Multigap Superconductivity and Strong Electron-Boson Coupling in Fe-Based Superconductors: A Point-Contact Andreev-Reflection Study of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ Single Crystals

Directional point-contact Andreev-reflection measurements in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ single crystals ($T_c=24.5\mathrm{K}$) indicate the presence of two superconducting gaps with no line nodes on the Fermi surface. The point-contact Andreev-reflection spectra also feature additional structures related to the electron-boson interaction, from which the characteristic boson energy ${\Omega }_b(T)$ is obtained, very similar to the spin-resonance energy observed in neutron scattering experiments. Both the gaps and the additional structures can be reproduced within a three-band $s\pm$ Eliashberg model by using an electron-boson spectral function peaked at ${\Omega }_0=12\mathrm{meV}\simeq {\Omega }_b(0)$.

Figure 1

Temperature dependence of the conductance curves in a $\mathrm{Ag}/{\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ $c$-axis point contact. The curves are vertically offset for clarity. The insets show the superconducting transition as seen by ac magnetic susceptibility (left) and dc resistance (right) measurements.

Figure 2

(a),(d) Sketch of $c$-axis and $ab$-plane contacts. (b),(c) Normalized conductance curves at 4.2 K for $c$-axis contacts (symbols) and their two-band fit (solid lines) with the relevant gap values ${\Delta }_1$ and ${\Delta }_2$. Arrows mark the structures related to the gaps and to the EBI. (e),(f) The same for two $ab$-plane contacts. In (b) and (e), a single-band fit is also shown (dashed lines) with the relevant gap amplitude $\Delta$.

Figure 3

Temperature dependence of the normalized conductance of Fig. 2c (symbols) and the relevant two-band BTK fits (lines). All curves except the bottom one are offset for clarity. Right inset: The gaps given by the fit (symbols) and the BCS-like temperature dependencies (lines). Left inset: Zoom of the curve at 4.2 K (symbols) with two possible one-gap BTK fits (dashed lines) and the best two-band BTK fit (solid line).

Figure 4

(a) An experimental AR spectrum (symbols) compared to the theoretical one (line) obtained from Eliashberg and BTK calculations (see text). (b) Experimental (full symbols) and theoretical (solid line) $-d^{2}I/d{V}^2$ vs $V$ curves obtained from the data in (a). Open symbols: The $-d^{2}I/d{V}^2$ curve averaged over 5 contacts. Dashed line: The electron-boson spectral function (shifted in energy by $\approx {\Delta }_{\max }$) used in the three-band Eliashberg calculations. (c) Temperature dependence of the $-d^{2}I/d{V}^2$ curves showing the displacement of the bosonic structures. The energy of the peak $E_p(T)$ and the corresponding characteristic boson energy ${\Omega }_b(T)$ are shown in (d). Lines are guides to the eye.