報告人：Haoliang Liu，University of Utah
時間：7月4日（週三）10:00
單位：中科院物理所
地點：M 樓 236會議室
As the limitations of electrical circuitry are reached, new paradigms are needed to develop future generations of fast and reduced energy dissipation (less lossy) miniature information processing devices. Nearly all existing electronic devices are based on controlling the electric chargeby electric fields. Electrons also possess a “spin” that gives them their magnetic properties. Spin density fluctuations in magnetically ordered materials such as ferromagnetic metals and insulators can propagate in the form of spin waves, i.e., quantized by magnons. Magnons are considered as a potential data carrier in information storage and transport,known as Magnonics. Magnonics is less prone of the inherent weakness of conventional electronics, i.e., Joule heating. Up to now, magnetic insulator, such as yttrium iron garnet (YIG),has been the main candidate for magnonic devices, which can transfer information over macroscopic distance due to its low Gilbert damping. However, YIG films used for magnonics are difficult to grow and integrate into device structures, since they are epitaxially grown as single crystals at very high temperature. Here we report an alternative candidate for magnonic devices, namely organic magnetic compounds. Attempts to measure magnons in an organic based magnet will be discussed.
報告人：Ajaya K. Nayak，National Institute of Science Education and Research (NISER)
地點：M樓253會議室
So far, magnetic materials with collinear spin alignments have dominated the field of spintronics. It was recently proposed that the back and forth motion of an atomically engineered chiral domain wall can provide a basis for novel high density solid-state storage memory devices - Racetrack Memories [Science 320, 190 (2008)]. However, the threshold current to move such non-collinear domain walls is much higher than that expected theoretically. In contrast, much lower threshold currents have been found for the motion of recently discovered skyrmions, which are topologically stable vortex-like objects with a twisted spin configuration [Nature 465, 901 (2010)]. Magnetic materials with broken inversion symmetry, where the asymmetric Dzyaloshinskii–Moriya interaction modifies the uniform magnetic state to a swirling state, are considered to be suitable candidates to achieve skyrmions. In this direction, the tetragonal Heusler compounds with D2d crystal symmetry are potential candidates for realizing antiskyrmions. This has been demonstrated in the recent work that show existence of antiskyrmion lattice over a wide temperature interval up to 400 K in Mn-Pt-Sn based Heusler materials [Nature 548, 561 (2017)]. I will also present some of the very recent results where we found a large topological Hall effect in some new Heusler materials and some potential candidates for antiferromagnetic antiskyrmions.
報告人：Tianquan Lian，美國埃默裡大學
時間：7月4日（週三）10:00
單位：中科院理化所
地點：1號樓407會議室
報告人：Chen-Liang Huang，UNLV
時間：7月4日（週三）12:00
單位：北京大學
地點：KIAA, 1st meeting room
Interacting supernovae, including type IIn and Ia-CSM, are supernovaethat show evidence of strong shock interaction between their ejectaand pre-existing circumstellar material (CSM), which may be ejectedfrom the unstable progenitor star before the explosion. After thesupernova shock wave has broken out of the progenitor star, theionizing radiation from the shock region is able to ionize thesurroundings. The CSM can have substantial optical depth to electronscattering and the continuum photosphere is in the unshocked CSM dueto the electron scattering opacity. The iconic feature of theinteracting supernovae are the broad wings (1000's of km\s) on narrowemission lines. In the talk, I'm going to explain the line formationof this feature, by adopting the idea that the narrow emissions arecreated in the preshock ionized CSM by recombination cascades, and theline profiles are broadened by the electron scattering for the photonsthat make their way out.
報告人：Jian-Min Yuan，Drexel University
時間：7月5日（週四）15:30
單位：北京師範大學
地點：物理樓106
Human diseases, such as cancers, diabetes, and heart diseases, are often related to networks of cellular signaling pathways. In view of their importance, we have applied thermodynamic, physical, and engineering methodologies in the investigation of mathematical models of signaling pathways. The goals are to understand the dynamics, the effects of cross-talks and feedback loops, and design principles of these pathways. Related to cancers, we have analyzed a mathematical model of the coupled MAPK and PI3K pathways, two better-known pathways of the Ras network. For diabetes, on the other hand, we have studied phenomenological models of Quon, et al. Time-dependent sensitivity analysis has been used to reveal information about the time scales and the interaction strengths of protein-protein interactions, intricate interplays of cross-talks and feedback loops, as well as the rankings of vulnerable nodes of these pathways. On the other hand, non-equilibrium thermodynamic concepts, such as affinities, fluxes, energy dissipated rates, and efficiency, allow us to probe deeper into the design principles of these pathways.
報告人：Eric D. Feigelson，Penn State Univ
時間：7月5日（週四）16:00
單位：北京大學
地點：KIAA-PKU Auditorium
While photometric surveys of normal stars for exoplanet transits have revealed many candidate planets, the effort is limited by non-Gaussian noise. This typically arises from stellar magnetic activity for space-based missions (Kepler, Corot, TESS) and from atmospheric effects for ground-based projects (WASP, HAT, AST3-1). Most treatments of these extraneous noise use nonparametric techniques, but we have found that parametric autoregressive ARIMA models are often very effective. This talk reviews the fitting of ARIMA models to reduce autocorrelated noise, the development of a Transit Comb Filter to find periodic transits in the ARIMA residuals, and application of a Random Forest classifier to reduce False Positives. Application to the Kepler mission data set discovers several dozen new planetary candidates. Preliminary examination indicates the method should allow planet detection for densely-cadenced ground-based surveys like HAT-S and AST3-1. ARIMA modeling may also valuable for studies of autocorrelated astrophysical phenomena like stellar activity and systems where the light is dominated by accretion onto compact objects.
報告人：Thomas Mehen，Duke University
時間：7月6日（週五）10:00
單位：中科院理論物理所
地點：Conference Room 322, ITP main building
This talk will describe new tests of quarkonium production using quarkonia that are produced within jets. We study the distribution in the fraction z of a jet's longitudinal momentum carried by the quarkonium. The z distribution is sensitive to the underlying NRQCD production mechanism. Analytic calculations of the z distributions in SCET that incorporate Next-to-Leading-Log (NLL) resummation disagree with default PYTHIA predictions. We describe a modified simulation method which agrees well with NLL analytic calculations. This method is then successfully applied to recent LHCb measurements of J/psi within jets. We discuss the implications of this measurement for extractions of NRQCD long-distance matrix elements. Finally, we discuss other observables involving quarkonium within jets which may be useful for discriminating between NRQCD production mechanisms.
報告人：Raphaël Duque
時間：7月6日（週五）12:00
On August 17 2017, gravitational waves from the inspiral phase of a binary neutron star merger were detected. Then, with a slight delay, a weak short gamma-ray burst GRB170817A was observed. These two historical detections triggered a hitherto unseen series of astronomical observations. First, follow-up ground-based searches inside the uncertainty region inferred by the gravitational wave signal promptly found an optical transient--the first observed kilonova--in the spheroidal galaxy NGC4993. Then, at the position of this optical transient was later discovered a multi-wavelength afterglow emission, which is still detected today. This afterglow likely originates from shock-accelerated electrons during the deceleration by the external medium of matter ejected from the merger event. In this talk, we will briefly describe these multi-messenger observations, and review some teachings the afterglow emission brings regarding the merger event: geometry and dynamical structure of the ejected matter, characteristics of the external medium, the possibility for a relativistic jet to have been produced, and finally the formation of GRB170817A and short gamma-ray bursts from merger events in general.
報告人：Grigory Kolesov，Harvard University
時間：7月6日（週五）14:00
Titanium dioxide is one of the best studied photocatalytic materials with numerous proposed applications, ranging from water photooxidation for hydrogen production to cleanup of environmental pollutants and self-cleaning coatings. Photocatalytic methoxy splitting on rutile TiO2 (110) surfaces that leads to formation of formaldehyde has previously been observed in STM and TPD experiments. In this work we computationally study non-adiabatic excited-state dynamics of methoxy splitting on TiO2 surfaces. Such computations are expensive, and in order to perform them we developed an efficient methodology based on time-dependent density functional theory (TDDFT). We use this methodology to study the excited-state trajectories that lead to conversion of metoxy to formaldehyde. We examine in detail electron and hole trajectories that lead to this reaction. We observe the hole migrating to adsorbed methoxy molecule and driving the reaction, while the remaining electron polarizes the crystal lattice and settles in a polaronic state. Polarons are distinctive feature of the rutile titania. We study them in bulk rutile TiO2 both experimentally, using Raman spectroscopy of photo-excited samples, and computationally, with the TDDFT mixed quantum-classical method. From simulation we find that small polaron formation in rutile titania is a strongly non-adiabatic process with the characteristic time scale of about 55 fs. In both experiment and theory we observe an unexpected stiffening of the A1g phonon mode under UV illumination. We computationally analyze the polaron structure and explain the observed effect. The resulting form of the potential in respect to oxygen atoms and t2g-orbitals of central Ti atom offers a possible explanation for an anomalous temperature-dependence of the Hall mobility in rutile titania, which, in contrast to drift mobility, is decreasing with the increasing temperature.
報告人：Eugene Demler
時間：7月6日（週五）16:00
單位：中科院理論物理所
地點：Conference Room 6620, ITP new Building
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