Global symmetry is a key factor in determining the universality class  of the long-distance, low-energy physics for quantum many-body systems. In recent years, it has been appreciated that unlocking the  full power of symmetry requires the study of topological defects. In the first part of the talk, we will discuss how symmetry and the  associated notion of anomaly explain the subtle interplay between  lattice effects and continuum field theory in quantum spin systems.  Using the example of a spin-1/2 chain, we will show how the continuum limit of a lattice model is properly described in terms of a field  theory with topological defects. In the second part, we will describe  how measuring “disorder parameter”, the expectation value of a defect  operator, provides new probes into various quantum phases of matter, such as defect quantum dimensions in gapped phases, and universal  logarithmic corrections at quantum critical points.

报告人简介：

Meng Cheng is an Assistant Professor of Physics at Yale University.  He received his undergraduate degree from Nanjing University (China).  In 2013, he obtained his Ph.D degree from the University of Maryland,  College Park, where he studied topological superconductivity and its  applications in quantum information processing. He then moved to a  postdoctoral researcher position in Microsoft Research, Station Q in  Santa Barbara, during which he investigated the interplay of global  symmetry and topological quantum order. He joint the Department of  Physics at Yale University in 2016.