Pauli-limited superconductivity and antiferromagnetism in the heavy-fermion compound CeCo(In_(1-x)Zn_x)_5

摘要

We report on the anisotropic properties of Pauli-limited superconductivity (SC) and antiferromagnetism (AFM) in the solid solutions CeCo(In_(1-x)Zn_x)_5 (x ≤ 0.07). In CeCo(In_(1-x)Zn_x)_5, the SC transition temperature T_c 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 T_N ～ 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 μ_0H_(c2) for both the tetragonal a and c axes, while it monotonically reduces the SC transition temperature T_c. In particular, the magnitudes of μ_0H_(c2) 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 T_c 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 AFM modulations involved in the Ce115 compounds.