Weak superconducting pairing and a single isotropic energy gap in stoichiometric LiFeAs

TL;DR

This study reveals that stoichiometric LiFeAs is a weakly coupled superconductor with a single isotropic energy gap, exhibiting well-ordered vortex lattice properties and aligning with BCS theory predictions.

Contribution

The paper provides the first comprehensive analysis combining SANS and ARPES to characterize the superconducting gap and vortex lattice in stoichiometric LiFeAs.

Findings

LiFeAs has a Tc of 17 K and a single isotropic gap of about 3.0 meV.

The vortex lattice shows better ordering than in doped compounds.

The superconducting properties align with weak-coupling BCS theory.

Abstract

We report superconducting (SC) properties of stoichiometric LiFeAs (Tc = 17 K) studied by small-angle neutron scattering (SANS) and angle-resolved photoemission (ARPES). Although the vortex lattice exhibits no long-range order, well-defined SANS rocking curves indicate better ordering than in chemically doped 122-compounds. The London penetration depth of 210 nm, determined from the magnetic field dependence of the form factor, is compared to that calculated from the ARPES band structure with no adjustable parameters. Its temperature dependence is best described by a single isotropic SC gap of 3.0 meV, which agrees with the ARPES value of 3.1 meV and corresponds to the ratio 2Delta/kTc = 4.1, approaching the weak-coupling limit predicted by the BCS theory. This classifies LiFeAs as a weakly coupled single-gap superconductor, similar to conventional metals.