Chiral Phonons in Chiral Materials

The concept of chirality makes ubiquitous appearance in nature. Particularly, both a structure and its collective excitations may acquire well defined chiralities. In this work, we reveal an intrinsic connection between the chiralities of a crystal structure and its phonon excitations. We show that the phonon chirality and its propagation direction are strongly coupled with the lattice chirality, which will be reversed when a chiral lattice is switched to its enantiomorph. In addition, distinct from achiral lattices, propagating chiral phonons exist for chiral crystals also on the principal axis through the $\Gamma$ point, which strengthens its relevance to various physical processes. We demonstrate our theory with a 1D helix-chain model and with a concrete and important 3D material, the $\alpha$-quartz. We predict a chirality diode effect in these systems, namely, at certain frequency window, a chiral signal can only pass the system in one way but not the other, specified by the system chirality. Experimental setups to test our theory are proposed. Our work discovers fundamental physics of chirality coupling between different levels of a system, and the predicted effects will provide a new way to control thermal transport and design information devices.Comment: 5 pages, 5 figure

Phonon chirality manipulation mechanism in TMD interlayer-sliding ferroelectrics

As an ideal platform, both the theoretical prediction and first experimental verification of chiral phonons are based on transition-metal dichalcogenide materials. The manipulation of phonon chirality in these materials will have a profound impact on the study of chiral phonons. In this work, we utilize the sliding ferroelectric mechanism to study the phonon chirality manipulation mechanism in transition-metal dichalcogenide materials. Based on first-principles calculations, we study the different effects of interlayer sliding on the phonon properties in bilayer and four-layer MoS$_2$ sliding ferroelectrics. We find that sliding can regulate phonon chirality and Berry curvature, which further affects the phonon angular momentum and magnetization under a temperature gradient and the phonon Hall effect under a magnetic field. Our work connects two emerging fields and opens up a new route to manipulate phonon chirality in transition-metal dichalcogenide materials through the sliding ferroelectric mechanism.Comment: 15 pages, 4 figure

UniDexGrasp: Universal Robotic Dexterous Grasping via Learning Diverse Proposal Generation and Goal-Conditioned Policy

In this work, we tackle the problem of learning universal robotic dexterous grasping from a point cloud observation under a table-top setting. The goal is to grasp and lift up objects in high-quality and diverse ways and generalize across hundreds of categories and even the unseen. Inspired by successful pipelines used in parallel gripper grasping, we split the task into two stages: 1) grasp proposal (pose) generation and 2) goal-conditioned grasp execution. For the first stage, we propose a novel probabilistic model of grasp pose conditioned on the point cloud observation that factorizes rotation from translation and articulation. Trained on our synthesized large-scale dexterous grasp dataset, this model enables us to sample diverse and high-quality dexterous grasp poses for the object point cloud.For the second stage, we propose to replace the motion planning used in parallel gripper grasping with a goal-conditioned grasp policy, due to the complexity involved in dexterous grasping execution. Note that it is very challenging to learn this highly generalizable grasp policy that only takes realistic inputs without oracle states. We thus propose several important innovations, including state canonicalization, object curriculum, and teacher-student distillation. Integrating the two stages, our final pipeline becomes the first to achieve universal generalization for dexterous grasping, demonstrating an average success rate of more than 60\% on thousands of object instances, which significantly outperforms all baselines, meanwhile showing only a minimal generalization gap.Comment: Accepted to CVPR 202

Precessing jet nozzle connecting to a spinning black hole in M87

The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole. The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from General Relativity. With a baseline of 17 years of observations, there was a shift in the jet's transverse position, possibly arising from an eight to ten-year quasi-periodicity. However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years in the position angle variation of the jet. We infer that we are seeing a spinning black hole that induces the Lense-Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation.Comment: 41 pages, 7 figures, 7 table

Operating range of magnetic stabilization flow regime for magnetized fluidized bed with Geldart-B magnetizable and nonmagnetizable particles

The magnetic stabilization flow regime could also be created for Geldart-B nonmagnetizable particles provided some magnetizable particles are introduced and the magnetic field is applied. This study aimed to explore the size (dpM) and density (ppM) effects of magnetizable particles on its operating range. The upper limit (UmbH) could not be determined from the APb-Ug down arrow curve but could from analyzing the variation of APb-fluctuation with increasing Ug. Due to the variation of UmfH (lower limit) with dpM and ppM, both UmbH-UmfH and (UmbH-UmfH)/UmfH were used to quantify the operating range of magnetic stabilization. UmbH-UmfH varied hardly with dpM but increased significantly with decreasing ppM. (UmbH-UmfH)/UmfH increased as dpM or ppM decreased. It was more difficult for the nonmagnetizable particles to escape from the network formed by the smaller/lighter magnetizable particles. For the same magnitude of change, dpM had a stronger effect than ppM on (UmbH-UmfH)/UmfH. Neither UmbH-UmfH nor (UmbH-UmfH)/UmfH varied monotonously with the minimum fluidization velocity of the magnetizable particles, indicating that no straightforward criterion for matching the magnetizable particles to the given nonmagnetizable particles could be established based on their minimum fluidization velocities to maximize the operating range of magnetic stabilization. (c) 2021 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved

Determining minimum fluidization velocity in magnetized fluidized bed with Geldart-B particles

This work aimed to solve the controversies about defining and determining the minimum fluidization velocity (U-mf) in the magnetized fluidized bed (MFB). It should be defined rigorously from the physics behind the secondary phenomena such as bed pressure drop and particle motion. Since U-mf demarcates the transition from the fixed-bed to the fluidization flow regime, it could not be determined from the Delta P-b-U-g down arrow curve but could from the Delta P-b-U-g up arrow curve. Moreover, the initial fixed bed for measuring this curve must meet its definition. For the MFB with purely magnetizable particles, it could be obtained via decreasing U-g gradually from full fluidization to zero at H = 0 kA/m to minimize the wedging effect. For the MFB with magnetizable and nonmagnetizable particles, such a decrease of U-g should be quick to simultaneously minimize the wedging effect and segregation. The as-determined U-mf remained invariant with H in both types of MFBs. (C) 2021 Elsevier B.V. All rights reserved

Segregation in magnetized fluidized bed with Geldart-B magnetizable and nonmagnetizable particles

For magnetized fluidized beds with Geldart-B magnetizable and nonmagnetizable particles operated under the magnetization-LAST mode, segregation occurs and deepens as the magnetic field intensity increases. This work aimed to explore the effects of particle size and density difference on such segregation. The field intensities (His and Hcs) for the segregation to initiate and complete increased as the gas velocity (Ug) increased. Moreover, their increases were steeper at higher gas velocities. The increase of Ug raised not only the collision to the magnetic chains but also their mixing tendency with the nonmagnetizable particles. For the given nonmagnetizable particles and Ug, His and Hcs increased as the size or density of the magnetizable particles decreased, indicating that it was more difficult for the segregation to occur and complete. Additionally, the smaller/lighter the magnetizable particles, the steeper the variations of His and Hcs with Ug. A much stronger magnetic field was required for more of such magnetizable particles to form the desired magnetic chains and keep the desired size. Segregation could only be postponed but completely prevented under the magnetization-LAST operation mode however the two types of particles were matched

Mass transfer intensification between fluidizing gas and Geldart-B nonmagnetizable particles in magnetized fluidized bed

This work aimed to intensify the mass transfer between fluidizing gas and Geldart-B nonmagnetizable particles by simultaneously introducing magnetizable particles and applying the magnetic field. The mere addition of magnetizable particles hardly affected the interphase mass transfer. Moreover, such mass transfer was not improved by the subsequent application of the magnetic field under the magnetization-LAST operation mode. However, the mass transfer was significantly enhanced by the magnetic field in the magnetic stabilization and transition flow regimes under the magnetization-FIRST operation mode. Apparently, the mass transfer intensification depended not only on the magnetic field intensify (H) but also on the operation mode. The gas-solid contact performance in the magnetic stabilization flow regime was always comparable to that in fixed beds. Hence, further increases in H could not raise the magnitude of intensification but only increase the operating range and stability of the magnetic stabilization, which was in metastable equilibrium

Modification of Lime-Fly Ash-Crushed Stone with Phosphogypsum for Road Base

In order to increase the recycling of phosphogypsum waste, this study explored the feasibility of using phosphogypsum to replace some of the lime and aggregate in the lime-fly ash-crushed stone mixture which is a widely used road base material in China. For this purpose, compaction, compressive strength, composition structures, wetting-drying cycle tests, and shrinkage tests were carried out on the lime-fly ash-phosphogypsum-crushed stone composite to investigate its performance. The results indicate that lime-fly ash-crushed stone modified with phosphogypsum has the required strength of the road base material and favourable performances in environment (wetting-drying cycle) stability. The image processing analysis and shrinkage tests demonstrated that phosphogypsum can significantly improve the compactness and shrinkage performance of lime-fly ash-crushed stone mixture. A suitable content of phosphogypsum and a reasonable content of fine aggregate are conducive to improving the roadway engineering properties (i.e., decreasing shrinkage cracks and increasing compressive strength) of lime-fly ash-phosphogypsum-crushed stone composites

State dependence of magnetized fluidized bed reactor on operation mode

There exist four typical modes to operate the magnetized fluidized bed (MFB) reactor: magnetization-FIRST, de fluidization, magnetization-LAST, and de-magnetization. This work aimed to compare the bed states obtained under these operation modes for both the MFB with purely magnetizable particles and the MFB with the binary admixture of magnetizable and nonmagnetizable particles. It was found that the covered operating range could be divided into four zones. Particularly in zones I to III, the bed state depended not only on the magnetic field intensity (H) and superficial gas velocity (Ug) but also on the operation mode, i.e., the bed state was a path function. Such a path-dependence feature resulted from that the MFB therein could have different equilibrium states at the same H and Ug. From our perspective, such a polymorphic characteristic was caused by the internal friction inside the bed. Furthermore, many of these bed states were demonstrated to be metastable. The metastable states in zones II and III behaved like the metastates of water. They evolved directly and quickly into the corresponding stable states after relatively large perturbations. The metastable states in zone I behaved like the amorphous/glass state. They resided at another metastable state during the evolution towards the stable state after relatively large perturbations. In zone I, the metastable states formed from the sharp increase of internal friction with decreasing Ug. The particles lost the ability to move before reaching the stable positions and were entrapped at the intermediate and metastable positions