Local Structure Around Sn in CeCoIn$_{5-x}$Sn$_x$

CeCoIn$_5$ is an ambient pressure superconductor with a transition temperature of 2.3 K. Recent experiments have shown that superconductivity is easily destroyed with the replacement of In with Sn. The role of Sn on the superconducting state in the CeCoIn$_{5-x}$Sn$_x$ alloy has been investigated by the extended x-ray absorption fine structure (EXAFS) technique. Our fit results for $x$ = 0.09, 0.12, 0.18, and 0.24 show a major fraction of the dopant Sn atoms preferentially residing on the planar In(1) site. This result is consistent with the two-dimensional superconducting state in CeCoIn$_5$ and will be discussed in relation to the observed rapid suppression of superconductivity in the CeCoIn$_{5-x}$Sn$_x$ system.