We report results of systematic de Haas-van Alphen (dHvA) studies on Ce 1−x Yb x CoIn 5 single crystals with varying Yb concentration. For x = 0.1, the well-known Fermi surfaces and the heavy effective masses of CeCoIn 5 (x = 0) have changed only slightly. We start to observe changes of the Fermi-surface topology at x = 0.2 leading to a drastic reconstruction above x = 0.55. At these concentrations, the effective masses are reduced considerably to values between 0.7 and 2.6 free electron masses. For both YbCoIn 5 and CeCoIn 5 , the angular-resolved dHvA frequencies can be very well described by conventional density-functional theory calculations. Projection of the Bloch states onto atomic Yb-4f orbitals yields a 4f occupation of 13.7 electrons, in agreement with previous experimental results indicating an intermediate Yb valence of +2.3.

The phase diagram of the filled skutterudite CeOs4Sb12 has been mapped in fields H of up to 60 T and temperatures T down to 0.5 K using resistivity, magnetostriction, and MHz conductivity. The valence transition separating the semimetallic, low-H, low-T , L phase from the metallic high-H, high-T H phase exhibits a very unusual, wedge-shaped phase boundary, with a non-monotonic gradient alternating between positive and negative. This is quite different from the text-book "elliptical" phase boundary usually followed by valence transitions. Analysis of Shubnikov-de Haas oscillations within the H phase reveals an effective mass that increases as H drops toward the H − L phase boundary, suggesting proximity to a quantum-critical point. The associated magnetic fluctuations may be responsible for the anomalous H, T dependence of the valence transition at high H, whereas the low−H, high−T portion of the phase boundary may rather be associated with the proximity of CeOs4Sb12 to a topological semimetal phase induced by uniaxial stress.

We report the observation of multigap superconductivity in TaRh 2 B 2 . We show TaRh 2 B 2 is a bulk type-II superconductor with a transition temperature T c = 6.00(5) K. We present transverse-field muon spin relaxation data where the superconducting gap can be fit using a two-gap (s + s)-wave model. We also report the zero-field electronic specific heat in the superconducting state that is best described by the same (s + s) model, providing further evidence of multiband behavior in this superconductor. Zero-field muon spin relaxation measurements show time-reversal symmetry is preserved in the superconducting state. We demonstrate that TaRh 2 B 2 has an upper critical field of 15.2(1) T, which is significantly higher than previously reported and exceeds the Pauli limit.