Multilayer Josephson Junctions Research Articles

Field theoretical model of multilayered Josephson junction and dynamics of Josephson vortices

Multi-layered Josephson junctions are modeled in the context of a field theory, and dynamics of Josephson vortices trapped inside insulators are studied. Starting from a theory consisting of complex and real scalar fields coupled to a U(1) gauge field which admit parallel $N-1$ domain-wall solutions, Josephson couplings are introduced weakly between the complex scalar fields. The $N-1$ domain walls behave as insulators separating $N$ superconductors, where one of the complex scalar fields has a gap. We construct the effective Lagrangian on the domain walls, which reduces to a coupled sine-Gordon model for well-separated walls We then construct sine-Gordon solitons emerging in an effective theory in which we identify Josephson vortices carrying singly quantized magnetic fluxes. When two neighboring superconductors tend to have the same phase, the ground state does not change with the positions of domain walls. On the other hand, when two neighboring superconductors tend to have $\pi$-phase differences, the ground state has a phase transition depending on the positions of domain walls; when the two walls are close to each other, frustration occurs because of the coupling between the two superconductors besides the thin superconductor. Focusing on the case of three superconductors separated by two insulators, we find for the former case that the interaction between two Josephson vortices on different insulators changes its nature, i.e., attractive or repulsive, depending on the positions of the domain walls. In the latter case, there emerges fractional Josephson vortices when two degenerate ground states appear due to spontaneous charge-symmetry breaking, and the number of the Josephson vortices varies with the position of the domain walls. Our predictions should be verified in multilayered Josephson junctions.

Dielectric losses in multi-layer Josephson junction qubits

We have measured the excited state lifetimes in Josephson junction phase and transmon qubits, all of which were fabricated with the same scalable multi-layer process. We have compared the lifetimes of phase qubits before and after removal of the isolating dielectric, SiNx, and find a fourfold improvement of the relaxation time after the removal. Together with the results from the transmon qubit and measurements on coplanar waveguide resonators, these measurements indicate that the lifetimes are limited by losses from the dielectric constituents of the qubits. We have extracted the individual loss contributions from the dielectrics in the tunnel junction barrier, AlOx, the isolating dielectric, SiNx, and the substrate, Si/SiO2, by weighting the total loss with the parts of the electric field over the different dielectric materials. Our results agree well with and complement the findings from other studies, demonstrating that superconducting qubits can be used as a reliable tool for high-frequency characterization of dielectric materials. We conclude with a discussion of how changes in design and material choice could improve qubit lifetimes by up to a factor of 4.

Overdamped characteristics of multilayered MgB2/AlN/Al/MgB2 Josephson junction

MgB 2 / Al / AlN / MgB 2 multilayered Josephson junctions were fabricated on c-plane sapphire substrates. The measured current-voltage characteristics were well fitted with a resistively and capacitively shunted junction model. For a junction with 0.56-nm-thick AlN and 10-nm-thick Al layers, the current density was 740 A/cm2 and the ICRN product was 210 μV. The Josephson currents were found to be ideally modulated in accordance with theoretical calculations by an external magnetic field. Clear Shapiro steps were observed under irradiation at 95.622 GHz, and fourth step was obtained. Shapiro step heights were consistent with the resistively and capacitively shunted junction model.

Multilayer Josephson Junctions as a Multiple Quantum Well Structure

The macroscopic quantum model in conjunction with the transfer matrix method and Bloch wave analysis have been used to compute the energy dispersion equation for Cooper pairs in finite superconducting quantum well (SQW) structures. The energy dispersion equation has been numerically solved by the Cauchy Integration method to obtain the energy levels of quantum states in SQW. The possibility of energy levels and subbands formation for Cooper pairs within the bulk energy gap has been shown. Energy gap squeezing has been derived when the Bragg condition for Bloch wave number is satisfied.

Collective oscillations of the superconducting gap of multilayered Josephson junctions

Collective modes involving the gap amplitude $\ensuremath{\Delta}$ and phase $\ensuremath{\Phi}$ in a multilayered superconducting junction are considered. The junction is formed by a sequence of ``clean'' superconducting layers $S$ separated by insulating barriers $I$. Using the quasiclassical Green function method, we derive the self-consistent linear equations describing the collective amplitude $\ensuremath{\delta}$ and phase $\ensuremath{\varphi}$ oscillations. We find that the $\ensuremath{\delta}$ and $\ensuremath{\varphi}$ modes in ``clean'' multilayered Josephson junctions may be long lived for a range of magnitudes of the barrier transparency and Cooper coupling strength. We find that the frequency of the resonances in a multilayered junction can be tuned by applying a bias supercurrent across the adjacent subjunction.

Microstructural studies of YB 2Cu 3O 7− δ/Nd 2CuO 4/YB 2Cu 3O 7− δ Josephson junctions with a Nd 2CuO 4 buffer layer grown on YSZ substrate

The effect of Nd 2CuO 4 (NCO) buffer layer on the microstructures of YB 2Cu 3O 7− x /Nd 2CuO 4/YB 2Cu 3O 7− x Josephson junctions grown on a YSZ substrate was investigated by high-resolution X-ray diffraction, small angle X-ray reflection, X-ray reciprocal space maps, atomic force microscopy and scanning electron microscopy. The results show that the YBCO films were well oriented in the [0 0 L] direction perpendicular to the substrate surface. No intermediate layer was formed between the YBCO and NCO films. The FWHM of the YBCO film with a NCO buffer was smaller comparing to that of the YBCO film deposited directly onto YSZ substrates. Moreover, its surface morphology was more compact and the average size of grains on the surface was also much smaller, forming a solid base for fabrication of YBCO/NCO/YBCO multilayer Josephson junctions.

Preparation of continually graded barriers of YPrBaCuO for HTS Josephson junctions

We report preparation of a novel barrier structure for high T/sub c/ superconducting multilayer Josephson junctions using a simple composite target technique. Such a barrier consists of Y/sub x/Pr/sub 1-x/Ba/sub 2/Cu/sub 3/O/sub y/ with a continually varied concentration of yttrium. In this barrier no lattice mismatch and other incompatibility problems occur between adjacent layers. Thus the formation of interfacial defects and structural strain can be mostly prevented. The Josephson coupling takes place at the electrically formed interfaces rather than the structural interfaces. A particular feature of these junctions is that the effective thickness of the barrier strongly depends on the measuring temperature and the concentration gradient. The absence of the structural interface in the weak link region greatly enhances the reproducibility and the performance of these junctions.

Simulation ofI−Vhysteresis branches in an intrinsic stack of Josephson junctions in high-Tcsuperconductors

$I\ensuremath{-}V$ characteristics of the high-${T}_{c}$ superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{1}{\mathrm{C}}_{2}{\mathrm{O}}_{8}$ shows a strong hysteresis, producing many branches. The origin of hysteresis jumps is studied by use of the model of multilayered Josephson junctions proposed by one of the authors (T.K.). The charging effect at superconducting layers produces a coupling between the next-nearest-neighbor phase differences, which determines the structure of hysteresis branches. It will be shown that a solution of phase motions is understood as a combination of rotating and oscillating phase differences, and that, at points of hysteresis jumps, there occurs a change in the number of rotating phase differences. Effects of dissipation are analyzed. The dissipation in insulating layers works to damp the phase motion itself, while the dissipation in superconducting layers works to damp relative motions of phase differences. Their effects to hysteresis jumps are discussed.

Interfacial microstructures in ramp type multilayer Josephson junctions studied by TEM

The microstructures of high T/sub c/ ramp type Josephson junctions were studied by using transmission electron microscopy. The work was emphasized at the interfacial defects and the influence of the ramp slope. The results show that for the ramp slope angles of 15/spl deg//spl sim/40/spl deg/, the epitaxy was still remained through all layers at the ramp region without the formation of big grain boundaries. No amorphous layers and secondary phases were observed at the barrier interfaces. For a gentle ramp junction, small misoriented grains appeared in some portions of the barrier. The substrate ramp formed during the ion etching process had little influence on the growth of the upper layers. In junctions with a steep ramp, defects increased near the interface, although the epitaxy of the barrier was of good quality. The results demonstrate that the slope angle of the junction ramp is an important factor that influences the performance of the Josephson junctions.

Superconducting films and devices

Simple superconducting circuits and sensors can be fabricated in the new cuprate high temperature superconductors and conventional low-T c components can be integrated at a large scale. Reliable multilayer depositions and Josephson junctions need to be developed in high-T c materials for commercial success. The field has had approximately 12 years to grow since the discovery of ceramic superconductors that enable approximately 10 times higher temperature operation than was previously possible. Compared to more mature technologies, ceramic superconductors should now be about half-way to commercial realization. Niche markets, in which specialized devices strongly out perform the competition, will be the first to develop, however, large-scale superconducting electronics should be no serious threat to semiconducting electronics within the next 10 to 12 years.

Submillimeter-band high-power generation using multilayered Josephson junctions

We report on numerical simulations of 10- and 19-layer stacked Josephson junctions. Simulations are performed using the experimentally feasible parameters for Nb/Al–AlOx/Nb junctions. With an appropriate choice of parameters such as magnetic field and coupling, the current–voltage curves of N-layer stacks systematically indicate in-phase locking among the inner N−2 junctions. The simulation results suggest that multilayer Josephson junctions are very promising as submillimeter-band high-power oscillators.

Microstructural study of ramp type multilayer Josephson junctions with different ramp slopes

The microstructures of YBa 2Cu 3O 7− δ /PrBa 2Cu 3O 7− δ /YBa 2Cu 3O 7− δ ramp type Josephson junctions with different ramp slopes fabricated on (100) SrTiO 3 substrates were studied by means of transmission electron microscopy. The results show that for the ramp slope angles of 15°–40°, the epitaxy was still remained through all layers at the ramp region without the formation of big grain boundaries. No amorphous layers and secondary phases were observed at the barrier interfaces. For a gentle ramp junction, small misoriented grains appeared in some portions of the barrier. The substrate ramp formed during the ion etching process had little influence on the growth of the upper layers. For a steep ramp junction, a dimple was created in the substrate, which caused the bending in the film and the nucleation of misoriented grains, although the epitaxy of the barrier was of good quality. The results demonstrate that the slope angle of the junction ramp is an important factor that influences the performance of the Josephson junctions.

Quasiparticle dynamics and phase locking in a S-I-S multilayer Josephson junction

New dynamical equations describing the Josephson effect and nonequilibrium quasiparticle distribution in a multilayer Josephson tunnel structure at T∼Tc are obtained, starting from the microscopic theory. It is shown that quasiparticle dynamics has a strong influence on the Josephson effect. Novel regimes with giant charge-imbalance oscillations are found. A new type of hysteresis on the current-voltage characteristic is predicted.

Properties of a YBCO/insulator/YBCO trilayer and its application to a multilayer Josephson junction

We have examined the properties of a trilayer high-temperature superconductor (HTS)/insulator/HTS structure and incorporated this structure with multilayer Josephson ramp edge junctions to form a SQUID with a ground plane. The lower and upper (YBCO) films in the trilayer structure showed zero-resistance temperatures of 89 K and 85 K respectively. A directly coupled ground-planed SQUID using ramp edge Josephson junctions with a normal barrier layer (PBCO) showed a voltage modulation of at 9 K and operated up to 50 K.

X-ray photon detection with multilayered Josephson junctions

Abstract Superconducting tunnel junctions can be configured as high resolution X-ray spectrometers however, because they are thin, STJs suffer from inherent photoabsorption inefficiency. A common approach to enhance quantum efficiency is to couple the junctions to superconducting absorbers and employ quasiparticle traps. An alternative approach is to use vertical stacks of tunnel junctions as detectors. These multilayered superconducting tunnel junctions (MSTJs) need not be thin and may have additional advantages including increased signal to noise ratio. We report on the fabrication of multilayered tunnel junctions and successful X-ray detection by an MSTJ.

Multilayer Josephson junction flux quantum devices

We describe the properties of flux quantum circuitry employing the relatively young technology of multilayer Josephson junctions with n superconductor-insulator (SI) layers. Multilayer junctions can be employed as both passive and active devices to increase circuit integration density, allow for new logic/voltage thresholds and higher impedances, and improve thermal noise stability. We present the results from numerical simulations of a conventional RSFQ circuit and two novel circuits with multilayer junction designs. Neural circuitry is a focus of our novel multilayer designs. We also discuss layout and fabrication issues, considering the recent progress in the fabrication of Nb multilayer junctions with AlN tunnel barriers, which exhibit intrinsic overdamping at the level of each SI layer. Included in this discussion is a long term assessment of a multilayer approach in view of deep sub-micron and high T/sub c/ technologies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Josephson transmission lines with multilayer Josephson junctions

We present a new type of Josephson transmission line (JTL) in which discrete multilayer Josephson junctions are employed instead of the standard single tri-layer junction technology. It is shown via numerical simulation that such a structure supports the propagation of multiple flux quantum solitons, which are represented in this system by a 2πn kink in the phase difference across a multilayer junction of n superconductor-insulator-superconductor layers. As a practical aspect, parallel and series connected multilayer junctions in a JTL can significantly reduce the need for microstrip line inductance, thereby providing the optimal conditions for large scale integration of JTL-based logic and memory circuitry. In this article we investigate in detail through numerical simulation the properties of JTL designs with multilayer junctions, incorporating expected variations in junction characteristics. A brief description of the current fabrication of multilayer junctions for flux quantum circuitry will be provided.

High quality a-Si/Nb and a-SiN/NbN artificial multilayers for Josephson applications

A high resolution transmission electron microscopy study of multilayer films prepared by magnetron sputtering shows that the morphology of the growing interface in a-Si/Nb and a-SiN/NbN multilayers is remarkably uniform and smooth; this is in contrast to the polycrystalline AlN/NbN multilayers grown under similar conditions, which exhibit columnar grain structure with rough interfaces. For proper sputtering parameters, the amorphous layers seem to periodically restore a relatively smooth initial interface condition for the successive Nb (or NbN) layer growth, consequently interrupting the tendency toward increased roughness due to mechanisms such as columnar growth. Artificial multilayers having very flat interfaces could stimulate applications based on multilayer Josephson junctions.

Preparation of MgO Thin Films by Dual Ion Beam Sputtering

1 Introduction Recently much attention has been devoted to the study on the deposition and properties of oxide films. Oxide thin films, such as MgO, SiO_2 and ZrO_2, not only can be used as insulating layers in electronic devices but also act as buffer layers which can effectively obstruct the interface reaction between substrates and films or different layers. Moreover, in the technology of multilayer structures and Josephson junctions, it is also needed to grow insulating layers with perfect epitaxial structure. Up to now, it has been experimentally proved that a variety of oxide thin films can be used as buffer layers or

Multilayer high-temperature superconductors

Abstract Using the BCS approach and a tightbinding description of the band structure, we explore some superconducting properties of layered superconductors. Particular emphasis is devoted to the interlayer pairing interaction g 3 . Negative values of g 3 lead to a staggered order parameter, having alternating signs from layer to layer, while g 3 positive leads to one sign only. Adopting a superconducting loop, interrupted by a multilayer, it is shown that the sign of g 3 can be determined experimentally. The same holds for a DC SQUID configuration, consisting of two multilayer Josephson junctions in a loop, and for an AC SQUID.