Phase-Incoherent Superconducting Pairs in the Normal State of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of $\mathrm{Ba}({\mathrm{Fe}}_{0.92}{\mathrm{Co}}_{0.08}{)}_2{\mathrm{As}}_2$ and a silver tip can be seen in the normal state of the film up to temperature $T\sim 1.3T_c$, where $T_c$ is the critical temperature of the superconductor. Andreev reflection far above $T_c$ can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.

Figure 1

Point-contact data on a $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ epitaxial film with $x\sim 0.08$. (a) The conductance $G_s$ normalized to its value $G_N$ at 100 mV at $T=5.8\mathrm{K}$. (b) Point-contact spectra as a function of temperature $T$. The black line in (b) corresponds to the data at $T=23.7\mathrm{K}$, the midpoint of the resistive transition.

Figure 2

(a) Four probe resistivity of the epitaxial film as a function of temperature. inset: The temperature dependence of resistivity close to the transition. (b) Resistivity data zoomed between 20 K and 35 K. The black arrow ($T_{\mathrm{PC}}$) points to the temperature where features associated with Andreev reflection disappear.

Figure 3

(a) Simulation of the point-contact conductance of a superconductor with three gaps including a normal state gap using a modified BTK model as described in the text. (b) The blue and the red dots indicate the assumed BCS-like gaps and the green dots indicate the high-bias gap used for the simulation. The black line shows the calculated curve at 23.3 K, close to the assumed $T_c$.