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Topological superconductivity is a long-sought state of matter in bulk materials, and the odd-parity superconductor UTe_{2} is a prime candidate. The recent observation of a field-trainable spontaneous Kerr signal in UTe_{2} at the onset of superconductivity provides strong evidence that the superconducting order parameter is multicomponent and breaks time-reversal symmetry. Here, we perform Kerr effect measurements on a number of UTe_{2} samples—grown via both chemical vapor transport and the molten-salt-flux methods—that show a single superconducting transition between 1.6 K and 2.1 K. Our results show no evidence for a spontaneous Kerr signal in zero-field measurements. This implies that the superconducting state of UTe_{2} does not intrinsically break time-reversal symmetry. Instead, we observe a field-trainable signal that varies in magnitude between samples and between different locations on a single sample, which is a sign of inhomogeneous magnetic regions. Our results provide an examination of representative UTe_{2} samples and place strong constraints on the superconducting order parameter of UTe_{2}.

At ambient pressure and zero field, tetragonal CeAuSb_{2} hosts stripe antiferromagnetic order at T_{N}=6.3 K. Here, we first show via bulk thermodynamic probes and x-ray diffraction measurements that this magnetic order is connected with a structural phase transition to a superstructure that likely breaks C_{4} symmetry, thus signaling nematic order. The temperature-field-pressure phase diagram of CeAuSb_{2} subsequently reveals the emergence of additional ordered states under applied pressure at a multicritical point. Our phenomenological model supports the presence of a vestigial nematic phase in CeAuSb_{2} akin to iron-based high-temperature superconductors; however, superconductivity, if present, remains to be discovered.

The effects of uniaxial compressive stress on the normal state ^{17}O nuclear-magnetic-resonance properties of the unconventional superconductor Sr_{2}RuO_{4} are reported. The paramagnetic shifts of both planar and apical oxygen sites show pronounced anomalies near the nominal a-axis strain ϵ_{aa}≡ϵ_{v} that maximizes the superconducting transition temperature T_{c}. The spin susceptibility weakly increases on lowering the temperature below T≃10 K, consistent with an enhanced density of states associated with passing the Fermi energy through a van Hove singularity. Although such a Lifshitz transition occurs in the γ band formed by the Ru d_{xy} states hybridized with in-plane O p_{π} orbitals, the large Hund’s coupling renormalizes the uniform spin susceptibility, which, in turn, affects the hyperfine fields of all nuclei. We estimate this “Stoner” renormalization S by combining the data with first-principles calculations and conclude that this is an important part of the strain effect, with implications for superconductivity.

Recent efforts at Los Alamos to discover strongly correlated superconductors and hard ferromagnets are reviewed. While serendipity remains a principal engine of materials discovery, design principles and structural building blocks are beginning to emerge that hold potential for predictive discovery. Successes over the last decade with the so-called “115” strongly correlated superconductors are summarized, and more recent efforts to translate these insights and principles to novel hard magnets are discussed. While true “materials by design” remains a distant aspiration, progress is being made in coupling empirical design principles to electronic structure simulation to accelerate and guide materials design and synthesis.

We report ^115 In nuclear quadrupolar resonance (NQR) measurements on the heavy-fermion superconductor PuCoIn _5 , in the temperature range 0.29K ≤ T ≤ 75K. The NQR parameters for the two crystallographically inequivalent In sites are determined, and their temperature dependence is investigated. A linear shift of the quadrupolar frequency with lowering temperature below the critical value T _c is revealed, in agreement with the prediction for composite pairing . The nuclear spin–lattice relaxation rate T _1 ^−1 ( T ) clearly signals a superconducting (SC) phase transition at T _c ≃ 2.3 K, with strong spin fluctuations, mostly in-plane, dominating the relaxation process in the normal state near to T _c . Analysis of the T _1 ^−1 data in the SC state suggests that PuCoIn _5 is a strong-coupling d -wave superconductor.

We have carried out a careful magnetic neutron scattering study of the heavy fermion compound URu _2 Si _2 to probe the possible existence of a small magnetic moment parallel to tetragonal basal plane in the ‘hidden-order’ phase. This small in-plane component of the magnetic moment on the uranium sites S _∥ has been postulated by two recent models (rank-5 superspin/hastatic order) aiming to explain the hidden-order phase, in addition to the well-known out-of-plane component S _⊥ ≈ 0.01–0.04 μ _B /U. In order to separate S _∥ and S _⊥ , we take advantage of the condition that for magnetic neutron scattering only the components of the magnetic structure that are perpendicular to the scattering vector Q contribute to the magnetic scattering. We find no evidence for an in-plane magnetic moment S _∥ . Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is S ^max _∥ ⩽ 1 × 10 ^−3 μ _B /U.