Origin of superconducting carriers in “non-doped” T′-(La,RE)2CuO4 (RE = Sm, Eu, Gd, Tb, Lu, and Y) prepared by molecular beam epitaxy

We have performed a systematic investigation of the variations of the lattice constants with substituent rare-earth element concentration x in the nominally undoped superconductors T′-La2-x3+REx3+CuO4 (RE = Sm, Eu, Gd, Tb, Lu, and Y), which we have recently discovered using MBE. The results show both the in-plane and out-of-plane lattice constants (a0 and c0 linearly decrease with x, whose extrapolation to x = 2 agrees well with the reported a0 and c0 values for each T′-RE2CuO4. This behavior is what one would expect simply from the ionic size difference between La3+ and RE3+. The absence of the Cu-O bond stretching due to electron-doping, which is commonly observed in electron-doped T′ and infinite-layer superconductors, implies that electron-doping via oxygen deficiencies is, at least, not a main source of charge carriers.