Crystal growth and metal-insulator transition in two-dimensional layered rare-earth palladates

We synthesized high-quality, single-crystalline thin films of layered rare-earth (R) palladates R2PdO4 (R = La, Nd, and Sm) and Ce substituted Nd2-xCexPdO4 by reactive molecular beam epitaxy. For La2PdO4, Nd2PdO4, and Sm2PdO4, we find that the electronic conduction is independent of R elements. Doping charge carriers into the PdO2 planes of Nd2-xCexPdO4 increases the electronic conduction and this effect is enhanced by vacuum annealing. The enhanced electronic conduction originates not solely from the doped charge carriers but is superimposed by Pd vacancies. X-ray photoelectron spectroscopy combined with inductively coupled plasma mass spectrometry revealed Pd deficiencies in Nd2-xCexPdO4 and these defects play a crucial role for the electronic conduction. We observe a monotonic enhancement of the electronic conduction in Nd2-xCexPdO4 thin films induced by Ce substitution and vacuum annealing. The estimated charge carrier concentration necessary for metallic conduction (x approximate to 0.45) at T = 0 K is far beyond the experimentally accessible solubility limit.