AF-Spin fluctuations and its possible relation with the superconducting pairing mechanism and the superconducting energy gap symmetry of the stoichiometric iron-pnictide superconductor LiFeAs (Tc~18K).*

In superconductivity (SC) discovery also understanding provide principal archetype, recently in 2008 discovery of iron-pnictide with several families, including spontaneous gauge symmetry breaking and Anderson-Higgs mechanism remain outstanding issue and opened new avenue of research in condensed matter physics. T_c raised in excess of 55ºK, bringing insights into bad-metal behavior, magnetism, and their striking interplay with unconventional SC.  Normal state AF-spin fluctuations and its possible relation with SC pairing mechanism and symmetry of SC energy gap function are important issue in iron-pnictide family of SC and are probed by lower critical field deduced from vortex penetration, specific heat measurements of SC single crystals and reversible magnetization of polycrystalline LiFeAs sample. LiFeAs seems to be almost isotropic and can be fit as a s-wave two-gap SC with the superfluid density strongly affected by the smaller gap. The specific heat of LiFeAs single crystals reveals s-multigap feature with a small gap of about 0.7 meV dominating low-temperature electronic quasiparticle excitations. A significant contribution from Einstein phonons is observed, as well as a noticeable residual linear term γ₀. Also, I will discuss magnetic properties, H_c1 anisotropy, anisotropic gaps, electronic correlations, and the role of orbital degrees of freedom. Developments and intense activity ensued commonalities and differences between high-T_c Fe-based, Cuprate, Heavy-fermion and Organic SC.