REBa 2 Cu 3 O x ((REBCO), RE= rare earth) superconductor tapes with moderate levels of
dopants have been optimized for high critical current density in low magnetic fields at 77 K,
but they do not exhibit exemplary performance in conditions of interest for practical
applications, ie, temperatures less than 50 K and fields of 2–30 T. Heavy doping of REBCO
tapes has been avoided by researchers thus far due to deterioration in properties. Here, we
report achievement of critical current densities (J c) above 20 MA/cm 2 at 30 K, 3 T in heavily …