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We report results of systematic de Haas-van Alphen (dHvA) studies on Ce 1−x Yb x CoIn 5 single crystals with varying Yb concentration. For x = 0.1, the well-known Fermi surfaces and the heavy effective masses of CeCoIn 5 (x = 0) have changed only slightly. We start to observe changes of the Fermi-surface topology at x = 0.2 leading to a drastic reconstruction above x = 0.55. At these concentrations, the effective masses are reduced considerably to values between 0.7 and 2.6 free electron masses. For both YbCoIn 5 and CeCoIn 5 , the angular-resolved dHvA frequencies can be very well described by conventional density-functional theory calculations. Projection of the Bloch states onto atomic Yb-4f orbitals yields a 4f occupation of 13.7 electrons, in agreement with previous experimental results indicating an intermediate Yb valence of +2.3.

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We have investigated the effect of Yb substitution on the Pauli limited, heavy fermion superconductor, CeCoIn5. Yb acts as a non-magnetic divalent substituent for Ce throughout the entire doping range, equivalent to hole doping on the rare earth site. We found that the upper critical field in (Ce,Yb)CoIn5 is Pauli limited, yet the reduced (H,T) phase diagram is insensitive to disorder, as expected in the purely orbitally limited case. We use the Pauli limiting field, the superconducting condensation energy and the electronic specific heat coefficient to determine the Wilson ratio (RW ), the ratio of the specific heat coefficient to the Pauli susceptibility in CeCoIn5. The method is applicable to any Pauli limited superconductor in the clean limit.

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