qubit is to be expected. Recent researches on the quantum property of Josephson junctions in high-Tc superconductors indicate that the low energy quasiparticle excitation is weak enough to observe the macroscopic quantum tunneling. Therefore, a detailed study on the quantum property of high-Tc Josephson junctions becomes more important for applications. We show our experimental results of the macroscopic tunneling of current biased intrinsic Josephson junctions in Bi-2212 and its resonant activation in the presence of microwave radiation.

Theory of macroscopic quantum tunneling in high-Tc cuprate

To reveal macroscopic quantum tunneling (MQT) in high-Tc superconductor Josephson junctions is an important issue since there is a possibility to fabricate a superconducting quantum bit by use of high Tc junctions. Using the functional integral and the instanton theory, we analytically obtain the MQT rate (the inverse lifetime of the metastable state) for the c-axis twist Josephson junctions. In the case of the zero twist angle, the system shows the super-Ohmic dissipation due to the presence of the nodal quasiparticle tunneling. Therefore, the MQT rate is suppressed compared with the finite twist angle cases. Furthermore, the effect of the zero energy bound states (ZES) on the MQT in the in-plane junctions is theoretically investigated. We obtained the analytical formula of the MQT rate and showed that the presence of the ZES at the normal/superconductor interface leads to a strong Ohmic quasiparticle dissipation. Therefore, the MQT rate is noticeably inhibited compared with the c-axis junctions in which the ZES are completely absent.

Influence of coupling parameter on current–voltage characteristics of intrinsic Josephson junctions in high-Tc superconductors

We study the current–voltage characteristics of intrinsic Josephson junctions in high-Tc superconductors by numerical calculations and in framework of capacitively coupled Josephson junctions model we obtain the total number of branches. The influence of the coupling parameter α on the current–voltage characteristics at fixed parameter β (β2=1/βc, where βc is McCumber parameter) and the influence of α on β-dependence of the current–voltage characteristics are investigated. We obtain the α-dependence of the branch’s slopes and branch’s endpoints. The presented results show new features of the coupling effect on the scheme of hysteresis jumps in current–voltage characteristics of intrinsic Josephson junctions in high-Tc superconductors.

Effect of zero-energy bound states on macroscopic quantum tunneling in high-Tcsuperconductor junctions

The macroscopic quantum tunneling (MQT) in the current biased high-Tc superconductor Josephson junctions and the effect of the zero energy bound states (ZES) on the MQT are theoretically investigated. We obtained the analytical formula of the MQT rate and showed that the presence of the ZES at the normal/superconductor interface leads to a strong Ohmic quasiparticle dissipation. Therefore, the MQT rate is noticeably inhibited in compared with the c-axis junctions in which the ZES are completely absent.

Macroscopic quantum tunneling and quasiparticle dissipation ind-wave superconductor Josephson junctions

We examine the macroscopic quantum tunneling (MQT) in high-Tc superconductor Josephson junctions with a d-wave order parameter. Using microscopic Hamiltonian and the functional integral method, we analytically obtain the MQT rate (the inverse lifetime of the metastable state) for the c-axis twist Josephson junctions. In the case of the zero twist angle, the system shows the super-Ohmic dissipation due to the presence of the nodal quasiparticle tunneling. Therefore, the MQT rate is strongly suppressed in compared with the finite twist angle cases.

Coupling of intrinsic Josephson junctions and subgap structure in the current–voltage characteristics of high-Tc superconductors

The subgap structure in current–voltage (I–V) characteristics of a stack of intrinsic Josephson junctions in high-Tc superconductors is studied. An analytical formula for the I–V characteristics is obtained which had taken into account the influence of the dynamically breaking of charge neutrality (DBCN) in CuO2 layers on the subgap structure. It is shown that DBCN does not affect the positions and the amplitudes of the subgap peaks, but changes the curvature of the branches in the I–V characteristics. As a possible manifestation of the non-equivalence of the junction, the experimental I–V characteristics of intrinsic Josephson junctions are presented.This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

Subharmonic Shapiro steps and noise in high-T c superconductor Josephson junctions

An experimental investigation is made of the subharmonic Shapiro steps observed on the I-V curves of high-T c superconductor Josephson junctions and on the bias-voltage dependences of the rf noise and detector response when the junctions are subjected to external submillimeter radiation. Structures of this type are ordinarily described by a nonsinusoidal current-phase relation, which is why subharmonic steps appear. Numerical modeling of the processes occurring in a Josephson junction by means of a simple current-phase relation, as in the case of an SNS junction, gives good agreement with experiment. The width of the characteristic Josephson generation line of the junction was estimated on the basis of the noise dependences and the selective detector response. The width can be explained by taking into account the shot noise of the tunneling component of the conductivity. A model of the conductivity of a high-T c superconductor Josephson junction, consisting of a tunnel junction with microshorts possessing metallic conductivity, is discussed.

Large Shapiro steps and wide junction behaviour in HTS Josephson junctions at nitrogen temperatures

Abstract We have fabricated bicrystal high-Tc superconductor (HTS) Josephson junctions which, under microwave radiation, exhibited first current steps (1. Shapiro steps) with an amplitude of 1 mA at 80 K. This was possible because of a considerable increase of the critical current Ic of the junctions by increasing the HTS film thickness up to 700 nm. However, above a certain Ic threshold, the junction behaviour deviated from the simple resistively shunted junction (RSJ) model. We show that the junctions can be described by the wide junction model. Both the static (dependence of Ic on magnetic field) and the dynamic characteristics (response to microwave radiation) of our junctions can be well fitted in the frame of the wide junction model. We present data of junctions with relative widths w/λj=5 and 7 (w-physical width of the junction, λj-Josephson penetration depth), i.e. with w/λj lying in the intermediate interval where the behavior of the junctions is most sensitive to changes in w/λj. This allowed us to fit experimental data with relatively good accuracy. We discuss the consequences of wide junction behavior on the maximal first Shapiro step and the impact on metrological applications of HTS Josephson junctions.