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A general picture of photonic eigenmodes of dielectric rings with a rectangular cross section is presented, which is fundamentally different from those of whispering gallery modes of the disc. The optical spectrum of a rectangular dielectric ring consists of an infinite set of individual galleries, each starting with broad transverse radial or axial Fabry-P\'erot resonances due to two pairs of side faces. Each gallery continues with a set of equidistant longitudinal modes with exponentially increasing quality factors. Transverse radial and axial modes exhibit strict periodicity of line contours in the Fano-Lorentz-Fano-Lorentz... sequence due to the periodicity of cylindrical harmonics. Theoretical and experimental results are in perfect agreement., Comment: 4 pages, 5 figures, Supplementary material

Periodicity is usually assumed to be the necessary and sufficient condition for the formation of band gaps, i.e., energy bands with a suppressed density of states. Here, we check this premise by analyzing the band gap properties of three structures that differ in the degree of periodicity and ordering. We consider a photonic crystal, disordered lattice, and ordered but nonperiodic quasicrystalline structure. A real-space metric allows us to compare the degree of periodicity of these different structures. Using this metric, we reveal that the disordered lattice and the ordered quasicrystal can be attributed to the same group of material structures. We examine the density of their photonic states both theoretically and experimentally. The analysis reveals that despite their dramatically different degrees of periodicity, the photonic crystal and the quasicrystalline structure demonstrate an almost similar suppression of the density of states. Our results give new insight into the physical mechanisms resulting in the formation of band gaps., Comment: 6 figures

An experimental demonstration of directivities exceeding the fundamental Kildal limit, a phenomenon called superdirectivity, is provided for spherical high-index dielectric antennas with an electric dipole excitation. A directivity factor of about 10 with a total efficiency of more than 80\% for an antenna having a size of a third of the wavelength was measured. High directivities are shown to be associated with constructive interference of particular electric and magnetic modes of an open spherical resonator. Both analytic solution for a point dipole and a full-wave rigorous simulation for a realistic dipole antenna were employed for optimization and analysis, yielding an excellent agreement between experimentally measured and numerically predicted directivities. The use of high-index low-loss ceramics can significantly reduce the physical size of such antennas while maintaining their overall high radiation efficiency. Such antennas can be attractive for various high-frequency applications, such as antennas for the Internet of things, smart city systems, 5G network systems, and others. The demonstrated concept can be scaled in frequency.

The work is devoted to the study of the processes of formation and analysis of the parameters of a functional nanostructure -- a superconducting spin valve, which is a multilayer structure consisting of ferromagnetic cobalt nanofilms separated by niobium superconductor films. The studies were carried out using molecular dynamics simulations. The atomic structure of individual nanolayers of the system is considered., Comment: Invited talk of prof. A.Sidorenko at XXIV International Symposium "Nanophysics and Nanoelectronics" in Nyjnii Novgorod, 2020, in russian

Employment of the non-trivial proximity effect in superconductor/ferromagnet (S/F) heterostructures for the creation of novel superconducting devices requires accurate control of magnetic states in complex thin-film multilayers. In this work, we study experimentally in-plane transport properties of microstructured Nb/Co multilayers. We apply various transport characterization techniques, including magnetoresistance, Hall effect, and the first-order-reversal-curves (FORC) analysis. We demonstrate how FORC can be used for detailed in situ characterization of magnetic states. It reveals that upon reduction of the external field, the magnetization in ferromagnetic layers first rotates in a coherent scissor-like manner, then switches abruptly into the antiparallel state and after that splits into the polydomain state, which gradually turns into the opposite parallel state., Comment: arXiv admin note: text overlap with arXiv:2010.03454

The article considers the simulation of the formation of a multilayer nanocomposite, the combination of elements of which gives rise to the effect of a spin valve. The relevance and importance of the effects in the field of spintronics and related materials and devices are described. The composition and atomic structure of individual layers of the multilayer nanocomposite, as well as the composition and morphology of the interface of the nanocomposite layers are the object of research. A sample with a periodic superconductor - ferromagnetic structure consisting of more than 20 alternating layers of niobium and cobalt was analyzed as a sample. The deposition process took place under deep vacuum conditions. Modeling was performed by the molecular dynamics method using the potential of the modified submerged atom method. The formation of the layers was performed in the stacionar mode.

Magnetic state of exchanged biased CoO(20nm)/Co($d_F$) bilayer ($d_F$=5-20nm) was studied by means of polarized neutron reflectometry. By spacing of CoO/Co bilayer and Al$_2$O$_3$ substrate with Nb(20nm) layer we created waveguide structure which allowed us to significantly enhance intensity of spin-flip (SF) scattering in the position of optical resonances. For the trained sample with thinnest Co(5nm) we detected strong SF scattering at the resonance position (up to 30\% of incoming intensity) speaking about high non-collinearity of the system. As $d_F$ increases, the intensity of SF scattering linearly decreases. At the same time we observed asymmetry of up-down and down-up scattering channels at the resonance positions. We attribute this asymmetry to the Zeeman splitting of neutrons energies with different initial polarization taking place in high external field. Analysis, however, shows that the applied in the PNR experiment external field is not enough to quantitatively explain the observed asymmetry for the samples with $d_F > $ 5nm and we have to postulate presence of additional magnetic field produced by sample. We attribute this additional field to the stray field produced by chiral Bloch domain walls. The chirality of the domain walls can be explained by Dzyaloshinskii-Moriya interaction arising at the CoO/Co interface. Our results can be useful for designing of spintronic devices using exchange bias effect.

We study Superconductor/Ferromagnet/Superconductor junctions with CuNi, PtNi, or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be significantly larger than through diluted alloys. The phenomenon is attributed to the dirtiness of disordered alloys leading to a short coherence length despite a small exchange energy. To the contrary, pure Ni is clean resulting in a coherence length as long as in a normal metal. Analysis of temperature dependencies of critical currents reveals a crossover from short (dirty) to long (clean) range proximity effects in Pt1-xNix with increasing Ni concentration. Our results point out that structural properties of a ferromagnet play a crucial role for the proximity effect and indicate that conventional strong-but-clean ferromagnets can be advantageously used in superconducting spintronic devices., Comment: 14 pages, 8 figures

Being a general wave phenomenon, bound states in the continuum (BICs) appear in acoustic, hydrodynamic, and photonic systems of various dimensionalities. Here, we report the first experimental observation of an accidental electromagnetic BIC in a one-dimensional periodic chain of coaxial ceramic disks. We show that the accidental BIC manifests itself as a narrow peak in the transmission spectra of the chain placed between two loop antennas. We demonstrate a linear growth of the radiative quality factor of the BICs with the number of disks that is well-described with a tight-binding model. We estimate the number of the disks when the radiation losses become negligible in comparison to material absorption and, therefore, the chain can be considered practically as infinite. The presented analysis is supported by near-field measurements of the BIC profile. The obtained results provide useful guidelines for practical implementations of structures with BICs opening new horizons for the development of radio-frequency and optical metadevices., Comment: 9 pages, 6 figures

We have investigated the structural, magnetic and superconducting properties of [Nb(1.5nm)/Fe(x)]$_{10}$ superlattices deposited on a thick Nb(50nm) layer. Our investigation showed that the Nb(50nm) layer grows epitaxially at 800$^\circ$C on Al$_2$O$_3$(1$\bar{1}$02) substrate. Samples grown at this condition posses a high residual resistivity ratio of 15-20. By using neutron reflectometry we show that Fe/Nb superlattices with $x<$ 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has properties of a weak ferromagnet. Proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by co-existing superconducting and normal-state domains. By increasing the thickness of the Fe layer to $x$ = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the periodic Fe/Nb structure.

We present a new study of magnetic structures with controllable effective exchange energy for Josephson switches and memory. As a basis for a weak link we propose to use a periodic structure comprised of ferromagnetic (F) layers spaced by thin superconductors (s). Our calculations based on Usadel equations show that switching from parallel (P) to antiparallel (AP) alignment of neighboring F layers can lead to a significant enhancement of the critical current through the junction. To control magnetic alignment we propose to use periodic system where unit cell is a pseudo spin-valve $F_1$/s/$F_2$/s with $F_1$ and $F_2$ two magnetic layers having different coercive fields. In order to check feasibility of controllable switching between AP and P states through the \emph{whole} periodic structure we prepared a superlattice [Co(1.5nm)/Nb(8nm)/Co(2.5nm)/Nb(8nm)]$_6$ between two superconducting layers of Nb(25nm). Neutron scattering showed that parallel and antiparallel alignment can be organized by using of magnetic fields of only several tens of Oersted.

The predictions of Moore's law are considered by experts to be valid until 2020 giving rise to "post-Moore's" technologies afterwards. Energy efficiency is one of the major challenges in high-performance computing that should be answered. Superconductor digital technology is a promising post-Moore's alternative for the development of supercomputers. In this paper, we consider operation principles of an energy-efficient superconductor logic and memory circuits with a short retrospective review of their evolution. We analyze their shortcomings in respect to computer circuits design. Possible ways of further research are outlined., Comment: OPEN ACCESS

We investigated the upper critical magnetic field, $H_{c}$, of a superconductor-ferromagnet (S/F) bilayer of Nb/Cu$_{41}$Ni$_{59}$ and a Nb film (as reference). We obtained the dependence of $H_{c\perp}$ and $H_{c\parallel}$ (perpendicular and parallel to the film plane, respectively) on the temperature, $T$, by measurements of the resistive transitions and the dependence on the inclination angle, $\theta$, of the applied field to the film plane, by non-resonant microwave absorption. Over a wide range, $H_{c\perp}$ and $H_{c\parallel}$ show the temperature dependence predicted by the Ginzburg-Landau theory. At low temperatures and close to the critical temperature deviations are observed. While $H_{c}(\theta)$ of the Nb film follows the Tinkham prediction for thin superconducting films, the Nb/Cu$_{41}$Ni$_{59}$-bilayer data exhibit deviations when $\theta$ approaches zero. We attribute this finding to the additional anisotropy induced by the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like state and propose a new vortex structure in S/F bilayers, adopting the segmentation approach from high-temperature superconductors., Comment: Accepted to Phys. Rev. B

An asymptotic investigation of monochromatic electromagnetic fields in a layered periodic medium is carried out under the assumption that the wave frequency is close to the frequency of a stationary point of the dispersion surface. We find solutions of Maxwell equations by the method of two-scale asymptotic expansions. We establish that the principal order of the expansion of a solution dependent on three spatial coordinates is the sum of two differently polarized Floquet-Bloch solutions, each of which is multiplied by a slowly varying envelope function. We derive that the envelope functions satisfy a system of differential equations with constant coefficients. In new variables, it is reduced to a system of two independent equations, both of them are either hyperbolic or elliptic, depending on the type of the stationary point. The envelope functions are independent only in the planar case. Some consequences are discussed., Comment: 36 pages

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core of a superconducting spin valve. The switching effect of the spin valve is based on interference phenomena occurring due to the proximity effect at the S/F interfaces. A remarkable effect is only expected if the core structure exhibits strong critical temperature oscillations, or most favorable, reentrant superconductivity, when the thickness of the ferromagnetic layer is increased. The core structure has to be grown on an antiferromagnetic oxide layer (or such layer to be placed on top) to pin by exchange bias the magnetization-orientation of one of the ferromagnetic layers. In the present paper we demonstrate that this is possible, keeping the superconducting behavior of the core structure undisturbed., Comment: 22 pages, 12 figures, 1 table

We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve structure, the resistive state of which depends on the preceding magnetic field polarity. The effect is based on a strong exchange bias (about -2 kOe) on a diluted ferromagnetic copper-nickel alloy and generation of a long range odd in frequency triplet pairing component. The difference of high and low resistance states at zero magnetic field is 90% of the normal state resistance for a transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic states of the structure do not require biasing fields or currents in the idle mode., Comment: 9 pages, 4 figures, Accepted to Applied Physics Letters

The upper critical magnetic field H_{c2} in thin-film FSF trilayer spin-valve cores is studied experimentally and theoretically in geometries perpendicular and parallel to the heterostructure surface. The series of samples with variable thicknesses of the bottom and of the top Cu_{41}Ni_{59} F-layers are prepared in a single run, utilizing a wedge deposition technique. The critical field H_{c2} is measured in the temperature range $0.4-8$ K and for magnetic fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of the superconducting transition temperature versus F-layers thickness, induced by an external magnetic field, has been observed for the first time. In order to properly interpret the experimental data, we develop a quasiclassical theory, enabling one to evaluate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of the system parameters. A fairly good agreement between our experimental data and theoretical predictions is demonstrated for all samples, using a single set of fit parameters. This confirms adequacy of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu_{41}Ni_{59}/Nb/Cu_{41}Ni_{59} spin-valve core trilayers., Comment: 16 pages, 7 figures; published version

The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu41Ni59/nc-Nb/Co/CoOx spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a collinear alignment of the Cu41Ni59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature Tc for NCA is lower than the fixed measuring temperature. The existence of a minimum Tc, at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation., Comment: 7 pages, 4 fgures, Submitted to Physical Review Letters

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature. Both phenomena have been realized experimentally in the Cu 41 Ni 59 /Nb/Cu 41 Ni 59 trilayers investigated in the present work., Comment: 22 pages, 6 figures

We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state establishing due to the proximity effect in superconducting Nb/Cu41Ni59 bilayers. Using a special wedge-type deposition technique, series of 20-35 samples could be fabricated by magnetron sputtering during one run. The layer thickness of only a few nanometers, the composition of the alloy, and the quality of interfaces were controlled by Rutherford backscattering spectrometry, high resolution transmission electron microscopy, and Auger spectroscopy. The magnetic properties of the ferromagnetic alloy layer were characterized with superconducting quantum interference device (SQUID) magnetometry. These studies yield precise information about the thickness, and demonstrate the homogeneity of the alloy composition and magnetic properties along the sample series. The dependencies of the critical temperature on the Nb and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant thickness dF of the magnetic alloy layer and dS of the superconducting layer, respectively. All types of non-monotonic behaviors of Tc versus dF predicted by the theory could be realized experimentally: from reentrant superconducting behavior with a broad extinction region to a slight suppression of superconductivity with a shallow minimum. Even a double extinction of superconductivity was observed, giving evidence for the multiple reentrant behavior predicted by theory. All critical temperature curves were fitted with suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F spin-switch core structure were calculated using these parameters. Finally, superconducting spin-switch fabrication issues are discussed in detail in view of the achieved results., Comment: 34 pages, 9 figures

The theoretical description of the Fulde-Ferrell-Larkin-Ovchinnikov like state establishing in nanostructered bilayers of ferromagnetic (F) and superconducting (S) material leads to critical temperature oscillations and reentrant superconductivity as the F-layer thickness gradually increases. The experimental realization of these phenomena is an important prerequisite for the fabrication of the Ferromagnet/Superconductor/Ferromagnet core structure of the superconducting spin-valve. A switching of the spin-valve is only expected if such non-monotonic critical temperature behavior is observed in F/S bilayers as well as in the S/F bilayers, a combination of which the spin-valve core structure can be regarded to consist of. In our former investigations we could demonstrate the required non-monotonic behavior of the critical temperature in S/F bilayers. In this study we succeeded in the preparation of F/S bilayers, where the superconducting material is now grown on top of the ferromagnetic metal, which show deep critical temperature oscillations as a function of the ferromagnetic layer thickness as well as an extinction and recovery, i.e. a reentrant behavior, of superconductivity. Especially, the latter is necessary to obtain a spin-valve with a large critical temperature shift between the parallel and antiparallel configurations of magnetizations in the F layers., Comment: 18 pages, 5 figures