Superconducting and spin-wave orders in Ba 0.6 K 0.4 Fe 2 As 2 probed by point contact spectroscopy

The doping dependent phase diagram of the iron pnictide systems displays diverse electronic ground states including unconventional superconductivity and magnetic ordering. From previous bulk measurements, it was argued that the superconducting phase of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_x{\mathrm{Fe}}₂{\mathrm{As}}₂$ might be described within a two band formalism where superconductivity emerges with significantly different magnitudes of the pairing amplitude in the different bands. We have performed point contact Andreev reflection spectroscopy on the optimally doped system $(x=0.4)$ where we found features that misleadingly mimic the signature of multiple gap amplitudes with large energy difference, when the point contacts are away from the ballistic regime. Closer to the ballistic regime, we found two types of spectra. In one type, a single superconducting gap with unusual broadening was found. The broadening might be due to the presence of multiple gap amplitudes with small energy spacing. The other kind of spectra displayed spectral features at $\ensuremath{\sim}30$ meV in the normal state that gradually diminished with increasing temperature and eventually disappeared at 140 K, the spin density wave transition temperature of parent ${\mathrm{BaFe}}₂{\mathrm{As}}₂$. We attribute the 30 meV spectral feature to a characteristic electron-magnon interaction energy scale in the system.