**Quantum over brunch** is an informal journal club series in the Physics Department at George Mason University. We meet once or twice a month to discuss topics of current research interest in the general field of condensed matter physics. This is a branch of physics which seeks to understand **quantum mechanics** of macroscopically many interacting particles.

The meetings are organized by the faculty of the **condensed matter theory group** at GMU, Predrag Nikolic, Erhai Zhao and Indu Satija. We welcome all graduate students at GMU interested in quantum physics. Our goal is to create a forum in which we can have conversations about the hottest current research topics, and both stimulate and satify our curiosity about the unexplained phenomena in quantum physics.

The format for this club is somewhat open: it will not be seminar style, but a discussion session. We will start every topic at a very basic level. By asking questions, we all hope to learn the subject. We believe that students interested in quantum physics will set the stage for the level of discussion. Since our plan is to learn each topic in full detail and depth, we will read and discuss scientific paper and our sessions will tend to be technical.

QOB informal seminar

Science and Technology I, Room 306, 11am (+ lunch after noon)

**Predrag Nikolic, GMU**

**Charge and spin fractionalization in strongly correlated topological insulators**

The recently discovered two-dimensional topological insulators (TI) with time-reversal symmetry are closely related to integer quantum Hall states in which electron spin plays the role of charge. The appearance of protected edge states in these systems can be understood by describing the spin-orbit coupling as the source of an SU(2) (spin-dependent) magnetic flux. However, the absence of spin conservation cripples the quantum spin-Hall effect. In this talk we will explore the possibility of obtaining strongly correlated TIs with fractional quasiparticles. Such states are the SU(2) analogues of fractional quantum Hall states, but with modified topological orders due to the spin non-conservation. We will discuss two heterostructure designs featuring a “conventional” TI quantum well that could host a fractional TI state of Cooper pairs or excitons. These devices exploit a quantum critical point for electron localization to provide a fragile spectrum that can be dramatically reshaped by the strong spin-orbit coupling. Then, we will discuss a topological field theory of fractional TIs and explain how the spin-orbit coupling can produce a combined charge and spin fractionalization despite the spin non-conservation.

### Physics and astronomy colloquium

Science and Technology I, Room 306, 11am (+ lunch after noon)

**Predrag Nikolic, GMU**

**Charge and spin fractionalization in strongly correlated topological insulators**

The recently discovered two-dimensional topological insulators (TI) with time-reversal symmetry are closely related to integer quantum Hall states in which electron spin plays the role of charge. The appearance of protected edge states in these systems can be understood by describing the spin-orbit coupling as the source of an SU(2) (spin-dependent) magnetic flux. However, the absence of spin conservation cripples the quantum spin-Hall effect. In this talk we will explore the possibility of obtaining strongly correlated TIs with fractional quasiparticles. Such states are the SU(2) analogues of fractional quantum Hall states, but with modified topological orders due to the spin non-conservation. We will discuss two heterostructure designs featuring a “conventional” TI quantum well that could host a fractional TI state of Cooper pairs or excitons. These devices exploit a quantum critical point for electron localization to provide a fragile spectrum that can be dramatically reshaped by the strong spin-orbit coupling. Then, we will discuss a topological field theory of fractional TIs and explain how the spin-orbit coupling can produce a combined charge and spin fractionalization despite the spin non-conservation.