厉害了，咱的北斗！| 本周物理学讲座

1厉害了，咱的北斗！

报告人：徐颖，中国科学院空天信息研究院

时间：1月23日（周三）14:30

单位：中科院物理研究所

地点：M234会议室

北斗系统是我国自主建设的卫星导航系统，已经在2018年12月27号开始为全世界提供服务。本次讲座将介绍：北斗系统是怎么发展到现在的，北斗三号具备哪些特有的优势，北斗系统和其他的卫星导航定位系统的区别。并展望未来的北斗我们还有哪些可以期待的发展。从大航海时代到大航天时代，北斗会有什么我们意想不到的应用呢？让我们一起回顾过去，认识现在，展望将来。

2An Artificial Neural Network Approach to the Analytic Continuation Problem

报告人：QuanSheng Wu，EPFL, Switzerland

时间：1月22日（周二）10:00

单位：中科院物理研究所

地点：M830

Inverse problems are encountered in many domains of both mathematics and physics, with analytic continuation of the imaginary Green’s function into the real frequency domain being a particularly important example. However, the analytic continuation problem is ill-defined and currently no analytic transformation for solving it is known. We present a general framework for building an artificial neural network that efficiently solves this task with a supervised learning approach, provided that the forward problem is sufficiently stable for creating a large training dataset of physically relevant examples. By comparing with the commonly used maximum entropy approach, we show that our method can reach the same level of accuracy for low-noise input data while performing significantly better when the noise strength increases. We also demonstrate that adding an unsupervised denoising step significantly improves the accuracy of the maximum entropy predictions for noisy inputs. The computational cost of the proposed neural network approach is reduced by almost three orders of magnitude compared to the maximum entropy method. Although the inverse problem we studied is a typical one, our neural network solution is not only limited to the analytic continuation problem, but also can be extended to many other inverse problems.

3Axion electrodynamics and the quantized topological magnetoelectric effect in topological insulators

报告人：N. Peter Armitage，The Johns Hopkins University

时间：1月22日（周二）14:00

单位：中科院物理研究所

地点：M楼253报告厅

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here we lower the chemical potential of three-dimensional (3D) Bi2Se3 films to 30 meV above the Dirac point, and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time- domain terahertz polarimetry. For fields higher than 5 T, we observed quantized Faraday and Kerr rotations, whereas the DC transport is still semi-classical.A non-trivial Berry phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine structure constant based on a topological invariant of a solid-state system.   I’ll also discuss our most recent measurements on topological insulator single crystals that give evidence for a half quantized Hall effect on the TI surfaces.

4Effect of the hyperon coupling constants on the properties of the massive neutron stars

报告人：赵先锋，西南石油大学

时间：1月22日（周二）15:00

单位：中科院理论物理所

地点：Room 322, ITP MAIN BUILDING

5Novel Properties of Single Layers of Transition Metal Dichalcogenides

报告人：TAI-CHANG CHIANG

时间：1月23日（周三）16:00

单位：北京大学

地点：Room W563, Physics Building

6Determination of resonance properties from the lattice energy levels using chiral effective field theory

报告人：郭志辉，河北师范大学

时间：1月24日（周四）10:00

单位：中科院理论物理所

地点：Room 322, ITP MAIN BUILDING

Remarkable progress on the two-body scattering in the lattice QCD has been made recently. A large amount of lattice finite-volume spectra obtained at unphysically large quark masses from the coupled-channel, both in the center of mass and moving frames, are obtained. These precise finite-volume spectra encode valuable information of the dynamics of the two-body system. This talk aims at extracting the physical scattering amplitudes and possible resonance information from the lattice energy levels by using the chiral effective field theory. Two examples shall be presented, including the pi-eta, K-Kbar and pi-eta’ coupled-channel scattering and the a0(980) resonance, and the D-pi, D-eta, Ds-Kbar coupled-channel scattering and the D*0(2400) resonance. 

7Magnetoresistance from Fermi Surface Topology

报告人：吴泉生，洛桑联邦理工学院

时间：1月24日（周四）10:00

单位：清华大学

地点：物理系理科楼C302

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