Significance: Sr 2 RuO 4 is distinctive among unconventional superconductors, in that in addition to exhibiting evidence for strong correlations, it is stoichiometric and extremely clean. As a result, its electronic structure is unusually well characterized, rendering it an ideal platform for developing a deep understanding of the mechanism behind the emergence of the superconducting state from a Fermi liquid. Toward that end, an unambiguous determination of the pairing symmetry is an essential step. For more than 2 decades, the preponderance of evidence pointed to a triplet spin pairing state and only recently has this interpretation been challenged. By field-dependent NMR Knight shift measurements, we eliminate from further consideration all candidate purely odd-parity triplet pairing states.

A gapped spin liquid: Quantum spin liquids avoid conventional magnetic ordering down to the lowest temperatures. Among the candidates for this state of matter, organic salts such as κ-(BEDT-TTF) ₂ Cu ₂ (CN) ₃ have been prominent. Miksch et al. studied this material using electron spin resonance to elucidate the nature of its ground state. Instead of the expected gapless state, the temperature dependence of spin susceptibility suggests the formation of a spin gap. Science , this issue p. 276

Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m^⋆. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe–Regel–Mott limit is accompanied by a ‘displaced Drude peak’ in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition.

A strongly frustrated ordered state can be induced in Y ₃ Cu ₉ (OH) ₁₉ Cl ₈ by slightly modifying the perfect kagome lattice YCu ₃ (OH) ₆ Cl ₃ .