Hidden in not-so-plain sight: altermagnets
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Spin and orbital magnetism by light in rutile altermagnets
While the understanding of altermagnetism is still at a very early stage, it is expected to play a role in various fields of condensed matter research, for example spintronics, caloritronics and superconductivity. In the field of optical magnetism, it is still unclear to which extent altermagnets as a class can exhibit a distinct behavior. Here we choose RuO2, a prototype metallic altermagnet with a giant spin splitting, and CoF2, an experimentally known insulating altermagnet, to study the light-induced magnetism in rutile altermagnets from first-principles. We demonstrate that in the non-relativisic limit the allowed sublattice-resolved orbital response exhibits symmetries, imposed by altermagnetism, which lead to a drastic canting of light-induced moments. On the other hand, we find that inclusion of spin-orbit interaction enhances the overall effect drastically, introduces a significant anisotropy with respect to the light polarization and strongly suppresses the canting of induced moments. Remarkably, we observe that the moments induced by linearly-polarized laser pulses in light altermagnets can even exceed in magnitude those predicted for heavy ferromagnets exposed to circularly polarized light. By resorting to microscopic tools we interpret our results in terms of the altermagnetic spin splittings and of their reciprocal space distribution. Based on our findings, we speculate that optical excitations may provide a unique tool to switch and probe the magnetic state of rutile altermagnets.
Structure, control, and dynamics of altermagnetic textures
We present a phenomenological theory of altermagnets, that captures their unique magnetization dynamics and allows modeling magnetic textures in this new magnetic phase. Focusing on the prototypical d-wave altermagnets, e.g., RuO2, we can explain intuitively the characteristic lifted degeneracy of their magnon spectra, by the emergence of an effective sublattice-dependent anisotropic spin stiffness arising naturally from the phenomenological theory. We show that as a consequence the altermagnetic domain walls, in contrast to antiferromagnets, have a finite gradient of the magnetization, with its strength and gradient direction connected to the altermagnetic anisotropy, even for 180° domain walls. This gradient generates a ponderomotive force in the domain wall in the presence of a strongly inhomogeneous external magnetic field, which may be achieved through magnetic force microscopy techniques. The motion of these altermagentic domain walls is also characterized by an anisotropic Walker breakdown, with much higher speed limits of propagation than ferromagnets but lower than antiferromagnets.
Altermagnetic perovskites
Altermagnet is a class of antiferromagnets, which shows a staggered spin ordering with wave vector q = 0, while its net magnetization is canceled out in the limit of zero relativistic spin-orbit coupling. The simplest case is when the up and down spins are ordered on two crystallographically equivalent sublattice sites within the unit cell that are not connected by translation, and consequently, the system breaks the macroscopic time-reversal symmetry. Accordingly, it exhibits non-relativistic spin splitting in the energy band and characteristic cross-correlation phenomena between spin, charge, and lattice (orbital) degrees of freedom. This is in contrast to conventional Néel-type antiferromagnets with q ≠ 0 conserving the macroscopic time-reversal symmetry, where the time-reversal operation flipping of spins combined with translation can make the system identical to the original state. Altermagnetism is universally latent in various magnetic materials that have been considered simple collinear-type antiferromagnets. In this article, we focus on perovskites with the chemical formula ABX3, which are typical playgrounds for strongly correlated electron systems, and overview their altermagnetic aspects that have been overlooked in the past researches, based on microscopic model studies revealing the mechanisms of their properties. We display that a combination of a variety of antiferromagnetic ordering and the commonly-seen lattice distortions in perovskites gives rise to a non-relativistic spin splitting whose mechanism does not rely on the spin-orbit coupling and its consequent spin current generation, and the anomalous Hall effect in the presence of the spin-orbit coupling.
Airborne optical imaging technology: a road map in CIOMP
Airborne optical imaging can flexibly obtain the intuitive information of the observed scene from the air, which plays an important role of modern optical remote sensing technology. Higher resolution, longer imaging distance, and broader coverage are the unwavering pursuits in this research field. Nevertheless, the imaging environment during aerial flights brings about multi-source dynamic interferences such as temperature, air pressure, and complex movements, which forms a serious contradiction with the requirements of precision and relative staticity in optical imaging. As the birthplace of Chinese optical industry, the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) has conducted the research on airborne optical imaging for decades, resulting in rich innovative achievements, completed research conditions, and exploring a feasible development path. This article provides an overview of the innovative work of CIOMP in the field of airborne optical imaging, sorts out the milestone nodes, and predicts the future development direction of this discipline, with the aim of providing inspiration for related research.
Uneven diffusion: a multi-scale analysis of rural settlement evolution and its driving forces in China from 2000–2020
In recent years, the spatial and temporal patterns of rural settlement expansion in China have shifted significantly due to rapid urbanization and industrialization. This study examines rural settlement expansion in China from 2000 to 2020, using the Landscape Expansion Index (LEI) and GIS spatial analysis to assess changes in land use scale and related factors. The findings reveal that: (1) From 2000 to 2020, China saw a rapid and large-scale expansion of rural settlements, with the total area increasing by 40,322.74 km², 87.42% of which resulted from outlying expansion, indicating a clear diffusion trend. (2) The movement of rural settlements has predominantly followed a southeast–northwest axis, focusing on the middle reaches of the Yangtze River, with a clockwise rotation shift. (3) Settlement expansion has been primarily concentrated in low-elevation, waterfront, and road-adjacent areas, where GDP per capita and population density significantly influence settlement patterns. These results offer valuable insights for optimizing the spatial distribution and industrial restructuring of rural settlements, as well as for guiding rural spatial planning and industrial policy development.
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