Xin Li

Dynamic charge transfers, or charge flux oscillations, generated by anharmonic phonon coupling, have attracted increasing interest in cuprate superconductors. In this article, a new computational method is developed to analyze such charge fluxes along all atomic bonds for a given material, which unveils a surprising fact that cuprate materials with high superconducting transition temperature show a strong tendency to support global charge flux flows beyond local charge oscillations. Such fluxes further show a strong correlation with both the maximum superconducting transition temperature of different cuprate families and the strong magnetic fluctuations as well. Motivated by these findings, we construct a charge flux model derived from quantum field theory to evaluate the effective interactions mediated by these flux flows. Finally, we discuss the implications of this flux-driven pairing mechanism for the design of new high-T c superconductors, offering a potential strategy for discovering higher T c superconductive materials.

AC loss is a critical concern for the stability of superconducting device. Various experimental methods are available for measuring AC loss in superconductors, with electrical measurement techniques being widely adopted. However, these methods only measure average loss and often disregard variations in superconductor inductance with current, which may lead to inaccuracy. This study introduces a finite-data-window least squares method to measure instantaneous AC loss in coils wound with YBCO tapes. By comparing the results with those obtained through the integral method, it is demonstrated that the proposed approach effectively measures instantaneous AC loss for coils with limited turns. Furthermore, the pattern of superconducting coil inductance in relation to current variations is examined.

Abstract AC loss has significant impact on the design and safe operation of superconducting power equipment. While there have been numerous simulation studies on AC loss, experimental measurement has proven challenging due to its relatively small magnitude compared to reactive power. In our previous work, a method based on fixed forgetting factor recursive least squares for measuring instantaneous AC loss is proposed. However, its applicability is limited due to the varying characteristics of each parameter in superconductors. This paper introduces four recursive least squares with variable forgetting factor algorithms and measures AC loss in ten different superconducting coils. The accuracy of these algorithms is analysed and compared using the integral method as benchmark. The results demonstrate that the recursive least squares with leverage-based multiple adaptive forgetting factors offers the widest range for AC loss measurements.