Nanostructured Superconductors Research Articles

Magnetotransport of fluxoids in intermediate-phase type-II superconducting NbN thin films around Tc

The mixed state transport properties of type-II superconductors are strongly influenced by the dynamic behavior of quantized magnetic fluxoids around the critical temperature (Tc), where a combination of normal and superconducting properties is exhibited. To understand the mixed state transport properties of type-II superconducting NbN ultrathin films (2D) we measured sheet resistance (RxxM) and Hall resistance (RxyM) of a 5-nm-thick NbN film around Tc (10.75 K) at temperatures 10.40, 10.68, and 10.77 K. Hall resistance (HR) was measured in external out-of-plane and in-plane magnetic fields up to 6 T, using 100 µA and 1 mA driving current in Van der Pauw geometry. The electric field of applied bias and Lorentz force of applied external magnetic field causes a movement of the normal conducting electrons within each fluxoid. The moving fluxoids cause dissipation and generation of Hall voltage. We developed a macroscopic analysis of the Hall resistance arising from fluxoids, to advance the differentiation between dissipating current and superconducting currents in type-II superconductors at Tc. We have extracted the number of normal conducting carriers per fluxoid and areal density and mobility of the fluxoids in dependence on the external magnetic field. This differentiation provides valuable insights into the dissipation mechanisms observed during transport measurements, e.g., after localized heating due to single photon absorption in nanostructured type-II superconductors. Furthermore, the developed macroscopic analysis of Hall resistance of fluxoids shows promising potential for investigating the fundamental aspects of fluxoid-defect interactions in type-II superconductors. Published by the American Physical Society 2024

High-frequency vortex dynamics and losses in the mixed state of nanostructured superconductors with columnar defects

We suggest a theoretical model for the microwave response of Abrikosov vortices in nanostructured superconductors with columnar defects, which serve as effective pinning sites for vortices that are considered elastic strings. For this case, we suppose that microwave losses related to the vortex oscillations in a microwave field arise caused by viscous oscillations of vortex kinks, which connect different parts of the vortex line pinned on adjacent columnar defects and vortex segments pinned on these defects. We consider the case of inclined magnetic field H, which creates vortex staircases consisting of pinned vortex segments and vortex kinks inside the superconductor. In this case, rf losses due to viscous kinks oscillations under the microwave current action arise even at T = 0 if the inclination angle of the magnetic field θΗ, concerning the columnar pinning site axis (z axis), exceeds some locking angle value θL.

Microwave surface resistance in nanostructured high-Tc superconductor films

The impact of artificially created defects nanostructure, formed by implanted dielectric nanoparticles or irradiation defects, on microwave properties of high-Tc superconductor films is analyzed in the framework of phenomenological theory for microwave response of type-II superconductors. We have calculated the surface resistance for such a kind of nanostructured type-II superconductor film and investigated conditions for the emergence of nonlinear response caused by the entrance of microwave-induced vortices in the film's interior through its edges. The obtained results indicate that artificial defect nanostructure in the film's interior formed by point-like or columnar structural defects can significantly improve its microwave characteristics in both the Meissner and mixed states and also increase the threshold for the onset of nonlinear response.

Chemical and Microstructural Nanoscale Homogeneity in Superconducting YBa2Cu3O7-x Films Derived from Metal-Propionate Fluorine-free Solutions.

Research involved in developing alternative energy sources has become a necessity to face global warming. In this context, superconductivity is an appealing solution to enhance clean electrical energy provided that lower production costs can be attained. By implementation of chemical solution deposition techniques and high-throughput growth methods, low-cost nanostructured epitaxial cuprate superconductors are timely candidates. Here, we present a versatile and tunable solution method suitable for the preparation of high-performance epitaxial cuprate superconducting films. Disregarding the renowned trifluoroacetate route, we center our focus on the transient liquid-assisted growth (TLAG) that meets the requirement of being a greener chemical process together with ultrafast growth rates beyond 100 nm/s. We developed a facile, fast, and cost-effective method, starting from the synthesis of metal-propionate powders of Y, Ba, and Cu of high purity and high yields, being the precursors of the fluorine-free solutions, which enable the chemical and microstructural nanoscale homogeneity of YBa2Cu3O7-x (YBCO) precursor films. These solutions present endured stability and enable precise tunability of the composition, concentration, porosity, and film thickness. Homogeneous precursor films up to thicknesses of 2.7 μm through eight layer multidepositions are demonstrated, thus establishing the correct basis for epitaxial growth using the fast kinetics of the TLAG process. YBCO films of 500 nm thickness with a critical current density of 2.6 MA/cm2 at 77 K were obtained, showing the correlation of precursor film homogeneity to the final YBCO physical properties.

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering.

In situ MgB2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, Jc, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 105 A/cm2 in the self-field and above 104 A/cm2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB2 nanograins in connection to the presence of MgO precipitates and areas rich in boron.

Magneto-optical measurements of mesoscopic Nb superconducting structures using a ferromagnetic metal indicator layer

Imaging local magnetic fields produced by nano- and micrometer-scale superconductors has become a vital tool that can not only reveal crucial information on the vortex dynamics and order parameters of the superconducting materials but also visualize the working mechanism of superconducting devices made for quantum information. Here, we performed measurements of the magnetic field distributions of mesoscopic superconducting structures with various geometries by combining a thin ferromagnetic metal layer as a magneto-optical sensing element that responds to the environmental magnetic fields and a custom-made sensitive Sagnac interferometer. The sensitivity of the technique enables the observation of magnetic flux jumps due to individual vortex entries into nanostructured superconductors. In addition, with the control of incident power at a focused laser spot, we induce thermally driven movement of vortices that leaves a trace of a microscopic optical heating pattern.

Phase formation and enhanced performance of Nb3Al prepared by spark plasma sintering and subsequent annealing

Although the Nb3Al wire prepared by the rapid heating and quenching (RHQ) technique has achieved excellent superconducting critical current density (Jc), various new preparation techniques have still been continuously tried to improve its performance. In this paper, the phase formation and superconducting properties of Nb3Al prepared with spark plasma sintering (SPS) was studied. It is found that the SPS in the temperature range of 1200–1400 °C can quickly produce high-density, nano-structured Nb3Al superconductor, which has a transition temperature around 16 K. Due to its tight and clean grain connection, the sample shows a high-performance current-carrying capacity in self-field. After post-annealing, the flux pinning force and the superconducting performance are significantly improved. The study clarified that the phase formation mechanism of the SPSed Nb3Al is similar to that directly formed from high temperature liquid phase rather than the transformation from bcc phase to A15 phase. The high temperature liquid phase may originate from the local high temperature induced by the plasma heating in the early stage of SPS process. Such a unique phase formation mechanism results in very few structural defects existing inside the sample. The results of this study suggest that chemical doping or other means which can introduce high density structure defects in Nb3Al should be considered to further improve the in-field Jc for the SPSed Nb3Al.

The morphologic correlation between vortex transformation and upper critical field line in opal-based nanocomposites

In this study, we investigate metallic nanocomposites to elucidate the properties of nanostructured conventional superconductors. Liquid tin, indium, and mercury are loaded into opal matrices by high pressure up to 10 kbar. The opal templates preserve the 3D dendritic morphology of confined superconducting metals to model a dendritic second phase with particular grain shape in bulk superconductors observed by a DualBeam microscope. We carry out measurements of the dc and ac magnetizations to study the superconducting phase diagrams, vortex dynamics, and impact of grain morphology in the opal composites. Besides, we apply the small-angle neutron scattering (SANS) to deny a regular vortex structure. The phase diagrams reveal an enhanced upper critical field Hc2(0) and curvature crossover in the upper critical field line. We also calculate the vortex activation barriers Ua and observe a transformation in the vortex system. According to the field dependence of Ua, the vortex structure transformation highly correlates with the curvature crossover in the upper critical field line. Our observations suggest that the similarity in the normalized phase diagrams and field dependences of Ua in the three nanocomposites is owing to their particular morphology of confinement.

Dependences of local density of state on temperature, size, and shape in two-dimensional nano-structured superconductors

In this paper, we investigate a local density of state (LDOS) in a two-dimensional nano-structured superconductor. We solve the Bogoliubov-de Gennes equations self-consistently with the two-dimensional finite element method. In the nano-structured superconductor, the LDOS as a function of energy has many discrete peaks. Discretization of the LDOS comes from discretization of energy levels due to the quantum confinement effect in the nano-structured system. When temperature increases, a width of a peak in the LDOS is spread to a large energy range and neighbor peaks are overlapped. On the other hand, for the fixed temperature, the behavior of the LDOS is different between nano-scaled rectangular and square systems. In the nano-scaled rectangular system, when only a length of a long side increases, a contribution of the quantum confinement effect from the long side is suppressed, while the contribution from a length of a short side remains large. Then, some peaks are left in the LDOS even when the length of the long side is very large. These peaks form a periodic structure and can be regarded as gaps in a multi gap structure due to the quantum confinement effect. On the other hand, energy levels in the square system tend to arrange equally. Then, in the square system, peaks in the LDOS which exist in the rectangular system are small. Also, the period between peaks in the LDOS in the square system is smaller than that in the rectangular system.

Local density of States in Two-Dimensional Nano-Structured Superconducting Systems with Superconductor-Normal Metal Interfaces

In nano-structured superconductors, it is important to consider a discreteness of energy levels instead of continuous energy levels due to the quantum confinement effect. This discreteness causes an appearance of many peaks in a density of state (DOS). In this paper, the effect of the discreetness of energy level or the effect of the quantum confinement on the DOS in superconducting systems with superconductor / normal metal (S/N) interfaces are focused. In particular, in nano-sized finite systems, a local density of state (LDOS) becomes strongly spatial dependent because of non-uniform spatial distributions of a gap energy and a pair amplitude. We investigate the pair amplitude and the LDOS by solving the Bogoliubov-de Gennes equations self-consistently. The proximity effect leads to a penetration of the pair amplitude into the normal metal region. Then, the pair amplitude in the normal metal decays non-monotonously because of the effect of the nano-sized finite system. On the other hand, the spatial-averaged LDOS plots as a function of the energy have many peaks in both superconductor and normal metal regions. Also, a contribution of the normal metal to the superconductor causes an appearance of peaks of the LDOS at the energy below the gap energy. In the SNS junction, when width of the normal metal increases, these peaks at the energy below the gap energy appear clearly.

Experimental investigations of the problem of the quantum jump with the help of superconductor nanostructures

The quantum theory, that we now know, arose in the process of sharp disputes between its creators. One of the results of these disputes was the emergence in recent years of fundamentally new areas of investigation and technology – quantum information and quantum computing. One of the subjects of controversy between the creators of quantum theory was the quantum jump. We draw the attention to the possibility of experimental investigation of this problem with the help of quantum superconductor nanostructures. The first results of experiments are presented, the paradoxicality of which indicates the relevance of the problem.

Magic-Angle Bilayer Graphene Nanocalorimeters: Toward Broadband, Energy-Resolving Single Photon Detection.

Because of the ultralow photon energies at mid-infrared and terahertz frequencies, in these bands photodetectors are notoriously underdeveloped, and broadband single photon detectors (SPDs) are nonexistent. Advanced SPDs exploit thermal effects in nanostructured superconductors, and their performance is currently limited to the more energetic near-infrared photons due to their high electronic heat capacity. Here, we demonstrate a superconducting magic-angle bilayer graphene (MAG) device that is theoretically capable of detecting single photons of ultralow energies by utilizing its record-low heat capacity and sharp superconducting transition. We theoretically quantify its calorimetric photoresponse and estimate its detection limits. This device allows the detection of ultrabroad range single photons from the visible to sub-terahertz with a response time around 4 ns and energy resolution better than 1 THz. These attributes position MAG as an exceptional material for long-wavelength single photon sensing, which could revolutionize such disparate fields as quantum information processing and radio astronomy.

Upper Frequency Limits for Vortex Guiding and Ratchet Effects

The guided and rectified motion of magnetic flux quanta are important effects governing the magnetoresistive response of nanostructured superconductors. While at low ac frequencies these effects are rather well understood, their manifestations at higher ac frequencies remain poorly investigated. Here, we explore the upper frequency limits for guided and rectified net motion of superconducting vortices in epitaxial $\mathrm{Nb}$ films decorated with ferromagnetic nanostripes. By combining broadband electrical spectroscopy with resistance measurements, we reveal that the rectified voltage vanishes at a geometrically defined frequency of about 700 MHz. By contrast, a vortex-guiding-related low-ac-loss response persists up to about 2 GHz. This value corresponds to the depinning frequency ${f}_{d}^{s}$ associated with the washboard pinning potential induced by the nanostripes, which exhibits peaks for commensurate vortex-lattice configurations. By applying a sum of dc and microwave ac currents at an angle $\ensuremath{\alpha}$ with respect to the nanostripes, the angle dependence of ${f}_{d}^{s}(\ensuremath{\alpha})$ is found to be correlated with the angle dependence of the depinning current. In all, our findings suggest that superconductors with higher ${f}_{d}^{s}$ should be favorable for efficient vortex manipulation in the gigahertz ac frequency range.

P.302 Healthy women who use oral contraceptives show lower brain serotonin 4 receptor binding relative to nonusers

Critical state field profiles and thermomagnetic instabilities are studied in two kinds of three-dimensional nanostructured superconductors. We find that the critical state field profiles in some simple bi-layer systems are not simple superpositions of critical states in the two layers. Competition between the divergence of the local field at the edges of the film and the shielding by the neighboring layer makes novel critical state field profiles. We also studied flux avalanches in shifted strip arrays (SSAs) with layer numbers up to six. Various forms of avalanches either perpendicular or parallel to the strip are observed when the overlap between strips in neighboring layers is large. We also find that introduction of asymmetry in various forms to SSA affects the shape of flux avalanches sensitively.

Microwave Response of Nanostructured High-Tc Superconductor Thin Films

A model for the microwave response of a nanostructured high-Tc superconductor (HTS) film, with implanted nanoparticles and nanorods of a dielectric material or point-like and columnar irradiation defects with a nano-sized cross-section is developed. In this case, the microwave surface resistance Rs(T,H,ω) is calculated both for the Meissner and mixed states of a superconductor film in an applied dc magnetic field. The obtained results indicate that the implantation of dielectric nanoparticles or point-like radiation defects can significantly improve superconductor characteristics at microwave frequencies. Namely, these nano-sized structural defects can decrease the surface resistance in the Meissner state and eliminate the oscillations of Abrikosov vortices and the related microwave energy losses, thus decreasing the contribution of Abrikosov vortices to the Rs value in the mixed state of a HTS film.

Impurity effects on nano-structured dirty superconductors: violation of Anderson’s Theorem

It is shown that in nano-structured s-wave superconductor, transition temperature Tc depends on random impurity potentials. This means violation of the Anderson’s theorem, which states that non-magnetic impurity does not affect the Tc of s-wave superconductor. We determine the impurity effects on Tc for nano-structured superconductor, using the finite element method to solve the Bogoliubov-de Gennes equations under spatially a random impurity potential. We show that some impurity potentials increase Tc but other impurity potentials decrease Tc. We find that the superconductor with localized order parameter shows increased Tc, which is contrary to expectation. Our results show that a dirty superconductor does not always mean weak superconductivity.

Transmission EBSD (t-EBSD) to determine grain and grain boundary properties on nanostructured superconductor samples

The knowledge of the properties of the grains and their grain boundaries is essential for the further optimization of sample performance of various high-Tc samples (YBa2Cu3Oy (YBCO), Bi-based cuprates, MgB2, iron-based materials). In these materials either the grains are of nanometer dimensions or the various additions of secondary phase(s) to improve the flux pinning are required to be of nanometer dimensions, so a high achievable resolution and better imaging properties are important to obtain reasonably high image quality of the recorded Kikuchi patterns to enable automated orientation mapping using the EBSD technique. Therefore, the newly developed transmission-EBSD (t-EBSD) technique is the method of choice to analyze the grain and the grain boundary properties in such nanostructured superconducting materials. Several results obtained on MgB2, melt-textured, infiltration-growth processed YBCO and electrospun Bi2Sr2CaCu2O8 nanowires are presented.

Computer simulation of an electronic device for generation of electric oscillations by supercooled in electric field superconductors

We present computer simulations of the work of an electronic device for generation of electric oscillation by negative differential conductivity of a superconductor supercooled below the critical temperature. The superconductor is cooled below its critical temperature in the normal state at a small constant electric voltage. To simulate the device, we use formerly derived explicit analytical expressions for the conductivity of nanostructured superconductors supercooled below the critical temperature in an electric field. Computer simulations reveal that the negative differential conductivity region of the current–voltage characteristic leads to excitation of electric oscillations. We simulate a circuit with distributed elements as a first step to obtain gigahertz frequency. This gives a hint that a hybrid device of nanostructured superconductors will work in terahertz frequencies. If the projected setup is successful, we consider the possibility for it to be put in a nanosatellite (such as a CubeSat). The study of layered high-Tc superconductors and EM waves emitted from them in space vacuum would be an important task for future space technology.

Transition temperature enhancement of nano-structured superconductors: Shape dependence and effects of Cooper pair distribution

Enhancement of superconducting critical temperature Tc for nano-structured square and rectangular plates is investigated, using the Gor’kov equations and the finite element method. Tc increases with decreasing system size and shows sawtooth behavior as the function of system size. We find the enhancement of Tc becomes larger for narrower rectangular plate. We discuss mechanisms of these behaviors of Tc, investigating quasi-particle eigen-energies and spatial order parameter distributions.

Stability of degenerate vortex states and multi-quanta confinement effects in a nanostructured superconductor with Kagome lattice of elongated antidots

In the present work, we directly visualize the multi-quanta cages (MQCs) consisting of the giant vortices pinned by the elongated antidots using low-temperature scanning Hall probe microscopy. The periodic but sufficiently isolated MQCs, observed at various magnetic fields, are in a good agreement with the simulated vortex states based on the time-dependent Ginzburg–Landau (tdGL) equations. Due to the competition between the interstitial vortices and the pinned giant vortices, the formation and collapse of the MQCs can be tuned by varying magnetic field. The experimental statistics of the interstitial vortices confined in the MQCs show that the interstitial vortex patterns become more disordered at higher magnetic fields. The stability of the degenerate vortex states and the multi-quanta confinement effects under an external current are also investigated by using the tdGL simulations. The splitting of the free energy of the degenerate vortex states indicates that applying external current can eliminate parts of the degenerate vortex states.