We report on the anisotropic properties of Pauli-limited superconductivity (SC) and antiferromagnetism (AFM) in the solid solutions CeCo(In1−xZnx)5 (x ≤ 0.07). In CeCo(In1−xZnx)5, the SC transition temperature Tc is continuously reduced from 2.3 K (x = 0) to ∼ 1.4 K (x = 0.07) by doping Zn, and then the AFM order with the transition temperature of TN ∼ 2.2 K develops for x larger than ∼ 0.05. The present thermal, transport and magnetic measurements under magnetic field B reveal that the substitution of Zn for In yields little change of low-temperature upper critical field µ0Hc2 for both the tetragonal a and c axes, while it monotonically reduces the SC transition temperature Tc. In particular, the magnitudes of µ0Hc2 at the nominal Zn concentration of x = 0.05 (measured Zn amount of ∼ 0.019) are 11.8 T for B || a and 4.8 T for B || c, which are as large as those of pure compound though Tc is reduced to 80% of that for x = 0. We consider that this feature originates from a combination of both an enhanced AFM correlation and a reduced SC condensation energy in these alloys. It is also clarified that the AFM order differently responds to the magnetic field, depending on the field directions. For B || c, the clear anomaly due to the AFM transition is observed up to the AFM critical field of ∼ 5 T in the thermodynamic quantities, whereas it is rapidly damped with increasing B for B || a. We discuss this anisotropic response on the basis of a rich variety of the AFM modulations involved in the Ce115 compounds.

The effect of off-plane impurity on superconductivity and non-Fermi-liquid (NFL) behavior in the layered heavyfermion compound CeCo 1−x Ni x In 5 is investigated by specific heat, magnetization, and electrical resistivity measurements. These measurements reveal that the superconducting (SC) transition temperature T c monotonically decreases from 2.3 K (x = 0) to 0.8 K (x = 0.20) with increasing x, and then the SC order disappears above x = 0.25. At the same time, the Ni substitution yields the NFL behavior at zero field for x = 0.25, characterized by the − ln T divergence in specific heat divided by temperature, C p /T , and magnetic susceptibility, M/B. The NFL behavior in magnetic fields for x = 0.25 is quite similar to that seen at around the SC upper critical field in pure CeCoIn 5 , suggesting that both compounds are governed by the same antiferromagnetic quantum criticality. The resemblance of the doping effect on the SC order among Ni-, Sn-, and Pt-substituted CeCoIn 5 supports the argument that the doped carriers are primarily responsible for the breakdown of the SC order. The present investigation further reveals the quantitative differences in the trends of the suppression of superconductivity between Ce(Co,Ni)In 5 and the other alloys, such as the rates of decrease in T c , dT c /dx, and specific heat jump at T c , d(∆C p /T c )/dx. We suggest that the occupied positions of the doped ions play an important role in the origin of these differences.

We demonstrate a close connection between observed field-induced antiferromagnetic (AFM) order and quantum critical fluctuation (QCF) in the Zn7%-doped heavy-fermion superconductor CeCoIn5. Magnetization, specific heat, and electrical resistivity at low temperatures all show the presence of new field-induced AFM order under the magnetic field B of 5-10 T, whose order parameter is clearly distinguished from the low-field AFM phase observed for B < 5 T and the superconducting phase for B < 3 T. The 4f electronic specific heat divided by the temperature, Ce/T , exhibits − ln T dependence at B ∼ 10 T (≡ B0), and furthermore, the Ce/T data for B ≥ B0 are well scaled by the logarithmic function of B and T : ln[(B − B0)/T 2.7 ]. These features are quite similar to the scaling behavior found in pure CeCoIn5, strongly suggesting that the field-induced QCF in pure CeCoIn5 originates from the hidden AFM order parameter equivalent to high-field AFM order in Zn7%-doped CeCoIn5.