# <B>Faculty Candidate Seminar</B><br><i>Topological Defects in a Topological Insulator/Physics Beyond the Laudau's Paradigm in Quantum Magnets</i>

Speaker: Dr. Ying Ran (UC Berkeley)

I will discuss two different topics of my past works. In the first part, I will talk about topological defects in a novel band insulator believed to be realized in the bulk material Bi$_{0.9}$Sb$_{0.1}$. Topological defects, such as domain walls and vortices, have long fascinated physicists. For example, cosmic strings which are vortices of the Higgs field, may be tied to propagating fermion modes. Can analogous phenomena occur in crystalline solids that host a plethora of topological defects? I will show that indeed dislocation lines are associated with one dimensional fermionic excitations in a `topological insulator'. In contrast to electrons in a regular quantum wire, these modes are topologically protected, and not scattered by disorder. In the second part, I will talk about a completely different class of systems: quantum magnets. For a long time people believed that the Landau's symmetry breaking theory is sufficient to describe all phases.

The discovery of the fractional quantum hall effect, however, shows that phases of the same symmetry can be fundamentally different due to quantum mechanics. Are there other experimental systems where such novel quantum phases can be realized? I will show that the strong quantum fluctuations in quantum magnets can in fact stabilize novel phases. I will focus on the recently discovered Herbertsmithite ZnCu3(OH)6Cl2 Kagome spin-1/2 system, and present the experimental properties and signatures of a possibly realized gapless spin liquid phase.