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As shown previously, a relation between the superconducting transition temperature and some characteristic distance in the crystal lattice holds, which enables the calculation of the superconducting transition temperature, Tc, based only on the knowledge of the electronic configuration and of some details of the crystallographic structure. This relation was found to apply for a large number of superconductors, including the high-temperature superconductors, the iron-based materials, alkali fullerides, metallic alloys, and element superconductors. When applying this scheme called Roeser-Huber formula to Moiré-type superconductivity, i.e., magic-angle twisted bi-layer graphene (tBLG) and bi-layer WSe2, we find that the calculated transition temperatures for tBLG are always higher than the available experimental data, e.g., for the magic angle 1.1∘, we find Tc≈ 4.2–6.7 K. Now, the question arises why the calculation produces larger Tc’s. Two possible scenarios may answer this question: (1) The given problem for experimentalists is the fact that for electric measurements always substrates/caps are required to arrange the electric contacts. When now discussing superconductivity in atomically thin objects, also these layers may play a role forming the Moiré patterns. The consequence of such substrate-induced super-Moiré patterns is that the resulting Moiré pattern always will show a larger cell size, and thus, a lower Tc of the final structure will result. (2) A correction factor to the Roeser-Huber formalism may be required to account for the low charge carrier density of the tBLG. Here, we test both scenarios and find that the introduction of a correction factor η enables a proper calculation of Tc, reproducing the experimental data. We find that η depends exponentially on the value of Tc.

Superconducting YBa2Cu3Oy (YBCO) foams were prepared using commercial open-cell, polyurethane foams as starting material to form ceramic Y2BaCuO5 foams which are then converted into superconducting YBCO by using the infiltration growth process. For modelling the superconducting and mechanical properties of the foam samples, a Kelvin-type cell may be employed as a first approach as reported in the literature for pure polyurethane foams. The results of a first modelling attempt in this direction are presented concerning an estimation of the possible trapped fields (TFs) and are compared to experimental results at 77 K. This simple modelling revealed already useful information concerning the best suited foam structure to realize large TF values, but it also became obvious that for various other parameters like magnetostriction, mechanical strength, percolative current flow and the details of the TF distribution, a refined model of a superconducting foam sample incorporating the real sample structure must be considered. Thus, a proper description of the specific microstructure of the superconducting YBCO foams is required. To obtain a set of reliable data, YBCO foam samples were investigated using optical microscopy, scanning electron microscopy and electron backscatter diffraction (EBSD). A variety of parameters including the size and shape of the cells and windows, the length and shape of the foam struts or ligaments and the respective intersection angles were determined to better describe the real foam structure. The investigation of the foam microstructures revealed not only the differences to the original polymer foams used as base material, but also provided further insights to the infiltration growth process via the large amount of internal surface in a foam sample.

Recently, an improvement in the flux pinning performance ofREBa2Cu3O7−δ(REBCO) bulk was achieved employing high energy ultrasonic irradiation (up to 300 W and 60 min) prior to the infiltration-growth process. Here, we demonstrate that a higher ultrasonic power and shorter duration treatment (450 W for 30 min) of theRE2BaCuO5(RE211,RE= Y, Gd) powder produces individual, nanometer-sized and surface damagedRE211 particles (RE211Ultra). We study the growth of YBCO and GdBCO systems via the top-seeded melt-growth method, with the addition of 30 mol% ofRE211Ultra, which were pre-treated by high-energy ultrasonication and compared with the conventional method. Isothermal growth experiments clarified that the addition ofRE211Ultraparticles enables faster and more effective crystal growth via an improved peritectic reaction due to their size in the nanometer range and presence of sharp edges, which are crucial for growing large, single grain bulks ofREBCO. Microstructural investigations by scanning electron microscopy indicated the presence of two differently sizedRE211 inclusions within the bulk GdBCO and YBCO superconductors. All samples showed an onset of superconductivity at ∼92 K or above. Utilizing the effective growth temperature window and the addition ofRE211Ultra, single grain bulks of GdBCO and YBCO were fabricated by the slow-cooling method. The field dependent critical current density (Jc) of the bulk samples usingRE211Ultrawere found to exhibit superior performance over the standard GdBCO/YBCO samples due to significant changes of sample growth and of their microstructures. Here, we discuss various factors affecting the addition ofRE211Ultraon the development of the microstructure, growth of bulk, single grain materials and further, the superconducting properties of differentREBCO superconductors.

Several properties of Earth’s magnetic field (field vectors, time dependence) are measured in various locations using a smartphone/tablet magnetic sensor. To enable a proper use of the magnetic sensor as a classroom tool, the exact location of the sensor in the device and its resolution must be identified in a first step. Then, students may perform several measurements on their own, allowing a comparison of the recorded field vectors and a discussion of the possible deviations. We also show that long-time measurements of the local magnetic field are possible using the smartphone sensors, which can be compared to available internet data. All the information obtained enables a better understanding of the properties of the Earth’s magnetic field and of the requirements for real applications of magnetic sensor elements.