  We report the synthesis of new superconducting cuprates T'-La2-xRExCuO4 (RE =
Sm, Eu, Tb, Lu, and Y) using molecular beam epitaxy. The new superconductors
have no effective dopant, at least nominally. The substitution of isovalent RE
for La was essentially performed to stabilize the T' phase of La2CuO4 instead
of the T phase. The maximum Tconset is ~ 25 K and Tczero is ~ 21 K. The keys to
our discovery are (1) the preparation of high-crystalline-quality La-based T'
films by low-temperature (~ 650C) thin film processes, and (2) more thorough
removal of impurity oxygen at the apical site, which is achieved by the larger
in-plane lattice constant (a0) of T'-La2-xRExCuO4 than other T'-Ln2CuO4 (Ln =
Pr, Nd, Sm, Eu, Gd) with the aid of large surface-to-volume ratio of thin
films.

Superconductivity