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Abstract Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergence of a plethora of novel quantum matter states such as unconventional superconductivity, electronic-nematic states, hidden order and most recently topological states of matter such as topological Kondo insulators and Kondo semimetals and putative chiral superconductors. The outstanding challenge is that the archetypal Kondo lattice model that captures the underlying electronic dichotomy is notoriously difficult to solve for real materials. Here we show, using the prototypical strongly-correlated antiferromagnet CeIn3, that a multi-orbital periodic Anderson model embedded with input from ab initio bandstructure calculations can be reduced to a simple Kondo-Heisenberg model, which captures the magnetic interactions quantitatively. We validate this tractable Hamiltonian via high-resolution neutron spectroscopy that reproduces accurately the magnetic soft modes in CeIn3, which are believed to mediate unconventional superconductivity. Our study paves the way for a quantitative understanding of metallic quantum states such as unconventional superconductivity.

Abstract We perform nuclear magnetic resonance (NMR) measurements of the oxygen-17 Knight shifts for Sr2RuO4, while subjected to uniaxial stress applied along [100] direction. The resulting strain is associated with a strong variation of the temperature and magnetic field dependence of the inferred magnetic response. A quasiparticle description based on density-functional theory calculations, supplemented by many-body renormalizations, is found to reproduce our experimental results, and highlights the key role of a van-Hove singularity. The Fermi-liquid coherence scale is shown to be tunable by strain, and driven to low values as the associated Lifshitz transition is approached.

The fate of high-energy degrees of freedom, such as spin-orbit interactions, in the coherent state of Kondo lattice materials remains unclear. Here, the authors use resonant inelastic x-ray scattering in CePd3 to show how Kondo-quasiparticle excitations are renormalized and develop a pronounced momentum dependence, while maintaining a largely unchanged spin-orbit gap.

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}.

Insights into the behavior of quantum materials are only possible because of the development of suitable experimental probes. Modic et. al. develop the theoretical and experimental basis for resonant torsion magnetometry—a technique to measure anisotropic magnetic responses with high sensitivity.

Weyl semimetals should exhibit unusual electronic behaviour but conditions where these effects dominate are difficult to achieve. Ramshaw et al. use high magnetic fields to drive TaAs into the quantum limit, finding evidence for the predicted chiral anomaly and an unanticipated increase in resistivity at the highest fields.

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.