__物理系学术报告____ Physics
Department Colloquium__

**12**月**26**日，周一，**16:00-17:00**，教**12-201**

**Nonperturbative**** effects of topological terms in Nonlinear
sigma models and Gauge Theories**

Cenke Xu

University of California at Santa
Barbara

摘要

Topological terms can
arise in condensed matter systems that involve topological fermion band
structure. We will first discuss the effect of the topological Theta-terms of
2+1 dimensional nonlinear sigma models (NLSM). Using nonperturbative
arguments, we conclude that the Theta term will strongly affect the disordered
phase of this NLSM: when Theta = Pi, the disordered phase is either a gapless
conformal field theory, or gapped and two fold degenerate. This gapless
conformal field theory induced by the Theta term is a stable 2+1 dimensional
algebraic spin liquid with power law correlations between physical quantities.
We will also generalize this result to other NLSMs and gauge theories with the
topological Theta term using a unified formalism.

**许岑科教授简介**

**Cenke**** Xu** graduated from Tsinghua
University with a Bachelor’s degree in 2003. In 2007 he received Ph.D. in
physics from University of California, Berkeley. After that Cenke
Xu joined Harvard University Society of Fellows as a Junior fellow. In 2009 Cenke Xu started his current position in UC Santa Barbara as an
assistant professor. In the last one year Cenke Xu has received the Alfred P. Sloan Fellowship, and OCPA
Outstanding Young Researcher Award. Cenke Xu’s research interests have been focusing on strongly
correlated systems, especially the topological effects in strongly correlated
systems.