fruitful discussions, Guanghan Cao and Zhicheng Wang for assisting with 3 He-SQUID measurements, and Xiaoyan Xiao for assistance with single crystal x-ray diffraction.
Unconventional superconductivity and other previously unknown phases of matter exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at absolute zero. Intensive theoretical and experimental investigations on itinerant systems have shown that metallic ferromagnets tend to develop via either a first-order phase transition or through the formation of intermediate superconducting or inhomogeneous magnetic phases. Here, through precision low-temperature measurements, we show that the Grüneisen ratio of the heavy fermion metallic ferromagnet YbNi(4)(P(0.92)As(0.08))(2) diverges upon cooling to T = 0, indicating a ferromagnetic QCP. Our observation that this kind of instability, which is forbidden in d-electron metals, occurs in a heavy fermion system will have a large impact on the studies of quantum critical materials.
Neutron diffraction experiments have been performed on a magnetically ordered CeCu2Si2 single crystal exhibiting A-phase anomalies in specific heat and thermal expansion. Below T(N) approximately 0.8 K antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the A phase in CeCu2Si2 suggests that a spin-density-wave instability is the origin of the quantum critical point in CeCu2Si2.
Abstract. We report measurements of the pressure-dependent superconducting transition temperature Tc and electrical resistivity of the heavy-fermion compound CeCoIn 5 . Pressure moves CeCoIn 5 away from its proximity to a quantum-critical point at atmospheric pressure. Experimental results are qualitatively consistent with theoretical predictions for strong-coupled, d-wave superconductivity in an anisotropic 3D superconductor. Although heavy-fermion superconductivity has been known for over two decades, a microscopic theory of this broken-symmetry state has remained elusive. Experiments have established, however, that the unconventional superconductivity in heavyfermion compounds most likely is mediated by magnetic fluctuations, which also are responsible for enhancing the effective mass of itinerant quasiparticles by two to three orders of magnitude. [1,2,3] In spite of 20 years of searching, there has been, until very recently, only one example of a Ce-based heavy-fermion compound, CeCu 2 Si 2 , that is superconducting at atmospheric pressure. [4] In contrast, several examples of Cebased heavy-fermion antiferromagnets have been found in which superconductivity appears as their Néel temperature is tuned toward T = 0 with the application of pressure. [5,6,7,8] This recent series of discoveries re-enforces the belief not only that a magnetic interaction is responsible for superconductivity in these materials but also that heavy-fermion superconductivity may be favored at a particular 'soft-spot' in phase space, i.e., near a quantum-critical point
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