Enhanced superfluid stiffness, lowered superconducting transition temperature, and field-induced magnetic state of the pnictide superconductor LiFeAs

Transverse-field muon-spin rotation measurements performed on two samples of LiFeAs demonstrate that the superfluid stiffness of the superconducting condensate in relation to its superconducting transition temperature is enhanced compared to other pnictide superconductors. Evidence is seen for a field-induced magnetic state in a sample with a significantly suppressed superconducting transition temperature. The results in this system highlight the role of direct Fe-Fe interactions in frustrating pairing mediated by antiferromagnetic fluctuations and indicate that, in common with other pnictide superconductors, the system is close to a magnetic instability.

Figure 1

The temperature dependence of the superconducting contribution to the field width of the $\mu \text{SR}$ line shape measured in a transverse field of 40 mT for the two samples of LiFeAs. The curves are a guide to the eyes.

Figure 2

(a) The magnetic field dependence of the superconducting contribution to the $\mu \text{SR}$ line width for sample 1 measured at 1.6 K. The field dependence is very weak and the $ab$ penetration depth can easily be extracted from the data by fitting to the field dependence of the GL model (solid curve). (b) The field dependence for sample 2 (with lower $T_c$). The low field behavior can again be fitted to the GL model, but a marked deviation sets in at fields above 0.1 T, and the overall fit (solid line) needs to include a field-induced magnetic contribution ${\sigma }_M\propto B^n$ in addition to the VL contribution (dotted line).

Figure 3

(a) Spectral distribution of the internal field in sample 1 at 40 mT obtained by Fourier transformation. The dotted line is the normal state and the solid line is the SC state. The dashed line represents a Gaussian broadened powder profile detailed in (b), where the unbroadened powder profile is shown as the solid curve. Panel (c) is as for (a) but for a field of 600 mT.

Figure 4

As Fig. 3 but for sample 2 which shows evidence of field-induced magnetism. The VL linewidth is narrower here than for sample 1, and the additional field-induced broadening is clearly seen on both sides of the main peak when comparing the 600 mT data in (c) with the 40 mT data in (a).

Figure 5

An Uemura plot of the superconducting transition temperature $T_c$ versus the low temperature superfluid stiffness. Data obtained here for the two LiFeAs samples are compared with previously reported data for doped members of the ${\text{LnFeAsO}}_{1-x}{\text{F}}_x$ family: $\text{Ln}=\text{La}$ (Ref. 11), Nd (Ref. 13) and Sm Refs. 12, 15, and the ${\text{Ba}}_x{\text{K}}_{1-x}{\text{Fe}}_2{\text{As}}_2$ family (Refs. 18, 19). The data taken from the previous reports have been restricted to the highest $T_c$ sample in each study. The lower dashed line indicates the trend line for electron-doped cuprates, which closely corresponds to the behavior of the LiFeAs system obtained here, in contrast to the other pnictide superconductors, which sit closer to the trend for hole-doped cuprates. The symbol size represents the typical estimated uncertainty of the data points.