单分子层和扭曲双层石墨烯的磁性和超导相关性 | 本周物理学讲座
1Magnetic and superconducting correlation in monolayer and twisted bilayer graphene
报告人：马天星，北京师范大学
时间：5月23日（周四）16:00
单位：中科院物理研究所
地点：M楼830
Using exact quantum Monte Carlo method, we identify the phase diagram of the half filled, the lightly doped and heavily doped graphene, which shows a rather rich physical properties. At half filling, the system is driven to a Mott insulator with antiferromagnetic long range order by increasing interaction, and a transition from a d+id pairing to a p+ip pairing is revealed, depends on the next-nearest hoping and the electronic fillings. We also examine the recent novel electronic states seen in magic-angle graphene superlattices. From the Hubbard model on a double-layer honeycomb lattice with a rotation angle θ=1.08 , we reveal that an antiferromagnetically ordered Mott insulator emerges beyond a critical Uc at half filling, and with a small doping, the pairing with d+id symmetry dominates over other pairings at low temperature. The effective d+id pairing interaction strongly increase as the on-site Coulomb interaction increases, indicating that the superconductivity is driven by electron-electron correlation. Our non-biased numerical results demonstrate that the twisted bilayer graphene share the similar superconducting mechanism of high temperature superconductors, which is a new and idea platform for further investigating the strongly correlated phenomena.
2Exciton management in TADF OLEDs and their advanced devices
报告人：Chihaya Adachi，Kyushu University
单位：北京大学
时间：5月21日（周二）10:00
地点：物理楼西楼202室
While the demonstration of the first OLED emitting thermally activated delayed fluorescence (TADF) in Advanced Materials in 2009 by our group gave a glimpse of TADF's potential, our successive report in Nature in 2012 of a TADF-based OLED with an internal quantum efficiency of nearly 100% grabbed the attention of researchers world-wide and established TADF as a truly promising technology. First, we mention about the recent advanced TADF OLEDs aiming for blue and high stability by engineering molecular structures and device architectures. Further, we will introduce our recent efforts on the advanced technologies such as long persistent luminescence (LPL), singlet fission and organic semiconductor laser diodes (OSLDs).
3Unusual superconducting state in the nearly ferromagnetic compound UTe2
报告人：Sheng Ran，University of Maryland
时间：5月21日（周二）10:30
单位：中科院物理研究所
地点：M253会议室
Our recent discovery of the novel spin-triplet superconductivity in UTe2 has inspired a lot of interests in the community. Superconducting state of UTe2 closely resembles that of ferromagnetic superconductors, but the normal state is paramagnetic and shows no indication of magnetic ordering. UTe2 exhibits an extremely large, anisotropic upper critical field Hc2, temperature independent NMR Knight shift in the superconducting state, and a large residual normal electronic density of states. All these results strongly indicate that the superconductivity in UTe2 is carried by spin-triplet pairs. Even more striking, superconductivity reenters in the magnetic field of 45 tesla and persists up to 65 tesla, which is the upper limit of magnetic field in our current study. These extreme properties reflect a new kind of exotic superconductivity rooted in magnetic fluctuations. In my talk, I will review our recent results on UTe2.
4Gold for Plasmonics - from condensed matter physics to life sciences
报告人：Hongdan Yan，Braunschweig University of Technology
时间：5月21日（周二）14:00
单位：中科院物理研究所
地点：M253会议室
Periodic gold nanowire arrays are prepared and several light nanoparticle molecule interaction phenomena related to such structures are discussed. A strong coupling between surface plasmons and molecular excitons are experimentally demonstrates, and their hybrid states using a quantum model by quantizing the collective plasmonic behavior are theoretically described. With respect to Surface enhanced Raman Scattering, an approach based on light induced molecular degradations to identify the enhancement contribution from chemical mechanisms are introduced. This work could improve the understanding of surface plasmon-molecule interaction phenomena in metallic nano structures and improve the design of surface plasmon-molecule interaction based sensors, devices and techniques.
报告人：David Ian Jones，University of Southampton
时间：5月21日（周二）14:00
单位：清华大学
地点：蒙民伟科技南楼S727
Magnetars are very strongly magnetised neutron stars, with magnetic field strengths in excess of 10^14 Gauss. In this talk I will describe the physics that determines the spin evolution of rapidly rotating (millisecond) magnetars, early in their lives. The results have implications for their gravitational wave emission, and for the distributions of spins and magnetic field geometries observed in the Galactic magnetar population.
6Gold for Plasmonics - from condensed matter physics to life sciences
报告人：Hongdan Yan，Braunschweig University of Technology
时间：5月21日（周二）14:00
单位：中科院物理研究所
地点：M253会议室
Periodic gold nanowire arrays are prepared and several light nanoparticle molecule interaction phenomena related to such structures are discussed. A strong coupling between surface plasmons and molecular excitons are experimentally demonstrates, and their hybrid states using a quantum model by quantizing the collective plasmonic behavior are theoretically described. With respect to Surface enhanced Raman Scattering, an approach based on light induced molecular degradations to identify the enhancement contribution from chemical mechanisms are introduced. This work could improve the understanding of surface plasmon-molecule interaction phenomena in metallic nano structures and improve the design of surface plasmon-molecule interaction based sensors, devices and techniques.
7Chemical Reaction Revisited by in situ Electron Microscopy and Field Gradient NMR
报告人：Huan Wang，Institute for Basic Science, South Korea
时间：5月21日（周二）15:30
单位：中科院物理研究所
地点：M253会议室
I will discuss unexpected observations and new questions that arise when we apply two techniques conventionally not used for studying chemical reactions. In the first case, inspecting base-pairing of single-stranded DNA at a single-molecule level by using real-time observation electron microscopy in aqueous environment, we observe some anticipated pathways and others that are surprising. In the second case, we find from field gradient NMR measurements and single molecule fluorescence spectroscopy that chemical reaction collectively agitates its surrounding fluids suggesting broad existence of chemistry-induced motility.
8What can be learned from tau -> 3 pi nu_tau for the axial form factor of the nucleon
报告人：Sergi Gonzalez-Solis，Indiana University Bloomington
时间：5月22日（周三）10:00
单位：中科院物理所
地点：Conference Room 322, ITP North Building
Electroweak form factors provide valuable information on the internal structure of the existing composite hadrons, and their understanding is of utmost importance in any theory or model of strong interactions. For the nucleon, the electromagnetic form factors are by far best understood, both theoretically and experimentally, than the axial and the induced pseudoscalar form factors. While the rho(770) and omega(782) vector mesons play an important role in the description of the electromagnetic form factors of the nucleon, the a1(1260) meson presumably dominates the axial-vector current transition. In this talk, we will investigate the axial-vector weak hadronic current by analyzing the 2013 ALEPH experimental data on the tau->3pi nu_tau axialvector spectral function. The latter provides a rather clean benchmark to study the axial-vector form factor since the 3pi system is predominantly in a JPC = 1++ state produced through the a1(1260) axial-vector meson. As a side result, we will discuss what can be learned for the low-Q2 behavior of the axial form factor of the nucleon.
报告人：Dorian Abbot，University of Chicago
时间：5月22日（周三）12:00
单位：北京大学
地点：KIAA 1st meeting room
The ice-albedo feedback on rapidly-rotating terrestrial planets in the habitable zone can lead to abrupt transitions(bifurcations) between a warm and a snowball (ice-covered) state, bistability between these states, and hysteresis in planetary climate.This is important for planetary habitability because snowball events may trigger rises in the complexity of life, but could also endangercomplex life that already exists. This raises the question of how the Snowball Bifurcation might work on tidally influenced planets in thehabitable zone orbiting M and K dwarf stars. We investigate this question using analytical theory, an ocean-atmosphere global climatemodel, and an intermediate complexity global climate model coupled to an active carbon cycle. We findthat planets locked in a 1:1 synchronous rotation state are likely to experience a smooth transition to global glaciation rather than abifurcation. This is important because it means that tidally locked planets with an active silicate-weathering feedback loop should nottend to stay in the snowball state (they would just pop out of it if they ever entered it because weathering would go to near zero whileCO2 outgassing would continue).
报告人：Shuang-Yong Zhou，USTC
时间：5月22日（周三）14:30
单位：中科院理论物理所
地点：ITP South (New) Building 6420
Locality, unitarity and analyticity are some of the most fundamental properties of quantum field theory. I will show how requiring a local, analytic, unitary UV completion for a low energy effective theory (EFT) can impose positivity bounds on the Wilson coefficients in the EFT. These positivity bounds take the form of constraints on combinations of the pole subtracted scattering amplitude and its derivatives. I will first demonstrate the main idea with the simple case of a massive scalar field, and then discuss the technical subtleties that arise when generalising to cases with nonzero spins. These positivity pounds can be applied to any EFTs. As an illustration, I will apply these positivity bounds to a few well-konwn EFTs, including galileon, standard model effective field theory and massive gravity.
报告人：David Ian Jones
时间：5月22日（周三）16:00
单位：北京大学
地点：KIAA-PKU Auditorium
Magnetars are very strongly magnetised neutron stars, with magnetic field strengths in excess of 10^14 Gauss. In this talk I will describe the physics that determines the spin evolution of rapidly rotating (millisecond) magnetars, early in their lives. The results have implications for their gravitational wave emission, and for the distributions of spins and magnetic field geometries observed in the Galactic magnetar population.
12Femto-second light and electron pulses to switch magnetisation
报告人：S.Mangin，University of Lorraine
时间：5月23日（周四）09:30
单位：中科院物理研究所
地点：M楼253会议室
Since the first observation of magnetization switching in ferrimagnetic GdFeCo alloy films using femtosecond laser pulses in 2007, understanding the mechanism behind all-optical switching (AOS) is becoming a topic of huge interest in the magnetism community.Moreover ultrafast magnetization switching in magnetic material thin film without any applied external magnetic field is drawing a lot of attention for the development of future ultrafast and energy efficient magnetic data storage and memories. Two type of all optical switching have then ben distinguished: Helicity Independent – All Optical Switching (HI-AOS) and Helicity Dependent – All Optical Switching (HD-AOS). HI-AOS has only been demonstrated for GdFeCo based material and is observed after a single laser pulse. After one pulse the magnetization is reversed in the opposite direction independently of the light helicity. On the other hand, HD-AOS has been observed for a large variety of magnetic material such as ferrimagnetic alloy,ferrimagnetic multilayer, ferromagnet, and granular media. However several studies shows that HD-AOS is only observed after multiple pulses. During the presentation I will present experimental results showing that the number of pulses can be reduced significantly in order to switch ferromagnetic [Co/Pt] multilayers using only several light pulses. Those results can be explained by considering the transfer of heat and angular from light to the sample’s electron bath. In all the previously reported experiments light is used to manipulate magnetization. However, recently we have engineered multilayer structures in order to create hot electrons femto second pulses. We have demonstrated that the magnetization of GdFeCo can be switched using a femto-second hot electron pulse with no direct light interaction which confirm the work from Wilson et al. Indeed they reported the switching of GdFeCo/Au bilayer via hot electrons generated by single pulse femtosecond laser. Moreover we have studied the magnetization reversal in a GdFeCo / Cu / [Co/Pt] spin valve structure. We observed single shot switching of both the ferrimagnetic GdFeCo and the ferromagnetic [Co/Pt] layer. The magnetisation switching is found to be mediated by spin polarized hot electron transport.
报告人：Lei Hao，SHAO
时间：5月23日（周四）14:00
单位：清华大学
地点：蒙民伟科技南楼S727
Formation and evolution of galaxies is one of the most fundamental aspects of astrophysics. With the advent of integral field spectrographs (IFSs), 2D spectroscopic observations now allow us to study the evolution of galaxies via many physical parameters in great detail. In this talk, I will describe progresses in our understanding on the gas cycling of galaxies and its relationship with the structures and star-formation processes of galaxies and AGNs.
报告人：李卫东，山西大学理论物理研究所
时间：5月23日（周四）14:30
单位：北京大学
地点：物理学院中楼215
A clear consensus on how long it takes a particle to tunnel through a potential barrier has never been so urgently required, due to the attoscecond time resolution in strong-field ionization experiments. A naïve quantum travel time is introduced by the ratio of the travel distance to the expected value of the velocity operator under the barrier. The validity of this definition is checked by considering the analytical solvable mode, rectangular barrier tunneling. In the case of attoclock experiment, the tunneling delay time could be interpreted as a travel time spent by the electron to tunnel from a point under the barrier to the tunnel exit. In addition, a peculiar oscillation structure in the wavelength dependent tunneling delay time in deep tunneling regime is predicted, which is beyond the scope of adiabatic tunneling picture. This oscillation structure can be attributed to the interference between the ground state tunneling channel and the excited states tunneling channels.
15From Band Topology to Density Waves: the Power of High-Dimensional Scattering Spectroscopy
报告人：Yuan Li，Peking University
时间：5月23日（周四）16:00
单位：清华大学
地点：理科楼郑裕彤讲堂
In this talk, I will present two of our recent studies where we have used scattering methods with neutrons and resonant X-rays as probing particles. By performing pixel-power-enabled mapping-styled measurements, which provide data coverage in the three-dimensional reciprocal space plus an extra dimension in energy or real time, we have obtained unprecedented insights into magnetically and charge-ordered systems.
In the first study, we establish the existence of magnon topological “semimetal” bands (with Dirac points) in a spin-1/2 antiferromagnet, Cu3TeO6, which turns out to be surprisingly “classical” in its magnetism because of long-range exchange interactions. The demonstrated topological band structure is new to the zoo of band topology.
In the second study, we take a very close look at the birth of charge density waves (CDW) in a prototypical Peierls CDW material, ZrTe3. We demonstrate a new and rigorous route to differentiate incipient CDW against electronic standing waves, also known as Friedel oscillations (FO). Our result reveals that the CDW formation is actually promoted by self-amplifying FO, and it brings new insights to related phenomena in cuprate high-Tc superconductors.
报告人：Daniel Stark，Steward Observatory
时间：5月23日（周四）16:00
单位：北京大学
地点：KIAA-PKU Auditorium
Over the past decade, deep infrared images have pushed the cosmic frontier back to just 500 million years after the Big Bang, delivering the first large sample of galaxies at redshifts 7<z<10. Sometime in this redshift window the hydrogen in the intergalactic medium transitioned from mostly neutral to ionized. The emergence of the first detailed spectra of z>7 galaxies have begun to sharpen our understanding of this reionization process while also providing a glimpse of the physical nature of early galaxies. The spectral features we are detecting at z>7 are unlike what has been seen at lower redshifts and are proving challenging to interpret, owing in large part to our incomplete knowledge of low metallicity massive stellar populations. In this talk, I will review the latest progress in our understanding of the galaxies in the first billion years of cosmic history and discuss remaining challenges that must be addressed in advance of JWST.
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