I should have added, yttrium barium copper oxide was the first high temperature superconductor to exhibit superconductivity above the boiling point of liquid nitrogen (T=77K, -196C). It goes superconducting at 93K (-180C).
Do you happen to know what the critical magnetic field is on the YBa2Cu3O7-x family?
What I really want to know is the supportable current density. Does a temp lower than 77K help in this regard?