High impact of spin-nematic order on the lattice domains in thin films of iron-based superconductors

The spin-nematic scenario proposed for the structural instability of iron-based superconductors is studied theoretically within a Ginzburg-Landau approach in epitaxial thin films. The transition temperature and the domains that appear due the interface with the substrate are computed analytically by following a variatonal procedure. It is predicted a cross-over thickness $l_{\ast }$ that separates the Kittel regime, in which the size of the domains goes as the square root of the film thickness $l$, from a new ultrathin regime in which the system tends to reach the single-domain state as $l\to 0$. The experimental observation of this cross-over is therefore proposed as a direct probe of spin nematiciy.