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

image The ab-plane resistivity under magnetic fields for two different crystallographic directions of a LiFeAs single crystal grown by the Sn-flux method with Tc=18.2 K. The crystal structure and a typical picture of the grown crystals are also displayed.

Iron-based superconductors, discovered in early 2008 and exhibiting superconducting transition temperatures Tc 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 Tc = 18.2 K with a narrow transition width ΔTc = 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 Tc, 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.

Single-crystal growth and superconducting properties of LiFeAs
Bumsung Lee, Seunghyun Khim, Jung Soo Kim, G. R. Stewart and Kee Hoon Kim, EPL 91, 67002 (2010)