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Crystal lattices with tetragonal or hexagonal structure often exhibit structural transitions in response to external stimuli 1 . Similar behaviour is anticipated for the lattice forms of topological spin textures, such as lattices composed of merons and antimerons or skyrmions and antiskyrmions (types of vortex related to the distribution of electron spins in a magnetic field), but has yet to be verified experimentally 2 , 3 . Here we report real-space observations of spin textures in a thin plate of the chiral-lattice magnet Co 8 Zn 9 Mn 3 , which exhibits in-plane magnetic anisotropy. The observations demonstrate the emergence of a two-dimensional square lattice of merons and antimerons from a helical state, and its transformation into a hexagonal lattice of skyrmions in the presence of a magnetic field at room temperature. Sequential observations with decreasing temperature reveal that the topologically protected skyrmions remain robust to changes in temperature, whereas the square lattice of merons and antimerons relaxes to non-topological in-plane spin helices, highlighting the different topological stabilities of merons, antimerons and skyrmions. Our results demonstrate the rich variety of topological spin textures and their lattice forms, and should stimulate further investigation of emergent electromagnetic properties. A magnetically induced two-dimensional square lattice of merons and antimerons is observed in real space, along with its transformation into a hexagonal lattice of skyrmions at room temperature.

Electrons in conventional metals become less mobile under the influence of electron correlation. Contrary to this empirical knowledge, we report here that electrons with the highest mobility ever found in known bulk oxide semiconductors emerge in the strong-correlation regime of the Dirac semimetal of perovskite CaIrO 3 . The transport measurements reveal that the high mobility exceeding 60,000 cm 2 V −1 s −1 originates from the proximity of the Fermi energy to the Dirac node (ΔE < 10 meV). The calculation based on the density functional theory and the dynamical mean field theory reveals that the energy difference becomes smaller as the system approaches the Mott transition, highlighting a crucial role of correlation effects cooperating with the spin-orbit coupling. The correlation-induced self-tuning of Dirac node enables the quantum limit at a modest magnetic field with a giant magnetoresistance, thus providing an ideal platform to study the novel phenomena of correlated Dirac electron. Electron correlation normally makes electrons less mobile, but it is still not clear when correlation becomes very strong in Dirac semimetals. Here, Fujioka et al. report a very high electron mobility exceeding 60,000 cm 2 V −1 s −1 in correlated Dirac semimetal of perovskite CaIrO3, due to the enhanced electron correlation nearby the Mott transition.

From magnetism, ferroelectricity and superconductivity to electrical and thermal properties, oxides show a broad range of phenomena of fundamental as well as practical relevance. Reviewed here are the emergent phenomena arising at the interface between oxide materials, which have attracted considerable interest based on advances in thin-film deposition techniques. Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.

Electrical rectification is usually achieved by layering p-type and n-type materials, but experiments now demonstrate rectification in a bulk polar semiconductor that has inversion-symmetry breaking and strong Rashba spin–orbit coupling. Noncentrosymmetric conductors are an interesting material platform, with rich spintronic functionalities 1 , 2 and exotic superconducting properties 3 , 4 typically produced in polar systems with Rashba-type spin–orbit interactions 5 . Polar conductors should also exhibit inherent nonreciprocal transport 6 , 7 , 8 , in which the rightward and leftward currents differ from each other. But such a rectification is difficult to achieve in bulk materials because, unlike the translationally asymmetric p–n junctions, bulk materials are translationally symmetric, making this phenomenon highly nontrivial. Here we report a bulk rectification effect in a three-dimensional Rashba-type polar semiconductor BiTeBr. Experimentally observed nonreciprocal electric signals are quantitatively explained by theoretical calculations based on the Boltzmann equation considering the giant Rashba spin–orbit coupling. The present result offers a microscopic understanding of the bulk rectification effect intrinsic to polar conductors as well as a simple electrical means to estimate the spin–orbit parameter in a variety of noncentrosymmetric systems.

The Weyl semimetal (WSM), which hosts pairs of Weyl points and accompanying Berry curvature in momentum space near Fermi level, is expected to exhibit novel electromagnetic phenomena. Although the large optical/electronic responses such as nonlinear optical effects and intrinsic anomalous Hall effect (AHE) have recently been demonstrated indeed, the conclusive evidence for their topological origins has remained elusive. Here, we report the gigantic magneto-optical (MO) response arising from the topological electronic structure with intense Berry curvature in magnetic WSM Co 3 Sn 2 S 2 . The low-energy MO spectroscopy and the first-principles calculation reveal that the interband transitions on the nodal rings connected to the Weyl points show the resonance of the optical Hall conductivity and give rise to the giant intrinsic AHE in dc limit. The terahertz Faraday and infrared Kerr rotations are found to be remarkably enhanced by these resonances with topological electronic structures, demonstrating the novel low-energy optical response inherent to the magnetic WSM. The evidence of topological origin for the recently observed anomalous Hall effect remains elusive. Here, the authors report that the resonance of the optical Hall conductivity resulted from topological electronic structure gives rise to the large intrinsic anomalous Hall effect in the magnetic Weyl semimetal Co 3 Sn 2 S 2 .

An engineered topological insulator-based heterostructure is reported to show transport properties consistent with the realization of an axion insulator. The axion insulator which may exhibit an exotic quantized magnetoelectric effect 1 , 2 , 3 , 4 , 5 , 6 is one of the most interesting quantum phases predicted for the three-dimensional topological insulator (TI). The axion insulator state is expected to show up in magnetically doped TIs with magnetizations pointing inwards and outwards from the respective surfaces. Towards the realization of the axion insulator, we here engineered a TI heterostructure in which magnetic ions (Cr) are modulation-doped only in the vicinity of the top and bottom surfaces of the TI ((Bi,Sb) 2 Te 3 ) film 7 . A separation layer between the two magnetic layers weakens interlayer coupling between them, enabling the magnetization reversal of individual layers. We demonstrate the realization of the axion insulator by observing a zero Hall plateau (ZHP) (where both the Hall and longitudinal conductivity become zero) in the electric transport properties, excluding the other possible origins for the ZHP 8 , 9 , 10 . The manifestation of the axion insulator can lead to a new stage of research on novel magnetoelectric responses in topological matter.

A magnetic skyrmion is a topologically stable particle-like object that appears as a vortex-like spin texture at the nanometer scale in a chiral-lattice magnet. Skyrmions have been observed in metallic materials, where they are controllable by electric currents. Here, we report the experimental discovery of magnetoelectric skyrmions in an insulating chiral-lattice magnet Cu₂OSeO₃ through Lorentz transmission electron microscopy and magnetic susceptibility measurements. We find that the skyrmion can magnetically induce electric polarization. The observed magnetoelectric coupling may potentially enable the manipulation of the skyrmion by an external electric field without losses due to joule heating.

Giant coupling between magnetism and phonons — or the giant thermal Hall effect — is reported in the insulating polar magnet (Zn x Fe 1− x ) 2 Mo 3 O 8 . Multiferroics 1 , 2 , in which dielectric and magnetic orders coexist and couple with each other, attract renewed interest for their cross-correlated phenomena 3 , 4 , offering a fundamental platform for novel functionalities. Elementary excitations in such systems are strongly affected by the lattice–spin interaction, as exemplified by the electromagnons 5 , 6 and the magneto-thermal transport 7 , 8 , 9 , 10 . Here we report an unprecedented coupling between magnetism and phonons in multiferroics, namely, the giant thermal Hall effect. The thermal transport of insulating polar magnets (Zn x Fe 1− x ) 2 Mo 3 O 8 is dominated by phonons, yet extremely sensitive to the magnetic structure. In particular, large thermal Hall conductivities are observed in the ferrimagnetic phase, indicating unconventional lattice–spin interactions and a new mechanism for the Hall effect in insulators. Our results show that the thermal Hall effect in multiferroic materials can be an effective probe for strong lattice–spin interactions and provide a new tool for magnetic control of thermal currents.

Dissipation-less electric control of magnetic state variable is an important target of contemporary spintronics. The non-volatile control of magnetic skyrmions, nanometre-sized spin-swirling objects, with electric fields may exemplify this goal. The skyrmion-hosting magnetoelectric chiral magnet Cu 2 OSeO 3 provides a unique platform for the implementation of such control; however, the hysteresis that accompanies the first-order transition associated with the skyrmion phase is negligibly narrow in practice. Here we demonstrate another method that functions irrespective of the transition boundary. Combination of magnetic-susceptibility measurements and microwave spectroscopy reveals that although the metastable skyrmion lattice is normally hidden behind a more thermodynamically stable conical phase, it emerges under electric fields and persists down to the lowest temperature. Once created, this metastable skyrmion lattice remains without electric fields, establishing a bistability distinct from the transition hysteresis. This bistability thus enables non-volatile electric-field control of the skyrmion lattice even in temperature/magnetic-field regions far from the transition boundary. Magnetoelectric material Cu 2 OSeO 3 hosts topologically-protected skyrmion magnetization textures, however only in an impractically narrow temperature range. Here, the authors demonstrate how the metastable skyrmion phase may be extended to low temperatures by an applied electric field.

Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.