Prediction of first-order martensitic transitions in strained epitaxial films

TL;DR

This paper predicts that strained epitaxial films can exhibit first-order martensitic transitions characterized by abrupt changes in out-of-plane lattice parameters, affecting properties like superconductivity.

Contribution

It introduces a theoretical framework predicting discontinuities in lattice parameters of strained films, revealing new possibilities for property switching in epitaxial materials.

Findings

Discontinuities in $c_{min}$ occur in V, Nb, Ru, La, Os, and Ir films.

Abrupt changes in $c_{min}$ can switch material properties.

Superconducting temperature of V can jump by 20% at a transition.

Abstract

Coherent epitaxial growth allows to produce crystalline films with strained structures which are unstable in the bulk. Thereby, the relationship between the lattice parameters of the overlayer in the interface plane, $(a,b)$, and its minimum-energy out-of-plane lattice parameter, $c_{\text{min}}(a,b)$, need not be continuous. This general statement is proven by examples of total energy calculations. As a consequence, $c_{\text{min}}$, which is determined by the choice of the substrate, and $c_{\text{min}}$-dependent intrinsic properties of the overlayer cannot always be tuned in a continuous way as one may aim to do by means of strained epitaxy. Employing the model of the epitaxial Bain path we predict that such discontinuities occur in films of the elements V, Nb, Ru, La, Os, and Ir. The abrupt change of $c_{\text{min}}$ can be exploited to switch properties specific to the overlayer material. This is demonstrated for the example of the superconducting critical temperature of a V film which we predict to jump by 20% at a discontinuity of $c_{\text{min}}$