Point-contact spectroscopy in Co-doped CaFe2As2: nodal superconductivity and topological Fermi surface transition

Abstract

We performed point-contact Andreev reflection spectroscopy measurements in Ca(Fe_{1-x}Co_x)2As2 single crystals with effective x=0.060 +- 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5-6 meV. Their fit with the three-dimensional Blonder-Tinkham-Klapwijk model (making use of a analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Fermi surface pockets centered at the Z point in the Brillouin zone. These results nicely fit into the theoretical picture for the appearance of nodal superconductivity in 122 compounds.

arXiv comment11 pages, 3 figures

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• Specific heat of Ca${}_{0.32}$Na${}_{0.68}$Fe${}_2$As${}_2$ single crystals: unconventional s${}_{\pm }$ multi-band superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings
• Doping and critical-temperature dependence of the energy gaps in Ba(Fe_{1-x}Co_x)_2As_2 thin films
• Fermi-surface topological phase transition and horizontal order-parameter nodes in CaFe${}_2$As${}_2$ under pressure
• Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance
• Point contact spectroscopy in Fe-based superconductors: recent advancements and future challenges
• The order-parameter symmetry and Fermi surface topology of 122 Fe-based superconductors: a point-contact Andreev-reflection study