  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.
