统计物理与机器学习 | 本周物理学讲座

1统计物理与机器学习

报告人：张潘，理论物理研究所

时间：9月27日（周五）18:30

单位：中国科学院大学

地点：雁栖湖校区教1-107

如何准确地计算多位子系统的自由能，热力学量，以及给出系统的无偏采样，是统计物理的核心问题之一。自上世纪初以来，统计物理学家建立了各种各样的理论和方法来处理此类问题，其中包括马尔科夫链蒙特卡洛，张南网络，重整化群，平均场近似及消息传递算法等等。在实际应用中，这些方法各有其优劣。本报告将介绍一种新的统计力学问题的计算方法一一变分自回归神经网络(VARIATIONAL AUTOREGRESSIVE NETWORK）。这个方法拓展了传统意义上的平均场占法，用自回归神经网络构建变分的概率分布，并通过强化学习的方法训练网络，进而可以同时计算系统的自由能上界，测量系统的能最，以及各种热力学量，并直接无偏地对系统采样。报告首先会从更广的角度描述统计物理的基本问题和机器学习中的非监督学习所具有的天然的联系，然后介绍变分自回归网络，如何利用它求解统计力学的基本问题，以及此方法在伊辛模型，自旋玻璃以及统计物理反问题中的应用。

2What is beyond the “visible” galaxy?

报告人：Jiangtao Li

时间：9月24日（周二）12:00

单位：北京大学

地点：KIAA 1st meeting room

Beyond the stellar light of the galaxy which is visible in broad-band optical emission, there exist multi-phase gases, dust, cosmic ray, and magnetic field, which more or less fill in the dark matter halo or even to larger scale. These matters are often called the circum-galactic medium (CGM), which play a critical role in the co-evolution of galaxies and their environments. I will briefly overview the basic observational methods and key results of existing multi-wavelength observations of the multi-phase CGM. I will show that although the CGM is ubiquitous around nearby galaxies, the physics related to the CGM, as well as the role of CGM is galaxy formation and evolution, are still poorly understood. Some new observations with future space- and/or ground-based facilities will help us to better understand the CGM. 

3 Classical, quantum and fractional statistics:From Boltzmann to Haldane

报告人：管习文，中科院武汉数学与物理研究所

时间：9月24日（周二）15:00

单位：中科院理论物理送

地点：Conference room 6620

4Quantum metal states in 2D Superconductors and Quantum Hall Effect without Landau Levels

报告人：Jian Wang，Peking University

时间：9月26日（周四）16:00

单位：清华大学

地点：理科楼郑裕彤讲堂

After decades of explorations, suffering from the subtle nature and sample quality, whether a metallic ground state exists in a two-dimensional system (2D) beyond Anderson localization is still a mystery. Our work reveals how quantum phase coherence evolves across bosonic superconductor-metal-insulator transitions via magneto-conductance quantum oscillations in high-Tc superconducting films with patterned nanopores. A robust intervening anomalous metallic state characterized by both resistance and oscillation amplitude saturations in the low temperature regime is detected, which suggests that the saturation of phase coherence plays a prominent role in the formation of the anomalous metallic state. Furthermore, we carried out a systematic transport study on the macro-size ambient-stable ultrathin crystalline PdTe2 films grown by molecular beam epitaxy. Remarkably, in perpendicular magnetic field, the film undergoes the quantum phase transition from quantum metal to weakly localized metal with the presence of intermediate quantum Griffiths singularity. Our findings lead to a global phase diagram of 2D superconducting system with strong spin-orbit coupling.

The quantum Hall effect (QHE) without Landau levels (LLs) has become a long-pursuit research topic since the QHE was discovered around 40 years ago. Previous theoretical proposals and experiments based on two dimensional (2D) topological systems with time-reversal symmetry broken have revealed the QHE without LLs with Chern number C=1 at ultralow temperatures. Now the key issues of the QHE without LLs are how to increase the working temperature and realize high Chern number with more dissipationless chiral edge states (C>1) for emerging physics and low-dissipation electronics. We discovered the high Chern number (C=2) QHE without LLs in the nine-septuple-layer magnetic MnBi2Te4 nano-device and C=1 Chern insulator state in the seven-septuple-layer nano-device displaying nearly quantized Hall resistance plateau at record-high temperatures up to 60 K. The thickness-dependent topological quantum phase transition from C=2 to C=1 is uncovered. To our knowledge, this is the first work to report high Chern number QHE without LLs above the liquid helium temperature and this is also the first time that the nearly quantized Hall resistance plateau is detected at the temperature up to 60 K for QHE without LLs.

5A crucial but unexplored chromosphere line for measuring stellar helium abundances

报告人：Mingjie Jian

时间：9月27日（周五）12:00

单位：北京大学

地点：1st meeting room, KIAA

Stellar helium abundance is an important stellar parameter which can shed light on the formation history of globular cluster and even galactic bulge.However, it is hard to be measured since only the helium 10830A chromospheric line presents in A-K type stars. To test its potential to become a helium an abundance tracer of these stars, I will introduce the formation mechanism of this helium line, the cross-match result of previous equivalent width measurement with recent surveys. 

更多报告信息：中国物理学会期刊网学术讲座列表

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