Magnetic-Field Induced Superconductor-Antiferromagnet Transition in Lightly Doped RBa2Cu3O6+x (R = Lu, Y) Crystals

The remarkable sensitivity of the c-axis resistivity and magnetoresistance in cuprates to the spin ordering is used to clarify the doping-induced transformation from an antiferromagnetic (AF) insulator to a superconducting (SC) metal in RBa2Cu3O6+x (R = Lu, Y) single crystals. The established phase diagram demonstrates that the AF and SC regions apparently overlap: The superconductivity in RBa2Cu3O6+x, in contrast to La2-xSrxCuO4, sets in before the long-range AF order is completely destroyed by hole doping. Magnetoresistance measurements of superconducting crystals with low T-c <= 15-20K give a clear view of the magnetic-field induced superconductivity suppression and recovery of the long-range AF state. What still remains to be understood is whether the AF order actually persists in the SC state or just revives when the superconductivity is suppressed, and in the former case, whether the antiferromagnetism and superconductivity reside in nanoscopically separated phases or coexist on an atomic scale.