We have studied the electronic structure of the nonmagnetic LiFeAs (Tc$\sim${}18\,\,K) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of 3, high density of states at the Fermi level caused by a van Hove singularity, and no evidence for either a static or a fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations suggest that these electronic properties capture the majority of ingredients necessary for the superconductivity in iron pnictides.

Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit Néel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (T_s=90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here we report inelastic neutron-scattering experiments that reveal both stripe and Néel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the Néel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ∼60% larger than that in the iron pnictide BaFe₂As₂. Our results suggest that FeSe is a novel S=1 nematic quantum-disordered paramagnet interpolating between the Néel and stripe magnetic instabilities.

The electronic structure of the iron chalcogenide superconductor ${\text{FeSe}}_{1-x}$ was investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of ${\text{FeSe}}_{1-x}$, DFT calculations, and existing data on ${\text{FeSe}}_x{\text{Te}}_{1-x}$. The bands undergo a pronounced orbital-dependent renormalization, different from what was observed for ${\text{FeSe}}_x{\text{Te}}_{1-x}$ and any other pnictides.

Neutron diffraction and bulk measurements are used to determine the nature of the low-temperature ordered state in ${\mathrm{LiCu}}_2{\mathrm{O}}_2$, a $S=1/2$ spin-chain compound with competing interactions. The spin structure is found to be helimagnetic, with a propagation vector $(0.5,\zeta ,0)$, $\zeta =0.174$. The nearest-neighbor exchange constant and frustration ratio are estimated to be $J_1=5.8\mathrm{meV}$ and $J_2/J_1=0.29$, respectively. For idealized quantum spin chains, these parameter values would signify a gapped spin-liquid ground state with commensurate spin correlations. The observed temperature dependence of the magnetic propagation vector in ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ is attributed to a competition between incommensurate helimagnetism in the classical spin model and commensurability in the quantum case. It is also proposed that long-range ordering in ${\mathrm{LiCu}}_2{\mathrm{O}}_2$ is facilitated by intrinsic nonstoichiometry.