Thermodynamic approach for enhancing superconducting critical current performance

The addition of artificial pinning centers has led to an impressive increase in the critical current density ( J c ) of superconductors, enabling record-breaking all-superconducting magnets and other applications. The J c of superconductors has reached ~0.2–0.3 J d , where J d is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying λ and/or ξ, the penetration depth and coherence length, respectively, we can increase J d . For (Y 0.77 Gd 0.23 )Ba 2 Cu 3 O y ((Y,Gd)123), we can achieve this by controlling the carrier density, which is related to λ and ξ. We can also tune λ and ξ by controlling the chemical pressure in Fe-based superconductors, i.e., BaFe 2 (As 1− x P x ) 2 films. The variation in λ and ξ leads to an intrinsic improvement in J c via J d , allowing extremely high values of J c of 130 MA/cm 2 and 8.0 MA/cm 2 at 4.2 K, consistent with an enhancement in J d of a factor of 2 for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe 2 (As 1− x P x ) 2 films, showing that this new material design is useful for achieving high critical current densities in a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached ~3.17 TN/m 3 at 4.2 K and 18 T ( H || c ), the highest values ever reported for any superconductor. A very active research area in superconductivity is the performance improvementofthe current carrying capability ( J c ). Here, we show that the depairing current ( J d ) can beincreased by controlling the penetration depth (λ) and coherence length (ξ) in the two most representative materials: in REBa 2 Cu 3 O y we increased the carrier density to decrease λ and ξ, and in BaFe 2 (As 1- x P x ) 2 we tuned the chemical pressure. The combination ofthese thermodynamic and flux-pinning optimization routes leads to a dramatic increase of J c to J c ~150 MA/cm 2 at 4.2 K (32.4% of J d ) for REBa 2 Cu 3 O y , the highest value ever reported for any superconductor.