Experimental elucidation of the origin of the `double spin resonances' in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study combines neutron scattering and ARPES to investigate spin resonances and superconducting gaps in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, revealing a quasi-2D spin resonance linked to spin excitons and clarifying doping effects.

Contribution

It provides a new interpretation of double spin resonance energy scales as spin excitons and anisotropic excitations, differing from previous gap-based explanations.

Findings

Spin resonance energy scales with the superconducting gap.

Identification of anisotropic low energy spin excitations.

Doping-dependent trend explained systematically.

Abstract

We report a combined study of the spin resonances and superconducting gaps for underdoped ($T_c=19$ K), optimally doped ($T_c=25$ K), and overdoped ($T_c=19$ K) Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with inelastic neutron scattering and angle resolved photoemission spectroscopy. We find a quasi two dimensional spin resonance whose energy scales with the superconducting gap in all three compounds. In addition, anisotropic low energy spin excitation enhancements in the superconducting state have been deduced and characterized for the under and optimally doped compounds. Our data suggest that the quasi two dimensional spin resonance is a spin exciton that corresponds to the spin singlet-triplet excitations of the itinerant electrons. However, the intensity enhancements of the anisotropic spin excitations are dominated by the out-of-plane spin excitations of the ordered moments due to the suppression of damping in the superconducting state. Hence we offer a new interpretation of the double energy scales differing from previous interpretations based on anisotropic superconducting energy gaps, and systematically explain the doping-dependent trend across the phase diagram.