Structural Goldstone modes in 111-strained perovskite SrMnO₃

Epitaxial strain has been extensively explored to enhance existing and enable new functional properties in perovskite thin films. While most of this work has been done on 001-oriented heterostructures, 111-oriented films can have very different properties. Here we use DFT calculations to predict structural Goldstone modes in the (111)-strained perovskite SrMnO₃. Massless Goldstone modes are found in high energy particle physics as well as in low energy systems like superconductors and superfluid helium, while structural Goldstone modes are rare. Here we predict acoustic Goldstone phonon modes under compressive strain resulting from coupling between two in-plane rotational instabilities, giving the characteristic Mexican hat shaped energy surface. Large tensile strain is found to induce in-plane polar instabilities giving rise to a continuous polar ground state. Such phonon modes with U(1) symmetry could emulate structural condensed matter Higgs modes, where the mass of this boson is given by the shape of the Mexican hat energy surface, which is tunable by epitaxial strain.