Physica C-superconductivity and Its Applications最新文献

c-axis Josephson tunnelling in atomically thin twisted flakes of quasi-two-dimensional cuprate superconductors is one of the experimental techniques which can be used to reveal the pairing symmetry in HTS cuprates. Until very recent experiments reported by Zhao et al. (Science 382, 1422 (2023)), there was a widely accepted consensus that the c-axis Josephson currents, I_c(T), in twisted cuprate junctions manifest the s-wave gap symmetry for c-axis gap component. Here, I highlighted that this consensus was based on the analysis of the I_c(T) data by using the Ambegaokar-Baratoff equation, where the weak-coupling s-wave gap equation was utilized. In this study, I introduced a more accurate gap equation in the AB model and reanalyzed the I_c(T) data in atomically thin twisted Bi₂Sr_2-xLa_xCuO_6+y (Bi-2201) and Bi₂Sr₂CaCu₂O_8+x (Bi-2212) flakes. In the result, the analysis revealed evidences for d-wave pairing symmetry. Beside this, the analysis of the R_nI_c(T) data reported by Zhao et al. (Science 382, 1422 (2023)) by a piecewise AB model, where a linear low-T dependence of the I_c(T) is used, and where the threshold temperature T_M is a free-fitting parameter, confirmed the d-wave symmetry in these twisted Bi-2212 junctions.

Solar cells, electronics, and the semiconductor industry all need high-quality silicon crystals. The Czochralsky technique has been widely used to make a high-quality single silicon crystal from the silicon melt. However, this technique needs a strong magnetic field to stabilize the melt and control the temperature. In this paper, two coils with a curved shape have been optimized and simulated to produce a transverse magnetic field for the Czochralsky technique grower. The coils face each other and have an opening angle of 150°, and a NbTi superconducting wire was used to wind the coils with 5110 turns. The current was 106 A and flowing in the same direction. The finding results indicated that the magnetic field at the silicon melt center is 0.4 T, and the peak field in coils is 2.95 T. The optimized curve shape coils were compared with conventional coils (circle shape coils). The comparison indicated that the curve shape coils produced a highly uniform magnetic field, required less current, and significantly reduced the outer diameter of the superconducting magnet. In addition, heat generation by the current leads was calculated. In the case of the curve-shaped coils, less heat leakage has been achieved; as a result, the cooling capacity, magnet size, weight, and cost will be reduced. Furthermore, mechanical analysis was performed, and the results showed that the stress in the coils is within the permissible range for NbTi at 4.2 K.

The growth orientation of YBa₂Cu₃O_7−x (YBCO) is extremely sensitive to the growth atmosphere and the heat-treatment temperature. However, according to the existing literature, to obtain YBCO with preferred a-axis orientation, a buffer layer needs to be prepared on the substrate. In this work, YBCO films were grown via pulsed laser deposition, and the influence of N₂ and O₂ deposition atmospheres on the growth orientation of the YBCO films was studied. It was found that high-quality YBCO films with preferred c-axis orientation can be grown in O₂ atmosphere, but it is difficult to grow a-axis-oriented YBCO films in O₂ atmosphere without using a buffer layer; however, YBCO films with a high degree of preferred a-axis orientation can be grown in N₂ atmosphere without using a buffer layer. Furthermore, an a/c-oriented homogeneous YBCO bilayer with c-axis orientation in the lower layer and a-axis orientation in the upper layer was prepared. X-ray diffraction and scanning transmission electron microscopy results show that the bilayer is epitaxial with a good a/c orientation, and the resistance–temperature curve shows the occurrence of two transitions at temperatures of 91 and 71 K. We believe that these temperatures correspond to the superconducting onset transition temperatures of YBCO in the c- and a-axis growth orientations, respectively. The superconducting current usually flows within the Cu–O plane. With its sudden change in the direction of the current at the interface, the homogeneous junction prepared in this work is expected to provide new ideas for the research of high-temperature superconducting junction devices.

Y_0.5Gd_0.5Ba₂Cu₃O_7-σ (YGBCO) and Ho_0.5Gd_0.5Ba₂Cu₃O_7-σ (HGBCO) targets with similar densities were prepared by solid-phase reaction. Pulsed laser deposition (PLD) technology was used to fabricate superconducting films with these targets. During the experiments in this paper, we varied the laser energy by adjusting the optical lens. The structure and texture of RE_0.5Gd_0.5Ba₂Cu₃O_7-σ (REGBCO, where RE = Y, Ho) thin films were analyzed by X-ray diffraction (XRD). The surface morphology was observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and the superconducting critical current was measured at 77 K using the standard four-probe method. The laser energy density and different doping elements can affect the properties of REGBCO films. This may be related to the ejection process of target particles and the size of rare earth atoms. Additionally, we prepared a 510-meter long second-generation high-temperature superconducting tape with the optimized parameters. The critical current of the tape is 350 A, and the current density is 3.9 × 10⁶ A/cm².

Two types of first-order gradiometers equipped with 50 mm diameter pickup coils were made out of an REBaCuO High-T_c superconducting (HTS) flexible tape. The HTS tapes were formed into the HTS gradiometers by ``cut-and-wind'' method without cable jointing.

The HTS gradiometers were examined at 77 K for magnetic sensitivity to a gradient field applied by an external coil. The axial-type HTS gradiometer eliminated the magnetic field output of the device induced by a uniform field, and enhanced the magnetic field output induced by a gradient field on the axis of the pickup coil. The planar-type HTS gradiometer made it possible to detect a 20 mm diameter steel cylinder placed beside the pickup coil in a uniform field of 100 µT by detecting the spatial field gradient in the vicinity of a ferromagnetic object.

Iron-based superconductors are regarded as prospective candidates for high magnetic field applications since they have very high upper critical field and low anisotropy. For practical applications, it is important to develop superconducting wires with strong current carrying capability. In this work, Cu/Ag composite sheathed (Ba, K)Fe₂As₂ (Ba-122) iron-based superconducting wires were fabricated by a two-axial rolling deformation process and a subsequent hot isostatic pressing (HIP) heat treatment. Compared with the previously reported Cu/Ag/Ba-122 wires prepared by groove rolling, the two-axial rolling allowed to use much thinner Cu/Ag composite sheath for wire fabrication, thus greatly increasing the filling factor of superconducting materials by about 5 times to 24 %, and lowering the volume fraction of silver down to 16 % in wires. By a comparative study on the microstructure and superconducting properties between the wires made by two-axial rolling and groove rolling processes, it is found that besides the significantly increased engineering critical current density, the former also exhibits improved homogeneity of mass distribution and uniformity of Ba-122/Ag interfaces, resulting in significantly enhanced n-values over 40 in magnetic fields up to 14 T. Our work suggests that two-axial rolling, combined with HIP processes, presents a promising approach to prepare iron-based superconducting wires with high filling factor, high uniformity and low cost for practical applications.

A preliminary design of a Rutherford cable (Rfc) consisting of a copper core and 10 Quasi-isotropic Strands (Q-ISs) with symmetrical geometry is proposed. The current sharing temperature (T_cs), minimum quench energy (MQE), and normal zone propagation velocity (NZPV) are significant for determining the thermal stability performance of the superconducting cable. Firstly, an electric model of the conductor equivalent circuit is established, and the algebraic equations are derived. The model is validated with the empirical formula by calculating the T_cs of a single Q-IS. Using the validated model, the T_cs of Rfc fabricated by Q-ISs operating in liquid helium temperature at different magnetic fields are obtained. Then, to quantitatively characterize the effect of Q-ISs located at different positions on Rfc after a heating disturbance, the MQE and NZPV of Rfc are numerically simulated by the finite element method, which uses a 3-D thermal model with a homogenization procedure and coupled with the previous electrical model. The analyzed results provide a preliminary assessment of the thermal stability of the high-current superconducting cable and provide important guidance for subsequent experiments and prospective engineering applications.

The Bi₂Sr₂CaCu₂O_8+x (Bi-2212) material, a high-temperature superconductor, holds significant promise for future high-field applications because of its multifilamentary, twistable production capabilities, coupled with a high upper critical field. Bi-2212 round wire (RW) is made by the powder-in-tube (PIT) technique, and Bi-2212 coils are usually being developed using the wind-and-react method and a high temperature and over-pressure (OP) heat treatment (50 atm, 890℃) is then required to achieve the high current performance. However, over-pressure (OP) heat treatment can lead to a reduction in wire diameter, compromising the stability of the coil structure. Addressing this issue, the pre-overpressure (pre-OP) heat treatment is used and proves effective in mitigating the reduction in Bi-2212 wire diameter. In this paper, to comprehensively investigate the impact of pre-OP heat treatment on the microstructure of Bi-2212 wire, a metallographic analysis of two kinds of Bi-2212 wires was carried out by the scanning electron microscope (SEM) and the ImageJ software. It was found that larger-area filaments experienced a greater reduction in area after pre-OP heat treatment, that pre-OP heat treatment increases the aspect ratio of filaments, and polygonal bundle structure helps maintain filament structure stability during pre-OP heat treatment. Additionally, the impact of pre-OP heat treatment on the performance of Bi-2212 wires was analyzed through critical current testing under high fields.

We found a significant difference between the central and end magnetic field delays during the testing of a metal-insulation (MI) high-temperature superconducting (HTS) magnet. In this paper, we first analyzed the reasons for the difference in the field, and the results show that it is mainly due to the difference in the characteristic resistance of each double pancake (DP) coil. We further evaluated the effect of the difference on the electromagnetic analysis results of the MI HTS magnet. Subsequently, we developed an equivalent circuit model that considers the difference in the characteristic resistance of the DP coils and used the model to analyze the effect of this difference on the fields and losses generated by the MI HTS magnets, especially for AC conditions. Finally, based on our analytical results, a coil arrangement strategy considering the characteristic resistance of each DP coil was proposed. This work can guide us in evaluating the magnetic field more accurately in future engineering applications and provide a coil arrangement strategy for the MI HTS magnet, which requires to excite rapidly or generate an alternating magnetic field.

CORC superconducting cable is a compact and flexible composite superconductor. It is widely used in large power systems because of its good flexibility and high engineering current density. In these applications, reducing the AC loss generated by the superconductor is a key factor in studying the electromagnetic field of the superconducting conductor, which affects the operational stability of the entire superconducting power device. Currently, the use of striated tape-wound cables not only allows for flexible wiring at design time but also effectively reduces AC losses. This technology has been widely used in large superconducting magnet equipment. In this paper, the finite element model of single superconducting tape and CORC superconducting cable is established based on the finite element idea. The electromagnetic properties and mechanical changes are numerically calculated by H-method. The results show that both the current density and the magnetic field of a single superconducting tape show a gradual penetration from the two sides to the center. The area of maximum current density and magnetic field distribution is at the tape boundary. The distribution of current density, magnetic field and Lorentz force of three-dimensional CORC superconducting cable is similar. All of them are gradually attenuated from the maximum value at the boundary to the center position. It is worth noting that the current density, magnetic field and Lorentz force of the striated superconducting cables are smaller than the values without striations. The calculation results of the finite element model in this paper show that the striations weaken the current density, magnetic field and Lorentz force of the CORC superconducting cable by about 8.77%, 16.21% and 9.23%. Moreover, the presence of striations can effectively reduce the AC loss of superconductors.