Weak Superconducting Pairing and a Single Isotropic Energy Gap in Stoichiometric LiFeAs

Abstract

We report superconducting (SC) properties of stoichiometric LiFeAs (T(c)=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 lambda(ab)(0)=210+/-20 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. The temperature dependence of lambda(ab) is best described by a single isotropic SC gap Delta(0)=3.0+/-0.2 meV, which agrees with the ARPES value of Delta(0)(ARPES)=3.1+/-0.3 meV and corresponds to the ratio 2Delta/k(B)T(c)=4.1+/-0.3, approaching the weak-coupling limit predicted by the BCS theory. This classifies LiFeAs as a weakly coupled single-gap superconductor.

Eilenberger Approach to the Vortex State in Iron Pnictide Superconductors

“…In contrast, the ξ h /ξ c2 curve shifts downward with impurity scattering in s ++ model. The high values of ξ h observed in μSR measurements in LiFeAs [81] supports the s ± pairing.…”

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

“…Superconducting gap values at T → 0 determined using ARPES and other measurements are listed in Table XV. 162,350,[355][356][357][358][359][360][361][362][363][364][365][366][367][368][369][370] From the table, the maximum gaps determined by optics measurements are generally larger than from other measurements on the same material.…”

“…180,181,185,355,361,362,370,419,420,[422][423][424][425][426][427][428] A survey and comparison of the low-T behaviors of the penetration depths in a variety of Fe-based superconductors measured by the TDR technique is shown in Fig. 79(a).…”

Crossover from weak to strong pairing in unconventional superconductors

“…A closer look at the Fe-based superconductors reveals a wide spread of gap ratios, from weak BCS-like values in nonmagnetic LiFeAs [37][38][39][40][41][42] to twice larger values in high-T c ferropnictides with strong antiferromagnetic (AFM) correlations, such as optimally-doped Ba 1−x K x Fe 2 As 2 (BKFA) [15][16][17][43][44][45][46][47][48] or various 1111-compounds. 14,[49][50][51][52][53][54] We would like to emphasize that despite all the uncertainties in the published values, these differences are established beyond any doubt, as they have been confirmed by many complementary experiments, at least for several most studied materials (Table I).…”

“…Despite its relatively high T c , LiFeAs is characterized by weak coupling barely above the BCS limit, [37][38][39][40][41][42]80,81 possibly related to the absence of notable Fermi surface nesting in its band structure 37 or even a different pairing mechanism. 82,83 In NaFeAs, on the contrary, superconductivity with T c ≈ 10 K coexists with antiferromagnetism.…”