Band- and momentum-dependent electron dynamics in superconducting $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ as seen via electronic Raman scattering

We present details of carrier properties in high quality $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition, highlighting the importance of complex band-dependent interactions. The presence of low-energy spectral weight deep in the superconducting state suggests a gap with accidental nodes, which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band- and momentum-dependent carrier dynamics indicate that the iron arsenides may have several competing superconducting ground states.

Figure 1

(Color online) Crystal structure and Raman selection rules for ${\text{BaFe}}_2{\text{As}}_2$. (a) The relevant cell of the Fe plane is smaller by a factor of 2 and is rotated by $45°$ (dashes) with respect to the crystal cell (full line). (b) Brillouin zone (BZ) of the unit cell (full line) and first quadrant of the Fe plane (dashed line). The light polarizations are indicated symbolically with respect to the basal plane shown in panel (a). The electronic $A_{1g}$ and $B_{2g}$ spectra project the $\alpha$ and $\beta$ bands, respectively.

Figure 2

(Color online) Symmetry-dependent Raman response $R{\chi }^{″}\left(\Omega ,T\right)$ of $\text{Ba}{\left({\text{Fe}}_{0.939}{\text{Co}}_{0.061}\right)}_2{\text{As}}_2$. (a) In $B_{1g}$ symmetry, there is a phonon at $214{\text{cm}}^{-1}$ from Fe vibrations. (b) In $A_{1g}$, there is a small increase toward $\Omega \to 0$ since it is always measured with parallel polarizations, where the laser is less efficiently suppressed. The analysis demonstrates that the $A_{2g}$ signal can safely be neglected (Ref. 25).

Figure 3

(Color online) Raman response $R{\chi }^{″}\left(\Omega ,T\right)$ of $\text{Ba}{\left({\text{Fe}}_{0.939}{\text{Co}}_{0.061}\right)}_2{\text{As}}_2$ in (a) $A_{1g}$ and (b) $B_{2g}$ polarizations. Spectra plotted with full lines are measured with a resolution of $5.0{\text{cm}}^{-1}$. For further clarifying the spectral shape at low energy and around the maximum, the superconducting $B_{2g}$ spectra were also measured with a resolution of $3.6{\text{cm}}^{-1}$ [(orange) points in (b), (e), and (f)]. The insets indicate power-law behavior for both symmetries. The finite spectral intensity at low energies supports gap nodes. (c) shows the gap forms used for the Raman spectra calculated in (d). The $A_{1g}$ and $B_{1g}$ spectra are multiplied by 0.5 and 10, respectively. (e) and (f) show the $B_{2g}$ spectra above and below the peak maximum on a logarithmic scale to highlight the divergence around ${\Omega }_{\text{max}}=69{\text{cm}}^{-1}$.

Figure 4

(Color online) $B_{2g}$ Raman-scattering response $R{\chi }^{″}\left(\Omega ,T\right)$ of $\text{Ba}{\left({\text{Fe}}_{0.915}{\text{Co}}_{0.085}\right)}_2{\text{As}}_2$. Spectra plotted with full lines [(orange) points] are measured with a resolution of $5.0{\text{cm}}^{-1}$ $\left(3.6{\text{cm}}^{-1}\right)$. The inset shows the phase diagram (Ref. 23) with the two samples studied here indicated by arrows. Instead of the sublinear energy dependence at 6.1% [Fig. 3b], a finite gap of approximately $10{\text{cm}}^{-1}$ is clearly resolved here. The logarithmic singularity is replaced by a broad maximum. Both the low- and the high-energy parts of the spectra follow naturally from the influence of impurities (Ref. 26).