Superconductivity and Non-Fermi-Liquid Behavior in the Heavy-Fermion Compound CeCo_1−xNi_xIn₅

Abstract

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.

Anisotropic magnetic-field response of quantum critical fluctuations in Ni-doped CeCoIn5

“…These NFL features are quite similar to those seen in pure CeCoIn 5 , strongly suggesting that the NFL anomaly observed in Ni-doped CeCoIn 5 also originates from the AFM quantum critical fluctuations. Furthermore, the effective magnetic moment for x ≤ 0.3, estimated from the Curie-Weiss law at high temperatures, is nearly independent of x and coincides well with that calculated from the J = 5/2 multiplet in the Ce 3+ ion [39], suggesting that the Ce 4f electrons are mainly responsible for the magnetic properties in pure and Ni-doped CeCoIn 5 .…”

“…A single crystal of CeCo 1−x Ni x In 5 with x = 0.25 was grown using an Indium flux technique, the details of which are described elsewhere [39]. The energy dispersive x-ray spectroscopy (EDS) and the inductively coupled plasma mass spectrometry (ICP-MS) measurements for the sample indicated that the actual Ni concentration approximately coincided with the starting (nominal) value within the deviation of ∆x/x ∼ 17%, including the experimental error.…”

“…In a recent study, we have revealed that in the mixed compound CeCo 1−x Ni x In 5 , the SC transition temperature T c monotonically decreases from 2.3 (x = 0) to 0.8 K (x = 0.20) with increasing x; subsequently, the SC order disappears above the critical Ni concentration x = 0.25 [39]. At this concentration, the NFL behaviors are realized around the zero field, characterized by the − ln T dependence in the specific heat divided by the temperature, the weak diverging behavior in the magnetization, and the nearly T -linear behavior of the electrical resistivity [39]. These NFL features are quite similar to those seen in pure CeCoIn 5 , strongly suggesting that the NFL anomaly observed in Ni-doped CeCoIn 5 also originates from the AFM quantum critical fluctuations.…”

“…In contrast, the substitutions of Sn for In [36][37][38] and Ni for Co [39] do not induce the AFM phase, but simply yield paramagnetic ground states through the suppres-sion of the SC phase. In a recent study, we have revealed that in the mixed compound CeCo 1−x Ni x In 5 , the SC transition temperature T c monotonically decreases from 2.3 (x = 0) to 0.8 K (x = 0.20) with increasing x; subsequently, the SC order disappears above the critical Ni concentration x = 0.25 [39].…”

“…In contrast, the substitutions of Sn for In [36][37][38] and Ni for Co [39] do not induce the AFM phase, but simply yield paramagnetic ground states through the suppres-sion of the SC phase. In a recent study, we have revealed that in the mixed compound CeCo 1−x Ni x In 5 , the SC transition temperature T c monotonically decreases from 2.3 (x = 0) to 0.8 K (x = 0.20) with increasing x; subsequently, the SC order disappears above the critical Ni concentration x = 0.25 [39]. At this concentration, the NFL behaviors are realized around the zero field, characterized by the − ln T dependence in the specific heat divided by the temperature, the weak diverging behavior in the magnetization, and the nearly T -linear behavior of the electrical resistivity [39].…”