Circular Josephson Junctions Research Articles

Rapid Single-Flux-Quantum Circuits Fabricated Using <inline-formula> <tex-math notation="TeX">$\hbox{20}\hbox{-}\hbox{kA}/\hbox{cm}^{2}$</tex-math></inline-formula> <inline-formula> <tex-math notation="TeX">$\hbox{Nb}/\hbox{AlO}_{x}/\hbox{Nb}$</tex-math></inline-formula> Process

We designed and fabricated rapid single-flux-quantum (RSFQ) circuits using a new 20-kA/cm 2 process based on conventional Nb/AlO x /Nb Josephson junction (JJ) technology. Circular JJs were fabricated to improve their uniformity. They had sufficiently low spread of the critical currents for large-scale RSFQ circuits. We selected the circuit design parameters carefully, including the bias voltage and McCumber-Stewart parameter, balancing the energy consumption, switching speeds, margins, and integration density. We demonstrated several logic gates and shift registers at a clock frequency of 154 GHz with the designed bias voltage of 2.5 mV, and demonstrated energy-efficient shift registers using the low-voltage RSFQ (LV-RSFQ) technique at 83 GHz.

The shape dependency of two-dimensional magnetic field dependence of a Josephson junction

Modulation characteristics of a Josephson current are usually measured by applying the external magnetic field parallel to the junction plane from one direction, and uniformity in tunnel barrier is discussed. So far, we have measured two-dimensional magnetic field dependence of a square Josephson junction by independently scanning the magnetic field (Hx,Hy) parallel to the junction plane from two directions. We can get a lot of information about spatial critical current distribution in a Josephson junction by observing the magnetic field dependence of a Josephson junction in two dimensions. This time, we have fabricated the different-shaped Josephson junctions and investigated the shape dependency of two-dimensional magnetic field dependence of a Josephson junction. We observed the Ic-(Hx,Hy) characteristics of triangular, hexagonal, and circular Josephson junctions quite different from the Ic-(Hx,Hy), characteristics of a square Josephson junction. Furthermore, we simulated two-dimensional magnetic field dependence of a Josephson junction by calculating the superconducting current density distribution in each magnetic field. The simulation results agreed well with experimental results.

20 ${\hbox{kA/cm}}^{2}$ Process Development for Superconducting Integrated Circuits With 80 GHz Clock Frequency

Results of the development of an advanced fabrication process for superconductor integrated circuits (ICs) with 20 kA/cm2 Nb/AlOx/Nb Josephson junctions is presented. The process has 4 niobium superconducting layers, one MoNx resistor layer with 4.0 Ohm per square sheet resistance for the junction shunting and circuit biasing, and employs circular Josephson junctions with the minimum diameter of 1 mum; total 11 photolithography levels. The goal of this process development is the demonstration of the feasibility of 80 GHz clock speeds in superconducting ICs for digital signal processing (DSP) and high performance computing. Basic components of rapid single flux quantum (RSFQ) logic such as DC/SFQ, SFQ/DC converters, Josephson transmission lines (JTLs), and simple digital circuits such as T-flip-flops and 4-bit digital counters have been fabricated and tested. The T-flip-flops were shown to operate up to 400 GHz with the widest margin of operation of plusmn13% at 325 GHz. Digital testing results on the 4-bit counters as well as the junctions, resistors, and other process parameters are also presented. Prospects for yet higher speeds and very large scale integration are discussed.

Rectification of harmonically oscillating magnetic fields in quarter circular Josephson junctions

A novel method for rectifying harmonically varying magnetic fields is demonstrated using fluxons in quarter circular Josephson junctions (JJs). A JJ with a quarter circular geometry terminated with a load resistor at one end is found to be capable of rectifying alternating fields when biased with a constant dc current. An external magnetic field applied parallel to the dielectric barrier of the junction interacts with the edges of the junction and make asymmetric boundary conditions. These asymmetric boundary conditions facilitate fluxon penetration under a dc bias from one end of the junction in alternate half cycles of the applied field. Thus effective rectification of the field can be achieved using quarter circular JJs. This unique phenomenon is specific to this geometry and can be exploited for making superconducting magnetic field rectifiers. This proposed device is expected to have important applications in millimeter and sub-millimeter radio wave astronomy.

Disorder-induced desynchronization in a2×2circular Josephson junction array

Analytical results are presented which characterize the behavior of a dc-biased, two-dimensional circular array of overdamped Josephson junctions subject to increasing levels of disorder. It is shown that high levels of disorder can abruptly destroy the synchronous functioning of the array. We identify the transition boundary between synchronized and desynchronized behavior, along with the mechanism responsible for the loss of frequency locking. Comparisons with recent results for arrays with rectangular lattice geometries are described.

Interference and transmission of quantum fluxons through a Josephson ring.

The influence of quantum interference on the transmission of a fluxon through an ideal long circular Josephson junction (a ``Josephson ring'') is studied. In the low-temperature regime the transmission is a periodic function of a gauge charge applied along the ring with a period 2e. Around points of full period of both the gauge charge and the optical path, the transmission shows resonances as a function of the gauge charge and ``antiresonances'' as a function of the optical path. These resonances and antiresonances are associated with energy levels of the circular junction and with short dwelling time of the fluxon in the ring. In the high-temperature regime the interaction with plasmons dephases the fluxon wave function completely. The transmission probability in this regime is calculated in a stationary picture and in a dynamical picture and two different results are obtained. The discrepancy between the two pictures is explained via the ratio of the dwelling time to the time the thermal bath needs to change the plasmons' microscopical state. A general method that retrieves the two results is presented.

Quantum dynamics of a fluxon in a long circular Josephson junction.

Quantum dynamics of a fluxon in a long circular Josephson junction.