Potential Major Improvement in Superconductors for High-Field Magnets
Fusion reactors are limited by the magnetic field available to confine their plasma. The commercial fusion industry uses the larger magnetic field and higher operating temperature of the cuprate superconductor YBa₂Cu₃O_(7−δ) (YBCO) in order to confine their plasma into a dense volume. A superconductor is a macroscopic quantum state that is protected from the metallic (resistive) state by an energy gap. Unfortunately, YBCO has an anisotropic gap, known as D-wave because it has the shape of a d_(x²−y²) chemical orbital. This D-wave gap means that poly-crystalline wire cannot be made because a few degree misalignment between grains in the wire leads to a drastic loss in its supercurrent carrying ability, and thereby its magnetic field limit. The superconductor industry has responded by growing nearly-single-crystal superconducting YBCO films on carefully prepared substrate tapes kilometers in length. Heroic development programs have made such tapes commercially available, but they are very expensive and delicate. MRI magnet superconductors, such as NbTi and Nb3Sn, are formed into poly-crystalline wires because they have an isotropic gap in the shape of an s chemical orbital (called S-wave) that makes them insensitive to grain misalignment. However, these materials are limited to lower magnetic fields and liquid-He temperatures. Here, we modified YBCO by doping the Y site with Ca and Ce atoms to form (Y₁₋ₓ₋ᵧCaₓCeᵧ)Ba₂Cu₃O_(7−δ), and show evidence that it changes to an S-wave gap. Its superconducting transition temperature, T꜀, of ∼70K, while lower than that of D-wave YBCO at ∼90K, is easily maintained using common, economic cryogenic equipment.
Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) We thank Thomas E. Sutto for suggesting Ce as a potential +4 oxidation state impurity atom that will reside at the Y site. This work was partially funded by the Office of Naval Research under Contract Number N00014-18-1-2679. AUTHOR CONTRIBUTIONS. J.T.-K. and C.A.M. conceived the project and designed experiments for thermopower and transition temperature. J.T.-K performed the room-temperature thermopower measurements, devised and performed the penetration depth experiments, devised the coax-correction and current-distribution correction protocols described in the supplement, and did all the theory calculations. Additional annealing at lower temperatures was done by J.T.-K. and C.A.M. They also wrote the paper. T.H. synthesized the materials and contributed to the interpretation of the XRD, SEM, and EDX data. M.L. performed the XRD, SEM, and EDX measurements, and did the materials characterization. M.L. also contributed to the materials characterization between sinters. M.S.O. designed and performed the PCAR experiment. The authors declare they have no competing financial interests.
Submitted - 2304.06171.pdf