We have succeeded in growing single crystals of the heavy-fermion superconductor CeCo(In 1−x Zn x ) 5 with x ≤ 0.07. Measurements of specific heat, electrical resistivity, dc magnetization and ac susceptibility revealed that the superconducting (SC) transition temperature T c decreases from 2.25 K (x = 0) to 1.8 K (x = 0.05) by doping Zn into CeCoIn 5 . Furthermore, these measurements indicate a development of a new ordered phase below T o ∼ 2.2 K for x ≥ 0.05, characterized by the reduced magnetization and electrical resistivity in the ordered phase, and the enhancement of specific heat at T o . This phase transition can be also recognized by the shoulder-like anomaly seen at H o ∼ 55 kOe in the field variations of the magnetization at low temperatures, which is clearly distinguished from the superconducting critical fields H c2 = 49 kOe for x = 0.05 and 42 kOe for x = 0.07. We suggest from these results that the antiferromagnetic (AFM) order is generated by doping Zn, and the interplay between the SC and AFM orders is realized in CeCo(In 1−x Zn x ) 5 .The nature of unusually enhanced fluctuations and ordered states related to the quantum critical phenomena has been attracting much interest in the physics of the heavy-fermion systems. These features are observed in the vicinity of the quantum critical point (QCP), corresponding to the phase transition at zero temperature, which is generated by suppressing the magnetically ordered state via applying pressure, magnetic field and doping ions. Near the QCP, the non-Fermiliquid (NFL) properties evolve in temperature variations of thermodynamic and transport quantities, indicating the existence of new types of low-energy excitations different from those expected in usual Fermi-liquid state. Furthermore, the unconventional superconductivity (SC) often emerges near the QCP, and it is therefore believed that the magnetic fluctuation enhanced near the QCP plays a crucial role in the formation of the Cooper pairs.Among the heavy-fermion systems showing the quantum critical phenomena, the heavy-fermion superconductor CeCoIn 5 (the HoCoGa 5 -type tetragonal structure) is one of the most intensively investigated compounds. The SC transition of CeCoIn 5 is characterized by an anomalously large specific-heat jump ∆C/γT c = 4.5 at the transition temperature T c = 2.3 K.1) A strong Pauli-limited effect gives rise to a first-order transition at the SC critical field H c2 below 0.7 K.2-4) In addition, above H c2 the NFL behavior is observed in the temperature variations of the bulk quantities, which is considered to be due to the effect of quantum critical fluctuation induced in the vicinity of antiferromagnetism. 5)In fact, an antiferromagnetic (AFM) order is generated by substituting ions for Co and In. In the mixed compounds * E-mail address: makotti@mx.ibaraki.ac.jp CeRh 1−x Co x In 5 , 6) it is revealed that substituting Rh for Co reduces T c down to ∼ 1.5 K at x ∼ 0.8, and the AFM order develops in the Co concentrations below x c ∼ 0.8. A continuous increase of the AFM trans...

Low-temperature magnetic and electronic properties of Sr 1−x La x RuO 3 for x ≤ 0.4 have been investigated by means of ac susceptibility χ and specific heat C p measurements to clarify the relationship between these properties and suppression of ferromagnetic (FM) order. A clear divergence associated with the FM transition is observed in the temperature variations of χ for x = 0. It changes into broad peaks by doping La, and the FM transition temperature is reduced from 161 K (x = 0) to 8.4 K (x = 0.4). Furthermore, peak-top temperature and magnitude of the peak in χ are found to significantly vary by the frequency of applied ac fields for x ∼ 0.4. Specific heat divided by temperature C p /T roughly follows a T 2 function at low temperatures for all the x range presently investigated, and substituting La for Sr increases the electronic specific heat coefficient γ from 30 mJ/K 2 mol (x = 0) to 85 mJ/K 2 mol (x = 0.4). We suggest from these results that the spatially inhomogeneous clusters of the spin-polarized Ru 4d electrons evolve as the long-range FM order breaks down, although the Ru 4d electrons still have itinerant nature.