Origin of superconducting carriers in ``non-doped'' T$^{\prime}$-(La, RE)$_{2}$CuO$_{4}$ (RE = Sm, Eu, Gd, Tb, Lu, and Y)

We have reported the isovalently-substituted new superconductors T'-La$_{2-x}$\textit{RE}$_{x}$CuO$_{4}$ ($T_{c}\sim $20-25K) prepared by MBE with no effective dopant. As regards the origin of the carriers in these nominally non-doped superconductors, there seems to be two possible scenarios: (i) oxygen deficiencies at the regular oxygen sites serve as a source of effective electron carriers, and (ii) they are not Mott insulators and have intrinsic carriers. Since precise information on the site- specific occupancy of oxygen is very difficult to obtain, alternatively, we investigated the in-plane lattice constant $a_{0}$ with changing \textit{RE} concentration $x$, with a view to examining possible $a_{0}$ expansion due to electron doping. In each \textit{RE} substitution, the $a_{0 }$of the T'-La$_{2-x}$\textit{RE}$_{x} $CuO$_{4 }$linearly decreases with increasing $x$, whose extrapolation to $x$=2 agrees well with the reported value for bulk T'-\textit{RE}$_{2}$CuO$_{4}$. This$_{ } $variation can simply be understood based on the difference in the ionic radius of \textit{RE}$^{3+ }$vs La$^{3+}$, suggesting that these superconductors$_{ }$are not electron-doped, at least substantially, and that the second scenario is the more plausible. This conclusion is also supported by the results of transport and photoemission experiments.