Evidence of a multiple boson emission in Sm$_{1-x}$Th$_x$OFeAs

Abstract

We studied a reproducible fine structure observed in dynamic conductance spectra of Andreev arrays in Sm$_{1-x}$Th$_x$OFeAs superconductors with various thorium concentrations ($x = 0.08 - 0.3$) and critical temperatures $T_c = 26-50$\,K. This structure is unambiguously caused by a multiple boson emission (of the same energy) during the process of multiple Andreev reflections. The directly determined energy of the bosonic mode reaches $\varepsilon_0 = 14.8 \pm 2.2$\,meV for optimal compound. Within the studied range of $T_c$, this energy as well as the large $\Delta_L$ and the small $\Delta_S$ superconducting gaps, nearly scales with critical temperature with the characteristic ratio $\varepsilon_0/k_BT_c \approx 3.2$ (and $2\Delta_L/k_BT_c \approx 5.3$, correspondingly) resembling the expected energy $\Delta_L + \Delta_S$ of spin resonance and spectral density enhancement in $s^{\pm}$ and $s^{++}$ states, respectively.