How to achieve the best performance superconductor based on Cu-1234

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The best performance superconductor should have the highest J c, and H irr and the lowest surface resistance R s at 77 K. Our original superconductor CuBa 2Ca 3Cu 4O 12− y (Cu-1234) has a high T c>117 K (=one and a half times of 77 K) even in over-doping state, a low superconducting anisotropy ( γ=1.6), a long coherence length along c-axis ( ξ c =1 nm) and a small penetration depth ( λ c =220 nm). Therefore, it is capable of becoming the best performance superconductor with a high J c {50 MA/cm 2 (77 K, 0 T), 0.5 MA/cm 2 (77 K, 10 T)}, a high H irr {30 T (77 K)}and a low R s {30 μΩ (77 K, 10 GHz)}. These superconducting properties are derived from its unique composition, lattice structure and electronic structure. The prediction is going to be proved with the experimental data of T c, J c, H irr and hole concentration, NMR, specific heat and photoemission measurements and band calculation. Sustainable high T c (>117 K) in over-doped Cu-1234 were achieved by the selective over-doping effect, and high T c's of 126 K in Cu 1− x Tl x -1234 and 132 K in Cu 1− x Tl x -1223 were achieved by homogeneous optimum-doping effect by band structure engineering. The results show the effectiveness of band theory and Fermi liquid theory for approaching to the best performance superconductor from over-doping side. The two kinds of superconducting transitions at T c=117 K and T c2=60 K have been observed for over-doped Cu-1234 by NMR and specific heat measurements indicating the weak coupling of two superconducting-order-parameters (d+is or d+id). These weak-coupling two order parameters in Cu-1234 are expected to contribute to the enhancement of superconducting performance. A new self-assembling epitaxy (SAE) techniques for the preparation of Cu(Tl)-1234 and -1223 thin films has been developed by combining Tl-1234 self-assembling effect and amorphous phase epitaxy (APE) effect. The thin films of Cu 1− x Tl x -1223 ( x∼0.5) have achieved a high J c=20 MA/cm 2 (77 K, 0 T) and J c=0.4 MA/cm 2 (77 K, 10 T), and its data extrapolate a high ( H irr) ab ∼30 T at 77 K.