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Abstract:

Magnets with isotropic easy-plane symmetry host Goldstone modes that can be leveraged for efficient spin transport. Here, we present a time-resolved optical polarimetry technique that allows us to detect and characterize such low-frequency modes, and use it to observe the Goldstone mode in the multi-Q broken helix phase of EuIn2As2. The strength of our technique comes from the ability to distinguish between nematic and magnetization dynamics in order to yield information about the mode structure, in addition to its frequency. We find that the nearly uniform spin precession characteristic of a Goldstone mode is realized only when a small magnetic field is used to unpin the broken helix from local strain generated during crystal growth. In this regime, the mode frequency scales linearly with the applied field due to the ground state C2z symmetry of the broken helix. Our work shows how optical polarimetry can be used to study the Goldstone modes of complex magnets.

Abstract:

We use spherical neutron polarimetry to determine the ground state magnetic structure of ${\mathrm{Mn}}_3\mathrm{Sn}$ . We find that ${\mathrm{Mn}}_3\mathrm{Sn}$ adopts an inverse triangular structure with spins parallel to $〈100〉$ (type III) rather than spins parallel to $〈110〉$ (type IV). Density functional theory calculations reveal no energy difference between these two structures, suggesting that the selection is caused by subtle effects such as sixth-order anisotropy. Partial control of the magnetic domain population through a moderate magnetic field is key to distinguishing between the two models. We find that three of the six domains are approximately equally populated, while the others have negligible population. Upon entering the low temperature incommensurate phase, the domain structure is lost. The domains in this phase are decoupled from the magnetic field and can therefore not be controlled by any known method.

Abstract:

We report a scalable, water-based methodology for the direct room-temperature synthesis of porous amorphous UiO-66-type metal–organic frameworks (aMOFs), enabling the incorporation of a range of functionalised terepthalate linkers without organic solvents during framework formation. Powder X-ray diffraction and scanning electron diffraction confirm the formation of truly topologically amorphous UiO-66 derivatives, while pair distribution function (PDF) analysis shows that the amorphous frameworks retain the local structural motifs of their crystalline analogues despite the loss of long-range order. Relative to crystalline UiO-66, the directly synthesised amorphous UiO-66 exhibits a reduced but permanent porosity (BET surface area 286 vs. 997 m2 g−1 CO2-accessible pore volume 0.196 vs. 0.519 cm3 g−1), together with a high concentration of defects, consistent with a cluster : linker ratio of 1 : 5.3 compared with 1 : 6 for the ideal crystalline framework. In the esterification of levulinic acid, amorphous UiO-66 reaches 87.7% conversion to methyl levulinate after 3 h, compared with 75.5% for crystalline UiO-66, and retains 95% of its initial activity after five catalytic cycles (vs. 86% for the crystalline analogue). These results demonstrate that direct, water-based synthesis provides access to functional, porous, and highly defective amorphous UiO-66 materials with catalytic performance comparable to or exceeding that of their crystalline counterparts under the conditions studied.