报告人：Peter Palffy-Muhoray，Kent State University
时间：7月31日（周二）16:00
单位：中科院理论所
地点：Conference Room 322, ITP main building
Both repulsive and attractive molecular interactions can give rise to orientational order. Using a density functional approach we have examined the relative contributions of these two interactions to nematic order in the high density regime. We find that in the purely steric case, the orientational probability distribution exhibits a unique feature: it has compact support in configuration space. In the presence of attractive interactions, the behavior changes, and at a critical interaction strength, Maier-Saupe type thermotropic behavior is regained. The value of the order parameter at the NI transition may indicate the relative contributions of repulsive and attractive interactions.
报告人：Fei Teng，Uppsala University
时间：8月1日（周三）14:00
单位：中科院理论所
地点：Conference Room 6420, ITP new building
Quantum field theories with four-derivative kinematic terms have ghostlike non-planewave states, in addition to usual physical planewave states. They are non-unitary, but often have better ultraviolet behavior, which might provide an ultraviolet completion of two-derivative theories. Examples of four-derivative theories include conformal gravity, (DF)^2 theory, etc. In this talk, I will start with factorization property of general four-derivative theories using the technique of mass deformation, which trades non-planewave states into massive planewaves. Then I will classify minimal versus non-minimal four-derivative theories, and discuss general features of the amplitudes. With the above knowledge in mind, I will use SU(1,1)/U(1) coset formalism to construct minimal conformal gravity Lagrangian, and most importantly, identify the non-minimal Lagrangian that produces the Berkovits-Witten amplitudes.
报告人：Binghai Yan
时间：8月2日（周四）09:30
单位：清华大学
地点：物理系理科楼C302
The past decade has witnessed intensive research aimed at investigating the role of band-structure topology and discovering topological materials. In this talk, I will introduce our most recent progress on novel topological phases discovered in a well-known family of compounds, transition metal dichalcogenides (WTe2 and MoTe2). These phases include the Weyl semimetal, the monopole nodal ring and the higher-order topological insulator. Further, their band-structure topology gives rise to an exotic nonlinear quantum Hall effect that exists with the time-reversal symmetry, distinct from the ordinary Hall effect.
报告人：Taejin Lee，Kangwon National University
时间：8月3日（周五）10:00
I will introduce the covariant string field theories and the Polyakov string path integrals in the proper-time gauge. In the proper-time gauge, the string path integral can be written as integrals over the proper-times in a way similar to the Schwinger's proper time representation of Feynman integrals of quantum field theory. For this reason, it becomes feasible in the proper-time gauge to identify the field theoretical expressions of the string path integrals which depict multiple string scatterings. I will discuss the four-massless-particle scattering amplitude in open string theory and the four-graviton scattering amplitude in the closed string theory. The covariant string field theories are useful to study the ultraviolet behavior and the entanglement entropy of strings which are quite different from their counterparts of point particles.
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