报告人:王楠林,北京大学
时间:11月15日(周四)16:00
单位:清华大学
地点:理科楼郑裕彤讲堂
光谱测量是研究光与物质相互作用的主要实验技术之一。对于固体电子系统,光谱探测在揭示电荷动力学响应、对称性破缺体系的单粒子和集体激发、电子关联效应以及验证理论模型等众多方面都发挥着重要作用。本报告将首先简介固体光谱的物理基础和测量技术,之后着重介绍近期我们利用强场超短脉冲激光激发对高温超导体开展的超快时间分辨太赫兹光谱方面研究工作。我们研究了铜氧化物高温超导体光激发所引起的c-轴方向电荷动力学变化,发现近红外或中红外超短脉冲激光激发会诱导铜氧化物高温超导体出现新的集体激发模式,使得体系从一个具有均匀Josephson耦合的超导态进入到一个新的具有不同Josephson耦合强度调制的亚稳超导态。
报告人:杨一玻,中科院理论所
时间:11月13日(周二)12:00
单位:中科院理论物理所
地点:理论物理所322报告厅
量子色动力学(QCD)是描述强相互作用及强子结构的基本理论。因为涉及强耦合,其严格求解是极具挑战性的理论难题。格点QCD是从QCD第一性原理出发研究夸克和胶子性质的数值规范场论方法。借助超级计算机的发展和计算技术的进步,多年来已经在计算强子谱与强子结构上获得了诸多进展。我将结合一些代表性的应用,介绍格点QCD作为紫外正规化方案和红外行为非微扰计算框架的基本概念。
报告人:Emanuele Daddi,Saclay Nuclear Research Centre
时间:11月13日(周二)16:00
单位:北京大学
地点:KIAA-PKU Auditorium
I will discuss recent attempts to better constrain the nature of star forming galaxies at cosmic noon (z~1-3), the relation between starformation activity and gas reservoirs, and the role played by galaxy mergers. This will include galaxies across the main sequence, displaying various level of activity: from most active starbursts, to typical star forming galaxies, to quiescent objects.Do all these follow a universal law, and which one ? This requires, critically, an understanding of how to measure gas masses,which is notably a difficult endeavour. New promising results suggest that Carbon lines, both atomic and ionized, might be helpfultracers. On the other hand, at high redshift both quiescent and main sequence galaxies reveal themselves different from what we had been typically expecting.
报告人:李金华,中科院地质与地球物理研究所
时间:11月14日(周三)14:00
单位:中科院物理研究所
地点:M楼253
地球有磁场和生命,地磁场约在35亿年前已经形成,而地球上的生命诞生于35亿年前后。地球上生命的繁衍生息是在地磁场中进行,与之协同演化并从中受益。研究表明,从低等的细菌到高等的鱼类、海龟、鸟类和哺乳类等一些种类的生物能够感知并利用地磁场进行定向或长距离迁徙。趋磁细菌是一类能在细胞内合成纳米尺寸磁性晶体颗粒-磁小体,并借此感知外界磁场从而沿磁力线游泳的原核微生物。趋磁细菌是生物地磁学和生物矿化研究的重要内容,对理解地球磁场-生命-环境三者协同演化过程有重要意义。作为生物成因纳米磁性颗粒,磁小体的功能化改造对开发新型生物纳米磁性材料有重要价值。
5Schrodinger, Klein-Gordon and Dirac equations for atoms and Operator Product Expansion
报告人:Yu Jia,IHEP
时间:11月14日(周三)14:30
单位:中科院理论物理所
地点:Conference Room 322, ITP main building
Effective field theory (EFT) and operator product expansion (OPE) are two modern theoretical tools in the realm of particle physics. In this talk, I show these powerful tools can also provide novel and deep insight on understanding short-range features of atomic wave functions. In the first half of the talk, I will start from the simple nonrelativistic Coulomb-Schrodinger EFT, then employ OPE technique to investigate the universal electron-nucleus coalescence behavior of the Schrodinger wave function for an arbitrary atom. An exact OPE relation is proved to all orders in perturbation theory. Our formalism can be readily extended to ascertain the multi-particle coalescence behaviors of atomic wave functions, which are otherwise difficult to achieve from the nonperturbative Schrodinger equation approach. In the second part of my talk, I will illustrate how to solve a 90-year-old puzzle in relativistic wave mechanics, which is related to the renowned Klein-Gordon equation and Dirac equation with a Coulomb potential, from the angle of EFT, OPE and renormalization group equation. In particular, I will demonstrate that, somewhat counterintuitively, the peculiar weakly divergent coalescence behaviors of the Klein-Gordon and Dirac wave functions for a S-wave hydrogen atom, can be best understood in the framework of a nonrelativistic EFT supplemented with certain types of relativistic corrections.
6Research on two-dimensional superconductivity using multifunctionality of electric double transistors
报告人:Tsutomu Nojima
时间:11月14日(周三)16:00
单位:北京大学
地点:Room W563, Physics Building
Recently, two-dimensional (2D) superconductivity with highly crystallinity has been an active research area accompanied with the technological advances in thin film fabrication, such as MBE, exfoliation, and ionic gating. Among them, the ionic gating by electric double layer transistor (EDLT) is a convenient technique to access the clean 2D superconducting state. In this seminar, I will discuss two topics of our recent progress in the research on 2D superconductors.
The first topic is the quantum phase transition (QPT) occurring at low temperature near zero, which is one of the remarkable phenomena of 2D superconductors. Such a QPT was predominantly investigated as a superconductor-insulator transition (SIT) tuned by magnetic field in dirty superconductors. Based on the detailed analyses of magneto-transport properties in ZrNCl- and Mo2-EDLT, we provide a totally different view of magnetic field?temperature (B-T) phase diagram, containing QPT, in highly crystalline 2D superconductors. These clean 2D superconducting systems commonly show the wide range of a quantum metallic state in out-of-plane magnetic field, which transfer to the weakly localized metal through the quantum critical point QCP characterized by quantum Griffiths singularity. I discuss such a phase diagram appears as a generic feature of highly crystalline 2D superconductors.
In the second topic, I introduce our recent progress in the study on ultrathin FeSe films, which have been uniquely evolved into a high transition temperature Tc superconductor above 40 K. We have developed an electrochemical etching technique, which is another functionality of EDLT, as a new access to the thin limit. Based on the thickness and gating material dependence of the critical parameters, such as Tc, Bc2 and Jc, we will discuss the robustness of high-Tc phase of FeSe in the thin limit and the increase in transport Tc around 50 K.
报告人:于晓东,University of Siege
时间:11月15日(周四)09:30
单位:北京理工大学
地点:中心教学楼501
Coherence is a basic phenomenon in quantum mechanics and considered to be an essential resource in quantum information processing. Although the quantification of coherence has attracted a lot of interest, the lack of efficient methods to measure the coherence in experiments limits the applications. We address this problem by introducing an experiment-friendly method for coherence and spectrum estimation. This method is based on the theory of majorization and can not only be used to prove the presence of coherence, but also result in a rather precise lower bound of the amount of coherence. As an illustration, we show how to characterize the freezing phenomenon of coherence with only two local measurements for any N-qubit quantum systems. Our approach also has other applications in quantum information processing, such as the characterization of distillability and entanglement transformations.
报告人:侯慧龙,美国马里兰大学材料科学与工程系
时间:11月15日(周四)10:00
单位:中科院物理研究所
地点:M楼238会议室
弹卡制冷是一类新兴的、具有理论高能效的制冷机制 (2012年兴起),与磁卡制冷(90年代兴起)一起成为取代蒸汽压缩制冷机制的潜在可行手段。目前,弹卡制冷材料与系统的研究正在全世界范围内快速发展。本报告的第一部分将介绍弹卡制冷系统的思考、设计与展示,着重解决弹卡材料的高应力驱动需求和磁卡材料的高磁场要求的问题, 实现多铁性复合材料器件在超低磁场下(0.16 T)降温4 K的功能。第二部分将介绍组合式合成与高通量表征在探索弹卡制冷合金中的相关应用,实现单次沉积、近二百次的测量,测量范围包括合金组分、晶体结构(与美国斯坦福直线加速器中心合作)、电阻和相变潜热(与美国国家标准和技术研究所合作)等。同时,针对高通量表征的实验结果,报告人将介绍自主研发的基于MATLAB 的可视化软件CombiView,可实现快速比对、分组、晶体相识别和晶体相比例分析。最后,报告人将对弹卡制冷与高通量表征的未来趋势做一些展望。
9Metamaterials and Topological Mechanics
报告人:Tom Lubensky
时间:11月15日(周四)10:00
单位:北京大学
地点:Room W563, Physics Building
Metamaterials are engineered to have properties not found in nature, such as a negative optical index of refraction, one-way light or vibration waves, or exotic elastic behavior. Made-to-order structures at length scales as short as a micron can now be fabricated with advanced materials processing like 3D printing. Topology is a unifying mathematical concept related to properties that do not change under continuous changes in parameters. Topological materials, like topological insulators, have bulk excitation spectra characterized by topological invariants, associated with the opening of bandgaps, that determine physical properties like the Hall conductivity or the nature of edge excitations. This talk will discuss mechanical metamaterials with topologically protected zero-energy edge states in an idealized limit. These states exist at every wavenumber on a surface so that any surface shape distortion costs no energy. Changing the topological class of the material causes one or more zero modes per wavenumber to move from one side of a sample to the opposite creating rigid and supersoft edges .
10Self-anisotropizing universe and gravitational waves in Horndeski theory
报告人:Hiroaki Tahara,University of Tokyo
时间:11月15日(周四)10:00
单位:中科院理论物理所
地点:Conference Room 6420, ITP new building
As opposed to Wald’s cosmic no-hair theorem in general relativity, we find that the Horndeski theory admits anisotropic inflationary attractors if the Lagrangian depends cubically on the second derivatives of the scalar field. We name such a solution a self-anisotropizing inflationary universe, because anisotropic inflation can occur without introducing any anisotropic matter fields such as a vector field. Next, we investigate perturbations in homogeneous anisotropic background, and find that speed of gravitational waves depend on direction of its propagation. This investigation reveals, however, that the speed of gravitational waves in some directions diverges in the self-anisotropizing inflationary background and it suggests unfitness for representing our present universe. On the other hand, it is also revealed that as long as our universe is anisotropic, speed of gravitational waves can have angular dependence. Such a nature is not seen in general relativity, and it shows us abundance of phenomenology in Horndeski theory.
11Serendipitous discovery of a dying AGN and searching for more fading/dying AGN(s)
报告人:Kohei Ichikawa
时间:11月15日(周四)12:00
单位:北京大学
地点:KIAA, 1st meeting room
Arp 187 is one of the fading active galactic nuclei (AGN), whose AGN activity is currently decreasing in luminosity, and is a good laboratory to obtain the crucial information on the accretion disk physics and the lifetime of AGN. We investigate the observational signatures of AGN in Arp 187, which trace various physical scales from less than 0.1 pc to the nearly 10 kpc, to estimate the longterm luminosity change over 10^4 years. The VLA 5 GHz, 8 GHz, and the ALMA 133 GHz images reveal bimodal jet lobes with ~5 kpc size and the absence of the central radio-core. The 6dF optical spectrum shows that Arp 187 hosts narrow line region with the estimated size of ~1 kpc, and the line strengths give the AGN luminosity of Lbol = 1.5x10^{46} erg/s. On the other hand, the current AGN activity estimated from the AGN torus emission gives the upper bound of Lbol < 2.2x10^{43} erg/s. The absence of the radio-core gives the more strict upper bound of the current AGN luminosity of Lbol < 8x10^{40} erg/s, suggesting that the central engine is already quenched. These multi-wavelength signatures indicate that Arp 187 hosts a ``dying'' AGN: the central engine is already dead, but the large scale AGN indicators are still observable as the remnant of the past AGN activity. The central engine has experienced the drastic luminosity decline by a factor of ~10^{3-5} fainter within ~10^4 years, which is roughly consistent with the viscous timescale of the inner part of the accretion disk within ~500 years. We also propose that those dying AGN could be found using the AGN indicators with a different physical scale such as 12 um band luminosity tracing AGN torus (R~10 pc) and the optical [O III]5007 emission line tracing narrow-line regions (R=10^{2-4} pc).
报告人:Thomas H. Reiprich,Bonn University
时间:11月15日(周四)16:00
单位:北京大学
地点:KIAA-PKU Auditorium
The massive dark matter halos that host galaxy clusters can be well traced by X-ray satellite observatories. The intracluster gas trapped in the deep potential wells gets heated to 10s of millions of Kelvin, emitting thermal bremsstrahlung at X-ray wavelengths. The latest cosmological constraints from X-ray selected galaxy clusters will be discussed. A new test to constrain the cosmological luminosity distance anisotropy using galaxy clusters is introduced and results indicating a violation of isotropy are shown. New X-ray selected very extended nearby galaxy groups and clusters have been discovered in ROSAT All-Sky Survey images at positions where no X-ray source was found previously; the properties of the cluster candidate sample (>1,000 in total) and implications for cosmological constraints from clusters are discussed. The current status of and prospects for the eROSITA telescope to be launched aboard the SRG mission early next year are outlined. The expectations for the ESA L-class mission Athena to discover and characterize early galaxy groups, massive and evolved enough to contain >10 million Kelvin gas, above redshift 2 are quantified.
13宇宙大爆炸锂问题
报告人:何建军,中科院国家天文台
时间:11月15日(周四)16:00
单位:北京师范大学
地点:物理楼106
1946年,乔治·伽莫夫首次提出了宇宙大爆炸理论。目前,它已成为广为人们接受的宇宙学标准模型。原初大爆炸核合成BBN始于宇宙大爆炸约3分钟之后,随着扩张着的宇宙温度和密度的逐渐降低,核反应约一个小时后熄灭,该原初核合成过程结束。该原初BBN过程产生的大部分是氢和氦,还有几种少量的轻核素,即2H,3He和7Li,而其他的核素含量微乎其微。这些遗迹为人们研究早期的宇宙提供了独一无二的窗口。
目前,利用标准BBN理论结合威尔金森微波各向异性探测器WMAP精确的宇宙重子密度结果可以对原初轻核的丰度做出严格的预言。其中D和4He的天文观测结果和理论预言符合得很好,但是,BBN+WMAP预言的原初7Li丰度要比观测的大3~4倍。这种在4~5σ置信度区间的不匹配或差异,被称之为宇宙锂问题。在过去的十几年里,人们做了很多尝试,但是传统的核物理都未能在标准的大爆炸模型下解决该问题。报告将介绍我们团队提出的一种解决宇宙锂问题新的思路和方法,期望能给年轻人一些启发。
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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