Local lattice dynamic correlation in FeSe$_x$Te$_{1-x}$

With the use of inelastic neutron scattering, the local lattice dynamics were determined for the new Fe-based superconductors, FeSe$_x$Te$_{1-x}$ with $x$=0.1, 0.5 and 0.9. The nature of the dynamic pair correlations was characterized above and below the phase transitions. In the $x$=0.1 sample that is not superconducting(SC), the nearest Fe-Te and Fe-Fe pair correlations gradually disappear with increasing energy by 35 meV. The same energy dependence is observed above and below the magnetic transition. This energy corresponds to the cut-off frequency of the phonon vibrational modes. On the other hand, in the SC $x$=0.5 and 0.9, the Fe-Fe correlations gain weight just above the elastic, only to be quickly suppressed by 15 meV. This effect is stronger below the transition than above. The Fe-Te correlations that overlap with the Fe-Fe bonds persist in $x$=0.5 (possibly in $x$=0.9 as well but are too weak). On the other hand, the Fe-Se correlations persist beyond this energy, and eventually disappear by 30 meV. These differences in the local lattice dynamics between the non-SC and SC might provide a clue towards understanding the phonon contribution to the mechanism of superconductivity in this system.