The long-accepted magnetic structure of $\mathrm{Fe}\mathrm{P}{\mathrm{S}}_3$ has recently been refuted through extensive powder neutron diffraction studies. Single-crystal neutron diffraction, using both quasi-Laue and monochromatic techniques, has now been conducted to reveal more information about the nature of the magnetic ordering in this compound. The magnetic unit cell was found to be twice as large as the crystallographic cell in both the $a$ and $b$ directions and around three times as large along the $c$ direction, giving a propagation vector of $\left[\frac{1}{2}\frac{1}{2}0.34\right]$. There is incomplete long-range magnetic order along the $c^{*}$ direction due to weak Fe-Fe interactions and large anisotropy in that direction. The variation of the spontaneous moment with temperature closely resembles that for an antiferromagnetic two-dimensional Ising model on a honeycomb lattice.

Neutron spectroscopy measurements have been performed on single crystals of the antiferromagnetic van der Waals compound $\mathrm{Ni}\mathrm{P}{\mathrm{S}}_3$. Linear spin-wave theory using a Heisenberg Hamiltonian with single-ion anisotropies has been applied to determine the magnetic exchange parameters and the nature of the anisotropy. The analysis reveals that $\mathrm{Ni}\mathrm{P}{\mathrm{S}}_3$ is less two-dimensional than its sister compounds, with a relatively large ferromagnetic exchange of $J^{\prime }=-0.3$ meV between the layered $\mathit{ab}$ planes. In-plane magnetic exchange interactions up to the third-nearest neighbor were required to fit the data. The nearest-neighbor exchange was ferromagnetic with $J_1=-2.6$ meV, the second neighbor was antiferromagnetic and small with $J_2=0.2$ meV, and the dominant antiferromagnetic third-neighbor exchange was $J_3=13.5$ meV. The anisotropy was shown to be largely $\mathit{XY}$-like with a small uniaxial component, leading to the appearance of two low-energy spin wave modes in the spin-wave spectrum at the Brillouin zone center. The analysis could reproduce the spin-wave energies, however, there are discrepancies with the calculated neutron intensities, hinting at more exotic phenomena.

Lead halide perovskites have emerged as promising materials for solar energy conversion and X-ray detection owing to their remarkable optoelectronic properties. However, the microscopic origins of their superior performance remain unclear. Here we show that low-symmetry dynamic nanodomains present in the high-symmetry average cubic phases, whose characteristics are dictated by the A-site cation, govern the macroscopic behaviour. We combine X-ray diffuse scattering, inelastic neutron spectroscopy, hyperspectral photoluminescence microscopy and machine-learning-assisted molecular dynamics simulations to directly correlate local nanoscale dynamics with macroscopic optoelectronic response. Our approach reveals that methylammonium-based perovskites form densely packed, anisotropic dynamic nanodomains with out-of-phase octahedral tilting, whereas formamidinium-based systems develop sparse, isotropic, spherical nanodomains with in-phase tilting, even when crystallography reveals cubic symmetry on average. We demonstrate that these sparsely distributed isotropic nanodomains present in formamidinium-based systems reduce electronic dynamic disorder, resulting in a beneficial optoelectronic response, thereby enhancing the performance of formamidinium-based lead halide perovskite devices. By elucidating the influence of the A-site cation on local dynamic nanodomains, and consequently, on the macroscopic properties, we propose leveraging this relationship to engineer the optoelectronic response of these materials, propelling further advancements in perovskite-based photovoltaics, optoelectronics and X-ray imaging.

${\mathrm{CuCrO}}_2$ is a manifestation of a two-dimensional triangular antiferromagnet which exhibits an incommensurate noncollinear magnetic structure similar to a classical ${120}^{\circ }$ ordering. Using the inelastic neutron scattering technique, direct evidence of a magnon-phonon coupling in ${\mathrm{CuCrO}}_2$ is revealed via the mixed magnon-phonon character of the excitation mode at 12.5 meV as well as a minimum at the zone boundary. A simple model Hamiltonian that incorporates an exchange-striction type magnon-phonon coupling accurately reproduces the observed features. Also, continuum excitations originating from the interaction between quasiparticles are observed with strong intensity at the zone boundary. These features of the magnetic excitations are key to an understanding of the emergent excitations in noncollinear antiferromagnetic compounds.

Three-dimensional, commensurate long-range magnetic order in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ quickly evolves to quasi-two-dimensional, incommensurate correlations upon doping with mobile holes, and superconductivity follows for $x$ as small as 0.05 in the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x/{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ family of superconductors. The onset of superconductivity in these systems is known to be coincident with a remarkable rotation of the incommensurate spin order from “diagonal stripes” below $x$ = 0.05 to “parallel stripes” above. However, little is known about the spin correlations at optimal and high doping levels, around and beyond the proposed quantum critical point for the pseudogap phase, $p^{*}$. Here, we present elastic and inelastic neutron scattering measurements on single crystals of ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x$ = 0.125, 0.19, 0.24, and 0.26 and show that two-dimensional, quasistatic, parallel spin stripes have an onset at temperatures such that the parallel spin stripe phase extends beyond $p^{*}$ and envelops the entirety of superconducting ground states in this system. We also show that the elastic order parameter for parallel spin stripes at optimum doping, $x$ = 0.19, displays an inflection point at superconducting $T_c$, while the low-energy dynamic spectral weight of parallel stripe fluctuations grows with decreasing temperature and saturates below $T_c$.