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CHY Formulae for Amplitudes from Higher-Dimensional Operators
报告人：张勇，北京师范大学
时间：6月7日（周三）15:30
单位：中科院理论物理研究所
地点：Conference Hall 322
We study tree-level amplitudes from higher-dimensional operators, including F^3 operator of gauge theory, and R^2 , R^3 operators of gravity, in the Cachazo-He-Yuan formulation. As a generalization of the reduced Pfaffian in Yang-Mills theory, we find a new, gauge-invariant object that leads to gluon amplitudes with a single insertion of F^3 , and gravity amplitudes by Kawai-Lewellen-Tye relations. When reduced to four dimensions for given helicities, the new object vanishes for any solution of scattering equations on which the reduced Pfaffian is non-vanishing. This intriguing behavior in four dimensions explains the vanishing of graviton helicity amplitudes produced by the Gauss-Bonnet R^2 term, and provides a scattering-equation origin of the decomposition into self-dual and anti-self-dual parts for F^3 and R^3 amplitudes.
6
A pressure-induced topological phase with large Berry curvature in Pb1-xSnxTe
报告人：LIANG Tian，Stanford University
时间： 6月8日（周四）10:00
单位：中科院物理研究所
地点：M楼236
The picture of how a gap closes in a semiconductor has been radically transformed by topological concepts. Instead of the gap closing and immediately re-opening, topological arguments predict that, in the absence of inversion symmetry, a metallic phase protected by Weyl nodes persists over a finite interval of the tuning parameter (e.g. pressure P). The gap re-appears when the Weyl nodes mutually annihilate. We report evidence that Pb1-xSnxTe exhibits this topological metallic phase. Using pressure to tune the gap, we have tracked the nucleation of a Fermi surface droplet that rapidly grows in volume with P. In the metallic state we observe a large Berry curvature which dominates the Hall effect. Moreover, a giant negative magnetoresistance is observed in the insulating side of phase boundaries, in accord with ab initio calculations. The results confirm the existence of a topological metallic phase over a finite pressure interval.
7
Collective modes in chiral (pseudo)relativistic matter
报告人：Igor Shovkovy，Arizona State University
时间： 6月8日（周四）10:00
单位：中科院高能物理研究所
地点：Theoretical Physics Division，319
8
Black Phosphorus and Beyond
报告人：Li Yang，the Washington University in St Louis
时间： 6月8日（周四）10:00
单位：北京师范大学物理系
地点：物理楼110
I will present our studies of the unusual electric, optical, thermal, and piezoelectric properties of a new family of two-dimensional (2D) materials, black phosphorus and its corresponding group IV-VI isoelectronic materials. For black phosphorus, we predict the intrinsic, fundamental properties such as quasiparticle band gaps, excitons, and anisotropic thermal conductance, and further engineered properties such as strain-tunable anisotropic electrical conductance and realizing Dirac cones and graphene electronics.
9
Road to Higher Tc Superconductivity
报告人：Shin-ichi Uchida，University of Tokyo
时间： 6月8日（周四）14:00
