Superconductivity phase diagrams for the electron-doped cuprates $R_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$ ($R=\mathrm{La}$, Pr, Nd, Sm, and Eu)

The superconductivity phase diagrams of electron-doped cuprates of the form $R_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$ (with $R=\mathrm{La}$, Pr, Nd, Sm, and Eu) have been determined for cerium compositions $0<x<0.36$ in a consistent series of epitaxial thin films grown by reactive molecular beam epitaxy. The use of epitaxial thin films allows the growth of materials away from thermodynamical equilibrium expanding the accessible phase space beyond the availability of bulk material. The superconducting phase space systematically increases with the rare-earth ionic size. The doping concentration where the maximal transition temperature occurs in ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$ is considerably shifted to lower doping $(x\sim 0.09)$ compared to ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ $(x\sim 0.15)$. At the same time, the width of the superconducting region is broadened.

Figure 1

(Color online) Typical thermodynamic phase stability diagram for electron-doped cuprates. Stability lines for CuO and ${\mathrm{Cu}}_2\mathrm{O}$ have been calculated using the commercially available program MALT®. Additionally, the equilibria oxidizing potential lines for ozone and oxygen are calculated. The numbered points describe a typical growth of the thin film. The border lines for the different copper valencies are thick red colored. Between them, all experimentally established stability lines for the $T^{\prime }$-structure compounds are lying. Points 1 and 2 represent the growth followed by annealing in vacuum (point 3) and afterwards cooled down to point 4.

Figure 2

(Color online) Superconductivity phase diagrams of ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$, ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$, and ${\mathrm{Pr}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$.

Figure 3

(Color online) Superconductivity phase diagrams of ${\mathrm{Sm}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$ and ${\mathrm{Eu}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$.

Figure 4

(Color online) Direct comparison of La-based cuprates with respect to electron and hole doping: Superconductivity phase diagrams of ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$ (thin film data) and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ (bulk data[2, 25]).