Xinsheng Yang

Bi-2223 high-temperature superconducting (HTS) tapes, with their relatively high upper critical field and current-carrying capability, the fine multi-filamentary microstructure, the excellent thermal stability under high magnetic fields and mature manufacturing processes, exhibit significant application potential in large-scale high-field magnets. The current distribution among Bi-2223 stacked tapes is a critical factor governing the stable operation of Bi-2223 stacked-tape cables. In this work, a five-layer Bi-2223 stacked-tape cable sample was investigated using an integrated measurement–inversion–simulation–validation approach, with a focus on the influence of joint resistance non-uniformity on the current distribution in multi-tape stacked cables. An annular Hall sensor array was employed to measure the circumferential magnetic field response of a short stacked-tape cable sample, and the current redistribution within the cable was identified and tracked through features of the derivatives of the magnetic field as a function of current. Taking account of the strongly nonlinear E–J characteristics and the anisotropy of the critical current density of the superconducting material, a two-dimensional finite element method (FEM) based on the H-formulation coupled with an equivalent circuit model (ECM) was used to investigate the effects of the non-uniformity of the joint resistance on the internal current distribution at different current ramping rates. A good agreement between experimental measurements and numerical simulations was obtained. The results provide experimental and theoretical support for joint resistance optimization, uniform current sharing, and current redistribution monitoring in HTS cables, and offer guidance for improving the current-carrying performance and operational stability of HTS stacked-tape cables for large-scale high-field magnets.

In practical applications, high-temperature superconducting (HTS) cables or magnets may carry AC with DC bias, such as in superferric magnets, which can increase the AC loss of the cables or magnets. When the DC bias current is high, the resulting high loss can lead to a significant temperature rise in the cable or magnet and may even cause quench. Furthermore, different waveforms of the alternating current also result in different losses and temperature rises. Therefore, it is essential to investigate the AC loss of the cable under different current waveforms and DC bias levels using an electromagnetic–thermal coupling method. In this paper, an electromagnetic–thermal coupling model is used to investigate the AC loss and temperature rise characteristics of four stacked REBCO tapes under four typical current waveforms and various DC bias levels. The actual multilayer structure of REBCO tapes is considered in the numerical simulation, which facilitates the analysis of current distribution among different layers and its contribution to the total loss of the stacked cable. The results show that under zero DC bias or a small DC bias (0.1I_dc), the square-wave current yields the largest AC loss, while the triangular-wave current results in the smallest AC loss. The losses generated by the sawtooth and sinusoidal currents are comparable and intermediate between those of the two aforementioned waveforms. When the DC bias current is moderate (0.5I_dc) and the amplitude of the alternating current is greater than 0.5I_cable, the loss of the cable increases rapidly. The loss generated by the square-wave current is the largest, followed by the sinusoidal current, while the sawtooth and triangular currents produce the smallest losses. When the DC bias current is high (0.9I_dc), even a small amplitude alternating current results in high AC loss in the cable.

Magnetic topological insulators (TIs) are promising candidates for realizing the quantum anomalous Hall effect (QAHE) and advancing the development of next-generation low-energy transistors and electronic devices. Doping Bi₂Se₃ with nano-carbon can introduce magnetic order and open the Dirac gap without introducing extrinsic magnetic impurities. In this work, the C_0.06Bi₂Se₃ single crystal was prepared using the Bridgman method, and their electrical and magnetotransport properties were systematically investigated. Temperature-dependent resistivity and magnetoresistance measurements revealed a magnetic-field-induced metal–insulator-like transition near 152 K. Angle-resolved photoemission spectroscopy (ARPES) detected an energy gap of about 43 meV at the Dirac point, confirming that carbon doping modulates the surface state and opens the gap. Pronounced Shubnikov–de Haas oscillations indicate high carrier mobility in C_0.06Bi₂Se₃. Furthermore, the temperature-dependent Kerr spectra shows that the spin relaxation behavior of C₀.₀₆Bi₂Se₃ differs significantly from that of pure Bi₂Se₃; the relaxation process of spin electrons from the surface state (τ_s) dominates the spin dynamics and exhibits distinct trends around 30 K and 150 K due to the interplay of the Dirac gap and impurity-induced states. These results demonstrate the potential of magnetic topological insulator C_0.06Bi₂Se₃ for novel electronic and spintronic applications.

Stacked high-temperature superconducting (HTS) cables, with their high current-carrying capacity and high structural strength, show significant advantages in strong electric applications. However, in an alternating magnetic field environment, the conductors will generate AC losses, which can cause local overheating. Filamentary technology can effectively reduce the AC losses. This work investigates the magnetization and transport losses of stacked HTS cables based on multi-filamentary REBCO tapes. Results directly demonstrates the fact of the multi-filamentary structure in reducing magnetization loss. As the twist angle increased, magnetization losses first decreased and then stabilized. At low magnetic fields, the difference in magnetization loss between the original cable and the multi-filamentary cable is relatively small. Under higher magnetic fields, the magnetization loss of the original cable is significantly higher than that of the multi-filamentary cable. The transport losses of the multi-filamentary cable are always lower than those of the original cable. The optimization effect of filamentization on magnetization losses is significantly stronger than that on transport losses.

Topological insulators (TIs) can be widely applied in the fields of ultrafast optical and spintronic devices owing to the existence of topologically protected gapless Dirac surface states. However, the study of ultrafast dynamics of carriers in TIs remains elusive. In this work, the carrier dynamics of Li-doped Bi_2−xSe₃ single crystals were investigated by femtosecond (fs) transient optical spectroscopy (ΔR/R(t) signals). The temperature dependence for the relaxation rates of the electron–electron interaction and electron–phonon coupling is consistent with the results of electrical transport, which indicates the carrier dynamics of TI is highly related with carrier concentrations. We find that the carrier type and concentration of Bi₂Se₃ can be tuned by Li doping, leading to a metal-insulation transition at low temperatures (T ≤ 55 K), indicating that electron–electron interactions are dominant at low temperature. For T > 55 K, electron–phonon coupling in the bulk carriers becomes the main electric transport mechanism.