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Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu2Si2 away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu2Si2 and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature Tcr. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature Tc and the valence instability parameter Tcr, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting Tc of a given sample is mainly controlled by the compound's Tcr and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general. Superconductivity: driven by valence fluctuations An analysis of electrical resistivity of several Ce-based heavy fermion compounds unveils the crucial role of critical valence fluctuations to enhance superconductivity. An international team led by Didier Jaccard from University of Geneva performed a comprehensive analysis of 17 data sets of high-pressure electrical resistivity from 9 different Ce-based heavy fermion compounds. They reveal a universal character of the relationship between the superconducting transition temperature (Tc) and the strength of the valence instability. Taking into account the superconducting pair-breaking effect of non-magnetic disorder, they identify two main parameters, a virtual valence transition temperature and the residual resistivity, which control Tc of a representative part of Ce-based heavy fermion superconductors. This study provides compelling evidence for the crucial role of critical valence fluctuations besides spin fluctuations. [ABSTRACT FROM AUTHOR]

In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the temperature-tuning parameter phase diagram, which is associated with a maximum of the superconducting pairing interaction. Proximity to antiferromagnetism suggests a relation between the disappearance of antiferromagnetic order and superconductivity. We combine muon spin rotation, neutron scattering, and x-ray absorption spectroscopy techniques to gain access to the magnetic and electronic structure of CeCo(In1−xCdx)5 at different time scales. Different magnetic structures are obtained that indicate a magnetic order of itinerant character, coexisting with bulk superconductivity. The suppression of the antiferromagnetic order appears to be driven by a modification of the bandwidth/carrier concentration, implying that the electronic structure and consequently the interplay of superconductivity and magnetism is strongly affected by hydrostatic and chemical pressure. [ABSTRACT FROM AUTHOR]

We present the pressure-temperature phase diagram of the antiferromagnet CeAg 2 Si 2 established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5 GPa. With increasing pressure, the Néel temperature [ T N ( p = 0 ) = 8.6 K] slowly increases up to T N = 13.4 K at 9.4 GPa and then vanishes abruptly at the magnetic critical pressure p c ∼ 13 GPa. For the first time, heavy fermion superconductivity is observed in CeAg 2 Si 2 . Superconductivity emerges at ∼ 11 GPa and persists over roughly 10 GPa. Partial- and bulk-transition temperatures are highest at p = 16 GPa, with a maximal T c bulk = 1.25 K. In the pressure region of superconductivity, Kondo and crystal-field splitting energies become comparable and resistivity exhibits clear signatures of a Ce-ion valence crossover. The crossover line is located at a rapid collapse in resistivity as function of pressure and extrapolates to a valence transition critical endpoint at critical pressure and temperature of p cr ∼ 17 GPa and T cr ∼ − 13 K , respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg 2 Si 2 . [ABSTRACT FROM AUTHOR]

In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the temperature-tuning parameter phase diagram, which is associated with a maximum of the superconducting pairing interaction. Proximity to antiferromagnetism suggests a relation between the disappearance of antiferromagnetic order and superconductivity. We combine muon spin rotation, neutron scattering, and x-ray absorption spectroscopy techniques to gain access to the magnetic and electronic structure of CeCo(In1−xCdx)5 at different time scales. Different magnetic structures are obtained that indicate a magnetic order of itinerant character, coexisting with bulk superconductivity. The suppression of the antiferromagnetic order appears to be driven by a modification of the bandwidth/carrier concentration, implying that the electronic structure and consequently the interplay of superconductivity and magnetism is strongly affected by hydrostatic and chemical pressure. [ABSTRACT FROM AUTHOR]