Optical conductivity and superconductivity in highly overdoped La2‑xCaxCuO4 thin films
Thin-film technology enables synthesis of superconducting material that is unstable in bulk form
Chemical substitution is widely used to modify the charge carrier concentration ("doping") in complex quantum materials. A prominent example is the "214" family of high-temperature superconductors, La2-x(Ba,Ca,Sr)xCuO4, where substitution of the trivalent La ions by divalent alkaline-earth ions releases positive charge carriers that form the supercurrents at low temperature. However, the randomness and disorder associated with chemical substitution affects the electronic properties in a manner that remains difficult to describe in the framework of quantitative theories. In particular, it remains unknown if disorder is responsible for the experimentally observed disappearance of high-temperature superconductivity at high charge carrier concentrations (see the figure). Current research therefore aims to develop a microscopic understanding of these effects, and then to optimize the functionality of quantum materials through "disorder management".
![Dependence of the superconducting transition temperature, Tc, on the charge-carrier concentration in bulk crystals of La2‑x(Ba,Sr)xCuO4 and in epitaxially stabilized films of La2‑xCaxCuO4.](/7521432/original-1630418474.jpg?t=eyJ3aWR0aCI6MjQ2LCJvYmpfaWQiOjc1MjE0MzJ9--f774daa855aaf740681408c0d1e42faa086c2933)
In the case of the 214 superconductors, these efforts have been hampered by the chemical instability of bulk La2‑xCaxCuO4, where disorder is minimal because the sizes of the Ca2+ and La3+ ions are nearly equal. We have overcome this challenge by using a substrate with a similar lattice structure as a template for high-quality La2‑xCaxCuO4 films. Measurements of the optical conductivity as well as other physical properties were then used to characterize the charge carrier concentration in these films. We found that high-temperature superconductivity is stable up to much higher doping levels than previously found for analogous bulk compounds with higher levels of disorder. The results imply that doping-induced disorder is the leading cause of the degradation of superconductivity for large charge-carrier concentrations, and they open up a previously inaccessible regime of the phase diagram of high-temperature superconductors to experimental investigation.