Shot-noise-driven escape in hysteretic Josephson junctions

We have measured the influence of shot noise on hysteretic Josephson junctions initially in the macroscopic quantum tunneling regime. The escape threshold current into the resistive state decreases monotonically with increasing average current through the scattering conductor, which is another tunnel junction. Escape is predominantly determined by excitation due to the wideband shot noise. This process is equivalent to thermal activation (TA) over the barrier at effective temperatures up to about 4 times the critical temperature of the superconductor. The presented TA model is in excellent agreement with the experimental results.     

Fabrication of high-quality submicron Nb/Al--AlOx/Nb tunnel junctions

Nb/Al-AlOx/Nb tunnel junctions are often used in the studies of macroscopic quantum phenomena and superconducting qubit applications of the Josephson devices. In this work, we describe a convenient and reliable process using electron beam lithography for the fabrication of high-quality, submicron-sized Nb/Al-AlOx/Nb Josephson junctions. The technique follows the well-known selective Nb etching process and produces high-quality junctions with Vm=100 mV at 2.3 K for the typical critical current density of 2.2 kA/cm^2, which can be adjusted by controlling the oxygen pressure and oxidation time during the formation of the tunnelling barrier. We present the results of the temperature dependence of the sub-gap current and in-plane magnetic-field dependence of the critical current, and compare them with the theoretical predictions.     

Josephson Effect in FS/I/N/I/FS Tunnel Junctions

The Bogoliubov de Gennes equation is applied to the study ofcoherence effects in the ferromagnetic superconductor/insulator/normalmetal/insulator/ferromagnetic/superconductor (FS/I/N/I/FS) junction. We calculated the Josephson current in FS/I/N/I/FS as a function of exchange field in ferromagnetic superconductor, temperature, and normal metal thickness. It is found that the Josephson critical current in FS/I/N/I/FS exhibits oscillations as a function of the length of normal metal. The exchange field always suppresses the Josephson critical current I_p for a parallel configuration of the magnetic moments of two ferromagnetic superconductor (FS) electrodes. In the antiparallel configuration, the Josephson critical current I_Ap at the minimum values of oscillation increases with the exchange field for strong barrier strength and at low temperatures.     

Preliminary results on tunnel junctions on YBCO bulk with an artificial barrier

YBCO-bulk based tunnel junctions employing a cadmium sulphide artificial barrier have been fabricated. Preliminary results on the role played by the CdS film thickness and different YBCO surface processing are discussed. dc Josephson effect has been detected in samples with a low tunneling resistance. The dependence of the critical current on the external magnetic field is reported.     

0-pi Josephson tunnel junctions with ferromagnetic barrier

We fabricated high quality Nb/Al2O3/Ni(0.6)Cu(0.4)/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a steplike thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microm (1.3lambda(J)) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying approximately 6.7% of the magnetic flux quantum Phi(0). The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.     

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]     

Nb/AlOx-Al/Nb超导隧道结制备氧化工艺与特性研究

提出一种氧的等离子氧化的方法改善结区边缘绝缘性能,降低超导隧道结的漏电流。对Al膜进行等离子氧化能够有效的改善氧化膜的绝缘性能,AES分析表明:氧化绝缘层均匀,界面清晰;应用此方法成功制备出较好性能的Nb隧道结。     

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).     

Josephson Current in f-Wave Superconductor/f-Wave Superconductor Junctions

We study the temperature T and the phase of the Josephson critical current I( φ ) by taking into account the roughness scattering effect at inerface in an f-wave superconductor (S)/Insulator layer (I)/f-wave superconductor (S) junction. It is found that the Josephson critical currents in f-wave S/f-wave S, the barrier strength and the roughness strength at inerface always suppress the Andreev reflection. When α =β, the phase dependence of theJosephson current I ( φ ) between two f-wave S is predicted to be sin φ; particularly, when α ≠β, the phase dependence of the Josephson current I( φ ) between two f-wave superconductors is not predicted to be sin φ and with the barrier strength increasing, the period of the I( φ ) turns decrease.     

Josephson effect through a ferromagnetic layer

We report transport measurements on Superconductor/Ferromagnet/Superconductor (S/F/S) junctions: Nb/Al/Gd/Al/Nb where gadolinium (Gd) is a weakly polarized ferromagnet. A sizeable critical current I _c is observed in the I(V) characteristics. This current can be modulated by a weak magnetic field, as expected for a Josephson current. With these experiments, we establish that superconducting coherent transport survives across a small ferromagnetic layer. The penetration depth of Cooper pairs in Gd has been measured. An extensive study of the Josephson critical current in temperature for different thicknesses of magnetic compounds is presented. A comparison of transport measurements with S/N/S junction is given through measurements made on Nb/Al/Y/Al/Nb, where yttrium (Y) is used as non magnetic rare earth metal. Received 20 September 2000 and Received in final form 22 February 2001     

Features of the effect of the Josephson medium parameters on vortex formation and critical current

The response of an intergranular Josephson junction to displacements of an Abrikosov vortex in a superconducting polycrystal is studied theoretically. The vortex filament in the vicinity of the junction excites a tunnel current in the junction and also generates a Josephson vortex with which it merges upon emergence at the surface of the junction. It is shown that the process of the Josephson vortex formation passes through a stage of overcoming a potential barrier, whose height depends on the distance between the Abrikosov vortex and the junction, as well as on the effective thickness of the junction, which is determined by the characteristic grain size, grain anisotropy, and the intensity of the intergranular coupling. The magnetic field dependence of the critical current of the intergranular Josephson junction is determined for various grain and intergranular parameters, as well as for the triangular and square configurations of the Abrikosov vortex lattice. The results indicate that a high degree of texturing in the grain size, anisotropy, and intensity of intergranular coupling is very important for obtaining high critical currents in pure polycrystalline materials.     

Are "pinholes" the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.     

Wideband detection of the third moment of shot noise by a hysteretic Josephson junction

We use a hysteretic Josephson junction as an on-chip detector of the third moment of shot noise of a tunnel junction. The detectable bandwidth is determined by the plasma frequency of the detector, which is about 50 GHz in the present experiment. The third moment of shot noise results in a measurable change of the switching rate when reversing polarity of the current through the noise source. We analyze the observed asymmetry assuming adiabatic response of the detector.     

A trilayer process for the fabrication of Al phase qubits

Herein we develop an Al/AlOx/Al trilayer process,feasible to fabricate complex circuits with wiring crossovers,for the preparation of Al junctions and phase qubits.The AlOx layer is obtained by in situ thermal oxidation,which provides high-quality junction tunnel barriers.The Al junctions show a considerably low leakage current and the Josephson critical current density can be conveniently controlled in the range of a few to above 100 A/cm2,which is favorable in the phase qubit application.Macroscopic quantum tunneling,energy spectrum,energy relaxation time,Rabi oscillation,and Ramsey interference of the Al phase qubits are measured,demonstrating clearly quantum coherent dynamics with a timescale of 10 ns.Further improvements of the coherent dynamic properties of the device are discussed.     

Nonequilibrium Josephson effect through helical edge states

We study Josephson junctions between superconductors connected through the helical edge states of a two-dimensional topological insulator in the presence of a magnetic barrier. As the equilibrium Andreev bound states of the junction are 4π periodic in the superconducting phase difference, it was speculated that, at finite dc bias voltage, the junction exhibits a fractional Josephson effect with half the Josephson frequency. Using the scattering matrix formalism, we show that his effect is absent in the average current. However, clear signatures can be seen in the finite-frequency current noise. Furthermore, we discuss other manifestations of the Majorana bound states forming at the edges of the superconductors.     

Josephson effects in dilute bose-einstein condensates

We propose an experiment that would demonstrate the dc and ac Josephson effects in two weakly linked Bose-Einstein condensates. We consider a time-dependent barrier, moving adiabatically across the trapping potential. The phase dynamics are governed by a "driven-pendulum" equation, as in current-driven superconducting Josephson junctions. At a critical velocity of the barrier (proportional to the critical tunneling current), there is a sharp transition between the dc and ac regimes. The signature is a sudden jump of a large fraction of the relative condensate population. Analytical results are compared with a numerical integration of the Gross-Pitaevskii equation, in an experimentally realistic situation.     

Low noise submillimeter SIS receiver with niobium nitride quasiparticle tunnel junctions

We have developed and tested a submillimeter waveguide SIS mixer with NbN-MgO-NbN quasiparticle tunnel junctions. The two junction array is integrated in a full NbN printed circuit. The NbN film critical temperature is 15 K and the junction gap voltage is 5 mV. The size of the junctions is 1.4 × 1.4 µm and Josephson critical current density is about 1.5 KA/cm² resulting in junction R_NC product about 40. The inductive tuning circuit in NbN is integrated with each junction in two junction array. A single non contacting backshort was tuned at each frequency in the mixer block.At 306 GHz the minimum DSB receiver noise temperature is as low as 230 K. The sources of the receiver noise and of the limits of the NbN SIS submillimeter mixer improvement are discussed.     

Josephson effect throughout the BCS-BEC crossover

We study the stationary Josephson effect for neutral fermions across the BCS-BEC (Bose-Einstein condensate) crossover, by solving numerically the Bogoliubov-de Gennes equations at zero temperature. The Josephson current is found to be considerably enhanced for all barriers at about unitarity. For vanishing barrier, the Josephson critical current approaches the Landau limiting value which, depending on the coupling, is determined by either pair-breaking or sound-mode excitations. In the coupling range from the BCS limit to unitarity, a procedure is proposed to extract the pairing gap from the Landau limiting current.     

Strong-coupling topological Josephson effect in quantum wires

We investigate the Josephson effect for a setup with two lattice quantum wires featuring Majorana zero energy boundary modes at the tunnel junction. In the weak-coupling regime, the exact solution reproduces the perturbative result for the energy containing a contribution ～ ± cos(?/2) relative to the tunneling of paired Majorana fermions. As the tunnel amplitude g grows relative to the hopping amplitude w, the gap between the energy levels gradually diminishes until it closes completely at the critical value gc [Formula: see text]. At this point the Josephson energies have the principal values [Formula: see text], where m =- 1,0,1 and σ =± 1, a result not following from perturbation theory. It represents a transparent regime where three Bogoliubov states merge, leading to additional degeneracies of the topologically nontrivial ground state with an odd number of Majorana fermions at the end of each wire. We also obtain the exact tunnel currents for a fixed parity of the eigenstates. The Josephson current shows the characteristic 4π periodicity expected for a topological Josephson effect. We discuss the additional features of the current associated with a closure of the energy gap between the energy levels.     

Quantum two level systems and kondo-like traps as possible sources of decoherence in superconducting qubits

We discuss the origin of decoherence in Josephson junction qubits. We find that two level systems in the surrounding insulator cannot be the dominant source of noise in small qubits. We argue that electron traps in the Josephson barrier with large Coulomb repulsion would emit noise that agrees both in magnitude and in temperature dependence with experimental data.