報告人：陸贇豪，浙江大學
時間：5月7日（周二）10:00
單位：北京航空航天大學
地點：物理學院主樓316會議室
鐵電性通常存在於不同元素組成的複合材料中，而鐵電材料，特別是二維材料中的鐵電性，由於其在信息存儲和電學控制領域的巨大應用前景，在最近幾年引起了人們的濃厚興趣。另一方面，幾乎所有報導的鐵電材料都是由不同種類的原子組成的化合物。據我們所知，單元素材料的鐵電性從未在文獻中報導過。在這裡，雖然很違反常識，但我們基於第一性原理計算，揭示了一系列單質二維體系及表面體系中的自發電極化和本徵鐵電性和反鐵電性。這些單元素材料的鐵電居裡溫度可能高於室溫，因此他們有望實現具有廣泛應用前景的超薄鐵電器件。與此同時，我們也構建一個通過模型來理解和搜索中新型二維鐵電和反鐵電材料。進一步，我們發現在某些單質體系中還有可能存在多鐵性，具有巨大的潛在應用價值。
報告人：彭毅，University of Minnesota
時間：5月7日（周二）14:30
單位：北京師範大學
地點：物理樓106
Phase transitions widely exist in nature and demonstrate colorful phenomena. However, we still lack the understanding of the dynamics of phase transitions at single particle level because atoms move too fast to be tracked in real time. Here we employ micron-sized colloidal particle and video microscopy to image the kinetic pathway of phase transitions in colloidal crystals and active fluids. Colloidal particles form disordered liquid and ordered crystal phases and exhibit phase transitions as volume fraction varies. We use temperature-sensitive colloids as a model system to study the kinetics of solid-solid transitions. The transition is a liquid-mediated two-step nucleation in quasi-equilibrium conditions while the liquid disappears and dislocation pairs induce the nucleation as external stress is applied. Active matter consists of self-propelled entities, such as swimming bacteria and motile cells, and thus shows more colorful collective behaviors. We map the phase diagram of bacterial swarming in experiment and use a simple model to show two-body hydrodynamic interaction is enough to predict the phase boundary. Furthermore, we employ gene-mutated light-controlled bacteria to study the kinetics of 『phase transitions』 in active matter. The transition is discontinuous and occurs in a nucleation manner. We compare the nucleation in active matter with the classical nucleation theory in equilibrium.
報告人：何志國，漢堡大學
時間：5月7日（周二）16:00
單位：北京師範大學
地點：物理樓106
In this talk, I will present our study on the double prompt heavy quarkonia production puzzle at the Large Hadron Collider (LHC). The cross sections are calculated within the framework of non-relativistic QCD (NRQCD) factorization at QCD leading order including both color-singlet and color-octet contribution. Two different ways of dealing with the parton distribution functions of the initial states are investigated. Moreover, at QCD next-to leading order, the NRQCD factorization is found to breakdown for the processes involving 2 heavy quarkonia. New type operators are introduced to cure the problem.
報告人：姚濤，中國科學大學
時間：5月9日（周四）10:00
單位：北京師範大學
地點：物理樓106
能源與環境是進入21世紀人類面臨最富挑戰性的兩大基本問題，發展高效的新能源技術將自然界中的水和二氧化碳轉變為清潔的可再生能源是解決能源危機和環境問題非常有潛力的途徑，但同時也面臨巨大挑戰。近年來，我們研究組發展了一系列同步輻射原位X射線吸收譜學技術，實現原子級水平監測能源材料工作狀態下結構演變的動態表徵。克服了催化材料與電解質溶液接觸的固-液表界面處的活性中心濃度低、以及活性位原子結構隨外加電場的動態變化等困難，從實驗上精確鑑別出單原子催化材料在電催化析氫反應過程中活性位點的真實結構和動態演化過程《Nature Catalysis 2019, 2, 134》；利用同步輻射X射線吸收譜技術設計並明確了氮化碳納米空間限域效應形成單位點鈷基光催化劑，實現了太陽光照條件下高效的全解水產氫性能《Angew. Chem. Int. Ed. 2017, 56, 9312》，工作被選為「Frontispiece」進行亮點報導；進一步利用同步輻射技術並結合第一性原理計算從原子尺度上揭示了光催化劑中「Co1-N4」單位點活性中心結構、及其加速產氫反應路徑的機理《Angew. Chem. Int. Ed.2017, 56, 12191》；基於同步輻射譜學技術，揭示了二維超薄材料表面不飽和配位和獨特的扭曲結構對提高光生電子-空穴對分離效率、提高電催化還原CO2活性的影響《Angew. Chem. Int. Ed. 2016, 55, 6716》，《Nat. Commun. 2016, 7, 12697》。這些研究工作擴大了同步輻射技術在能源材料領域的應用範圍，同時也為揭示光電催化劑可控構築和性能剪裁的內在本質，設計具有高效穩定和實用型的光電催化劑材料提供了理論基礎。
報告人：Jun Zhang，Shanghai Jiaotong Univ.
時間：5月9日（周四）14:00
單位：清華大學
地點：蒙民偉科技南樓S727
Large scale galaxy surveys have brought us unprecedented opportunities in exploring various mysteries of our Universe, e.g., dark matter and dark energy, which may lead to exciting leaps in our understanding of fundamental physics. Weak lensing is a direct probe of the large scale structure, as only gravitational physics is involved. We show that to achieve robust constraints on cosmology, weak lensing measurements in both the underdense and overdense regions, as well as shape measurement itself, can be done in untraditional ways to reduce their model-dependences. In the first part of my talk, I show a new way of defining the underdense regions, and how to use their background lensing signals to constrain the equation-of-state of dark energy. Secondly, regarding the halo mass function measurement with weak lensing, I discuss a way of avoiding the Eddington bias and possibly many other sources of errors. Finally, I report the status of our Fourier_Quad shear measurement pipeline, which is accurate, fast, and model-independent.
報告人：高宇，中科院高能所
時間：5月9日（周四）14:00
單位：清華大學
地點：物理系理科樓C302
The precision cosmic microwave background (CMB) measurement and a hint of neutral hydrogen 21cm line observation tell us a lot about the CMB propagation during the dark and reionization eras of the Universe. Beside a significant boost towards precision cosmology, this newly acquired understanding of the CMB signals can be turned into a powerful test of potential imprints on to the standard astrophysical/cosmic history from beyond the Standard Model physics: injection of the energy from the annihilation, decays or radiation from dark matter or black holes. Upcoming CMB polarization experiments and 21cm measurements on the cosmic ionization history will bring the prospective sensitivity to such new physics processes to a whole new level.
7針尖上的舞蹈---多鐵性納米陣列中的疇與疇壁
報告人：劉俊明，南京大學物理學院
時間：5月9日（周四）16:00
單位：清華大學
地點：理科樓鄭裕彤講堂
具有納米尺度極化空間分辨的壓電力顯微術(PFM)，藉助不斷集成的新方法，為研究各類材料中極性疇結構的基本性質和揭示微區新效應提供了有力支撐。作為 PFM 技術的一個初級用戶，報告人所在團隊利用這一技術對鐵電與多鐵性納米陣列結構中的疇、疇壁、多鐵異質結等體系開展了一些初級探索，涉及到拓撲中心疇、疇壁導電、電控磁性等問題。作為班門弄斧之作，整個報告內容略顯零散，姑且就教於各位老師和同學。
報告人：Sang Pyo Kim，Kunsan National University, Korea
時間：5月10日（周五）10:30
單位：北京師範大學
地點：教四樓110
Black hole physics near extremal black holes differs from nonextremal black holes. The surface gravity and thereby the Hawking temperature vanishes for extremal black holes and Hawking radiation exponentially suppressed to zero. This drastic change originates from the geometry near the event horizon. The near-horizon geometry of near-extremal charged Reissner-Nordstrom black hole has a structure of 〖AdS〗_2×S^2 while the geometry of nonextremal black hole has another structure of 〖Rindler〗_2×S^2 and identifies the Unruh temperature with the Hawking temperature. The near-horizon geometry of near-extremal charged Kerr-Newman has a warped 〖AdS〗_3.
Using the symmetry of the near-horizon geometry and applying quantum field theory of a complex scalar field, the spontaneous pair production of charged scalars is studied from near-extremal nonrotating and rotating, electrically and/or magnetically charged black holes. Analytical expressions for pair production, vacuum persistence, and absorption cross section are found, and the spectral distribution is given a thermal interpretation. The pair production in near-extremal black holes has a factorization into the Schwinger effect in AdS and another Schwinger effect in Rindler space, measuring the deviational from extremality. The associated holographical correspondence is confirmed at the 2-point function level by comparing the absorption cross section ratio as well as the pair production rate both from the gravity and the conformal field theories. The perspective of black hole physics in near-extremal black holes is advanced.
報告人：Matteo Bonnetti，University of Milan
時間：5月10日（周五）12:00
單位：北京大學
地點：first meeting room, KIAA
Massive black hole binaries (MBHBs) are expected to form at the centre of merging galaxies during the hierarchical assembly of the cosmic structure, and are expected to be the loudest sources of gravitational waves (GWs) in the low frequency domain. However, because of the dearth of energy exchanges with background stars and gas, many of these MBHBs may stall at separations too large for GW emission to drive them to coalescence within the Hubble time. Triple MBH systems are then bound to form after a further galaxy merger, triggering a rich dynamics that can eventually lead to MBH coalescences. I will show that the complex 3-body dynamics can lead to the formation of highly eccentric binaries, hence leading to copious GW energy losses. I will analyse the eccentricity distribution of 3-body induced coalescing binaries, paying particular care to those sources that can emit an observable GW signal in PTA or LISA band.
報告人：Daniel Steffensen，Niels Bohr Institute, University of Copenhagen
時間：5月10日（周五）16:00
單位：中科院理論物理所
地點：Seminar Room 322 Old ITP Building
Systems that inherently exhibit topological superconductivity are rare in nature, and the highly coveted Majorana fermions are therefore mainly pursued in engineered hybrid devices. In this talk, I will present how to harness the possible microscopic coexistence of superconductivity and magnetism, as an alternative pathway to craft intrinsic topological superconductors. I will focus on materials with spontaneous textured magnetic order driven by Fermi surface nesting. This work is motivated by the iron pnictides, in which such a coexistence has been shown experimentally, and a recent theoretical analysis has revealed that textured magnetism is also accessible. I will discuss the arising topological superconducting phases in layered multiband materials with magnetic spiral, whirl or skyrmion order, coexisting with various types of spin-singlet superconductivity. The diverse magnetic phases lead to a variety of flat, unidirectional, helical and chiral Majorana edge modes. As I will demonstrate, this multifaceted manifestation of Majorana fermion modes stems from the interplay of topological phases with both gapped and nodal bulk energy spectra.
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