Berry Phase Effects on Electronic Properties

Ever since its discovery, the Berry phase has permeated through all branches of physics. Over the last three decades, it was gradually realized that the Berry phase of the electronic wave function can have a profound effect on material properties and is responsible for a spectrum of phenomena, such as ferroelectricity, orbital magnetism, various (quantum/anomalous/spin) Hall effects, and quantum charge pumping. This progress is summarized in a pedagogical manner in this review. We start with a brief summary of necessary background, followed by a detailed discussion of the Berry phase effect in a variety of solid state applications. A common thread of the review is the semiclassical formulation of electron dynamics, which is a versatile tool in the study of electron dynamics in the presence of electromagnetic fields and more general perturbations. Finally, we demonstrate a re-quantization method that converts a semiclassical theory to an effective quantum theory. It is clear that the Berry phase should be added as a basic ingredient to our understanding of basic material properties.Comment: 48 pages, 16 figures, submitted to RM

High-cadence, High-resolution Spectroscopic Observations of Herbig Stars HD 98922 and V1295 Aquila

Recent observational work has indicated that mechanisms for accretion and outflow in Herbig Ae/Be star-disk systems may differ from magnetospheric accretion (MA) as it is thought to occur in T Tauri star-disk systems. In this work, we assess the temporal evolution of spectral lines probing accretion and mass loss in Herbig Ae/Be systems and test for consistency with the MA paradigm. For two Herbig Ae/Be stars, HD 98922 (B9e) and V1295 Aql (A2e), we have gathered multi-epoch (~years) and high-cadence (~minutes) high-resolution optical spectra to probe a wide range of kinematic processes. Employing a line equivalent width evolution correlation metric introduced here, we identify species co-evolving (indicative of common line origin) via novel visualization. We interferometrically constrain often problematically degenerate parameters, inclination and inner disk radius, allowing us to focus on the structure of the wind, magnetosphere, and inner gaseous disk in radiative transfer models. Over all timescales sampled, the strongest variability occurs within the blueshifted absorption components of the Balmer series lines; the strength of variability increases with the cadence of the observations. Finally, high-resolution spectra allow us to probe substructure within the Balmer series' blueshifted absorption components: we observe static, low-velocity features and time-evolving features at higher velocities. Overall, we find the observed line morphologies and variability are inconsistent with a scaled-up T Tauri MA scenario. We suggest that as magnetic field structure and strength change dramatically with increasing stellar mass from T Tauri to Herbig Ae/Be stars, so too may accretion and outflow processes.Comment: 34 pages, 52 figures, published in the Ap

Some stability properties of T. Chan’s preconditioner

AbstractA matrix is said to be stable if the real parts of all the eigenvalues are negative. In this paper, for any matrix An, we give some sufficient and necessary conditions for the stability of T. Chan’s preconditioner cU(An)

Control interface concepts for CHARA 6-telescope fringe tracking with CHAMP+MIRC

Cophasing six telescopes from the CHARA array, the CHARA-Michigan Phasetracker (CHAMP) and Michigan Infrared Combiner (MIRC) are pushing the frontiers of infrared long-baseline interferometric imaging in key scientific areas such as star- and planet-formation. Here we review our concepts and recent improvements on the CHAMP and MIRC control interfaces, which establish the communication to the real-time data recording & fringe tracking code, provide essential performance diagnostics, and assist the observer in the alignment and flux optimization procedure. For fringe detection and tracking with MIRC, we have developed a novel matrix approach, which provides predictions for the fringe positions based on cross-fringe information.Comment: 6 pages, 4 figures, published in SPIE conference proceedings (http://dx.doi.org/10.1117/12.926559

A Span-Extraction Dataset for Chinese Machine Reading Comprehension

Machine Reading Comprehension (MRC) has become enormously popular recently and has attracted a lot of attention. However, the existing reading comprehension datasets are mostly in English. In this paper, we introduce a Span-Extraction dataset for Chinese machine reading comprehension to add language diversities in this area. The dataset is composed by near 20,000 real questions annotated on Wikipedia paragraphs by human experts. We also annotated a challenge set which contains the questions that need comprehensive understanding and multi-sentence inference throughout the context. We present several baseline systems as well as anonymous submissions for demonstrating the difficulties in this dataset. With the release of the dataset, we hosted the Second Evaluation Workshop on Chinese Machine Reading Comprehension (CMRC 2018). We hope the release of the dataset could further accelerate the Chinese machine reading comprehension research. Resources are available: https://github.com/ymcui/cmrc2018Comment: 6 pages, accepted as a conference paper at EMNLP-IJCNLP 2019 (short paper

Electron energy-loss spectroscopy and ab initio electronic structure of the LaOFeP superconductor

The electronic band structures of the LaOFeP superconductor have been calculated theoretically by the first principles method and measured experimentally by electron energy loss spectroscopy. The calculations indicate that the Fe atom in LaOFeP crystal shows a weak magnetic moment and does not form a long-range magnetic ordering. Band structure, Fermi surfaces and fluorine-doping effects are also analyzed based on the data of the density functional theory. The fine structures of the EELS data have been carefully examined in both the low loss energy region and the core losses region (O K, Fe L2,3, and La M4,5). A slight bump edge at 44 eV shows notable orientation-dependence: it can be observed in the low loss EELS spectra with q parallel to c, but becomes almost invisible in the q vertical to c spectra. Annealing experiments indicate that low oxygen pressure favors the appearance of superconductivity in LaOFeP, this fact is also confirmed by the changes of Fe L2,3 and O K excitation edges in the experimental EELS data

Near Infrared View of Stellar Surfaces and Circumstellar Disks with an Upgraded Optical Interferometer.

Modern long-baseline optical interferometers can reach sub-milliarcsecond angular resolutions, allowing us to resolve the detailed structures over stellar surfaces as well as circumstellar disks. Through modeling and imaging two rapidly rotating stars observed in H band by the interferometer CHARA/MIRC, I will show how the rotation affects the stellar geometries, surface temperature distributions and evolutions. A high-eccentricity Be binary system was observed right after its periastron by the same interferometer, providing an opportunity to study whether the gravitational interference from the secondary would trig the gaseous disk formation around the primary. I have also worked on three instrumental projects to improve the scientific results from the interferometer. In order to reduce the uncertainty of visibility measurements from MIRC, I have developed Photometric Channels to directly measure the fluxes of individual beams in the real time. MIRC has further been expanded from a 4-beam combiner to a 6-beam combiner to improve the (u,v) coverage to image more complicated systems. Lastly we have been developing Wavefront Sensors of the Adaptive Optics systems on the CHARA telescopes to improve the sensitivity to study  fainter objects such as the innermost edges of YSO disks.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108941/1/xche_1.pd

Mao-Zedong At SemEval-2023 Task 4: Label Represention Multi-Head Attention Model With Contrastive Learning-Enhanced Nearest Neighbor Mechanism For Multi-Label Text Classification

The study of human values is essential in both practical and theoretical domains. With the development of computational linguistics, the creation of large-scale datasets has made it possible to automatically recognize human values accurately. SemEval 2023 Task 4\cite{kiesel:2023} provides a set of arguments and 20 types of human values that are implicitly expressed in each argument. In this paper, we present our team's solution. We use the Roberta\cite{liu_roberta_2019} model to obtain the word vector encoding of the document and propose a multi-head attention mechanism to establish connections between specific labels and semantic components. Furthermore, we use a contrastive learning-enhanced K-nearest neighbor mechanism\cite{su_contrastive_2022} to leverage existing instance information for prediction. Our approach achieved an F1 score of 0.533 on the test set and ranked fourth on the leaderboard