Inelastic cold-neutron scattering on ${\mathrm{LaCoO}}_3$ provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate $S=1$ spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$, the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

The perovskite ${\mathrm{LaCoO}}_3$ evolves from a nonmagnetic Mott insulator to a spin cluster ferromagnet (FM) with the substitution of ${\mathrm{Sr}}^{2+}$ for ${\mathrm{La}}^{3+}$ in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$. The clusters increase in size and number with $x$ and the charge percolation through the clusters leads to a metallic state. Using elastic neutron scattering on ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ single crystals, we show that an incommensurate spin superstructure coexists with the FM spin clusters. The incommensurability increases continuously with $x$, with the intensity rising in the insulating phase and dropping in the metallic phase as it directly competes with the commensurate FM, itinerant clusters. The spin incommensurability arises from local order of ${\mathrm{Co}}^{3+}\mathrm{\text{−}}{\mathrm{Co}}^{4+}$ clusters but no long-range static or dynamic spin stripes develop. The coexistence and competition of the two magnetic phases explain the residual resistivity at low temperatures in samples with metalliclike transport.

The local atomic structure of the new nonmagnetic superconducting system ${\text{LaO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ is investigated using neutron diffraction and the pair density function analysis. Evidence for local charge fluctuations linked to a charge disproportionation of the Bi ions in the distorted lattice of superconducting ${\text{LaO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ is presented. In-plane short-range distortions of sulfur atoms up to 0.3 Å in magnitude break site symmetry and create two distinct environments around Bi. Out-of-plane motion of apical sulfur brings it closer to the La-O/F doping layer with increasing $x$ that may lead to a charge transfer conduit between the doping layers and the superconducting ${\text{BiS}}_2$ planes. The mechanism for superconductivity may arise from the interplay between charge density fluctuations and an enhanced spin-orbit coupling suggested theoretically that induces spin polarization.

Uniaxial tensile strain changes the magnetoresistance according to the direction it is applied on in MoTe ₂ .

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe₂ possesses two stacking arrangements, the ferroelectric Weyl semimetal T_d phase and the higher-order topological insulator 1T′ phase. However, in thin flakes of MoTe₂, it is unclear if the layer stacking follows the T_d, 1T′, or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe₂ layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T′ and T_d domains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe₂ stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe₂ flakes. More broadly, this work suggests c-axis confinement as a method to influence layer stacking in other 2D materials.

We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide ${K}_{y}{\mathrm{Fe}}_{1.6+x}{\mathrm{Se}}_{2}$. In the high temperature paramagnetic state, both materials have a single phase with a crystal structure similar to that of the BaFe${}_{2}$As${}_{2}$ family of iron pnictides. While the insulating ${K}_{y}{\mathrm{Fe}}_{1.6+x}{\mathrm{Se}}_{2}$ forms a $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ iron vacancy ordered block antiferromagnetic (AF) structure at low temperature, the superconducting compounds spontaneously phase separate into an insulating part with $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ iron vacancy order and a superconducting phase with chemical composition of K${}_{z}$Fe${}_{2}$Se${}_{2}$ and BaFe${}_{2}$As${}_{2}$ structure. Therefore, superconductivity in alkaline iron selenides arises from alkali deficient K${}_{z}$Fe${}_{2}$Se${}_{2}$ in the matrix of the insulating block AF phase.