Electronic inhomogeneities in the superconducting phase of CaFe1.96Ni0.04As2 single crystals

TL;DR

This study uses scanning tunneling microscopy to reveal electronic inhomogeneities in underdoped CaFe1.96Ni0.04As2 single crystals, showing how local properties vary near the superconducting transition and relate to phase coexistence.

Contribution

It provides detailed local electronic property measurements of underdoped CaFe1.96Ni0.04As2, highlighting inhomogeneities associated with the coexistence of superconductivity and spin density wave order.

Findings

Superconductivity appears in about 1.2% of the sample below 15 K.

Spatial inhomogeneity in electronic spectra is observed below Tc.

Inhomogeneity diminishes above Tc and disappears far above it.

Abstract

Superconductivity in Iron-Arsenic based pnictides emerges in close proximity to an antiferromagnetic (AFM)ordered parent state and the AFM phase overlaps with superconducting (SC) phase in some pnictides for certain range of doping. CaFe2-xNixAs2 belongs to this category, where both the phases overlap. Here we use scanning tunneling microscopy and spectroscopy to investigate the local electronic properties of uderdoped CaFe1.96Ni0.04As2 single crystals in the vicinity of the boundary of the two phases. Both resistivity and magnetic measurements show that a tiny portion (~ 1.2 %) of this compound becomes superconductor below the SC onset temperature TC ~ 15 K. Topographic images show reasonably flat surface with signatures of atomic resolution. High temperature spectra are spatially homogeneous and show signatures of spin density wave (SDW) gap with a finite density of states near the Fermi energy. Below TC, spectra show significant spatial inhomogeneity with a SDW gap everywhere but at some locations we also see an asymmetric or symmetric depression in ~ +/- 5 meV energy range together with the SDW gap. Inhomogeneity reduces significantly as the temperature goes above TC and disappears completely far above TC. These observations are discussed in terms of an inhomogeneous electronic phase that may exist due to the vicinity of this composition to the SC dome boundary on the underdoped side of the phase diagram.