Workshop "NMR, µSR, Mössbauer spectroscopies in the study of Fe-based and other unconventional high-Tc superconductors"
57Fe-Mössbauer study of LiNH2-intercalated FeSe: Evidence for spin fluctuations connected with superconductivity
Vadim Ksenofontov, University of Mainz, Germany
Despite of intensive studies of Fe-based superconductors (s.c.) in the last years, the pairing mechanism is still an open question. Here we present a 57Fe-Mössbauer study of LiNH2-intercalated FeSe with a s.c. transition temperature, Tc = 43 K . This is the highest Tc in the family of FeSe-based s.c., which possess the same structural characteristics, namely FeSe layers, here separated by the intercalated LiNH2-moieties . The present Mössbauer study continues previous studies, starting with the most simple system, FeSe with Tc = 8.5 K, where high pressure could dramatically enlarge Tc up to 36 K  and demonstrate, using 57Fe-synchrotron phonon studies, that conventional electron-phonon pairing cannot explain this pressure-induced increase in Tc [2, 3]. 57Fe-Mössbauer studies enabled also to observe in Rb0.8Fe1.6Se2 with Tc = 32 K a phase separation into a magnetic phase and a non-magnetic s.c. phase, the latter with hyperfine parameters near to those of FeSe .
57Fe-Mössbauer spectra of the present LiNH2-intercalated FeSe sample exhibit at temperatures above 100 K only a quadrupole doublet with hyperfine parameters near to those of FeSe and the s.c. phase in Rb0.8Fe1.6Se2, but with characteristic changes pointing to modifications of the electronic band structure, which could be correlated with the increase in Tc. Most important is the observation that at lower temperatures, approaching Tc, we observe beside this dominant quadrupole doublet additional magnetic subspectrum of up to 25% of the Fe sites. Detailed analysis of this magnetic subspectrum points to its dynamic character, characteristic for spin fluctuations of Fe2+ ions, located in the s.c. FeSe layers. We will discuss these properties also with respect to high-pressure resistivity studies of the present sample up to 6 GPa, where we observe beside a reduction of Tc down to 28 K also characteristic changes in the resistivity curves indicating corresponding changes in the fluctuation rate of these magnetic Fe ions, similar to previous observations in FeSe under pressure . We compare the present results with the NMR studies  considering recent conclusions that also in FeAs-based s.c. systems antiferromagnetic spin fluctuations may act as “glue” for superconducting pairing. It is important especially with respect to the experimental observations and theoretical conclusions that s.c. and magnetism are closely related in the corresponding phase diagrams .
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