Nematic fluctuations and phase transitions in LaFeAsO: A Raman scattering study

Raman scattering experiments on LaFeAsO with distinct antiferromagnetic ($T_{\text{AFM}}=140$ K) and tetragonal-orthorhombic ($T_S=155$ K) transitions show a quasielastic peak (QEP) in $B_{2g}$ symmetry (2 Fe tetragonal cell) that fades away below $\sim T_{\text{AFM}}$ and is ascribed to electronic nematic fluctuations. A scaling of the reported shear modulus with the $T$ dependence of the QEP height rather than the QEP area indicates that magnetic degrees of freedom drive the structural transition. The large separation between $T_S$ and $T_{\text{AFM}}$ in LaFeAsO compared to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ manifests itself in slower dynamics of nematic fluctuations in the former.

Figure 1

(a) Schematic view of the Fe square lattice with the stripe antiferromagnetic structure and representations of the $XY,YY,X^{\prime }{Y}^{\prime }$, and $Y^{\prime }{Y}^{\prime }$ linear polarizations. The unit vectors e${}_i$ and e${}_s$ represent the polarizations of the incident and scattered photons, respectively. The edges and axes for the 2 Fe tetragonal and 4 Fe orthorhombic unit cells are also displayed. (b) Raman-active phonons accessible in the scattering geometry employed in this paper. The corresponding symmetries and observed frequencies at 290 K are indicated. (c,d) Raman spectra for distinct polarizations at $T=20$ K (c) and $T=290$ K (d). In (c) and (d), the symmetry associated with each polarization is given with respect to the corresponding orthorhombic and tetragonal unit cells.

Figure 2

(a–e) Raman response ${\chi }^{\prime \prime }(\omega ,T)$ in $B_{2g}$ symmetry for the tetragonal cell $(XY$ polarization) at selected temperatures. The thick lines are fittings to a model including a Lorentzian quasielastic peak (QEP, dashed line) and an additional linear contribution (thin solid line) (see text). (f) ${\chi }^{\prime \prime }(\omega ,T)/\omega$ at selected temperatures and corresponding fits to the QEP model.

Figure 3

$T$ dependence of (a) area $A$, (b,d) width $\mathrm{\Gamma }$, and (c) height $A/\mathrm{\Gamma }$ of $B_{2g}$ Lorentzian QEP (2 Fe tetragonal cell). Error bars, when not displayed, are smaller than the symbol sizes. The shaded areas mark the $T_{\text{AFM}}<T<T_S$ interval. The solid line in (c) shows a fit of the $B_{2g}$ QEP height to a Curie-Weiss-like behavior between $T_S$ and $\sim 200$ K, yielding ${\theta }_{\text{CW}}=137\left(3\right)$ K (see text). The solid line in (d) is a guide to the eyes.

Figure 4

Temperature dependence of the polycrystalline shear modulus taken from Ref. [12] (solid line), and attempted scalings of this curve to the $B_{2g}$ QEP area $A\left(T\right)$ (open and solid triangles) and height $A\left(T\right)/\mathrm{\Gamma }\left(T\right)$ (circles) extracted from Figs. 3 and 3, respectively.