Physics – Condensed Matter – Superconductivity
Scientific paper
2011-08-12
Physics
Condensed Matter
Superconductivity
10 pages, 7 figures, updated reference and acknowledgment
Scientific paper
We report results of $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments as well as $^7$Li NMR on different high quality samples of self flux grown LiFeAs and 5% Co doped LiFeAs single crystals, and a polycrystalline LiFeAs sample. We were able to distinguish the samples by their slightly different quadrupole frequencies, $\nu_Q$, which is a direct measure of the electric field gradient (EFG) at the As site. Interestingly, samples with a large quadrupole frequency appear to have a different superconducting ground state from those with a lower $\nu_Q$. For sample S1 which has the largest $\nu_Q$, we find constant Knight shift $\mathcal{K}$ across $T_c$ and a peculiar upturn of the spin lattice relaxation rate \slr\ below $T_c$, which is consistent with spin triplet superconductivity, whereas samples with a lower $\nu_Q$ exhibit usual singlet superconductivity indicated by a drop of $\mathcal{K}$ and \slr\ below $T_c$. Taking into account the high purity of our crystals, indicated by the very narrow $^{75}$As NQR spectrum, these results put LiFeAs close to a critical ferromagnetic instability, where tiny changes in stoichiometry can lead to Cooper pair states with very different spin and/or angular momentum.
Aswartham Saicharan
Baek Seung-Ho
Büchner Bernd
Fuchs Madeleine
Grafe Hans-Joachim
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