報告人：胡江平，中國科學院物理研究所
時間：5月16日（周四）16:00
單位：清華大學
地點：理科樓鄭裕彤講堂
至今距銅基高溫超導體的發現已經過去了30多年，鐵基高溫超導研究也已經有十個年頭，但是高溫超導機理依舊是凝聚態物理研究前沿的一個難題。本次講座通過回顧過去這方面的研究進展，總結問題形成的一些個人觀點，就當前存在的挑戰和機遇做一個概述。
報告人：Thomas Heine，Chair of Theoretical Chemistry
時間：5月14日（周二）10:30
單位：中科院物理研究所
地點：A樓234會議室
The discovery of graphene initiated an immense research effort in the field of two-dimensional (2D) crystals. Graphene shows extraordinary high stability, intriguing electronic, including topological, properties, and chemical inertness. Soon it was clear that 2D crystals can be formed from virtually all layered materials by top-down (exfoliation…), but also bottom-up (chemical vapor deposition…) approaches. The family of 2D materials contains a lot of remarkable phenomena, for example 2D semiconductors that get metallic when the number of layers is increased, 2D topological insulators, and 2D metals.
It is somewhat less known that graphene represents also the prototype 2D polymer. 2D polymers (also 2D covalent-organic frameworks – 2D COFs) are a rather new family of synthetic 2D crystals where molecular units are stitched together with strong bonds. This offers a regular crystal lattice and thus materials comprising all collective phenomena that are known from solid state physics, however, with a much richer diversity due to the essentially infinite number of molecules that can be considered. A recent breakthrough was the discovery of chemical coupling reactions that achieve full conjugation between the constituting molecules, which is the precondition for the formation of 2D semiconductors with ballistic transport properties.
For the 2D polymers I will to focus on structural diversity: while mathematically, 11 tilings are possible in two-dimensions (the so-called Kepler nets), nature offers much less structural diversity in crystalline 2D materials. By picking suitable molecular building units we can form lattices with structural topologies that impose, in turn, electronic topologies. One of the examples that I will highlight is the kagome structure, which produces both Dirac points and flat bands.
報告人：方海平，中國科學院上海高等研究院
時間：5月14日（周二）16:00
單位：中科院理論物理所
地點：Seminar Room 6520,ITP South Building
自然界中水絕大部分以鹽溶液的形式存在。芳香環結構（以碳原子為主體的環狀結構，例如苯環）廣泛的存在於碳基材料和生物分子中，如石墨烯、碳納米管、富勒烯、芳香環胺基酸、DNA和RNA等。芳香環結構與水分子之間僅存在較弱的van de Waals作用，因此被認為是疏水的。上世紀八十年代，人們發現富含π電子的芳香環與離子之間有強的非共價鍵作用（離子-π作用）。由於離子與水之間存在水合作用，這樣的競爭導致水溶液中的離子-π作用被大幅度減弱，從而一直被忽視。在這個報告中，將介紹我們在過去十年中，通過考慮碳基表面上的多芳香環效應和溶液中的多離子效應，重新審視水合離子-π作用，理解離子溶液中碳基表面的一些特性、發現相關的新物理並展示相應應用。<1>自2005年以來，實驗表明碳納米管具有很好的水滲透能力，預期其可以作為性能極佳的脫鹽材料，但至2016年依然不能在實驗上實現。我們認為離子與碳納米管上的芳香環之間的離子-π作用導致離子被吸附在碳納米管口，從而阻塞碳納米管。基於此機理提出的改進方案，其中一種得到最近實驗驗證。該工作被英國的Chemistry World 以「Nanotube desalination could be put back on track」為題專門報導。<2>精確控制（氧化）石墨烯膜的層間距，達到一納米大小和一埃的精度，是其在水處理、離子/分子分離以及電池/電容等應用的關鍵。但，對像紙一樣的石墨烯納米片，這樣的精確操控，非常困難。利用水合離子與石墨烯上的多芳香環的吸附作用，我們提出並從實驗上實現了用水合離子精確控制石墨烯膜的層間距，並展示了其出色的離子篩分性能。<3>我們日常吃的食鹽，氯化鈉晶體，由一份鈉對應一份氯構成。在石墨和石墨烯表面，在常溫常壓下我們實現了氯化二鈉（Na2Cl, 鈉氯元素比2:1）和氯化三鈉（Na3Cl，鈉氯元素比3:1）等具有反常化學計量比的二維晶體，我們稱之為"反晶"。反晶具有獨特的電子結構，因此具有與一般晶體完全不同的物理化學性質。對芳香環胺基酸和短肽，離子的存在提高了這些胺基酸和短肽中疏水芳環與水分子之間的親合力。實驗觀測驗證了這個預言，芳環胺基酸（色氨酸和苯丙氨酸）和短肽在二價過渡金屬離子中的溶解度顯著增加。
報告人：馮萬哲，北京師範大學
時間：5月15日（周三）10:00
單位：北京師範大學
地點：物理樓106
Three of the important puzzles in cosmology relate to the origin of baryon asymmetry in the Universe, the nature of dark matter and the cosmic coincidence that the amount of dark matter and visible matter are comparable. In this talk, I will discuss classes of models where baryon asymmetry and dark matter have a common origin within the framework of U(1) extensions of the standard model. In one intriguing model all of the fundamental interactions do not violate baryon or lepton number, and the total B-L in the Universe vanishes, which is quite different from previous mechanisms that baryon or lepton number are violated in generating the matter and anti-matter asymmetry. In addition, one may also generate a normal hierarchy of neutrino masses and mixings in conformity with the current data from Daya Bay reactor neutrino experiment. I will then discuss some recent progress of intersecting D-brane models which can give rise to much richer phenomenology than people used to think. Puzzles and anomalies in particle physics can be resolved in D-brane models purely by geometrical constructions of the intersecting D-branes. I will discuss two classes of D-brane models. One class of the models offers a scenario connecting the hidden sector physics using Stueckelberg U(1)'s. The other class offers a natural explanation of the recent LHCb anomaly.
報告人：施天從，高雄科技大學電子系
時間：5月15日（周三）10:00
單位：中科院半導體研究所
地點：半導體研究所學術會議中心
6Electron scattering for proton and exotic nuclei
報告人：Toshimi Suda，
時間：5月15日（周三）14:00
單位：北京航空航天大學
地點：物理學院學術交流廳（513）
報告人：Markus Deserno，Carnegie Mellon University
時間：5月15日（周三）16:00
單位：北京師範大學
地點：物理樓106
All lipid biomembranes are asymmetric: their two individual leaflets differ in their lipid composition. Such a state is only metastable, decaying over a time scale of (presumably) many hours into a symmetric distribution via lipid flip-flop between the leaflets. Cells hence must put work into maintaining asymmetry as a nonequilibrium steady state. Recent experimental advances have succeeded in artificially creating such asymmetric membranes, and to study their properties before the asymmetry decays. Such experiments suggest that asymmetric membranes have quite unexpected properties. For instance, they have been found to be much stiffer than what one would predict based on the curvature elasticity of their individual leaflets. In this talk I will try to explain these findings, using a combination of theory and coarse-grained molecular dynamics simulations. A key part of the puzzle will be that asymmetric membranes are not fully characterized by their two leaflet compositions alone. One needs at least one more observable, the differential stress between leaflets, to fully characterize the state. I will argue that it is not the compositional asymmetry but the differential stress which is directly responsible for the experimentally observed stiffening. However, lipid asymmetry is an indirect cause for that stress, which can be understood by looking at the processes by which asymmetric membranes are experimentally created.
報告人：趙川
時間：5月16日（周四）10:00
單位：北京航空航天大學
地點：物理學院學術交流廳（513）
報告人：Jinlin Han，NAOC
時間：5月16日（周四）14:00
單位：清華大學
地點：蒙民偉科技南樓S727
A variety of approaches are used to investigate the interstellar and intergalactic magnetic fields. Observational results of these magnetic fields are reviewed: The large-scale magnetic fields in the Milky Way have been best probed by Faraday rotation measures of a large number of pulsars and extragalactic radio sources. The coherent Galactic magnetic fields are found to follow the spiral arms and have their direction reversals in arms and interarm regions in the disk. The azimuthal fields in the halo reverse their directions below and above the Galactic plane; The orientations of the organized magnetic fields in nearby galaxies have been observed through polarized synchrotron emission. Magnetic fields in the intracluster medium have been indicated by diffuse radio halos, polarized radio relics, and Faraday rotations of the embedded radio galaxies and the background sources. Future observational perspectives are given to reveal the 3D tomography of the large-scale coherent magnetic fields in our Galaxy and nearby galaxies, a better description of intracluster field properties, and firm detections of intergalactic magnetic fields in the cosmic web. See Han (2017, ARA&A 55, 111) for a review and Han et al. (2018, ApJS 234, 11) for most recent progress.
報告人：王靖，Fudan University
時間：5月16日（周四）14:00
單位：清華大學
地點：物理系理科樓C302
Here, we predict the tetradymite-type compound MnBi2Te4 and its related materials host topologically nontrivial magnetic states. The magnetic ground state of MnBi2Te4 is an antiferromagetic topological insulator state with a large topologically non-trivial energy gap (0.2 eV). It presents the axion state, which has gapped bulk and surface states, and the quantized topological magnetoelectric effect. It has several advantages over the previous proposals on realizing the topological magnetoelectric effect. The intrinsic magnetic and band inversion further lead to quantum anomalous Hall effect in odd layer MnBi2Te4 thin film with combined inversion and time-reversal symmetry breaking.
The intrinsic magnetic topological states in MnBi2Te4 provides an ideal platform for studying the exotic physics of chiral Majorana fermion modes. In proximity to an s-wave superconductor with nontrivial band topology, an N=3 chiral topological superconductor is realized. A smoking gun signature of the Majorana nature of three chiral edge fermions is that the two-terminal electrical conductance of a quantized anomalous Hall-TSC junction obeys a unique distribution averaged to (2/3)e^2/h, which is due to the random edge mode mixing of chiral Majorana fermions and is distinguished from possible trivial explanations.
報告人：彭細洪，亞利桑那州立大學
時間：5月16日（周四）15:30
單位：北京師範大學
地點：物理樓106
「第一性」原理電子結構計算方法適用於解決許多學科的問題，包括物理，化學，材料科學與工程。它在現代科學研究和探索中發揮著至關重要的作用。在過去的十來年裡，我們科研組建立並使用了基於物理學的預測模型和「第一性」原理計算方法研究了多種納米結構和材料系統。在這次報告裡，我將概述我們近年來在以下兩個主要領域的科研成果。
（一）：基於半導體納米材料在納米技術中的潛在應用，納米結構在過去幾十年吸引了大量的科研力量。納米材料如納米線由於它們可以極大縮小器件尺寸和改變器件形態從而提高運行速度和能力，有可能在科學和技術方面引起巨大變革。我們探索了材料尺寸大小，表面鈍化和機械應變等因素，嘗試理解這些因素對半導體納米材料電子性質及性能的影響。
（二）：隨著對能源資源枯竭問題的日益關注，人們對開發新的清潔能源及安全工藝產生了濃厚的興趣。我們通過與實驗科研者緊密合作，開展了燃料電池，太陽能電池，鋰離子電池和氫氣儲存等領域的新型材料研究。我將概述我們研究組在這方面的主要成果。
報告人：Naoki Seto，Kyoto University
時間：5月16日（周四）16:00
單位：北京大學
地點：KIAA-PKU Auditorium
I discuss the prospects of gravitational wave astronomy around 0.1-10Hz. This frequency regime is just between the two bands probed byground-based detectors (LIGO, Virgo, KAGRA,..) and the proposed spacedetectors (LISA, TianQin, Taiji). I briefly introduce the Japaneseplan DECIGO targeting this band and then discuss its main sciencetargets, including primordial stochastic background and compactbinaries.
報告人：Yang Chen，Padova Observatory
時間：5月17日（周五）12:00
單位：北京大學
地點：KIAA 1st meeting room
After a brief history of PARSEC/Padova stellar evolution models, an overview of the progresses in recent years will be presented, including1) models calibrated T-tau relation with observed mass-radius relation, which improve greatly the isochrone fitting to the color-magnitude diagramof star clusters; 2) implementing the strong Eddington factor dependant stellar wind for very massive stars, which naturally explains the observed scarsity in the Humphreys-Davidson limit region; 3) a homogeneous and comprehensive publically available database of stellar bolometric corrections;4) updated TRILEGAL population synthesis tool; 5) detailed TP-AGB calibration with observation data for SMC, M31, etc; 6) PARSEC models with rotation; 7) PARSEC alpha-enhanced stellar models and atmosphere models; 8) PARSEC SSP models;
報告人：Adam Laizzi，Taibei University
時間：5月17日（周五）15:00
單位：北京師範大學
地點：物理樓106
Simple models of interacting quantum spins (like the Heisenberg model) are remarkable tools for understanding strong quantum fluctuations, but relatively few studies have considered the effects of external magnetic fields on these systems. I investigate the influence of magnetic fields in the J-Q model, an antiferromagnetic Heisenberg model with an added 4-spin interaction (Q). This model is known to harbor a direct, continuous phase transition between the Néel state and a valence-bond solid. This transition is believed to be an example of deconfined quantum criticality, where the excitations are exotic fractionalized particles known as spinons (S=1/2 bosons). We study the thermodynamics of the excitations and find direct evidence that they are indeed fractional. Separately, we also find that the four-spin term changes the nature of the saturation transition from "zero-scale-factor" universality to metamagnetism (magnetization jumps).
報告人：Jiadong Zang，University of New Hampshire
時間：5月17日（周五）16:00
單位：中科院理論物理所
地點：M樓253會議室
Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the Dzyaloshinskii-Moriya interaction, stimulates the formation of many novel topological spin textures. One typical example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability and gives out promise on future spintronic devices. A key transport signature of the skyrmion is the topological Hall effect, that is, the electron moves in sideway under the effect of real space Berry phase induced by spin chirality. In the first part of the talk, I will argue that the presence of topological Hall in many experiments is not enough to identify skyrmions. It could originated from atomic scale chirality from thermal fluctuations.
In the second part of the talk, I will generalize the skyrmion texture from two dimensions (2D) to three dimensions (3D), and discuss two relevant 3D spin textures in chiral magnets. One is the target skyrmion we recently observed, both theoretically and experimentally, in ultra-small nanodisks of chiral magnets. A target skyrmion consists of concentric helical rings and can be stabilized in the absence of external magnetic field. Switch between two types of target skyrmions can be enabled by a field loop. The other texture to be discussed is the magnetic hopfion. We propose the presence of zero-field hopfion in synthetic chiral magnetic multilayers. The transition from hopfion to the ground state, a monopole-antimonopole pair, can be fully understood as the topological transition between torus and sphere. These works could stimulate the development of 3D spintronics.
報告人：苟利軍研究員，毛淑德教授和虞駿（Steed）先生
時間：5月17日（周五）19:20
單位：清華大學
地點：清華大學西階教室
今年大年初一上映的《流浪地球》電影是一部成功的好萊塢式科幻電影，廣受好評。電影中出現了大量的天文學名詞與天文學現象，如電影劇情的災厄之源——太陽氦閃，劇情高潮——木星洛希極限等等，然而電影中的科學必然帶有科幻色彩，現實中我們的生命之源——太陽會不會突然爆發毀滅全人類呢？我們將邀請苟利軍研究員為我們講解在天文學中，這些現象的真實面貌。
如果地球駛出太陽系，那麼好奇的外星人怎樣找到正在流浪的地球呢？我們將邀請毛淑德教授為我們介紹在茫茫太空中尋找流浪行星的具體方法，探索宇宙中一個個不同的「流浪地球」。他們在看著我們，我們也在看著他們，地球是一艘飛船，承載著我們所有人在宇宙中航行。就算我們還沒有出去流浪，在地球飛船的甲板上，我們仍然可以遙望宇宙，看見遠遠超出我們生命時長的宏大事件。
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