# Control of dopant crystallinity in electrochemically treated cuprate thin films

## TL;DR

This paper introduces an electrochemical method to precisely control oxygen doping in La2CuO4+y thin films, enabling tuning of superconducting properties and inducing dopant crystallinity through a novel insertion mechanism.

## Contribution

It presents a new ex-situ electrochemical technique to manipulate dopant oxygen arrangement and lattice symmetry in cuprate thin films, expanding control over their superconducting characteristics.

## Key findings

- Superconducting transition temperature is adjustable via pH-controlled electrochemistry.
- Dopant oxygens can form disordered or crystalline phases.
- A new mechanism involves dopant insertion altering lattice symmetry.

## Abstract

We present a methodology based on \textit{ex-situ} (post-growth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide La$_2$CuO$_{4+y}$ grown epitaxially on substrates of isostructural LaSrAlO$_4$. The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and describe the resulting methodology as a platform to be explored in thin films of other transition metal oxides.