You can improve the current density a lot if you can make a single crystal, or at least make your crystal grains larger.
Impure superconductor samples often come out as a spongy mixture of superconducting and non-superconducting bits, the critical current is limited because less then 25% of the cross section is actually carrying current. When I was DIYing YBCO this is what happened most of the time. Every now and then you would get a good one.
edit: Actually if you look at the sample picture in the paper on page 7, it looks like spongy crap. Nobel prize winning spongy crap, but still. I would expect the numbers to improve as better crystal growing methods are found.
Impure superconductor samples often come out as a spongy mixture of superconducting and non-superconducting bits, the critical current is limited because less then 25% of the cross section is actually carrying current. When I was DIYing YBCO this is what happened most of the time. Every now and then you would get a good one.
See this patent for growing single crystals of YBCO. https://patents.google.com/patent/US6046139A/en
edit: Actually if you look at the sample picture in the paper on page 7, it looks like spongy crap. Nobel prize winning spongy crap, but still. I would expect the numbers to improve as better crystal growing methods are found.