Raman scattering study of the tetragonal magnetic phase in ${\mathrm{Sr}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$: Structural symmetry and electronic gap

We use inelastic light scattering to study ${\mathrm{Sr}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($x\approx 0.34$), which exhibits a robust tetragonal magnetic phase that restores the fourfold 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 a different electronic gap. The energy of this gap is substantially smaller than the gap observed in the orthorhombic magnetic phase, and the two gaps compete only weakly for spectral weight. These observations are consistent with the notion that the reentrant phase behavior is driven by spin-orbit interactions, which seem to dictate the manifestation of magnetism in Fe-based superconductors. No phonon back folding can be detected above the noise level, which implies that any lattice translation symmetry breaking in the tetragonal magnetic phase must be very weak.

Figure 1

$T$ dependence of (a) resistivity and (b) its derivative measured on three samples. (c) The doping range of our samples in the phase diagram of ${\mathrm{Sr}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ [21].

Figure 2

(a) Photograph of a single crystal of ${\mathrm{Sr}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. The insets display freshly cleaved $ab$ (top view) and $ac$ (side view) surfaces for our Raman measurements. (b) Raman spectra measured with perpendicular $ac$ polarizations at different temperatures, offset for clarity. (c) $T$ dependence of the fitted peak energies (see text). The inset illustrates the experimental geometry, with light polarizations indicated by double arrows.

Figure 3

Raman spectra measured with perpendicular $ab$ polarizations. (a) Spectra obtained between 10 and 60 K. (b) Intensity difference against the 60 K spectrum. The inset illustrates the polarization geometry (double arrows) along with the electronic Raman selection rules [7]. (c) Spectra obtained between 60 and 160 K, offset for clarity. The spectra in (a) and (c) have been normalized at 1100–1200 ${\mathrm{cm}}^{-1}$ to correct for uncontrolled fluctuations in the overall intensity. (d) $T$ dependence of integrated quasielastic scattering below intensity 50 ${\mathrm{cm}}^{-1}$ [shaded region in (a) and (c)].

Figure 4

$T$ dependence of (a) the energy and (b) the full width at half maximum (FWHM) of the $A_{1g}$ phonon peak, measured with parallel $cc$ polarization. The inset of (a) presents four representative spectra to demonstrate the nonmonotonic peak movement. (c) and (d) Raman spectra measured with parallel $cc$ and $aa$ polarizations, respectively, at selected temperatures. The insets are illustrations of the polarization geometry and zoom-in view of the 10 K spectra. Inverse triangles in (c) indicate polarization leakage from $E_g$ phonon signals which are unrelated to phonon back folding.