Pair breaking, “intrinsic”T c,and the upper limit ofT c in the cuprates

The presence of magnetic impurities leads to a drastic decrease inT _c in the overdoped region, gaplessness, and the usual temperature dependence ofH _c2. The magnetic moments are localized on the apical oxygen site, and this allows us to explain the increase inT _c with the increase in the number of Cu-O planes in the unit cell. Applied pressure can raiseT _c above the usual value at optimum doping, toward the “intrinsic”T _c. The cuprates as a class of compounds have an upper limit ofT _c in the rangeT _c,upp=160–170 K.