Experimental consequences of thes-wavecos(kx)cos(ky

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(kx) · cos(ky) nodeless order parameter recently derived by two of us 1 . 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(kx) · cos(ky) superconducting gap is largest on the smallest (hole) Fermi surface. For the 1/T1 NMR spin relaxation rate, the inter-band contribution is consistent with the current experimental results, including a (nonuniversal) 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 Tc in the clean limit.Introduction -The recent discovery of iron-based superconductors with a transition temperature as high as 55K has stimulated a flurry of experimental and theoretical activity 2,3,4,5,6,7,8,9,10,11 . However, a conclusive observation of the pairing symmetry still remains elusive, with both nodal and nodeless order parameters reported in experimental observations. Numerical and analytic research suggests that the antiferromagnetic exchange coupling between Fe sites is strong 12,13,14 . Owing to As-mediated hopping, antiferromagnetic exchange exists not only between the nearest neighbor (NN) Fe sites, but also between next nearest neighbor (NNN) sites. Moreover, the NNN coupling strength J 2 is stronger than the NN coupling strength J 1 . The J 1 − J 2 model produces half-filled magnetic physics consistent with experimental neutron data 15 . A nematic magnetic phase transition has been predicted in this model 16,17 , consistent with the experimental observation of a structural transition preceding the spin density wave (SDW) formation. This model suffers, however, from an important deficiency -it is an insulator whereas the real material is an, albeit bad, metal. We, however, believe that the spin-spin interaction insight is important to the physics of the iron-pnictides.In a recent paper 1 , two of us added electron itineracy to the problem and studied a t − J 1 − J 2 model without band renormalization. We found that the singletforming J 1 − J 2 interaction gives rise to four possible pairing symmetries: cos(k x ) ± cos(k y ), sin(k x ) · sin(k y ) and cos(k x ) · cos(k y ). The last two are strongly preferred from an interaction standpoint when J 2 > J 1 , but only cos(k x ) · cos(k y ) matches the symmetry...

The Fulde–Ferrell–Larkin–Ovchinnikov State in Pnictides

“…Theoretical results indicate the presence of s x 2 y 2 ∼ cos(k x ) · cos(k y ) (also called s ± ) pairing symmetry in FeSC [37][38][39][40][41][42][43]. In this case the OP exhibits a sign reversal between the hole pockets and electron pockets (Fig.…”

Robustness ofs-Wave Pairing in Electron-OverdopedA1−yFe2−xSe2Fang1, Wu2, Thomale3 et al. 2011Phys. Rev. XSelf Cite9751141View full textAdd to dashboardCite

“…Two of us showed that the pairing symmetry is determined mainly by the next-nearest-neighbor (NNN) antiferromagnetic (AFM) exchange coupling J 2 together with a renormalized narrow band width [11,46]. The superconducting gap is close to a cosðk x Þ cosðk y Þ form in momentum space.…”

NMR Study of Superconductivity and Spin Fluctuations in Hole-Doped Superconductor Ca 1− x Na x Fe 2 As 2 ( T

“…In many iron pnictides, ARPES studies shows multiple isotropic gaps with s-wave pairing symmetry [29,30] , and an s ± pairing has been proposed due to interband transitions [31,32] . The coherence peak for the s ± superconductivity may be suppressed by interband impurity scatterings [33][34][35][36][37] . In heavily hole-doped KFe 2 As 2 and the isovalent-doped BaFe 2 As 2-x P x , however, superconducting gaps with line nodes were proposed [7,38] , to account for the absence of the coherence peak [7] .…”