Optical conductivity and superconductivity in highly overdoped La _{2− x} Ca _x CuO ₄ thin films

Significance

Chemical substitution is widely used to modify the charge-carrier concentration (“doping”) in complex quantum materials, but the influence of the associated structural disorder on the electronic phase behavior remains poorly understood. We synthesized thin films of the high-temperature superconductor ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{C}\mathrm{a}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ with minimal structural disorder and characterized their doping levels through measurements of the optical conductivity. We find that superconductivity with $T_c$ = 15 to 20 K is stable up to much higher doping levels than previously found for analogous compounds with stronger disorder. The results imply that doping-induced disorder is the leading cause of the degradation of superconductivity for large carrier concentration, and they open up a previously inaccessible regime of the phase diagram of high-temperature superconductors to experimental investigation.

Abstract

We have used atomic layer-by-layer oxide molecular beam epitaxy to grow epitaxial thin films of ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{C}\mathrm{a}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ with $x$ up to 0.5, greatly exceeding the solubility limit of Ca in bulk systems ( $x\sim 0.12$ ). A comparison of the optical conductivity measured by spectroscopic ellipsometry to prior predictions from dynamical mean-field theory demonstrates that the hole concentration $p$ is approximately equal to $x$ . We find superconductivity with $T_c$ of 15 to 20 K up to the highest doping levels and attribute the unusual stability of superconductivity in ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{C}\mathrm{a}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ to the nearly identical radii of La and Ca ions, which minimizes the impact of structural disorder. We conclude that careful disorder management can greatly extend the “superconducting dome” in the phase diagram of the cuprates.