Title: Raman scattering study of tetragonal magnetic phase in Sr$_{1-x}$Na$_x$Fe$_2$As$_2$: structural symmetry and electronic gap,
Abstract: We use inelastic light scattering to study Sr$_{1-x}$Na$_x$Fe$_2$As$_2$
($x\approx0.34$), which exhibits a robust tetragonal magnetic phase that
restores the four-fold rotation symmetry inside the orthorhombic magnetic
phase. With cooling, we observe splitting and recombination of an $E_g$ phonon
peak upon entering the orthorhombic and tetragonal magnetic phases,
respectively, consistent with the reentrant phase behavior. Our electronic
Raman data reveal a pronounced feature that is clearly associated with the
tetragonal magnetic phase, suggesting the opening of an electronic gap. No
phonon back-folding behavior can be detected above the noise level, which
implies that any lattice translation symmetry breaking in the tetragonal
magnetic phase must be very weak.