[en] Heavy fermion superconductors are largely studied for their unconventional ground states. We present results on the skutterudite compound PrOs₄Sb₁₂ (T_c ∼ 1.8K), and on the heavy-fermion CeCoIn₅ (T_c ∼ 2.3K), where both the low field and the low temperature behaviour of their thermal conductivity yield evidence for multigap superconductivity.

[en] We report on an ongoing multiprobe experiment on a very high puritiy CeCu₂Si₂ single crystal at ambient and high pressure. The Nernst signal, which has not been measured in this compound so far, exhibits quite uncommon features and seems to be coupled to the antiferromagnetic phase, rather than to the vortex motion within the superconducting mixed state.

[en] Heavy fermion compounds are host to many interesting but poorly understood strong correlation effects, leading in exceptional cases to unconventional superconductivity. CeCoIn₅ is such an example of an ambient pressure heavy fermion superconductor with a T_c of 2.3K. Such compounds are best described as a Kondo lattice, where Ce, Yb or U ions form a periodic lattice of magnetic moments coupled to the conduction electrons via antiferromagnetic exchange interaction. Dilution, or substitution, of the Kondo ion with a lanthanide, actinide or alkali-earth element that does not participate in the Kondo lattice, results in the so-called ^Kondo hole^. The electron scattering off the Kondo hole leads to significant changes in both the normal and superconducting properties. We have investigated the suppression of superconductivity in CeCoIn₅ diluted with La, Yb, Y, Ca, Th, Gd, Er, Eu and Lu via specific heat measurements on well characterized single crystals. The suppression of the superconducting condensation energy exhibits a remarkable departure from the Abrikosov-Gorkov impurity pair breaking picture, emphasizing that the Kondo holes are no ordinary pair-breakers.

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[en] We report investigations of the effect of electron doping in FeGa₃ via electric resistivity, specific heat and magnetic susceptibility measurements in single crystals. FeGa₃ is a non-magnetic small gap semiconductor (Δ ∼ 0.3-0.4 eV). Low concentration of Co in FeGa₃ induces a crossover to a metallic-like behavior, also creating weakly coupled local moments. Electronic specific heat and resistivity suggest a mass enhancement of charge carriers. Thus, the low carrier density metal formed by doping FeGa₃ presents some physical properties that resemble heavy fermion metals.

[en] The heavy-fermion (HF) metals are very susceptible to chemical substitution. In these compounds the Kondo coupling between a lattice of local moments and the conduction band creates quasiparticle excitations with large effective masses and the dopants disrupt the coherent Kondo coupling. We study the effect of Yb doping on the Pauli-limited, HF superconductor, CeCoIn₅ via de Haas-van Alphen (dHvA) measurements. Yb acts as a non-magnetic divalent substitution for Ce, equivalent to hole doping on the rare-earth site. Our main goal consists in the systematic investigation of the dHvA oscillations on Ce_1-xYb_xCoIn₅ in order to elucidate the evolution of the Fermi surface as a function of Yb. The dHvA data were obtained on high-quality single crystals with different concentrations of Yb atoms. The experiment was performed in a top-loading dilution refrigerator by use of a capacitive torque cantilever technique at temperatures down to 20 mK in magnetic fields up to 18 T.

[en] The superconducting state of the heavy fermion PrOs₄Sb₁₂ was studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The temperature and field dependence of the thermal conductivity confirm the recently suggested multiband superconductivity scenario and point to fully open gaps on the whole Fermi surface

[en] We report on low-temperature thermal conductivity of the noncentrosymmetric superconductor LaRhSi₃ (T_C = 2.3 K), which is a paramagnetic analog of the pressure-induced superconductor CeRhSi₃. In the normal state (either in zero field above T_c, or in magnetic field above H_c2), the thermal conductivity k is mostly due to electrons, and shows a nearly T-linear dependence. In the superconducting state, k decreases exponentially below T_c, and crosses over to κ ∝ T² behavior at T << T_c. The temperature dependence of thermal conductivity of LaRhSi₃ is similar to that of a number of conventional s-wave superconductors. The field dependence of the residual linear term κ₀/T as a function of magnetic field suggests that LaRhSi₃ has no nodes in the superconducting gap.

[en] The intermetallic compounds RIn₃ and RTIn₅ (R= rare earth, T= transition metal) have attracted great interest for their large variety of anomalous ground states. Among these are the well-known heavy-fermion superconductors CeCoIn₅ and CeIrIn₅. We present here a dHvA study of YbCoIn₅ and LuCoIn₅, performed by use of a capacitive torque cantilever technique at temperatures down to 0.4 K in magnetic fields up to 13 T. In addition, one single crystal of LuCoIn₅ has been measured in magnetic field up to 34 T. Besides their angular-dependent Fermi-surface topologies, we have also determined the effective masses of the different bands by following the temperature-dependent amplitude changes of the dHvA oscillations. A large number of different dHvA frequencies has been observed for the main crystallographic directions. In contrast to CeCoIn₅ and CeIrIn₅, the cyclotron effective masses for these compounds are in the range from 0.7 to 2.0 m₀.

[en] The superconducting state of the heavy fermion PrOs₄Sb₁₂ was studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The temperature and field dependence of the thermal conductivity confirm the recently suggested multiband superconductivity scenario and point to fully open gaps on the whole Fermi surface