Title: Goldstone-like phonon modes in a (111)-strained perovskite,
Abstract: Goldstone modes are massless particles resulting from spontaneous symmetry
breaking. Although such modes are found in elementary particle physics as well
as in condensed matter systems like superfluid helium, superconductors and
magnons - structural Goldstone modes are rare. Epitaxial strain in thin films
can induce structures and properties not accessible in bulk and has been
intensively studied for (001)-oriented perovskite oxides. Here we predict
Goldstone-like phonon modes in (111)-strained SrMnO3 by first-principles
calculations. Under compressive strain the coupling between two in-plane
rotational instabilities give rise to a Mexican hat shaped energy surface
characteristic of a Goldstone mode. Conversely, large tensile strain induces
in-plane polar instabilities with no directional preference, giving rise to a
continuous polar ground state. Such phonon modes with U(1) symmetry could
emulate structural condensed matter Higgs modes. The mass of this Higgs boson,
given by the shape of the Mexican hat energy surface, can be tuned by strain
through proper choice of substrate.