纳米材料结构和性能的理论研究| 本周物理学讲座

1纳米材料结构和性能的理论研究

报告人：杨小宝，华南理工大学物理与光电学院

时间：7月17日（周二）16:30

单位：北京师范大学

地点：物理楼106

纳米材料结构多样，性能丰富，备受关注。本报告主要介绍我们研究组在材料结构稳定性和电子结构方面的工作进展，包括高通量计算、建立模型势、蒙特卡洛树搜索等。1）简单体系的结构演化。运用键能模型，我们研究了碳族、硫族纳米材料的结构稳定性，验证了实验观测到的金刚石量子点的幻数结构以及碲、硒纳米带结构；考虑了构型熵的贡献，解释了不同温度下硅锗纳米体系中原子的分布变化以及碲、硒纳米带的结构演化。2）基于结构识别的高通量第一原理计算。对于C60上的原子替换、硼平面及相应团簇，我们对结构编码实现快速识别，在遍历计算中避免重复结构，保证样本多样性；确定了不同尺寸下稳定的硼团簇结构，被后续的实验证实，讨论了在不同衬底下硼平面结构的演化，预测了一系列具有半导体特性的硼平面单层结构，为硼纳米结构研究提供新的思路，有可能使其成为媲美石墨烯的新型材料。 3）模型建立和结构搜索的普适方案。在高通量第一原理计算的基础上，我们提出了以原子局域环境为基础的能量拟合方法，可以对复杂合金体系进行系统的稳定性研究，并结合紧束缚方法对半导体的能隙进行拟合，采取蒙特卡洛树搜索方案，有效解决结构搜索问题，实现以性能为导向的材料设计。我们将进一步把方法推广到磁性体系，考虑体系构型熵和振动熵对稳定性、电子结构的影响。

2Charge and spin diffusion on the metallic side of the metal-insulator transition: a self-consistent approach

报告人：Rodolfo A. Jalabert，Strasbourg University

时间：7月17日（周二）10:00

单位：北京大学

地点：Room W563, Physics Building

A self-consistent theory, describing the spin and spatial electron diffusion in the impurity band of doped semiconductors under the effect of a weak spin-orbit coupling, is presented. The low-temperature spin-relaxation time and diffusion coefficient are calculated within different schemes of the self-consistent framework. It is shown that, for the case of the electron conductance in the impurity band, the Dresselhaus spin-orbit coupling provides the dominant spin relaxation mechanism in a wide class of zinc-blende semiconductors. The results are then universal and account for the measured spin-relaxation times of materials with very different physical parameters. The Dresselhaus coupling is shown to also play a key role in the spin-relaxation of wurtzite materials, yielding values that compare favorably with existing experiments and that are tunable through an external potential. Furthermore, the Dresselhaus coupling determines the electronic states and spin-relaxation rates in nanowire-based wurtzite semiconductor quantum dots.

3 Resonant x-ray scattering from quantum materials

报告人：Bernhard Keimer, Max Planck Institute for Solid State Research

时间：7月17日（周二）10:30

单位：清华大学

地点：理科楼郑裕彤讲堂

This talk will start by a general introduction to the field of “quantum materials”, including particularly metal oxides that exhibit a large variety of quantum collective phenomena such as superconductivity and complex magnetism. We will then describe how resonant elastic and inelastic x-ray scattering (REXS and RIXS) yield comprehensive, energy- and momentum resolved information about the ground state and excitation spectra these materials. The unprecedented sensitivity of RIXS now allows experiments on dispersive excitations in monolayers and microcrystals of complex quantum materials that were hard to imagine just a few years ago. We will go through several examples where REXS and RIXS have yielded qualitatively new insights: - spin and charge order and dynamics in copper oxides, and their influence on superconductivity, - orbital, spin, and charge order and dynamics in nickel-oxide films and superlattices, and - initial RIXS experiments on dispersive magnetic excitations in 4d metal-oxides.

4 Demystifying the Diverse IR Continuum SED Shapes among Type-1 AGNs

报告人：Jianwei Lyu，Arizona University

时间：7月17日（周二）12:00

单位：北京大学

地点：First floor meeting room, KIAA

In the classical unification scheme, type-1 (broad-line) AGNs are depicted as unobscured accreting supermassive black holes with similar face-on viewings of the circumnuclear optically thick tori. However, the dust SEDs of these systems can have significant variations, as seen among, e.g., the hot-dust-obscured galaxies, extremely red quasars, and hot-dust-free quasars, that cannot be easily explained in a uniform picture. In this talk, I will introduce a semi-empirical framework that has successfully reconciled the IR SEDs of type-1 AGNs seen at z~0 to z~6. In contrast with the complicated numerical torus (+polar dust) models, our simple approach greatly reduces the model degeneracy and provides useful insights on the possibly diverse dust environment among different populations of type-1 AGNs. Combining the knowledge of AGN SED variations with other techniques, such as mid-IR dust reverberation mappings and SED decomposition techniques, I will also talk what constraints we may put on the AGN torus structures and the possible features of z～5-6 quasar host galaxies. 

5Tunable phenomena in graphene: from quantum critical Dirac fluid to engineered Mott insulator

报告人：Feng Wang，physics department，UC Berkeley 

时间：7月17日（周二）13:30

单位：北京大学

地点：Room W563, Physics Building

The electronic structure of graphene depends sensitively on the layer number and stacking. Monolayer graphene is famously known to have massless Dirac electrons, while ABC stacked trilayer can have a remarkably large electron mass at low energy. In this seminar I will discuss unique phenomena that emerge in these two very different systems. Using an on-chip terahertz spectroscopy, we directly probe the quantum critical electrodynamics of massless Dirac fluid in boron nitride-encapsulated monolayer graphene. In ABC trilayer graphene, we exploit the Moire superlattice between graphene and boronnitride to realize strongly correlated behavior such as a tunable Mott insulator.

6最新牛津型动磁式线性压缩机

报告人：梁昆，英国萨塞克斯大学

时间：7月17日（周二）14:30

单位：中科院理化所

地点：1号楼403会议室

7特殊生境真菌复杂结构化合物的生物合成与调控机制研究

报告人：刘 钢，中科院微生物所

时间：7月17日（周二）15:00

单位：中科院物理所

地点：M楼238会议室

以青霉素为代表的微生物次级代谢产物为人类健康作出了巨大贡献。深入了解微生物次级代谢产物的生物合成与调控，是进一步挖掘新型药物先导化合物的基础。讲座将以来源于特殊生存环境的两株丝状真菌为例，介绍该课题组在新型复杂化合物生物合成和调控研究中的部分工作。

8High-fidelity transfer and storage of photon states in a single nuclear spin

报告人：Sen Yang，Chinese University of Hong Kong

时间：7月18日（周三）10:00

单位：北京大学

地点：Room W563, Physics Building

Quantum repeater is one of the key elements to realize long distance quantum communication. In the heart of a quantum repeater is quantum memory. There are a few requirements for this memory: it needs to couple to flying qubits: photon ; it needs to have long coherencetime, so quantum error correction algorithm can be performed in the quantum repeater nods; it needs to be stable under optical illuminations. Nitrogen nuclear spin is available for every nitrogen vacancy center(NV) in diamond. Besides it can be a robust quantum memory forspin qubit operation, nitrogen nuclear spin can couple to photon by taking advantage of optically resonant excitation of spin-selective transitionsin low temperature. Here we demonstrate the coherent storage of quantum information from photon into nuclear spin. We show this quantum memory fulfils requirements as quantum memory for quantum repeater. Coherent time beyond 10 seconds is measured in C13 natural abundant sample.

9Detection of single electron and nuclear spins using scanning tunneling microscopy

报告人：Yishay Manassen，Ben Gurion University

时间：7月18日（周三）10:00

单位：中科院物理所

地点：M楼253会议室

The possibility to detect hyperfine spectrum in ESR-STM experiments opens many possibilities in nano-chemical analysis and in single spin physics. Hyperfine spectra were observed on SiC surfaces (coupling with 29Si nuclei), on Cu atoms on silicon (coupling with 63Cu and 65Cu nuclei) and on Tempo molecules on Au (coupling with 14N nuclei). It is shown that the fingerprints of the levels of the individual nuclei can be observed through rf irradiation of the nuclear levels, monitoring the changes of the intensity of the hyperfine spectra. (STM-ENDOR).
We find that the lineshape of the spectrum can provide motional information as done macroscopically, and show how the affect of  the tunneling current affects the molecular motion, and the linewidth. Additional studies are performed with the purpose to get an ESR-STM spectrum from nonmagnetic molecules and species, and to detect the nuclear Zeeman frequencies of the atoms under the tip.

10Synthesis and applications of metal oxide/sulphide nanomaterials

报告人：Muhammad Aftab Rafiq，Pakistan Institute of Engineering and Applied Sciences

时间：7月18日（周三）10:00

单位：中科院物理所

地点：M楼 236会议室

In this talk synthesis of different nanomaterials (such as nanofibers, nanoparticles, nanoflakes) using simple chemical routes (Solid state reaction method, co-precipitation technique and electrospinning) will be discussed. The characterization results of these nanomaterials using different techniques such as XRD, SEM and TEM will be presented. Then applications of these nanomaterials for gas sensing, humidity sensing, photodetection and water cleaning will be discussed.

11Implication of Higgs Precision Measurement on New Physics

报告人：Yongcheng Wu, Carleton University

时间：7月18日（周三）10:30

单位：清华大学

地点：理科楼C302报告厅

Measurement of Higgs couplings and its properties will be one of the most important target in the current and future experiments. There are some proposals of future lepton collider as Higgs factories. In these machines, we can achieve much higher precision on these measurements. With such high precision, only considering the tree level coupling is not appropriate, loop effects should also be included to give the whole results. With these possible data, using Type-II 2HDM model as an explicit new physics example model, we perform a global fitting including 1-loop corrections to all relevant couplings within several different parameter choices. From the global fitting, we get the lower bound on the mass of the extra scalars in the model which is as expected from the naive effective operator analysis. We also find that with 1-loop correction included, the Higgs measurement can also constrain the mass difference between different scalars. With just hZZ coupling, the constraint will be similar to those coming from the S/T/U parameters. However, when including all other possible couplings, the constrained region will be much different and mis-aligned with that in the S/T/U parameter fittings. Hence, in this sense, the Higgs precision measurement is also complimentary to the Z-pole EW measurement.

12Alkali metals in exoplanet atmospheres

报告人：CHEN Guo，PMO

时间：7月18日（周三）12:00

单位：北京大学

地点：First floor meeting room, KIAA

Exoplanet atmospheres could carry chemical imprints (e.g. metallicity, C/O ratio) to link to planet formation and evolution history. While a large collection of exoplanets have exhibited the water absorption signature at 1.4 micron as probed by HST/WFC3, the determination of water abundances is somewhat hindered by the potential presence of clouds and hazes owing to the degeneracy between pressure and abundance. Fortunately, clear atmospheres could exhibit two major characteristics: Rayleigh scattering from molecular hydrogen and pressure-broadened absorption line wings of alkali metals, which are easily probed by optical transmission spectroscopy. In this talk, I will present the latest results from our transit spectroscopy survey using the low-resolution optical spectrograph at the 10.4 meter GTC telescope. One of the highlights is that we recently detected Na, K, and Li in a super-Neptune and spectrally resolved the pressure-broadened wings of Na and K. This is not only the first time that multiple alkali metals are robustly detected in an exoplanet, but also the first time that lithium is detected in an exoplanet. In addition, I will also present our efforts in using high-dispersion spectrographs (e.g. HARPS-N, CARMENES) to characterize the absorption line cores of alkali metals. 

13Quantum Advantages in Hypercube Games

报告人：段润尧, 百度研究院量子计算中心主任

时间：7月19日（周四）16:30

单位：清华大学

地点：西阶教室

段润尧教授在清华大学获得学士和博士学位，之后在悉尼科技大学任教。从2008年开始担任悉尼科技大学量子计算实验室主任。2018年3月8日，百度宣布成立量子计算研究所，悉尼科技大学量子软件和信息中心创办主任段润尧教授出任百度量子计算研究所所长，直接向百度总裁张亚勤汇报。

14Nucleon Tomography (核子断层扫描)

报告人：ZhihongYe，Argonne National Lab

时间：7月20日（周五）10:00

单位：中科院近代物理研究所

地点：5号楼1006会议室

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