Anisotropic effect of Cd and Hg doping on Pauli limited superconductor CeCoIn${}_5$

We investigated the effect of Cd and Hg doping on the first order superconducting (SC) transition and the high field-low temperature SC state of CeCoIn${}_5$ by measuring the specific heat of CeCo(In${}_{1-\mathrm{x}}$Cd${\rm x}$)$5$withx=0.0011,0.0022and0.0033andCeCo(In${\rm 1-x}$Hg${\rm x}$)$_5$ with x=0.00016, 0.00032, and 0.00048 at temperatures down to 0.1 K and fields up to 14 T. Cd substitution rapidly suppresses the cross-over temperature $T_0$, where the superconducting transition changes from second to first order, to $T$=0 K with x=0.0022 for $H\parallel$ [100], while it remains roughly constant up to x=0.0033 for $H\parallel$ [001]. The associated anomaly of the proposed FFLO state in Hg-doped samples is washed out by x=0.00048, while remaining at the same temperature, indicating high sensitivity of that state to impurities. We interpret these results as supporting the non-magnetic, possibly FFLO, origin of the high field - low temperature state in CeCoIn${}_5$.