Phonon chirality and Hall effect in Janus materials

We provide a topological understanding of the phonon Hall effect in dielectrics with Raman spin-phonon coupling. A general expression for phonon Hall conductivity is obtained in terms of the Berry curvature of band structures. We find a nonmonotonic behavior of phonon Hall conductivity as a function of the magnetic field. Moreover, we observe a phase transition in the phonon Hall effect, which corresponds to the sudden change of band topology, characterized by the altering of integer Chern numbers. This can be explained by touching and splitting of phonon bands.

Circulation in the liquid core and coupling with the mantle

Discovering compact, stable, and easily controllable nanoscale non-trivial topological magnetic structures, such as magnetic skyrmions, is the key to developing next-generation high-density, high-speed, and low-energy non-volatile information storage devices. Based on the topological generation mechanism, magnetic skyrmions can be generated through the Dzyaloshinskii–Moriya interaction (DMI) caused by breaking space-reversal symmetry. Two-dimensional (2D) non-centrosymmetric Janus structurecan generate vertical built-in electric fields to break spatial inversion symmetry. Therefore, seeking for 2D Janus material with intrinsic magnetism is fundamental to develop the novel chiral magnetic storage technologies. In this work, we combine detailed machine learning techniques and first-principle calculations to investigate the magnetism of the unexplored 2D Janus material. We first collect 1179 2D hexagonal ABC-type Janus materials based on the Materials Project database, and use elemental composition as feature descriptors to construct four machine learning models: random forest (RF), gradient boosting decision trees (GBDT), extreme gradient boosting (XGB), and extra trees (ET). These algorithms and models are constructed to predict lattice constants, formation energy, and magnetic moment, via hyperparameter optimization and ten-fold cross-validation. The GBDT exhibits the highest accuracy and best prediction performance for magnetic moment classification. Subsequently, the collected data of 82018 yet-undiscovered 2D Janus materials, are input into the trained models to generate 4024 high magnetic moment 2D Janus materials with thermal stability. First-principles calculations are employed to validate random sample of 13 Janus materials with high magnetic moment. This study provides an effective machine learning framework for classifying the magnetic moments and screening highthroughput 2D Janus structures, thereby accelerating the exploration of their magnetic properties. The datasets provided in this work are available from <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://doi.org/10.57760/sciencedb.j00213.00072">https://doi.org/10.57760/sciencedb.j00213.00072</ext-link>.

THERE is a strong (0.93) correlation1,2 between the change in the length of day and the change in westward velocity of the eccentric dipole or “magnetic centre” (as defined by Schmidt3) of the geomagnetic field, presumably because of the conservation of the angular momentum of the Earth. If the motion of the eccentric dipole is related to a motion somewhere in the core in which the magnetic field is “frozen” into the fluid, then a change in westward motion and angular momentum in the core must be compensated for by a change in angular momentum of the mantle. This appears as a change in the length of day, as determined astronomically.

When light propagates through a transparent medium, both the orbital and spin angular momenta of the light may change. It is shown by explicit calculation that the change in the angular momentum of the light is exactly equal and opposite to the change in the internal angular momentum of the atoms in the transparent medium. The centre-of-mass motion of the atoms does not play any significant role in this interchange of angular momentum.

Recent studies have demonstrated that the thermal Hall effect (THE) can originate from magnons (magnon Hall effect), phonons (phonon Hall effect), or their combination (magnon-polaron Hall effect). The magnon-polaron Hall effect, first observed in Fe2Mo3O8, is particularly intriguing as its thermal Hall signal can be remarkably large. In this study, we explore the THE in MnPS3, an insulating antiferromagnetic material exhibiting a spin-flop (SF) transition and significant magnetoelastic coupling, making it a strong candidate for studying the THE originating from spin-lattice coupling. We report an exceptionally large thermal Hall angle down to 4 K and show that it cannot be accounted for by standard calculations based on the intrinsic magnon-polaron Berry curvature. Our findings provide an in-depth analysis of the role of the SF transition in the thermal properties of MnPS3and call for further theory development on magnon-phonon coupling and scattering to reveal their influence on transverse heat transport.

A mechanism for the phonon Hall effect (PHE) in nonmagnetic insulators under an external magnetic field is theoretically studied. PHE is known in (para)magnetic compounds, where the magnetic moments and spin-orbit interaction play an essential role. In sharp contrast, we here discuss that the PHE also occurs in nonmagnetic band insulators subject to the magnetic field. We find that a correction to the Born-Oppenheimer approximation gives rise to a Raman-type interaction between the magnetic field and the phonons; this interaction gives rise to the Berry curvature of a phonon band. This Berry curvature results in the finite thermal Hall conductivity κ_{H} in nonmagnetic band insulators. The value of κ_{H} is calculated for square and honeycomb lattices. The order of the magnitude estimation for κ_{H} is given for Si at room temperature.

The article discusses problems of dynamic bending for beams of semi-infinite length. To solve such problems, the article uses a method based on the implementation of the conservation laws, namely, the law of energy conservation, the law of change in momentum and the law of change in angular momentum. The results obtained are compared with the analytical solution for the problem of a semi-infinite beam motion loaded at the free end with a transverse force. The peculiarity of this solution is that the change in the stress-strain state of the rod is characterized by a wave front. It is considered that all changes in the state of the beam occur at an infinite speed. All designed solutions are characterized by the presence of a wave front in the beam. It is shown that, in contrast to the transfer of longitudinal disturbances along the length of the beam, which occur at a constant speed, bending disturbances propagate at a variable speed, and, with increasing time, this speed decreases and tends to zero at an infinitely distant point of the beam. It was discovered that the propagation velocity of the wave front during the transfer of concentrated force and concentrated moment differs from each other. In this case, the speed of transverse force transfer is almost twice as high as the speed of the wave front from the bending moment.

Since the work of Zhao et al. in 2014, two-dimensional (2D) transition-metal monocarbides (TMMCs) have emerged as atomically thin materials. With unique metal–carbon bonding properties, 2D TMMCs exhibit enhanced structural stability and excellent electronic properties, making them ideal candidates for spintronics and flexible mechanical applications. However, the structural diversity and the phonon characteristics of 2D TMMCs remain underexplored. Here, we carry out a systematic first-principles investigation of five prototypical TMMC lattices, revealing 27 dynamically stable monolayers—seven of which host clean topological phonons and six of which host chiral phonons. The honeycomb-RuC and tetragonal-RuC (H-RuC and T-RuC) are selected as typical examples for detailed phonon spectral analyses and to assess their structural stabilities. Both H-RuC and T-RuC support topological phonons and phononic edge states, whereas chiral phonons arise exclusively at the K points in H-RuC due to its threefold rotational symmetry. Through phonon free-energy calculations coupled with thermodynamic modeling, T-RuC is preferred in the 2000 K temperature range and in biaxial compression, while H-RuC is more stable under tensile strain conditions. Our findings not only expand the 2D TMMC materials family but also illuminate the relationship between structural phases and phononic functionality, laying the groundwork for their integration into phononic applications in the future.

On the validity of the geostrophic approximation for calculating the changes in the angular momentum of the core

Due to the breaking of the time reversal symmetry and spatial inversion symmetry, hexagonal ferrovalley materials have intrinsic large valley polarization. Model analysis shows that tuning the two different band gaps of valleys can realize phase transitions between ferrovalley semiconductors, half valley metals, and valley-polarized quantum anomalous Hall semiconductors. Through first-principle calculations, monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br), which exhibit valley splitting at the top valence band and the bottom conduction band, are predicted to achieve this valley-dependent topological phase transition. Due to the different orbital proportions of d orbitals, the valley splitting at the top valence band is much greater than that at the bottom conduction band. Strain can regulate the interaction between orbitals, thus producing valley-dependent band inversion, leading to the quantum spin or valley Hall effect. The chiral edge states are demonstrated under appropriate biaxial strain. The topological phase transition is related to the inversion of the band structure and Berry curvatures at K and K′ valleys. These results have certain significance for the design of two-dimensional valley-dependent quantum materials and the application of valleytronic devices.

Germanene, though with Dirac valleys, is not deemed as a good valleytronic material due to its minute band gap, negligible spin–orbit coupling and spatial inversion symmetry. In comparison of interfacing germanene with MoS2, we proposed that forming heterostructure with Tl2S, an anti-MoS2 material with two outer heavy metal layers, could be more effective in raising spin–orbit coupling and band gap in germanene due to the direct Ge-metal contact. By carrying out first-principles calculations, we studied the valleytronic properties of germanene enhanced by monolayer Tl2S. It is found that the Ge–Tl direct interaction is strong to a proper extent so that the valleys of germanene still persist and simultaneously the valley gap is drastically increased from 23 to 370 meV. The valley spin splitting, being zero in pristine germanene, become 45 meV, which is opposite at inequivalent valleys owing to the time reversal symmetry. The inversion symmetry of germanene is broken by Tl2S, resulting in large Berry curvature near the valleys and hence laying the ground for Berry phase physics in germanene, e.g., valley spin Hall effect and valley–spin locking, as revealed in our study. The calculations found a perfect valley-selective circular dichroism, by which the valley and spin degrees of freedom can be manipulated selectively and correlatively.

Altermagnets exhibit nonrelativistic spin splitting with net-zero magnetic moments, making them advantageous for spintronic devices with miniaturized size and high integration. Developing general methods to design altermagnets, particularly in a low dimension, is highly desirable. Here, we propose that breaking the spatial inversion symmetry of crystals can produce altermagnetism in antiferromagnetic monolayers. By applying Janus structurization to two-dimensional (2D) FeSe-type monolayers, 41 polar altermagnets were successfully identified through first-principles calculations, confirming the feasibility of our proposed approach. Furthermore, via systematic screening, we obtained 29 altermagnets with significant spin splitting (>0.5 eV) and high Néel temperatures (above liquid nitrogen temperature). Moreover, using 2D Mn2PSe as an example, we revealed the mechanism of how polarity drives the transformation of antiferromagnets into altermagnets and demonstrated its anisotropic spin current generation and notable spin Hall effect. This work paves a way for realizing high-performance and multifunctional nanoaltermagnets.

Chiral phonons are desirable for applications in spintronics but their generation and control remains a challenge. Here we demonstrate the emergence of truly chiral phonons from selective magnon-phonon coupling in inversion-symmetric magnetic systems. Considering bcc Fe as an example, we quantitatively calculate hybridized magnon-phonon quasiparticle states across the entire Brillouin zone utilizing first-principles calculations. Our findings challenge conventional magnetoelastic interpretations and reveal finite zero-point phonon angular momentum and strong anomalous thermal Hall responses linked to finite (spin) Berry curvatures. Our results further establish that the existence of chiral phonons, particularly along high-symmetry directions, is common in many magnetic materials, offering promising avenues for novel spintronic and phononic devices.

The present chapter gives a model for the change in mass of spinning black holes w.r.t. the angular momentum in XRBs & AGN for maximum spin and concludes that the change in mass of the spinning black holes due to corresponding change in the angular momentum for maximum spinning rate of black holes (a*=1) decreases with increasing mass of the black holes.