低维单质材料中的铁电性和多铁性 | 本周物理学讲座

来源：中国物理学会期刊网

1低维单质材料中的铁电性和多铁性

报告人：陆赟豪，浙江大学

时间：5月7日（周二）10:00

单位：北京航空航天大学

地点：物理学院主楼316会议室

铁电性通常存在于不同元素组成的复合材料中，而铁电材料，特别是二维材料中的铁电性，由于其在信息存储和电学控制领域的巨大应用前景，在最近几年引起了人们的浓厚兴趣。另一方面，几乎所有报道的铁电材料都是由不同种类的原子组成的化合物。据我们所知，单元素材料的铁电性从未在文献中报道过。在这里，虽然很违反常识，但我们基于第一性原理计算，揭示了一系列单质二维体系及表面体系中的自发电极化和本征铁电性和反铁电性。这些单元素材料的铁电居里温度可能高于室温，因此他们有望实现具有广泛应用前景的超薄铁电器件。与此同时，我们也构建一个通过模型来理解和搜索中新型二维铁电和反铁电材料。进一步，我们发现在某些单质体系中还有可能存在多铁性，具有巨大的潜在应用价值。

2Phase transitions in equilibrium and non-equilibrium soft matter

报告人：彭毅，University of Minnesota

时间：5月7日（周二）14:30

单位：北京师范大学

地点：物理楼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.

3Double heavy quarkonia production at Large Hadron Collider

报告人：何志国，汉堡大学

时间：5月7日（周二）16:00

单位：北京师范大学

地点：物理楼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.

4同步辐射X射线吸收谱在能源材料研究中的应用

报告人：姚涛，中国科学大学

时间：5月9日（周四）10:00

单位：北京师范大学

地点：物理楼106

能源与环境是进入21世纪人类面临最富挑战性的两大基本问题，发展高效的新能源技术将自然界中的水和二氧化碳转变为清洁的可再生能源是解决能源危机和环境问题非常有潜力的途径，但同时也面临巨大挑战。近年来，我们研究组发展了一系列同步辐射原位X射线吸收谱学技术，实现原子级水平监测能源材料工作状态下结构演变的动态表征。克服了催化材料与电解质溶液接触的固-液表界面处的活性中心浓度低、以及活性位原子结构随外加电场的动态变化等困难，从实验上精确鉴别出单原子催化材料在电催化析氢反应过程中活性位点的真实结构和动态演化过程《Nature Catalysis 2019, 2, 134》；利用同步辐射X射线吸收谱技术设计并明确了氮化碳纳米空间限域效应形成单位点钴基光催化剂，实现了太阳光照条件下高效的全解水产氢性能《Angew. Chem. Int. Ed. 2017, 56, 9312》，工作被选为“Frontispiece”进行亮点报道；进一步利用同步辐射技术并结合第一性原理计算从原子尺度上揭示了光催化剂中“Co1-N4”单位点活性中心结构、及其加速产氢反应路径的机理《Angew. Chem. Int. Ed.2017, 56, 12191》；基于同步辐射谱学技术，揭示了二维超薄材料表面不饱和配位和独特的扭曲结构对提高光生电子-空穴对分离效率、提高电催化还原CO2活性的影响《Angew. Chem. Int. Ed. 2016, 55, 6716》，《Nat. Commun. 2016, 7, 12697》。这些研究工作扩大了同步辐射技术在能源材料领域的应用范围，同时也为揭示光电催化剂可控构筑和性能剪裁的内在本质，设计具有高效稳定和实用型的光电催化剂材料提供了理论基础。

5Towards Model-Independent Cosmological Probes in Weak Lensing

报告人：Jun Zhang，Shanghai Jiaotong Univ.

时间：5月9日（周四）14:00

单位：清华大学

地点：蒙民伟科技南楼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.

6CMB & 21cm on New Physics Energy Injections

报告人：高宇，中科院高能所

时间：5月9日（周四）14:00

单位：清华大学

地点：物理系理科楼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针尖上的舞蹈---多铁性纳米阵列中的畴与畴壁

报告人：刘俊明，南京大学物理学院

时间：5月9日（周四）16:00

单位：清华大学

地点：理科楼郑裕彤讲堂

具有纳米尺度极化空间分辨的压电力显微术(PFM)，借助不断集成的新方法，为研究各类材料中极性畴结构的基本性质和揭示微区新效应提供了有力支撑。作为 PFM 技术的一个初级用户，报告人所在团队利用这一技术对铁电与多铁性纳米阵列结构中的畴、畴壁、多铁异质结等体系开展了一些初级探索，涉及到拓扑中心畴、畴壁导电、电控磁性等问题。作为班门弄斧之作，整个报告内容略显零散，姑且就教于各位老师和同学。

8Black Hole Physics on Near-Extremal Horizons

报告人：Sang Pyo Kim，Kunsan National University, Korea

时间：5月10日（周五）10:30

单位：北京师范大学

地点：教四楼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.

9Massive black hole triplets: implications for PTA and LISA

报告人：Matteo Bonnetti，University of Milan

时间：5月10日（周五）12:00

单位：北京大学

地点：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.

10Intrinsic Topological Superconductivity Induced by Textured Magnetic Order

报告人：Daniel Steffensen，Niels Bohr Institute, University of Copenhagen

时间：5月10日（周五）16:00

单位：中科院理论物理所

地点：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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