Impact of the spin-density-wave order on the superconducting gap of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$

We report a doping-dependent electronic Raman-scattering measurements on iron-pnictide superconductor $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystals. The ${\text{B}}_{2g}$ Raman spectrum at optimal doping is consistent with a strongly anisotropic gap on the electron pocket. Upon entering the coexistence region between superconducting (SC) and spin-density-wave (SDW) orders, the effective pairing energy scale is strongly reduced. Our results are interpreted in terms of a competition between SC and SDW orders for electronic states at the Fermi level. Our findings advocate for a strong connection between the SC and SDW gaps anisotropies which are both linked to interband interactions.

Figure 1

(Color online) (a) Evolution of Raman intensity with temperature in the $B_{2g}$ symmetry for optimal doping $x=0.065$. Inset: integrated area and energy of the pair-breaking peak as a function of temperature. The dashed line is the BCS temperature dependence of $2\Delta$. (b) Raman response ${\chi }^{″}\sim {\left[1+n\left(\omega ,T\right)\right]}^{-1}\times I$, where $n\left(\omega ,T\right)$ is the Bose factor and $I$ is the measured Raman intensity, for different laser wavelengths $\lambda =514.52$ and 647.1 nm at $T=8\text{K}$. The theoretical fit in blue was obtained using an anisotropic gap (see text), a broadening $\gamma =0.1{\Delta }_{max}$ and a constant ${\text{B}}_{2g}$ Raman vertex around the electron pocket (Ref. 14). The theoretical fit in black was obtained using two isotropic gaps $2{\Delta }_1$ and $2{\Delta }_2$ (dashed lines), a broadening $\gamma =0.3{\Delta }_2$. (c) Sketches of an anisotropic $s$-wave gap around the $M$ electron pocket in the ($k_x$, $k_y$) plane (left) or along $k_z$ (right).

Figure 2

(Color online) (a) Raman response ${\chi }^{″}$ in the $B_{2g}$ symmetry above and below the SC transition for $x=0.055$, $x=0.06$, $x=0.065$, $x=0.075$, and $x=0.1$. (b) Phase diagram of Co-Ba122 showing the superconducting peak area, defined as ${\int }_{15{\text{cm}}^{-1}}^{125{\text{cm}}^{-1}}{\chi }^{″}\left(\omega \right)d\omega$ (empty triangles) as a function of doping. The overall intensity of the spectra were normalized with the response above $200{\text{cm}}^{-1}$, which was found to be weakly doping dependent between $x=0.055$ and $x=0.10$. (c) Doping evolution of the two component, $2{\Delta }_{max}$ and $2{\Delta }_{min}$ in an anisotropic gap picture. $2{\Delta }_{min}$ values are not reported for $x=0.075$ and $x=0.10$ because of the absence of a clear low-energy component in the spectra. (d) Raman response at low temperature for indicated doping. Inset: substraction of ${\chi }^{″}$ at $T=8\text{K}$ in the SC state and at $T=30\text{K}$ in the normal state for $x=0.055$ and $x=0.06$. (e) Sketch of the competition between SDW and SC gaps on the $M$ electron pocket.

Figure 3

(Color online) (a) Temperature dependence of the low-energy Raman intensity in the $B_{2g}$ symmetry for $x=0$ $\left(T_N=138\text{K}\right)$, $x=0.045$ $\left(T_N=64\text{K}\right)$, $x=0.065$, and $x=0.075$. The spectra were vertically translated for clarity. Several phonon anomalies are observed at the tetragonal to orthorhombic transition (Ref. 10). (b) Temperature dependence of the Raman continuum up to $600{\text{cm}}^{-1}$ for $x=0$. (c) Area of the Lorentzian QEP [full squares, highlighted in red in panel (a)] and Raman response at $30{\text{cm}}^{-1}$ (empty squares) as a function of temperature. Structural and magnetic transitions are indicated in dashed lines.