A very clean Fe-based superconductor might show up new ground states (Vol. 42, No. 1)

Iron-based superconductors, discovered in early 2008 and exhibiting superconducting transition temperatures T_c as high as 57 K, currently draw focused attention in the condensed matter community. Among various structural forms known, LiFeAs represents the prototype of the "111'' compounds with several unique properties: superconductivity without chemical doping, thus being subject to the least disorder effect, no Fermi surface nesting for inducing an antiferromagnetic spin density wave fluctuation as a possible pairing glue for superconductivity. Moreover, the structure contains a FeAs layer sandwiched by the double Li layers so that the cleaving can be easily done in the Li surface without having surface reconstruction, an important favorable condition for surface-sensitive investigations. Due to the highly volatile nature of Li, the high quality single crystal growth has only recently been achieved. The present research reports the first successful growth of a large area single crystal of LiFeAs by using Sn flux, producing T_c = 18.2 K with a narrow transition width ΔT_c = 1.1 K as well as a relatively large residual resistivity ratio, ~ 22-35. Upon measuring transport under high magnetic fields for two crystallographic directions, H // ab-plane and // c-axis, the system exhibits a moderate anisotropy of 2.3 near T_c, consistent with a prediction of a reduced anisotropy caused by correlation effects. Based on several recent proposals which point out the possible realization of a p-wave pairing symmetry and a Fulde, Ferrell, Larkin & Ovchinnikov state, it is envisioned that the present cleanest, large LiFeAs single crystal offers an unprecedented opportunity to find new, exotic ground states of correlated electron systems in the Fe-based superconductors.