Language：English   Publisher：Physical Review Materials  

Strain in materials changes their electronic structure, and the strain response realizes rich material properties and devices. Superconductivity under hydrostatic pressure and epitaxial strain suggests significant response to an external variable strain in a single sample, but this has not yet been demonstrated because the strain is usually a fixed parameter after sample fabrication. (La,Sr)2CuO4 films were fabricated on flexible metal substrates, and bending strain was applied to them to observe the critical temperature (Tc) and resistivity variation induced by strain. The compressive bending strain of -0.005 increased the Tc from 23.4 to 27.3 K. The magnitude of the Tc change by the bending strain is independent of the doping level and initial epitaxial strain. Furthermore, the irreversibility temperature was also improved by the compressive bending, and reasonable Tc variation with respect to the reversible strain was observed. Ab initio density functional calculation for the mother compound La2CuO4 clarified that the low-energy electronic structures are sensitive to the bending strain. While the carriers (holes) are preferentially injected into the in-plane orbitals of the CuO2 plane under the compressive strain, the tensile strain leads to the carrier injection into the perpendicular orbitals which is unfavorable to the superconductivity. The strain-sensitive high-Tc superconductor under the external strain highlights a new aspect for cuprate superconductors, which opens monitoring of the stress situation in the cryogenic systems such as superconducting magnet and liquid hydrogen container.

DOI： 10.1103/PhysRevMaterials.8.094802

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Authorship：Corresponding author   Language：Japanese   Publisher：The Institute of Electrical Engineers of Japan  

<p>The discovery of new superconductors has a significant impact on the scientific and engineering communities, unraveling interesting physical phenomena and providing unique applications in energy and devices. Superconductors with a high critical temperature are limited to a few families, such as cuprates, iron-based compounds, and hydrides under ultra-high pressure. In traditional studies, the exploration of new superconductors relies on theories, experiments, and simulations. However, recent advances in data science have made machine learning available in a variety of fields, including materials informatics. Utilizing superconductor databases and various regression methods, machine learning has proposed several new superconductors. The chemical descriptors are widely used, and the descriptor of the crystalline structure is being developed for more accurate prediction. In this review, the theoretical and experimental studies for the discovery of new superconductors are explained. The available database and data-driven studies are also shown. Furthermore, after reviewing the recent machine learning studies for the discovery of new superconductors and other materials, future aspects in this field are discussed.</p>

DOI： 10.1541/ieejfms.144.344

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CiNii Research

Authorship：Last author,　Corresponding author   Language：English   Publisher：Superconductor Science and Technology  

YBa2Cu3O7 coated conductors are a strategic material for superconducting applications such as high field magnets, fusion, and motors. Grain boundaries reduce the critical current density (Jc) even at a tilt angle as low as 5°, but the successful development of the highly oriented substrates seemed to overcome the weak link problem at grain boundaries. However, it reappears when we visit the homogeneity of the coated conductors. To suppress the weak link in the coated conductors, the Ca doping was investigated. The Ca-doped YBa2Cu3O7 films were fabricated on the moderately oriented substrates. While the grain boundaries in the moderately oriented substrates significantly degraded the Jc without Ca doping, the Ca doping improved the Jc especially at low temperature. This indicates that the tilt angle dependence of Jc was varied by the Ca doping. While the Jc for the moderately oriented substrate was 20 times smaller than that for the highly oriented substrate, the Ca doping restored 1/2 of the Jc for the highly oriented substrate at 40 K and 9 T. The vortex structure changed from Abrikosov Josephson vortices to the Abrikosov vortices with increasing the Ca content. The combination of Ca doping and moderate substrate texture is another design of coated conductors. The Ca doping can patch the local degradation of the substrate texture to mass produce the practical coated conductors with improved homogeneity.

DOI： 10.1088/1361-6668/ad68d7

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Authorship：Last author,　Corresponding author   Language：English   Publisher：ACS Applied Electronic Materials  

Fabrication of YBa2Cu3O7 nanocomposite films on silicon substrates is required for superconducting electromagnetic devices. Low-temperature deposition is effective in suppressing the chemical reaction between YBa2Cu3O7 and Si, while sufficient diffusion is required to form the well-defined nanocomposite structure. The fabrication of the YBa2Cu3O7 nanocomposite films on Si substrates should simultaneously satisfy these conflicting requirements. A YSZ (yttrium stabilized zirconia) buffer layer was epitaxially grown on Si substrates to suppress the chemical reaction. Then, the YBa2Cu3O7 + Ba2YbNbO6 films were fabricated on the YSZ/Si. The nanorods with diameters of 9-17 nm were elongated along the c-axis even at the low deposition temperature. The YBa2Cu3O7 + Ba2YbNbO6 nanocomposite film exhibited a critical temperature of 86.0 K, which is comparable to the critical temperature of 85.6 K in the pure film. The irreversibility temperature was slightly improved by the nanorods. Thus, we demonstrate the formation of nanorods in the YBa2Cu3O7 films on Si substrates without lowering the critical temperature. This opens the superconducting electromagnetic devices integrated with high-temperature superconductors and semiconductors.

DOI： 10.1021/acsaelm.4c00612

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Publisher：Superconductivity  

The pinning of quantized magnetic vortices in superconducting YBa2Cu3O7- δ(YBCO or Y123) thin films with Y2BaCuO5 (Y211) nanoinclusions have been investigated over wide temperature range (4.2–77 K). The concentration of Y211 nanoinclusions has been systematically varied inside YBCO thin films prepared by laser ablation technique using surface modified target approach. Large pinning force density values (Fp ∼ 0.5 TNm−3 at 4.2 K, 9 T) have been observed for the YBCO film with moderate concentration of Y211 nanoinclusions (3.6 area % on ablation target). In addition, uniform enhancement in critical current density (Jc) was observed in the angular dependent Jc measurement of YBCO+Y211 nanocomposite films. Y211 nanoinclusions have been found to be very efficient in pinning the quantized vortices thereby enhancing the in-field Jc values over a wide range of temperature. Increasing the concentration of Y211 secondary phase into Y123 film matrix results into agglomeration of Y211 phase and observed as increased Y211 nanoparticle size. These larger secondary phase nanoparticles are not as efficient pinning centers at lower temperatures as they are at higher temperatures due to substantial reduction of the coherence length at lower temperatures. Investigation of the temperature dependence of Jc for YBCO+Y211 nanocomposite films has been conducted and possible vortex pinning mechanism in these nanocomposite films has been discussed.

DOI： 10.1016/j.supcon.2024.100087

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