Large Josephson Junction Research Articles

Magnetic field dependence of the critical current in YBa2Cu3O7−δ bicrystal grain boundary junctions

We have performed a detailed study of the magnetic field dependence of the critical current, Ic(B), of YBa2Cu3O7−δ bicrystal grain boundary junctions (GBJs). GBJs with width W much larger than the Josephson penetration depth λJ show Ic(B) dependencies that are close to those of ideal large Josephson junctions with overlap geometry. The Ic(B) dependencies are symmetrical with respect to B=0 and Ic decreases linearly with increasing applied magnetic field for B≤BJ1, where BJ1 is the lower critical field of the GBJ. Furthermore, Ic(B=0) increases linearly with increasing width of the GBJs as expected for Josephson junctions with overlap geometry. From the measured Ic(B) dependencies the temperature dependence of BJ1 and the London penetration depth could be derived.

Dynamics of rotationally symmetric solitons in the near-SG field model with application to fluxon motion in a large area Josephson junction: a collective-coordinate description

The dynamics of the ( n+1)-dimensional rotationally symmetric sine-Gordon solitons of large radius under potential perturbations and dissipative pertubations as well, is considered by means of a perturbation treatment based on a collective-coordinate description of the solution and the Lagrangian variational method. The particle-like character of a soliton is emphasized. The equation of soliton motion and, in particular, the generalized momentum of the soliton are obtained in a canonical manner. All kinds of dynamical regimes in the phase space that correspond to various initial conditions are explored. In the presence of dissipation, the corrected equation of soliton motion is obtained from the generalized variation method. Finally the application of the theoretical treatment is considered for the solitons (or fluxons) in a circularly symmetric Josephson junction, and the analytical results are examined by direct numerical simulations with fairly good agreement.

Properties of superconducting networks with large Josephson junctions

We derive a new approach to study superconducting Josephson lattices whose size L is large compared to the London penetration depth λ L. When L is larger than the Josephson penetration depth δ, we find the critical magnetic field H c1, the energy E p of pinning and the frequency of free oscillations of vortices about centers of pinning. Josephson lattices with L ⪌ δ are described by the XY model taking into account self-consistently magnetic fields of the Josephson currents. Highly anisotropic Josephson lattices are described by the one dimensional Frenkel-Kontorova model. Finally, in the limit of a high magnetic field, H, the problem is reduced to the modified XY model whose coupling strength has an oscillating dependence on H.

Preparation of Large Area NbN/AlN/NbN Josephson Junctions

AℓN has been used as a barrier material in large josephson junctions. The chemical and structural compatibility of AℓN with NbN make it possible to fabricate NbN/AℓN/NbN junctions by sequential reactive sputtering in a common Ar and N2 atmosphere. In a junction with an area of about 1.0×1.0 mm2, having a transition temperature of 14.5K. the measured I–V and first derivative curves yield a sum gap value of about 3.0 meV.

Period doubling and chaos in large area Josephson junctions induced by rf signals

The influence of an applied rf signal on the emitted radiation from a large area Josephson junction is examined. A model of the system is presented in the framework of the one-dimensional sine-Gordon equation. The model linearizes for small and large values of the amplitude of the applied signal. In the intermediate regime lower and upper threshold values (for the amplitude) for transition to chaos are found. Feigenbaum period doubling (period-doubling bifurcation cascade) appears when chaos is approached for increasing amplitude. The findings are supported experimentally.

Some Aspects of Self-Field Effects in Large Vanadium-Based Josephson Junctions

Experiments concerning large V-VxOy-Pb Josephson junctions have been performed. Structures having an overlap-type geometry have been considered. Preliminary experimental results are justified in the framework of the linearized current-phase model.

THE STEP STRUCTURES EFFECT OF DC JOSEPHSON CURRENT WITHIN ONE FLUX QUANTUM PERIOD φ0(Ⅱ)—— JOSEPHSON JUNCTION WITH LARGER DIMENSIONS

We point out that by placing a large Josephson junction in high Q-value cavity whose rth eigen mode is excited by the Josephson frequency ω0=2cV0/h, the oscillating electromagnetic field will act on the juntion and one must recognize that the space part of the eigen mode of the field are not the same on the junction. We have obained theoretical result which is different from paper [2] and shows how the step structure current vary with the magnetic field. The match condition for eigen mode of the feedback field from the cavity and electromagnetic field radiated by Josephson junction has been worked out. Finally, the physical reason for the absence of step structure with period less than φ0 is given.

Magnetic field distribution in large two-dimensional Josephson junctions

Asymptotic approach to the theory of large ( a \ll \lambda_{J} ) two-dimensional Josephson junction is developed. As a result, the usual dc sine-Gordon equation for the rapidly changing Josephson phase difference φ is reduced to the much more simple hydrodynamic-type equations div\{A(k)\rightarrow{k}\} = 0 , curl \rightarrow{k} = 0 , for the slowly changing wave vector \rightarrow{k} (A is a simple function of k ). The reduced equations are applied for analysis of a square-shaped Josephson junction and are solved using the rigorous boundary conditions. The obtained dependence of the junction critical current on the applied magnetic field is discussed and compared with the recent experimental data.

On vortex configurations in two-dimensional sine-Gordon systems with applications to phase transitions of the Kosterlitz-Thouless type and to Josephson junctions

Vortex solutions of the two-dimensional sine-Gordon equation are found by a new method. Their properties and connections to the Kosterlitz-Thouless type of phase transitions are investigated. An interpretation of the single vortex configuration in large area Josephson junctions is given.

Zero field singularities in Josephson junctions

Abstract The maximum amplitude, the magnetic field dependence, and the temperature dependence of D.C. current singularities in large Josephson junctions are discussed by approximate analytical solutions of the Sine-Gordon equation with bias and losses.

大型ジョセフソン素子の作業及びその基礎特性の評価

Large Josephson tunnel junctions have been fabricated using the dc plasma anodization method and their basic properties have been studied by the comparison between theory and experiment. It is shown that the uniformity of the tunneling barrier layer can be evaluated by the quasi-particle tunneling current and the magnetic field dependence of the maximum zero-voltage current. Many interesting phenomena have been observed in dc current-voltage characteristics of large Josephson junctions in the presence of a microwave radiation, and it is shown that they result from vortex motion induced by the applied microwave.

Vortex motion modulated by a self field in a large Josephson junction

Motion of a vortex array (flux flow) in a large Josephson junction is studied both theoretically and experimentally. An expression for dc current-voltage ( I-V ) characteristics of a large junction in a flux-flow state, which is caused by the external dc magnetic field and current, is obtained by taking account of the self field due to the dc current through the junction as well as the large loss due to the surface resistance of junction electrodes. It is shown that the spacially uniform flux flow is crucially modulated by the self field in a high-velocity region of the flux flow. As a result, the magnitude of the current peak in I-V curves decreases significantly and saturates with respect to both the length and the quality factor of the junction, which is confirmed by experiments with large Pb-Pb x O y -Pb junctions.

Regulated AC power for Josephson interferometer latching logic circuits

We have developed an ac power distribution scheme which provides switched regulated power of alternating polarity (no rectification necessary) for Josephson interferometer latching logic circuits. Distribution from the power source relies on a tree system of thin film transformers having single primaries and multiple secondaries to maintain low current levels and small phase skews throughout the system. Power signals are clipped on chip to the desired voltage supply level by series connected large area Josephson junctions (regulator junctions) and distributed across the chip by voltage busses. Supply resistors between the regulated voltage bus and the logic circuits define the operating current level. A detailed design is presented for a specific interferometer latching circuit family. We report experimental data on the two major components of the power system design - the regulators and the transformers. A series string of four regulator junctions provides a trapezoidal waveform of 11 mV amplitude at > 66% logic duty cycle (that portion of the machine cycle in which logic operations can be performed). Thin film transformers have their designed inductance (350 pH) and coupling coefficient ( > .90).

Flux-flow characteristics of a large Josephson junction

In order to study the various aspects associated with flux flow in a large Josephson junction, a new equation which describes a balance of net forces acting on each vortex has been derived by a spatial average of the two-dimensional Josephson equation. This equation reveals the existence of two kinds of driving forces, i.e., a driving force due to density gradients of vortices and that due to a line tension of a vortex, and also the existence of two kinds of counterforces, i.e., a viscous drag force due to a normal current and an inertial force due to a junction capacitance. It is shown by this equation that there exist two propagation modes of envelope waves in a large Josephson junction in a flux-flow state. An analytical expression for flux-flow conductance is also obtained using this equation, which is in reasonable agreement with experimental results.

Current flow in large Josephson junctions

Some aspects of current flow in large Josephson junctions are discussed within the approximation of a linear current-phase relationship. The behavior of various geometrical junction configurations, widely employed in practice, has been investigated.

Dynamic behavior of Josephson tunnel junctions in the subnanosecond range

Transitions from the zero-voltage to the voltage state have been directly measured on large (≃104 μm2) Josephson junctions. By adjusting the current load, the switching times (10–90%) were varied from 60 to 400 psec. Good agreement was found between theoretical and experimental results. The times required to transfer current from junctions into superconductive inductances (10−11−10−10 H) were also investigated. The switching times ranged from 300 to 550 psec and agreed basically with our simple theoretical model. To explain the details of the voltage developed during current transfer, self-pumped junction resonances had to be invoked. Although these times are expected to be smaller in miniaturized devices, they represent to our knowledge the shortest times ever reported for circuits with a dissipation of less than 10−5 W in continuous operation.

Thermal effect in U–I characteristics of large Josephson junctions

Thermal effect in U–I characteristics of large Josephson junctions

Flux-Flow Effect in Josephson Tunnel Junctions

Experimental results exhibiting the displaced linear slope effect in the volt-ampere characteristics of large Josephson junctions are reported. These data substantially confirm theoretical predictions based on the effect of magnetic flux flow.