Neutron diffraction and inelastic spectroscopy is used to characterize the magnetic Hamiltonian of SrHo 2 O 4 and SrDy 2 O 4 . Through a detailed computation of the crystal-field levels we find site-dependent anisotropic single-ion magnetism in both materials, and diffraction measurements show the presence of strong one-dimensional spin correlations. Our measurements indicate that competing interactions of the zigzag chain, combined with frustrated interchain interactions, play a crucial role in stabilizing spin-liquid type correlations in this series.

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.

We have investigated the effect of Yb substitution on the Pauli limited, heavy fermion superconductor, CeCoIn5. Yb acts as a non-magnetic divalent substituent for Ce throughout the entire doping range, equivalent to hole doping on the rare earth site. We found that the upper critical field in (Ce,Yb)CoIn5 is Pauli limited, yet the reduced (H,T) phase diagram is insensitive to disorder, as expected in the purely orbitally limited case. We use the Pauli limiting field, the superconducting condensation energy and the electronic specific heat coefficient to determine the Wilson ratio (RW ), the ratio of the specific heat coefficient to the Pauli susceptibility in CeCoIn5. The method is applicable to any Pauli limited superconductor in the clean limit.

Magnetic frustration and low dimensionality can prevent long range magnetic order and lead to exotic correlated ground states. SrDy 2 O 4 consists of magnetic Dy 3+ ions forming magnetically frustrated zig-zag chains along the c-axis and shows no long range order to temperatures as low as T = 60 mK. We carried out neutron scattering and AC magnetic susceptibility measurements using powder and single crystals of SrDy 2 O 4 . Diffuse neutron scattering indicates strong one-dimensional (1D) magnetic correlations along the chain direction that can be qualitatively accounted for by the axial next-nearest neighbour Ising (ANNNI) model with nearestneighbor and next-nearest-neighbor exchange J 1 = 0.3 meV and J 2 = 0.2 meV, respectively. Three-dimensional (3D) correlations become important below T * ≈ 0.7 K. At T = 60 mK, the short range correlations are characterized by a putative propagation vector k 1/2 = (0,2 ). We argue that the absence of long range order arises from the presence of slowly decaying 1D domain walls that are trapped due to 3D correlations. This stabilizes a low-temperature phase without long range magnetic order, but with well-ordered chain segments separated by slowly-moving domain walls.