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We studied the anisotropy of the superconducting upper critical field Hc2 in the heavy-fermion superconductor UTe2 under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure pc ≈ 1.5 GPa, and a magnetically ordered state appears. The unusual Hc2(T) at low temperatures for \(H\parallel a\) suggests that the multiple superconducting phases which appear under pressure have quite different Hc2. For a field applied along the hard magnetization b axis Hc2(0) is glued to the metamagnetic transition Hm, which is suppressed near pc. The suppression of Hm with pressure follows the decrease of temperature \(T_{\chi }^{\text{max}}\) , at the maximum in the susceptibility along b. The strong reinforcement of Hc2 at ambient pressure for \(H\parallel b\) above 16 T is rapidly suppressed under pressure due to the increase of Tsc and the decrease of Hm. The change in the hierarchy of the anisotropy of Hc2(0) on approaching pc points out that the c axis becomes the hard magnetization axis. [ABSTRACT FROM AUTHOR]

The pressure-temperature phase diagram of the new heavy-fermion superconductor CeAu2Si2 is markedly different from those studied previously. Indeed, superconductivity emerges not on the verge but deep inside the magnetic phase, and mysteriously Tc increases with the strengthening of magnetism. In this context, we have carried out ac calorimetry, resistivity, and thermoelectric power measurements on a CeAu2Si2 single crystal under high pressure. We uncover a strong link between the enhancement of superconductivity and quantum-critical-like features in the normal-state resistivity. Non-Fermi-liquid behavior is observed around the maximum of superconductivity and enhanced scattering rates are observed close to both the emergence and the maximum of superconductivity. Furthermore we observe signatures of pressure- and temperature-driven modifications of the magnetic structure inside the antiferromagnetic phase. A comparison of the features of CeAu2Si2 and its parent compounds CeCu2Si2 and CeCu2Ge2 plotted as function of the unit-cell volume leads us to propose that critical fluctuations of a valence crossover play a crucial role in the superconducting pairing mechanism. Our study illustrates the complex interplay between magnetism, valence fluctuations, and superconductivity. [ABSTRACT FROM AUTHOR]

We report neutron scattering experiment results revealing the nature of the magnetic order occurring in the heavy fermion superconductor Ce0.95Nd0.05CoIn5. In this system, an antiferromagnetic state is stabilized at a temperature below the superconducting transition one. We evidence that it is an incommensurate order and its propagation vector is found to be identical to that of the magnetic field induced antiferromagnetic order occurring in the stoichiometric superconductor CeCoIn5, the so-called Q-phase. The commonality between these two cases suggests that d-wave superconductivity is playing a key role for the formation of this kind of magnetic order. The proposed mechanism is the enhancement of the nesting condition by an order parameter with nodes in the nesting area. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the upper critical field for CeCoIn5 under high pressure. We show that, consistent with other measurements, this system shows a decoupling between the maximum of the superconducting transition temperature Tc and the maximum pairing strength. We propose a model in which, in order to account for the discrepancy in pressure between the maximum of the upper critical field and the maximum of Tc, we introduce magnetic pair-breaking effects, already widely suggested by other measurements. We found that within the Eliashberg frame work, the unusual shape of Hc2(T ) can be completely reproduced when magnetic pair breaking is taken into account. Surprisingly, we found that the maximum of pair breaking and of pair coupling coincide in pressure, suggesting that both mechanisms originate from quantum criticality. Our model implies that CeCoIn5 is the first compound of its family that shows clear decoupling between the maximum of Tc and quantum criticality. [ABSTRACT FROM AUTHOR]

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 a detailed analysis of the upper critical field for CeCoIn5 under high pressure. We show that, consistent with other measurements, this system shows a decoupling between the maximum of the superconducting transition temperature Tc and the maximum pairing strength. We propose a model in which, in order to account for the discrepancy in pressure between the maximum of the upper critical field and the maximum of Tc, we introduce magnetic pair-breaking effects, already widely suggested by other measurements. We found that within the Eliashberg frame work, the unusual shape of Hc2(T ) can be completely reproduced when magnetic pair breaking is taken into account. Surprisingly, we found that the maximum of pair breaking and of pair coupling coincide in pressure, suggesting that both mechanisms originate from quantum criticality. Our model implies that CeCoIn5 is the first compound of its family that shows clear decoupling between the maximum of Tc and quantum criticality. [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]