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

Abstract

We synthesized high-quality, single-crystalline thin films of layered rare-earth (R) palladates R${}_{2}{\mathrm{PdO}}_{4}$ ($R$ = La, Nd, and Sm) and Ce substituted ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ by reactive molecular beam epitaxy. For ${\mathrm{La}}_{2}{\mathrm{PdO}}_{4}$, ${\mathrm{Nd}}_{2}{\mathrm{PdO}}_{4}$, and ${\mathrm{Sm}}_{2}{\mathrm{PdO}}_{4}$, we find that the electronic conduction is independent of R elements. Doping charge carriers into the ${\mathrm{PdO}}_{2}$ planes of ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ 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 ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ and these defects play a crucial role for the electronic conduction. We observe a monotonic enhancement of the electronic conduction in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ thin films induced by Ce substitution and vacuum annealing. The estimated charge carrier concentration necessary for metallic conduction ($x\ensuremath{\approx}0.45$) at $T=0$ K is far beyond the experimentally accessible solubility limit.