Chiral Phonons Arising from Chirality-Selective Magnon-Phonon Coupling.

The interplay of topology and chirality in non-symmorphic chiral crystals unveils novel quantum phenomena such as chiral anomalies and exotic fermionic excitations with extended Fermi arcs. In this study, we unify two seemingly distinct concepts topological phonons, arising from non-trivial band topology, and chiral phonons, associated with circular polarization and non-zero angular momentum through the lens of Weyl phonons in chiral CoGe phase. Usingfirst-principlescalculations, symmetry analysis, and effective modeling, we reveal the entanglement of chiral and topological phonons in theP213phase of CoGe. We show that crystal chirality dictates topological charge, mirror enantiomers host Weyl phonons with opposite charges, distinct surface states, and reversed arc connections. Remarkably, we identify quadruple two-fold degenerate Weyl nodes with a Chern number of ±4 the highest reported for phononic systems. Due to theC3rotational symmetry along the Γ-Rdirection, chiral phonon modes emerge naturally, with circular polarization reversing under mirror operations mirroring the topological charge reversal. We propose helicity-resolved Raman spectroscopy as a powerful tool to detect these chiral phonons, linking circular polarization to Berry curvature and phonon angular momentum. Additionally, we explore a distorted kagome lattice phase (space group P-62m) of CoGe, predicting Dirac nodal lines and triply degenerate phonon points. In this achiral structure,valley-selective chiral phononsemerge at ±K(±H) points, exhibiting opposite circular polarization. Our findings establish CoGe as a versatile platform to explore a wide range of phononic topological quasiparticles, including spin-1/2 Weyl, spin-1 Weyl, charge-2 Dirac, charge-4 Weyl, Dirac nodal lines,type-I and type-III quadratic nodal points, and triply degenerate nodal points.

Chiral symmetry breaking of phonons plays an essential role in emergent quantum phenomena owing to its strong coupling to spin degree of freedom. However, direct experimental evidence of the chiral phonon–spin coupling is lacking. In this study, we report a chiral phonon–mediated interlayer exchange interaction in atomically controlled ferromagnetic metal (SrRuO3)–nonmagnetic insulator (SrTiO3) heterostructures. Owing to the unconventional interlayer exchange interaction, we have observed rotation of spins as a function of nonmagnetic insulating spacer thickness, resulting in a spin spiral state. The chiral phonon–spin coupling is further confirmed by phonon Zeeman effect. The existence of the chiral phonons and their interplay with spins along with our atomic-scale heterostructure approach unveil the crucial roles of chiral phonons in magnetic materials.

Magnetic materials are of great importance for energy harvesting and conversion and information technologies, as exemplified by permanent magnets and spintronic applications, respectively. There is a strong impetus to develop new high performance magnets to satisfy the increasing requirements of such technologies. On the other hand, high-throughput (HTP) screening based on density functional theory calculations provides an efficiency way to predict novel materials with target properties. Therefore, it is meaningful to accelerate the development of functional magnetic materials by using the high-throughput screening method. In this thesis, we endeavored to perform HTP engineering of both 3D and 2D magnetic materials, focusing on spintronic and permanent magnet applications. For the spintronic applications, based on HTP density functional theory calculations, we performed a systematic screening by examining peculiar electronic structure. For instance, we have done HTP screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys XX'YZ. Following an empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we have identified 70 so far unreported SGSs, confirmed by explicit electronic structure calculations with proper magnetic ground states, of which 17 candidates have a distance to the convex hull smaller than 0.10 eV/atom. It is demonstrated that all four types of SGSs can be realized, defined based on the spin characters of the touching bands around the Fermi energy. Particularly, it is found that the type-II SGSs exhibit promising transport properties for spintronic applications, such as large anisotropic magnetoresistance and anomalous Nernst effects driven by spin-orbit coupling. On the other hand, as to 2D materials, we carried out a systematic HTP screening for in-plane ordered MXene (i-MXene), which can be obtained by etching the main group element (A) away from the recently synthesized in-plane ordered MAX (i-MAX) phase. Such 2D i-MXenes provide us a new playground for 2D magnetic materials. It is observed that the spin configurations, hence the magnetic anisotropy, can be tuned by strain. We found that five i-MXenes have a significantly large out-of-plane magnetic anisotropy energy (>0.5 meV/f.u.), and the four ferromagnetic candidates have high Curie temperature based on the 2D Ising model. Additionally, we found i-MXene can realize large Seebeck effect, antiferromagnetic topological insulator, and spin-gapless semiconductors, making them interesting for future studies. In terms of designing permanent magnets, we carried out a systematic HTP screening for rare-earth free permanent magnets by incorporating light interstitials (H, B, C, N) into magnetic full Heusler alloys. We successfully identified 32 candidates with an out-of-plane magnetic anisotropy larger than 0.4 MJ/m$^3$ as well as 10 cases with large in-plane anisotropy. Detailed analysis reveals that the interstitials are very efficient in inducing global tetragonal distortions, whereas the local chemical bonding and changes in the crystalline environment result in significant enhancement of MAE. We strongly believe this provides an efficient way to tailor MAE, and such newly predicted permanent magnet candidates are promising gap magnets between rare earth based Sm-Co and Nd-Fe-B and the transition metal based AlNiCo and ferrite. To summarise, in addition to screening over various types of functional magnetic materials, my work provides valuable solutions to the pending challenges of HTP design of magnetic materials. We have established automated workflows, which can be easily applied on the other magnetic materials. This paves the way for further design of advanced magnetic materials with optimal performance

Two-dimensional van der Waals materials have attracted considerable attention because of their promising applications in spintronic devices. This paper reports on first-principles calculations of the electronic structure and ferromagnetism of Cr2Ge2Te6 with surface Ge vacancies. These vacancies are found to remove the bandgap and induce metallicity in Cr2Ge2Te6 that persists with decreasing Ge-vacancy concentration. Meanwhile, the Ge vacancies cause an unexpected sharp increase in the magnetic anisotropy energy compared to that of perfect Cr2Ge2Te6. More importantly, how thickness affects the magnetic anisotropy energy is studied to show that the latter oscillates upon increasing the number of layers, thereby switching between the out-of-plane and in-plane magnetization directions. The present findings regarding a surface-vacancy-tuned bandgap and magnetism controlled by the layer thickness in a two-dimensional van der Waals magnet could lead to potential applications in next-generation magnetic memory storage, sensors, and spintronics.

The intrinsic anomalous Hall effect in ferromagnetic materials depends on the spin–orbit-coupling-induced effect in the electronic structure. The presence of anomalous Hall effect can be realized from the existence of the finite Berry curvature. Since the Heusler material Pt2MnGa is ferromagnetic in its collinear state and no report on its transport properties has yet been made, the anomalous Hall effect in this system is investigated using a combined approach of the density functional theory and maximally localized Wannier function. Analysis of the Berry curvature reveals positive and negative peaks along the high symmetry k-points. These peaks together lead to a significant negative anomalous Hall conductivity, which makes the material a promising candidate for applications in spintronics.

The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of $\sim$560 K has been widely used in microwave and spintronic devices. Anomalous features in the spin Seeback effect (SSE) voltages have been observed in Pt/YIG and attributed to the magnon-phonon coupling. Here we use inelastic neutron scattering to map out low-energy spin waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic field. By comparing the zero and 9.1 T data, we find that instead of splitting and opening up gaps at the spin wave and acoustic phonon dispersion intersecting points, magnon-phonon coupling in YIG enhances the hybridized scattering intensity. These results are different from expectations of conventional spin-lattice coupling, calling for new paradigms to understand the scattering process of magnon-phonon interactions and the resulting magnon-polarons.

The significant anomalous Nernst effect (ANE) in two-dimensional van der Waals magnetic materials provides potential applications for the design of novel thermoelectric devices and spintronic devices. Fe3GaTe2 is a newly discovered two-dimensional magnetic material, which has a similar structure to Fe3GeTe2 but a higher Curie temperature. In this study, we present the observation of the anomalous Hall effect (AHE) and ANE in Fe3GaTe2. It has an above-room-temperature Curie temperature of about 340K, and a Nernst thermopower of Sxy~1.2μV/K at 100K, which is much higher than those in ordinary magnets. Most importantly, the bipolarity of the AHE of Fe3GaTe2 holds the potential in thermoelectric and spintronic applications.

The anomalous transverse transport properties, including the anomalous Hall and Nernst effects, are crucial for probing the topological band features in magnetic materials. In this study, we present a comprehensive investigation of the magnetic, electrical, and thermal transport, as well as the electronic band structure, of the noncollinear ferromagnet ${\mathrm{PrMn}}_{2}{\mathrm{Ge}}_{2}$. Our findings reveal that ${\mathrm{PrMn}}_{2}{\mathrm{Ge}}_{2}$ exhibits prominent anomalous Hall and Nernst effects. Scaling analysis suggests that the anomalous Hall effect is predominantly influenced by the intrinsic Berry curvature contribution. Additionally, the large anomalous Nernst signal exceeds the magnetization scaling relations observed in conventional ferromagnets, and the calculated anomalous Nernst conductivity exhibits a $T\text{ln}T$ relation. These observations align with the behavior seen in other magnetic topological materials, further suggesting a substantial intrinsic Berry curvature effect. Complemented by detailed theoretical calculations, we clarify the electronic band properties and confirm a substantial net Berry curvature near the Fermi level in this compound. Our work elucidates that the large anomalous Hall and Nernst effects of ${\mathrm{PrMn}}_{2}{\mathrm{Ge}}_{2}$ are dominated by an intrinsic mechanism and presents it as a probable magnetic topological material with a significant Berry phase effect in the momentum space.

Quantum field theories with a complex action suffer from a sign problem in stochastic nonperturbative treatments, making many systems of great interest – such as polarized or mass-imbalanced fermions and QCD at finite baryon density – extremely challenging to treat numerically. Another such system is that of bosons at finite angular momentum; experimentalists have successfully achieved vortex formation in ultracold bosonic atoms, and have measured quantities of interest such as density profiles and the moment of inertia. However, the treatment of superfluids requires the use of complex bosons, making the usual numerical methods unusable. In this work, we apply complex Langevin, a method that has gained much attention in lattice QCD, to the calculation of basic properties of interacting bosons at finite angular momentum. We show preliminary results for the angular momentum and moment of inertia and benchmark calculations in the noninteracting limit.

The transverse voltage generated by a temperature gradient in a perpendicularly applied magnetic field, termed the Nernst effect, has promise for thermoelectric applications and for probing electronic structure. In magnetic materials, an anomalous Nernst effect (ANE) is possible in a zero magnetic field. We report a colossal ANE in the ferromagnetic metal UCo0.8Ru0.2Al, reaching 23 microvolts per kelvin. Uranium's 5f electrons provide strong electronic correlations that lead to narrow bands, a known route to producing a large thermoelectric response. In addition, uranium's strong spin-orbit coupling produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that in UCo0.8Ru0.2Al at least 148 Weyl nodes, and two nodal lines, exist within 60 millielectron volt of the Fermi level. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Two-dimensional (2D) magnetic materials exhibit a wide array of fascinating magnetic properties, making them highly attractive for spintronic applications such as high-density nonvolatile memories and multifunctional nano-devices. Recently, chromium tellurides (CrTex) have attracted significant attention due to their metallic band structure, strong magnetic anisotropy, and tunable exchange couplings. The unique tunability of magnetic properties in a metallic ground state makes CrTexa promising platform for generating, controlling, and manipulating spin currents. This review summarizes recent advances in large-scale 2D magnetic CrTexepitaxial thin films, emphasizing synthesis techniques that produce high-quality, large-area films. It explores the role of self-intercalation and heterostructure engineering in tailoring the magnetic and structural properties of these materials. We also review the band structure, magnetic characteristics, and spin dynamics of CrTex, with a particular emphasis on thickness-dependent band dispersion, magnetic anisotropy, and the emergence of skyrmions. Moreover, this review highlights the applications of CrTexin spintronics, covering the anomalous Hall effect, topological Hall effect, spin valves, and spin-orbit torque devices. The goal of this review is to furnish readers with a comprehensive overview of the intriguing properties of CrTexcompounds and to inspire further innovative studies on the vast potential of 2D magnetic materials for next-generation spintronic and quantum devices.

Spintronics, leveraging electron spin for information processing, promises substantial advancements in energy-efficient computing. Van der Waals (vdW) magnetic materials, with their unique-layered structures and exceptional magnetic properties, have emerged as pivotal components in this field. This report explores the current-based control of vdW magnets, focusing on the spin–orbit torque (SOT) mechanism, which is crucial for spintronic applications. Key studies on Fe3GaTe2/Pt and Fe3GaTe2/WTe2 heterostructures are highlighted, demonstrating efficient SOT switching at room temperature. The advantages of vdW magnets for SOT switching, including high spin-torque efficiencies and superior interface quality, are discussed. The report also examines future directions, such as wafer-scale growth techniques, materials design for enhanced Curie temperatures (Tc), and the development of magneto tunnel junctions using all-vdW materials. These advancements underscore the potential of vdW magnetic materials in developing scalable, high-performance spintronic devices, paving the way for significant breakthroughs in energy-efficient computing.Graphical abstract

Ultra-thin ferromagnets with large magnetic anisotropy by assembling MnCl3 superatoms on SbAs monolayer

Spin–orbit coupling (SOC) combined with broken inversion symmetry plays a key role in inducing Rashba effect. The combined spontaneous polarization and Rashba effect enables controlling a material's spin degrees of freedom electrically. In this work, we investigated an electronic band structure for several combinations of TiX2 monolayers (X = Te, S, and Se): TiTe2/TiSe2, TiTe2/TiS2, and TiSe2/TiS2. Based on the observed orbital hybridization between the different monolayers in these heterostructures (HSs), we conclude that the most significant Rashba splitting occurs in TiSe2/TiS2. Subsequently, we used fluorine (F) as an adatom over the surface of TiSe2/TiS2 at hollow and top sites of the surface to enhance the Rashba intensity, as the F adatom induces polarization due to the difference in charge distribution. Furthermore, by increasing the number of F atoms on the surface, we reinforced the band splitting, i.e., we observe Rashba splitting accompanied by Zeeman splitting at the valence-band edge states. Berry curvatures at K and K′ with equal and opposite nature confirm the existence of valley polarization. The computationally observed properties suggest that these HSs are promising candidates for spin–valley Hall effect devices and other spintronic applications.

Prediction of high spin polarization and perpendicular magnetic anisotropy in two dimensional ferromagnetic Mn2CXX’ (X, X′=F, Cl, Br, I) Janus monolayers