Proximity fingerprint of $s_{\pm}$ superconductivity

We suggest a straightforward and unambiguous test to identify possible opposite signs of superconducting order parameter in different bands proposed for iron-based superconductors ($s_{\pm}$-state). We consider proximity effect in a weakly coupled sandwich composed of a $s_{\pm} $-superconductor and thin layer of $s$-wave superconductor. In such system the $s$-wave order parameter is coupled differently with different $s_{\pm} $-gaps and it typically aligns with one of these gaps. This forces the other $s_{\pm}$-gap to be anti-aligned with the $s$-wave gap. In such situation the aligned band induces a peak in the $s$-wave density of states (DoS), while the anti-aligned band induces a dip. Observation of such contact-induced negative feature in the $s$-wave DoS would provide a definite proof for $s_{\pm}$-superconductivity.