We report the observation of heavy-fermion superconductivity in CeCoIn 5 at T c = 2.3 K. When compared to the pressure-induced T c of its cubic relative CeIn 3 (T c ∼ 200 mK), the T c of CeCoIn 5 is remarkably high. We suggest that this difference may arise from magnetically mediated superconductivity in the layered crystal structure of CeCoIn 5. Superconductivity is distinct in the correlation often evident between structure and properties: certain crystal structures or substructures favour superconductivity [1]. In particular, what underlies this relationship in the high-T c cuprates and heavy-fermion materials, which border so closely on magnetically ordered phases, is of essential interest both fundamentally and in the search for new superconducting materials [2, 3]. For example, fully half of the known heavyfermion superconductors crystallize in the tetragonal ThCr 2 Si 2 structure, which is also the structure type of the La 2 CuO 4 family of high-T c superconductors [4]. For the cuprates, there is no consensus on the origin of the superconductivity, but their quasi-2D structure and proximity to magnetic order have been shown to be particularly favourable for an unconventional form of superconductivity in which a pairwise-attractive interaction among quasiparticles is mediated by magnetic correlations [5]. Here, we report the discovery of a possible heavy-fermion analogue of the cuprates, a new layered superconductor CeCoIn 5 , with the highest known ambient-pressure superconducting transition temperature T c in the class of heavy-fermion materials. Heavy-fermion superconductors are materials in which superconductivity emerges out of a normal state with strong electronic correlations. The presence of an appropriate magnetic ion-in this case Ce-enhances the effective mass m * of conduction electrons by several orders of magnitude [6]. In the more than twenty years since the first heavy-fermion superconductor was discovered (CeCu 2 Si 2) [7], only one other Ce-based material has been found that unambiguously shows superconductivity at ambient pressure: CeIrIn 5 [8]. Both of these materials exhibit rather complex phenomena and/or metallurgy, making their study challenging. The ground state of CeCu 2 Si 2 can be either antiferromagnetic or superconducting depending on very small changes in unit-cell volume or composition [9]; CeIrIn 5 shows zero resistivity near 1 K but does not produce a thermodynamic signature of superconductivity until

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Abstract. -CeIrIn 5 is a member of a new family of heavy-fermion compounds and has a Sommerfeld specific heat coefficient of 720 mJ/mol-K 2 . It exhibits a bulk, thermodynamic transition to a superconducting state at T c =0.40 K, below which the specific heat decreases as T 2 to a small residual T-linear value. Surprisingly, the electrical resistivity drops below instrumental resolution at a much higher temperature T 0 =1.2 K.These behaviors are highly reproducible and field-dependent studies indicate that T 0 and T c arise from the same underlying electronic structure. The layered crystal structure of suggest that heavy-fermion superconductivity might be found in structurally-related materials.Experimental and theoretical study of the superconductivity in these heavyfermion materials has formed a substantial basis for understanding more broadly classes of unconventional superconductors, including the high-T c cuprates, in which the electronpairing interaction responsible for superconductivity may be mediated by spin fluctuations [1]. In spite of progress, the heavy-fermion problem and heavy-fermion superconductivity in particular remain challenges to experiment and theory [6]. Though heavy-fermion behavior has been found in several structure types, it appears that, like conventional BCS superconductivity, heavy-fermion superconductivity may be favored by particular crystallographic structures. Because of the limited number of examples, we know very little about relationships that should exist between the structure and properties of these materials. Any predictive understanding of how superconductivity can emerge in the highly correlated ground state has to be able to explain why it appears in one crystal structure and not another. This makes the discovery of a new prototype structure for heavy-fermion superconductivity of special interest. Here, we report a new ambientpressure Ce-based heavy-fermion superconductor that is isostructural to CeRhIn 5 , suggesting that this structure, like the ThCr 2 Si 2 structure, may be particularly favorable for superconductivity. Unlike CeCu 2 Si 2 , this new compound grows easily and reproducibly as large, very pure single crystals, opening the possibility for unprecedented study. Thermodynamic and transport properties of CeIrIn 5 at low temperatures are summarized in Fig. 2. Above 0.4 K, the specific heat divided by temperature C/T≡ γ=720 mJ/mole-K 2 and is nearly temperature independent. At T c =0.40 K, there is a jump in C/T

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Members of the RBiPt (R =Ce-Lu with the exceptions of Pm and Eu) series have been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on rcprcscntatives. Tue high temperature resistance uniformly changes from that of a small-gap semiconductor or semimetal seen in NdBiPt to that of a heavy-fermion meta! seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K. 2 mole.Further, the lighter rare earth members s how an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.

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Measurements of the electrical resistivity, magnetoresistance, ac and dc magnetic susceptibility, and specific heat on YbBiPt indicate this compound to be a very heavy-electron system. The lowtemperature Sommerfeld coefficient of 8 J/K'(mole Yb) is not affected by a phase transition at 0.4 K. We suggest that the heavy-mass state in YbBiPt is unconventional in that it develops from Bloch states in an electronic subsystem with low carrier concentration.