O. Chernyashevskyy

A Radically Configurable Six-State Compound

Radically Organic Metals such as manganese are relatively stable over a wide range of oxidation states. In contrast, purely organic compounds are rarely susceptible to incremental addition or removal of electrons without accompanying fragmentation or coupling reactions. Barnes et al. (p. 429; see the Perspective by Benniston) report a catenane (a compound comprising interlocked rings) in which the topological structure stabilizes six different states that successively differ by the presence or absence of one or two electrons in the framework. The hepta-oxidized state proved remarkably resilient to oxygen exposure. An interlocked-rings topology stabilizes a wide range of collective oxidation states in a metal-free organic compound. [Also see Perspective by Benniston] Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.

Microwave absorption measurements using a broad-band meanderline approach

We describe a technique that permits broad-band, field-dependent ferromagnetic and electron paramagnetic resonance absorption measurements that is applicable to thin films and patterned micro-/nanostructured arrays and is based on a wire-wound meanderline approach. Techniques to prepare meanderlines and perform microwave measurements are described along with some demonstrations involving an electron paramagnetic resonance calibration/test material, 2,2-diphenyl-1-picryl-hydrazyl, and a ferromagnetic cobalt thin film.

Superconducting-Ferromagnetic Transistor

We report experimental results on the dc and ac characterization of multiterminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively), which display transistor-like properties. We investigated two types of such superconducting-ferromagnetic transistors (SFTs): ordinary devices with a single acceptor (SIS) junction, and devices with a double acceptor. The devices with the single SIS acceptor were investigated and demonstrated a modulation of the maximum Josephson current as a function of the SFIFS current injection level. For devices of the second type, by applying an ac signal (in the kilohertz range) with a constant dc bias current to the injector (SFIFS) junction, we observed a voltage gain of about 25 on the double acceptor with the operating point chosen in the subgap region of the acceptor current-voltage characteristic. We also observed an excellent input-output isolation in our SFIFSIS devices. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications.

Control of Supercurrent in Hybrid Superconducting–Ferromagnetic Transistors

We report experimental results on characteristics of SFIFS junctions and multiterminal SFIFSIS and SISFIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The SFIFS (SFIS) junctions serve as injectors in the SFIFSIS (SISFIS) devices, which have transistorlike properties; for this reason, we call them superconducting-ferromagnetic transistors. We have found the F (Ni) thickness at which the SFIFS current-voltage characteristic becomes linear. For three-terminal SFIFSIS devices, we focused on studying the influence of the injection current through the SFIFS junction on the maximum Josephson current of the SIS acceptor. For four-terminal SISFIS devices, we studied dependence of the transport current through the middle Nb electrode on injection current level through the SFIS junction. In both cases, we found that the output superconducting current (either Josephson or transport) can be efficiently modulated by the quasiparticle injection from SFIFS or SFIS input junction. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications.

DC and RF measurements of superconducting-ferromagnetic multi-terminal devices

We report experimental results of the DC and RF characterization of multi-terminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively), which display transistor-like properties. Applying a 0.2 MHz signal in addition to a constant DC bias current to the injector (SFIFS) junction, we observe a voltage gain of ~1.25 on the acceptor SIS junction, if operation point of the SIS junction is chosen in the sub-gap region of its current-voltage characteristic. We also observed a good input-output isolation of our SFIFSIS devices of the order of 30 dBV. The experiments indicate that, after optimization of the geometry and improvement of junction quality, these devices can be used as input-output isolators and amplifiers for memory, digital and RF applications.

Broadband ferromagnetic resonance measurements of a micromagnetic disk array using a meander-line technique

A simple technique involving transmission through a copper-wire meander line is developed to study ferromagnetic resonance (FMR) in magnetic arrays. Using this technique, we carried out FMR measurements on a square array of submicrometer-size Permalloy magnetic disks for an external dc magnetic field parallel to the plane of the array, and found several magnetic excitation modes. The dominant mode obeys the Kittel formula for a plain film. The technique is suitable for studying the collective excitation modes in the magnetic nanoarrays over a broad range of frequencies.

Critical Current Gain in High-jc Superconducting-Ferromagnetic Transistors

We report experimental and theoretical results on the current gain in superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively]. The Josephson critical current density jca of the acceptor (SISF) junction in the devices is above 9 kA/cm2, which is higher than that in our previous devices. The critical current gain is defined as |δIca|/|δIi|, where a change |δIi| in the injector (SFIFS junction) current produces a change |δIca| in the acceptor maximum Josephson current. We observed a small-signal gain as high as 7.8 and a large-signal current gain of about 2.2. In addition, we found that the Josephson current of the acceptor junction is sensitive to the state of the ferromagnetic layers. A theoretical model is proposed to describe the nonequilibrium processes in the SFT devices, which agrees with the experimental observations. The calculation shows that, in the present device configuration, the dominant contribution to the gap suppression in the middle Nb electrode is due to the quasiparticle injection; spin injection plays a secondary role. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection. We conclude that SFT devices can be used as input/output isolators and amplifiers for memory, digital, and RF applications.

Multi-Terminal Superconducting Nonequilibrium Device With a Ferromagnetic Screen

We have fabricated and characterized double-barrier multi-terminal superconducting transistor-like devices with the Nb/Al/AlO_x/Al/Nb/Ni/Al/AlO_x/Al/Nb (S₁IS₂FIS₃) structure, where S, I, and F refer to superconducting, insulating, and ferromagnetic layers, respectively. A thin ferromagnetic Ni layer screens the superconductivity of the middle Nb layer on the injector-barrier side, so that the Nb/Ni bilayer manifests itself as a superconductor in the current-voltage characteristic (I-V curve) of the acceptor S₁IS₂ junction, but as a normal metal in the I-V curve of the injector S₂FIS₃ junction. It is shown that this property allows for considerable improvement of the input-output isolation of the quasiparticle-injection devices as compared with that for the formerly reported quiteron.

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 ...

Characteristics of Zr-based single- and multiple-barrier superconducting tunnel junctions

Using Zr oxide for the tunnel barrier formation, it is shown experimentally that the effect of pinholes is suppressed in the double-barrier versus single-barrier junction configuration. It is found that Josephson current in Nb∕Zr double-barrier junctions is much lower than in Nb∕Al junctions with comparable specific tunneling resistance and middle layer thickness. Multiple oxidation process is used to improve the quality of superconductor-insulator-superconductor Zr-based junctions.