Superconducting Junction Arrays Research Articles

Parity effect and single-electron injection for Josephson junction chains deep in the insulating state

We have made a systematic investigation of charge transport in one-dimensional chains of Josephson junctions where the characteristic Josephson energy is much less than the single-junction Cooper-pair charging energy, ${E}_{J}\ensuremath{\ll}{E}_{CP}$. Such chains are deep in the insulating state, where superconducting phase coherence across the chain is absent, and a voltage threshold for conduction is observed at the lowest temperatures. We find that Cooper-pair tunneling in such chains is completely suppressed. Instead, charge transport is dominated by tunneling of single electrons, which is very sensitive to the presence of BCS quasiparticles on the superconducting islands of the chain. Consequently, we observe a strong parity effect, where the threshold voltage vanishes sharply at a characteristic parity temperature ${T}^{*}$, which is significantly lower than the critical temperature ${T}_{c}$. A measurable and thermally activated zero-bias conductance appears above ${T}^{*}$, with an activation energy equal to the superconducting gap, confirming the role of thermally excited quasiparticles. Conduction below ${T}^{*}$ and above the voltage threshold occurs via injection of single electrons/holes into the Cooper-pair insulator, forming a nonequilibrium steady state with a significantly enhanced effective temperature. Our results explicitly show that single-electron transport dominates deep in the insulating state of Josephson junction arrays. This conduction process has mostly been ignored in previous studies of both superconducting junction arrays and granular superconducting films below the superconductor-insulator quantum phase transition.

Theoretical Simulation of the Mixing Performance of Distributed Superconducting Tunnel Junction Arrays at 1.2 THz

In this paper we focus on the numerical simulation of the mixing behaviors of distributed superconducting junction arrays at 1.2 THz. A novel type of superconducting tunnel junctions, i.e., NbN/AlN/Nb, which have a relatively high gap voltage (4.3 mV) and can reach a critical current density as high as several tens of kA/cm 2 , are proposed for this characterization along with conventional Nb/AlOx/Nb junctions. The former is incorporated with a NbN/SiO 2 /Al tuning circuit, and the latter with a Nb/SiO 2 /Al and a NbTiN/SiO 2 /Al tuning circuits. The noise performance, local-oscillator power requirement, IF bandwidth, and optimum embedding impedance are thoroughly characterized for the two types of distributed superconducting junction arrays.

Kink pairs unbinding on domain walls and the sequence of phase transitions in fully frustrated XY models.

The unbinding of kink pairs on domain walls in the fully frustrated XY model (on square or triangular lattices) is shown to induce the vanishing of phase coupling across the walls. This forces the phase transition, associated with unbinding of vortex pairs, to take place at a lower temperature than the other phase transition, associated with proliferation of the Ising-type domain walls. The results are applicable for a description of superconducting junction arrays and wire networks in a perpendicular magnetic field, as well as of planar antiferromagnets with a triangular lattice.

Interference effects in isolated Josephson junction arrays with geometric symmetries

Reducing the size of the junctions in a Josephson junction array takes the system from the classical to the quantum regime and turns the condensate phases on the islands into periodic quantum variables. Under suitable conditions such quantum arrays exhibit Aharonov-Bohm-Casher-type interference effects which change the spectrum of this macroscopic object by the addition of one Cooper pair to the array. We discuss the case of an isolated symmetric loop of N identical junctions encircling one half superconducting quantum of magnetic flux. We argue that the ground state is nondegenerate if the total number of Cooper pairs on the array is divisible by N, and doubly degenerate otherwise (after the stray charges are compensated by the gate voltages). This result is first established in the two opposite limits of dominating Josephson and charging energies, and is further explained using symmetry arguments. As long as the ground-state symmetry is preserved, its degeneracy remains unchanged in the entire range of charging-to-Josephson energy ratio. Such interference effects may be relevant in applications of superconducting junction arrays for quantum computing.

Lower critical field and vortices configuration in 2D superconducting junction arrays

In order to describe properly the magnetic status of a 2D superconducting junction array, one has to consider not only the effect of the screening currents but also that of the particular experimental protocol followed to measure the physical quantities of interest like, for example, the magnetization. We show that the value of the lower critical field,fc1, of the junction array depends strongly on the intensity of the screening currents,i.e. on the strength of the junction coupling,Ej, and that reliable results can be obtained only by considering the full-inductance matrix. We also show that the magnetic configuration of the vortices may depend on the particular experimental approach followed (static or dynamic) and, even, in some cases, on the initial configuration of the phases.

Critical Josephson current as a characterization tool for superconducting junction arrays

Arrays of Nb/Al-Al2O3/Nb junctions used for heterodyne detection in radioastronomy were measured. Observing the current-voltage curve for different values of the external magnetic field, and interpreting the dc Josephson current, we were able to distinguish the behavior of each junction, and to estimate the inhomogeneities in areas, in critical currents, or in leakage currents among them. This provides an indication of possible cancellation of the Josephson current which is a source of additional noise and polarization instabilities (drop-back) in high-frequency superconductor-insulator-superconductor heterodyne receivers.

Charge and vortex dynamics in arrays of tunnel junctions.

Arrays of tunnel junctions can be fabricated with small nearest-neighbor capacitances between the islands and with even smaller self-capacitances of the islands. It has been suggested by Mooij et al. that electric charges on the islands of such an array interact logarithmically over large distances, and as a consequence, a Kosterlitz-Thouless-Berezinskii transition can occur where charge-anticharge pairs dissociate. Furthermore, in superconducting junction arrays the more familiar vortex-unbinding transition can occur. We investigate the effect of single-electron and Cooper-pair tunneling on these transitions and discuss the competition between the charge and the vortex unbinding.

Fractal description of ferromagnetic glasses and random Josephson networks

The well-known analogies between X-Y ferromagnets and superconducting junction arrays are discussed in the light of data on critical exponents of the latter. The role of disorder is analysed. It is shown that, although the Harris criterion seemingly predicts that disorder is irrelevant, its application to fractal percolating clusters predicts exponent values in very good agreement with experiment.