Experimental Consequences of the S-wave $\cos(k_x) \cdot \cos(k_y)$ Superconductivity in the Iron-Pnictides

TL;DR

This paper analyzes the experimental signatures of the $ ext{cos}(k_x) ext{cos}(k_y)$ superconducting order parameter in iron-based superconductors, highlighting its effects on spectral properties, gap structure, and NMR relaxation, with implications for experimental tests.

Contribution

It provides a detailed theoretical analysis of the $ ext{cos}(k_x) ext{cos}(k_y)$ order parameter's experimental consequences, including spectral and NMR properties, in iron-pnictide superconductors.

Findings

Superconductor is fully gapped with line-zeroes not intersecting Fermi surfaces.

Doping affects the gap size and penetration depth decay.

NMR relaxation rates are consistent with experimental observations.

Abstract

The experimental consequences of different order parameters in iron-based superconductors are theoretically analyzed. We consider both nodeless and nodal order parameters, with an emphasis on the $\cos(k_x)\cdot \cos(k_y)$ nodeless order parameter recently derived by two of us. We analyze the effect of this order parameter on the spectral function, density of states, tunneling differential conductance, penetration depth, and the NMR spin relaxation time. This extended s-wave symmetry has line-zeroes in between the electron and hole pockets, but they do not intersect the two Fermi surfaces for moderate doping, and the superconductor is fully gapped. However, this suggests several quantitative tests: the exponential decay of the penetration depth weakens and the density of states reveals a smaller gap upon electron or hole doping. Moreover, the $\cos(k_x) \cdot \cos(k_y)$ superconducting gap is largest on the smallest (hole) Fermi surface. For the $1/T_1$ NMR spin relaxation rate, the inter-band contribution is consistent with the current experimental results, including a (non-universal) $T^{3}$ behavior and the absence of a coherence peak. However, the intra-band contribution is considerably larger than the inter-band contributions and still exhibits a small enhancement in the NMR spin relaxation rate right below $T_c$ in the clean limit.