Functional Nanostructures and Metamaterials for Superconducting Spintronics

The short review is devoted to the state of the art of a booming field of research in spintronics—superconducting spintronics. The spin valve properties of hybrid structures consisting of alternating layers of superconductor (S) and ferromagnetic (F) of nanoscale thicknesses and superconducting due to the proximity effect are considered in detail. The experimental data for the weak and strong ferromagnetic materials are analyzed; the role of the domain structure of the ferromagnet and the scattering of electrons with spin flip at the SF interfaces in the magnitude of the valve effect is considered. Theoretical works describing the effects of the spin valve for diffusive and clean limits are analyzed. The necessity of consideration of the multiplicity of configurations of the superconducting order parameter in multilayer SF heterostructures is underlined.

A heterostructure comprising several ferromagnetic and superconducting layers acquires functionality of managing the superconducting properties of a system applying external magnetic field. At non-collinear magnetic configurations of the ferromagnetic layers, spin-triplet pairings can be induced in these heterostructures. The triplet pairing channel brings additional degrees of freedom to manage superconducting transition temperature in proximity effect superconducting spin valves. Applied to Josephson junctions’ physics, a robust long-range pairing in ferromagnetic weak links produces spin-polarized Josephson currents available for manipulations with magnetic fields and currents. The unique features of the spin-triplet pairings in superconductor–ferromagnet heterostructures make them promising for superconducting spintronics (supertronics).

This chapter is devoted to the study of controllable proximity effects in superconductors (S), in terms of both fundamental aspects and applications. As a part of the work, theoretical description was suggested for a number of structures with superconducting electrodes and multiple interlayers with new physics related to the proximity effect and nanoscale φ-junctions. They are Josephson structures with the phase of the ground state φ _g , 0 <  φ _g  < π φ-junctions can be created on the basis of longitudinally oriented normal metal (N) and ferromagnetics (F) layers between superconducting electrodes. Under certain conditions, the amplitude of the first harmonic in the current-phase relation (CPR) is relatively small due to F layer. The coupling across N layer provides negative sign of the second harmonic. To derive quantitative criteria for realization of a φ-junction, we have solved two-dimensional boundary-value problem in the frame of Usadel equations for overlap and ramp geometries of different structures with NF bilayer. This chapter is focused on different geometries of nanoscale φ-structures of the size much less than Josephson penetration depth λ _J . At the same time, φ-state cannot be realized in conventional SNS and SFS sandwiches. Proximity effect between N and F layers limits minimal possible size of φ-junction. In the case of smaller junctions, NF bilayer becomes almost homogeneous, φ-state is prohibited, and junction exists in 0- or π-state. The conditions for realization of φ-junctions in ramp-type S–NF–S, overlap-type SFN–FN–NFS, and RTO-type SN–FN–NS geometries are discussed in the chapter. It is shown that RTO-type SN–FN–NS geometry is most suitable for practical realization. It is also shown in this chapter that the parameter range of φ-state existence can be sufficiently broadened. It allows to realize Josephson φ-junctions using up-to-date technology. By varying the temperature, we can slightly shift the region of 0-π transition and, consequently, we can control the mentioned phase of the ground state. Furthermore, sensitivity of the ground state to an electron distribution function permits applications of φ-junctions as small-scale self-biasing single-photon detectors. Moreover, these junctions are controllable and have degenerate ground states +φ and −φ, providing necessary condition for the so-called silent quantum bits.

We report on studies of heterostructure made of a cuprate superconductor YBa₂Cu₃O_7-d, a ruthenate/manganite (SrRuO₃/La_0.7Sr_0.3MnO₃) spin valve, and thin gold film (Au). It is shown that a magnetic moment is excited in the cuprate superconductor due to magnetic proximity effect, at the same time magnetic moment is suppressed in the ruthenate/manganite part. The measurements showed that magnetic moment penetration depth significantly exceeds the coherence length of the cuprate superconductor. The induced magnetic moment could be attributed to coupling of the Cu and Mn atoms by a covalent chemical bond resulting in a strong hybridization and orbital reconstruction. The mesa-structures with micrometer sizes were prepared by adding superconducting niobium film (Nb) adjacent to the gold, forming a second superconducting electrode. The DC superconducting current flowing across the mesa-structure was observed even in the case when interlayer thicknesses were much greater than the coherence lengths of the ferromagnets in heterostructure. The maximum of the critical current took place when the thicknesses of ferromagnetic films in spin valve were near to the coherence lengths of the ferromagnets. Obtained data agree with the theoretical predictions for occurrence of the spin-triplet pairing. We measured superconducting current when applied magnetic field was by two orders greater than the field level required for one magnetic flux quantum nucleation in the mesa-structure. Although theory for long-range spin-triplet pairing predicts a dominance of the second harmonic, our estimation of the second harmonic amplitude in the current-phase relation of superconducting current did not exceed 50% of the first one.

Normal metal–insulator–superconductor (NIS) tunnel junctions, as well as SIS junctions, are the main building blocks for superconducting electronics. Single NIS junctions and arrays are used in microwave bolometers, cryogenic thermometers, electron coolers, radiation detector. In such devices, junctions should fit different parameters for area, transparency, material properties and thermal characteristics. This leads to various fabrication methods and technique for measurements. Estimations made for equilibrium measurement conditions can be controversial in the case of microwave bolometers, ultra-low temperature thermometers, electron coolers. In terahertz bolometers, as well as in electron coolers, the energy distribution of electrons becomes different from Fermi distribution. In bolometers illuminated with terahertz radiation, the density of electrons will increase at higher energies, and in electron coolers, the distribution will be with reduced density at higher energies. Andreev reflection and proximity effect at the superconductor-normal metal interface induce changes in IV curve compared to simple SIN model. In this review, we present two levels of description for practical applications; first approximation is conventional with single-electron tunneling and equilibrium electron energy distribution, and the second taking into account non-Fermi distribution, Andreev currents, high-energy phonon creation and phonon escape.

Two types of planar multichroic antennas are reviewed. Frequency and beam characteristics of the antennas are calculated numerically. Both antennas are shown to have characteristics satisfying the ESA requirements.

In this chapter, we firstly introduce the fundamental radiation detection theory of the passive millimeter-wave imaging technique for human-body security inspections. A radiation temperature transfer model for the passive millimeter-wave near-field imaging is then proposed. The method to analyze the temperature contrast between the human body and the concealed objects under indoor and outdoor environments is presented accordingly. Furthermore, several key technologies involved in the passive millimeter-wave imaging systems are discussed in detail, including the millimeter-wave radiometer, the millimeter-wave feed antenna, the focusing antenna, and the quasi-optical theory. Finally, a prototype of a Ka-band passive millimeter-wave imaging system based on the focal plane array is manufactured. The system design process and the measurement results for some typical scenarios are elucidated.

Superconductor–ferromagnet nanoscale hybrid structures have attracted considerable theoretical and experimental efforts, both due to the fundamental question of the competition of superconductivity and magnetism, and possible applications in superconducting spintronics and quantum information processing. Most of the attention has recently been focussed on magnetic Josephson junctions and the triplet proximity effect, and several chapters in this book are devoted to this topic. In addition to triplet Cooper pairs, superconductor–ferromagnet hybrid structures feature spin-polarized quasiparticle transport. Quasiparticles are exclusively responsible for heat transport in superconductors, since the Cooper pairs carry no entropy. In this chapter, we discuss recent developments in the investigation of coupled quasiparticle spin and heat transport in nanoscale superconductors with a spin splitting of the density of states. The coupling of spin and heat transport leads to very slow spin relaxation and giant thermoelectric effects, which could lead to interesting applications in thermometry or microrefrigeration.

This chapter is devoted to special realizations of lasing on single artificial atoms. It is demonstrated that special properties of quantum systems, implemented as an electrical circuit, may be explored to repeat original quantum optic experiments and extend them to new regimes. As we will discuss, this can, for example, lead to the realizations of lasing that only requires two states of the artificial atom. There we make use of the relaxation and of special coupling properties that naturally are achieved in the field of the circuit quantum electrodynamics.

An overview is given about some of topological effects, owing to special geometries in real space, implemented by the high-tech self-organization techniques to fabricate micro- and nanoarchitectures. Self-assembled quantum volcanos, which are singly connected, surprisingly exhibit the Aharonov–Bohm behavior in experiment. This is explained by the fact that in a quantum volcano the electron wave functions are identical to the electron wave functions in a quantum ring from a topological point of view. Combination of a geometric potential and an inhomogeneous twist renders an observation of the topology-driven effects in the electron ground-state energy in Möbius rings at the microscale into the area of experimental verification. In inhomogeneous Möbius rings, a “Delocalization-to-localization” transition is found for the electron ground state. Advances in the high-tech roll-up fabrication methods have provided qualitatively novel curved superconductor micro- and nanoarchitectures, e.g., nanostructured microtubes, microhelices and their arrays. Vortex dynamics in open superconductor microtubes in the presence of a transport current are influenced by the interplay between the scalar potential and the inhomogeneous magnetic field component, which is normal to the surface. The rolled-up conical-shaped asymmetric microcavities provide a background to realize the spin–orbit interaction of light for the analysis of topological effects in the course of a non-Abelian evolution. Robustness of the topologically induced geometric phase of light opens novel ways of manipulating photons and thus implies promising perspectives of applications in on-chip quantum devices.

An overview of functional magnonic metamaterials is presented. We consider three types of the magnetic structures. First, we demonstrate the frequency-selective spin-wave transmission in irregular tapered magnonic strip with a periodical width modulation. By using space- and time-resolved Brillouin light scattering spectroscopy technique, we measured the features of the intermodal interaction and scattering at the boundaries of the periodical structures. In the vicinity of the band-gap frequency region, the spin-wave spatial patterns depend on the mode interaction in the width-modulated confined magnonic structure. We believe that these results are important for control of the spin-wave propagation in width-modulated magnetic structures for future spintronic and magnonic devices. Second, we consider the irregular magnetic strip with the tapered region. We show that the broken translational symmetry leads to the switching of the mode interferential pattern. We also demonstrate that the non-uniform magnetic field profile forms the conditions for the local three-magnon decay in the irregular magnetic strip. These findings are important for the planar magnonic network concept as the “Beyond CMOS” computing techniques. Next, we propose the side-coupled magnonic crystal with the defect area inside. The coupling of the defects leads to the complicated spin-wave transmission spectra due to the coexistence of the multiple defect modes inside the frequency range of the magnonic band gap. Finally, we present the results of the study of the transformation of dynamic magnetization patterns in a bilayer multiferroic structure. This phenomenon is described with a simple electrodynamic model based on the numerical finite-element method. The studied confined multiferroic strip can be utilized for the fabrication of integrated dual tunable functional devices for magnonic applications.

This chapter reviews the results of study of electronic quantum transport, superconductivity, and magnetic phenomena of bicrystals of Bi and BiSb alloy system at temperatures 1.8–100 K and magnetic fields up to 400 kOe. A similar Fermi surface (FS) consisting of crystallite interfaces (CIs) and bulk crystallites has been found. In the quantum transport oscillations spectrum, a number of new oscillation harmonics have been detected, characterizing the much larger cross-sectional areas of FS in CIs than in Bi single crystals. A number of quantum Hall plateaus were observed in inclination-type bicrystals. They vanish after the magnetic field reversal and thereby indicate that the flow of Dirac fermions is dependent on the field orientation. It has been found that the semiconductor–semimetal transition is induced in the central and adjacent layers of the CIs at different values of the magnetic field. Two/one superconducting phases with the onset of transition ≤ 36 K are observed at CIs of bicrystals, while the rhombohedral Bi and 3D topological insulator (TI) BiSb are diamagnetic and do not exhibit superconductivity. In large crystallite disorientation angle BiSb interfaces, both superconductivity and weak ferromagnetism were revealed simultaneously.