A ~(57)Fe Mossbauer study on the FeSe and Fe(Se,Te) superconductors: discontinuities in the hyperfine parameters at T_c

摘要

Polycrystalline samples of FeSe_(0.82) and FeSe_(0.5)Te_(0.5) were synthesized using a solid-state reaction route. Bulk superconductivity was confirmed using SQUID magnetometry. The onset of T_c was at 8.0 K for FeSe_(0.82) and 12.5 K for FeSe_(0.5)Te_(0.5). Paramagnetic ~(57)Fe Mossbauer spectra were recorded at temperatures between 5.4 and 320 K in transmission geometry. All spectra exhibited simple quadrupole splitting. For FeSe_(0.5)Te_(0.5) a small drop in the quadrupole splitting was observed about T_c upon cooling. Additionally, for both samples the isomer shift and the total absorption started to drop around T_c, indicating a softening of the lattice. The drop is estimated to correspond to at least 60 K from the original Debye temperatures.