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 ${\epsilon }_0=14.8\pm 2.2$\,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 ${\epsilon }_0/k_B{T}_c\approx 3.2$ (and $2{\Delta }_L/k_B{T}_c\approx 5.3$, correspondingly) resembling the expected energy ${\Delta }_L+{\Delta }_S$ of spin resonance and spectral density enhancement in $s^{\pm }$ and $s^{++}$ states, respectively.

10 pages, 4 figures