Electrically-controlled hybrid superconductor-ferromagnet cell for high density cryogenic memory

We report the fabrication and testing, at 4.2 K, of an SISFS device, where S, F, and I denote a superconductor (Nb), a ferromagnetic material (permalloy), and an insulator (AlOx), respectively. The F layer covers about one half of the top electrode of the SIS Josephson junction and is positioned off-center. Electric current, I _tr, along the S electrode can change the magnetization of the F layer in such a way that, for one direction of I_tr, a magnetic flux penetrates the junction perpendicular to the layers, whereas for the opposite direction, the perpendicular magnetic flux can be removed. In the former state, the modulation pattern of the Josephson critical current, Ic, in the magnetic field, H, may acquire minimum near H=0, and restores its usual shape with maximum in the second state. These states can be used for building a compact cryogenic memory compatible with single flux quantum electronics.Comment: 9 pages, 3 figure

Coherent effects in double-barrier Josephson junctions

The general solution for ballistic electronic transport through double-barrier Josephson junctions is derived. We show the existence of a regime of phase-coherent transport in which the supercurrent is proportional to the single barrier transparency and the way in which this coherence is destroyed for increasing interlayer thickness. The quasiparticle dc current at arbitrary voltage is determined.Comment: 4 pages, 2 figures, submitted to Phys. Rev.

Maximum velocity of a fluxon in a stack of coupled Josephson junctions

Dynamics of a fluxon in a stack of inductively coupled long Josephson junctions is studied analytically and numerically. We demonstrate that the fluxon has a maximum velocity, which does not necessarily coincide with any of the characteristic Josephson plasma wave velocities. The maximum fluxon velocity is found by means of numerical simulations of the quasi-infinite system. Using the variational approximation, we propose a simple analytical formula for the dependence of the fluxon's maximum velocity on the coupling constant and on the distribution of critical currents in different layers. This analysis yields rather precise results in the limit of small dissipation. The simulations also show that nonzero dissipation additionally stabilizes the fluxon.Comment: 8 pages, 5 figures, 1 table. submitted to Phys. Lett. A. Suggestions and criticism are welcom

Current-carrying states in superconductor/insulator and superconductor/semiconductor superlattices in the mesoscopic regime

We discuss some of the basic theoretical aspects of current-carrying states in superconducting superlattices with tunnel barriers in the mesoscopic regime, when the superconducting layer thickness is small compared to the BCS coherence length but large compared to the atomic scale. We establish the necessary conditions for the observation of the classical Josephson effect (with sinusoidal current-phase dependence) and derive self-consistent analytical expressions for the critical Josephson current. These expressions are proportional to an additional small factor and have unusual temperature dependence as compared with the single-junction case. For certain parameter values, the superconducting gap exhibits an exponential decrease due to pair-breaking effect of the supercurrent. The supercurrent can completely destroy the superconductivity of the system above a certain characteristic temperature (lower than the transition temperature of individual layers). In this paper, we also study the effect of intrabarrier exchange interactions. We show that this effect is strongly enhanced compared with the single- junction case and can manifest itself in an exponential decrease of the critical temperature.Comment: 13 pages, ReVTeX, no figure

Coherent electronic transport through a superconducting film

We study coherent quantum transport through a superconducting film connected to normal-metal electrodes. Simple expressions for the differential conductance and the local density of states are obtained in the clean limit and for transparent interfaces. Quasiparticle interference causes periodic vanishing of the Andreev reflection at the energies of geometrical resonances, subgap transport, and gapless superconductivity near the interfaces. Application of the results to spectroscopic measurements of the superconducting gap and the Fermi velocity is analyzed.Comment: 5 pages, 4 figure

Current–voltage characteristics of Nb–carbon–Nb junctions

We report on properties of Nb(/Ti)–carbon–(Ti/)Nb junctions fabricated on graphite flakes using e-beam lithography. The devices were characterized at temperatures above 1.8 K where a Josephson current was not observed, but the differential conductivity revealed features below the critical temperature of Nb, and overall metallic conductivity, in spite of a high-junctions resistance. Since the conductivity of graphite along the planes is essentially two-dimensional (2D), we use a theoretical model developed for metal/graphene junctions for interpretation of the results. The model involves two very different graphene “access” lengths. The shorter length characterizes ordinary tunneling between the three-dimensional Nb(/Ti) electrode and 2D graphene, while the second, much longer length, is associated with the Andreev reflections (AR) inside the junction and involves also “reflectionless” AR processes. The relevant transmission factors are small in the first case and much larger in the second, which explains the apparent contradiction of the observed behavior

Globular reduced graphene oxide-metal oxide structures for energy storage applications

In this work, we employed an in situ spray pyrolysis approach to fabricate metal oxide-graphene composites with highly porous morphologies.The materials exhibited unique globular structures comprising metal oxide nanoparticles embedded between graphene sheets with high capacitance

Vortex phase boundaries from ferromagnetic measurements in a patterned disc array

Using a recently developed broadband microwave measurement technique, we have studied the hysteretic appearance and disappearance with in-plane magnetic field of the uniform ferromagnetic resonance (FMR) mode of a patterned permalloy disk array. The observed features are consistent with our micromagnetic simulations (performed on an infinite array of such disk), which predict that on decreasing the magnetic field from a positively magnetized state at positive fields the array will: (i) pass continuously into a double-vortex state; (ii) followed by a discontinuous transition to a single-vortex state; and finally (iii) discontinuously into a negatively magnetized state at some negative field. The hysteretic counterpart occurs on reversing the field sweep and returning to positive fields. The FMR data are consistent with the hysteretic dc magnetization measurements performed earlier on samples patterned in an identical manner