Publisher：Physica C Superconductivity and Its Applications  

Even in REBa<inf>2</inf>Cu<inf>3</inf>O<inf>y</inf> (REBCO) films, which exhibit high critical current density (J<inf>c</inf>) under low-temperature and high-magnetic-field conditions, their performance deteriorates significantly at high-temperatures because of pronounced flux creep induced by thermal agitation. This degradation is particularly pronounced in thin superconducting films, as the thickness constrains the pinning correlation length. In this study, we examined the effect of YBCO film thickness on J<inf>c</inf> within the range of 0.94 to 3.7 μm. While some variations were observed depending on crystal orientation, thicker superconducting layers demonstrated a smaller reduction in J<inf>c</inf> with increasing magnetic field in high-temperature regions, resulting in superior J<inf>c</inf> values under high-temperature and high-magnetic-field conditions. Moreover, the apparent pinning potential U<inf>0</inf>^*, determined from magnetization relaxation measurements, was larger for thicker films in these conditions, indicating that U<inf>0</inf>^* is influenced by the thickness of the superconducting layer. These findings are further supported by the flux creep flow model analysis. Collectively, these results suggest that not only introducing strong pinning forces but also increasing the thickness of the superconducting layer is an effective strategy for enhancing performance near liquid nitrogen temperatures.

DOI： 10.1016/j.physc.2025.1354734

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Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publisher：Communications Engineering  

Process engineering of materials determines not only materials properties, but also cost, yield and production capacity. Although process design is generally based on the experience of process engineers, mathematical/data-science modeling is a key challenge for future process optimization. Here we create new opportunities for process optimization in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> film fabrication through data/model-driven process design. We show integrated modelling of substrate temperature and critical current density in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> films. Gaussian process regression augmented by transfer learning and physics knowledge was constructed from a small amount of data to show substrate temperature dependence of critical current density. Non-numerical factors such as chamber design and substrate material were included in the transfer learning, and physics-aided techniques extended the model to different magnetic fields. Magnetic field dependence of critical current density was successfully predicted for a given substrate temperature for a five-sample series deposited using different pulsed laser deposition systems. Our integrated process and property modelling strategy enables data/model-driven process design for YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> film fabrication for coated conductor applications.

DOI： 10.1038/s44172-025-00434-1

Open Access

Web of Science

Scopus

PubMed

Authorship：Corresponding author   Publisher：Journal of Applied Physics  

To improve the critical current density of c-axis oriented YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> (YBCO) thin films, much effort has been devoted to the introduction of pinning centers that disrupt vortex migration under magnetic fields. Threading dislocations are anticipated to act as one-dimensional pinning centers without reducing the superconducting regions. In this study, YBCO thin films were prepared on a SrTiO<inf>3</inf> (STO) + xCeO<inf>2</inf> (x = 0%, 30%, and 50%) buffer layer by pulsed-laser deposition, and their nanostructures were analyzed by transmission electron microscopy. The interface between the buffer layer and the YBCO thin film was flat for the pure STO, while it was roughened by numerous nanoparticles for the CeO<inf>2</inf>-doped buffer layer. Due to these nanoparticles, threading dislocations were introduced more densely in the YBCO films on the STO + CeO<inf>2</inf> buffer layer than in those on the pure STO buffer layer, depending on the volume percent of CeO<inf>2</inf>. Investigations of the critical current density as a function of the magnetic field revealed that the threading dislocations effectively act as pinning centers.

DOI： 10.1063/5.0249007

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Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publisher：Nanoscale  

Self-organization realizes various nanostructures to control material properties such as superconducting vortex pinning and thermal conductivity. However, the self-organization of nucleation and growth is constrained by the growth geometric symmetry. To realize highly controlled three-dimensional nanostructures by self-organization, nanostructure formation that breaks the growth geometric symmetry thermodynamically and kinetically, such as tilted or in-plane aligned nanostructures, is a challenging issue. A vertically aligned nano-checkerboard is typically formed from ZnMnGaO<inf>4</inf> with the twin domain vertically aligned by the stress from the MgO substrate. The change in the template structure is promising to form a different type of nanostructure. The cubic ZnMnGaO<inf>4</inf>/MgO films were annealed to form nanoscale tetragonal domains in the tilted direction from the surface, which is determined by lattice mismatch, lattice symmetry, and atomic bonding. On the other hand, as a result of free deformation, in-plane aligned twin domains were formed on the SrTiO<inf>3</inf> substrate with a thin MgO buffer layer, which does not induce stress in the ZnMnGaO<inf>4</inf> film. By annealing the ZnMnGaO<inf>4</inf>/MgO/SrTiO<inf>3</inf> film, the nano-checkerboard with a size of ∼10 nm and a length of ∼200 nm is elongated to the in-plane [100] or [001] direction. This study demonstrates the possibility of fabricating a nanostructure that breaks the growth geometric symmetry, which is not achieved by the previous self-organization. The phase separation with controlled template opens more complicated three-dimensional structures by self-organization.

DOI： 10.1039/d4nr04343j

Open Access

Web of Science

Scopus

PubMed

Publisher：Journal of Physics Conference Series  

YBCO superconductors offer high transition temperatures for applications. REBCO wires, made via pulsed laser deposition (PLD), are developed for various uses. However, optimizing PLD parameters is time-consuming. To resolve this, we aimed to develop the simulation of REBCO thin film growth with Monte Carlo algorithm and Bayesian optimization to find the optimal parameters. While current simulations show discrepancies with experiments, this approach sets the foundation for improved models and simulations.

DOI： 10.1088/1742-6596/3054/1/012020

Scopus

Publisher：Journal of Physics Conference Series  

This study used a depth camera to monitor the plume during YBa<inf>2</inf>Cu<inf>3</inf>O<inf>y</inf> (YBCO) thin film deposition by pulsed laser deposition (PLD). Plume characteristics (height, width, and color) were changed by varying oxygen pressure and laser energy. Correlations between parameters were analyzed using principal component analysis (PCA), suggesting a correlation between plume color and composition. These results suggest that monitoring the plume is an effective way to improve the reproducibility of YBCO thin films.

DOI： 10.1088/1742-6596/3054/1/012019

Scopus

Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers  

In depositing REBa2Cu3Oy (REBCO) superconducting thin films, the vapor-liquid-solid (VLS) technique offers advantages over usual pulsed laser deposition (PLD) for achieving high deposition rates and controlled crystal orientation. However, introducing BaMO3 (BMO) additions into VLS-REBCO thin films to enhance critical current density in magnetic fields can lead to distinct nanostructures compared to films prepared by PLD. While simulations have explored nanorod morphologies in PLD. However, a comprehensive understanding of BMO nanostructure self-organization in VLS growth remains elusive. This study presents a novel simulation approach that incorporates the liquid phase as droplets and motion of them within the VLS process. This model successfully reproduces experimental observed BMO nanostructures in VLS growth, providing valuable insights and a practical guideline for tailoring BMO nanostructures using the VLS technique.

DOI： 10.35848/1347-4065/ad7f39

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Scopus

Authorship：Lead author,　Corresponding author   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：Lead author,　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

Scopus

CiNii Research

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

YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> coated conductors are a strategic material for superconducting applications such as high field magnets, fusion, and motors. Grain boundaries reduce the critical current density (J<inf>c</inf>) 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 YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> films were fabricated on the moderately oriented substrates. While the grain boundaries in the moderately oriented substrates significantly degraded the J<inf>c</inf> without Ca doping, the Ca doping improved the J<inf>c</inf> especially at low temperature. This indicates that the tilt angle dependence of J<inf>c</inf> was varied by the Ca doping. While the J<inf>c</inf> for the moderately oriented substrate was 20 times smaller than that for the highly oriented substrate, the Ca doping restored 1/2 of the J<inf>c</inf> 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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Scopus

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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Scopus

Publisher：Superconductivity  

The pinning of quantized magnetic vortices in superconducting YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7- δ</inf>(YBCO or Y123) thin films with Y<inf>2</inf>BaCuO<inf>5</inf> (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 (F<inf>p</inf> ∼ 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 (J<inf>c</inf>) was observed in the angular dependent J<inf>c</inf> 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 J<inf>c</inf> 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 J<inf>c</inf> 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

Open Access

Web of Science

Scopus

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

To improve the thermoelectric properties of SnSe films, carrier control is required, but elemental doping is difficult due to the thermodynamic solubility limit. Isovalent elements may generate holes or electrons not in a direct manner but in the manner to form point defects. In this study, the Sn(Se,Te) films were fabricated by a pulsed laser deposition (PLD) method to control the carrier concentration by substituting isovalent Te for Se. The coexistence of the orthorhombic and cubic phases at the tens of nanometer scale in the SnSe<inf>0.5</inf>Te<inf>0.5</inf> film was clarified by the structural observation, which is consistent with the equilibrium phase diagram. In spite of the phase coexistence, the lattice parameters linearly increased with an increase in the Te content in the Sn(Se,Te) films. This demonstrates the metastable composition situation for each phase, namely, the carrier control beyond the thermodynamic limit due to the nonequilibrium growth in PLD. As a result, the Seebeck coefficient decreased, and the electrical conductivity increased to increase the power factor, especially in a low temperature near room temperature. The Te substitution in the nonequilibrium PLD increases the hole concentration beyond the thermodynamic solubility limit and thus is effective in controlling the carrier in the SnSe films where carrier doping is difficult.

DOI： 10.1021/acsaelm.3c01490

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：ACS Applied Energy Materials  

Thermoelectric energy conversion is promising for high-efficiency power generation. The p- and n-type thermoelectric materials are required to fabricate high-performance thermoelectric modules. SnSe is one of the highest-performance thermoelectric materials, and SnSe films should be fabricated for small-scale applications. While the p-type SnSe films can be easily prepared without precise control of the doping, it is difficult to control the electron concentration in the n-type SnSe films. In this study, the Bi-doped SnSe films were fabricated at low deposition temperatures by using the self-buffer layer, namely, the SnSe-based buffer layers. The low-temperature deposition on the buffer layer increased the concentration of doped Bi, although the contribution of the buffer layer to the electronic conduction was observed. The π-type thermoelectric module consisting of p-type SnSe and n-type SnSe/buffer layers was fabricated. The output power of 0.09 μW and the open-circuit voltage of 0.11 V were obtained at 155 °C at the cold end and 255 °C at the hot end. The feasibility of fabricating the π-type SnSe film module was demonstrated. By optimizing the buffer layer thickness and increasing the number of legs, we expect the module performance to be further improved.

DOI： 10.1021/acsaem.3c02683

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Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/1361-6668/acecad

Scopus

Authorship：Last author,　Corresponding author  

DOI： 10.35848/1347-4065/ad009f

Web of Science

DOI： 10.1063/5.0155145

Web of Science

DOI： 10.1063/5.0153801

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.7566/JPSCP.38.011044

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.jpcc.1c09324

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsaelm.2c00438

Authorship：Lead author,　Corresponding author   Language：English