# Untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ thin films on MgO substrates: a platform to study strain effects on the local orders in cuprates

## TL;DR

This study reports the growth of untwinned, strain-engineered YBa2Cu3O7−δ thin films on MgO substrates, providing a new platform to investigate how strain influences local electronic orders in cuprate superconductors.

## Contribution

The paper introduces a method to produce untwinned, highly strained YBCO films with controlled orientation and thickness, enabling detailed studies of strain effects on cuprate properties.

## Key findings

- Untwinned YBCO films with suppressed twinning domains.
- Strong uniaxial strain and buckling observed in the films.
- Strain effects become more pronounced in ultrathin structures.

## Abstract

We have grown untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) films on (110) MgO substrates that were pre-annealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear, in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO.