Condensed Matter Theory Workshop at KIAS on Friday, April 15th 2016.

We will have a condensed matter theory workshop. The location is tentatively room 1424, the 4th floor of building no.1 at KIAS (We will move to room 1503 if the nubmer of attendants is many). Everyone is invited to attend and participate. Please let us know(kkim@kias.re.kr) if you have any questions. More information on speakers and their talks are following:

10:30-11:15am. Speaker: Sung Bin Lee (KAIST).
Title: Space group fractionalization in two dimensional nonsymmorphic crystals
Abstract: Motivated by several materials that belong to nonsymmorphic space group, we explore the extended HOLSM theorem and study unique features of topologically ordered phases in 2D nonsymmorphic crystals. It turns out that trivial insulating phase is forbidden even in certain integer filling if the system is nonsymmorphic and thus the system develops topologically ordered phases. Focusing on two dimensional cases, we found that such topologically ordered phases have exotic low energy properties where deconfined topological excitations fractionalize glide-reflection symmetries. We accomplish this by generalizing the dichotomy between even and odd gauge theories to incorporate additional nonsymmorphicity.

11:15-12:00pm. Speaker: Gil Young Cho (KAIST).
Title: Lieb-Schultz-Mattis Theorem and Symmetry-Protected Topological Phases
Abstract: There are several microscopic data constraining the allowed structures of emergent long-distance and low-energy theories. The most fundamental inputs are the symmetries and appropriately-defined symmetry charge per unit cell. Given the data, Lieb-Schultz-Mattis theorem dictates that if the ground state is completely symmetric and the filling per unit cell is not integral, then the ground state cannot be a trivial single-degenerate insulator. On the other hand, exactly the same restriction arises on the boundaries of so-called symmetry-protected topological phases: the d-dimensional boundary of the (d+1)-dimensional symmetry-protected topological phase cannot have a trivial single-degenerate insulator if the symmetry is enforced strictly. Such constraint has been understood in terms of various quantum anomaly associated with the boundary theories. Here we uncover the connection between the Lieb-Schultz-Mattis theorem and the boundary of symmetry-protected topological phases and quantum anomaly associated with the Lieb-Schultz-Mattis theorem. We show this by the "duality" between the d-dimensional lattice systems with non-local translation symmetry and d-dimensional symmetry-protected topological phase with on-site symmetry, which is similar to the recent proposal of the duality between the boundary of the topological superconductor and half-filled Landau level systems.

Lunch

2:00-2:45pm. Speaker: Bohm Jung Yang (SNU).
Title: Classification of three-dimensional Dirac semimetals
Abstract: A three-dimensional (3D) Dirac semimetal (SM) is the 3D analogue of graphene having linear energy dispersion around Fermi points. Owing to the nontrivial topology of electronic wave functions, the 3D Dirac SM shows nontrivial physical properties and hosts various exotic quantum states such as Weyl SMs and topological insulators under proper external conditions. There are several kinds of Dirac SMs proposed theoretically and partly confirmed experimentally, but its unified picture is still missing. In this talk, we propose a general framework to classify stable 3D Dirac SMs in systems having the time-reversal, inversion and uniaxial rotational symmetries. We show that there are three distinct classes of 3D Dirac SMs depending on the underlying symmetry protecting the Dirac points. In class I, the Dirac SMs are protected by symmorphic symmetries, and have a pair of Dirac points created by band inversion. On the other hand, class II Dirac semimetals, protected by type-I nonsymmorphic symmetries, have a single Dirac point at a time-reversal invariant momentum on the rotation axis. Finally, class III Dirac semimetals are protected by type-II nonsymmorphic symmetries, and possess either line nodes or point nodes on the Brillouin zone boundaries.

2:45-3:30pm. Speaker: Suk Bum Chung (SNU).
Title: Rashba effect in transition metal perovskite oxide
Abstract: In the two-dimensional electron gas (2DEG) emerging at the transition metal oxide surface and interface, it has been pointed out that the Rashba spin-orbit interaction, the momentum-dependent spin splitting due to broken inversion symmetry and atomic spin-orbit coupling, can have profound effects on electronic ordering in the spin, orbit, and charge channels, and may help give rise to exotic phenomena such as ferromagnetism-superconductivity coexistence and topological superconductivity. Although a large Rashba splitting is expected to improve experimental accessibility of such phenomena, it has not been understood how we can maximally enhance this splitting. Here, we present a promising route to realize significant Rashba-type band splitting using a thin film heterostructure. Based on first-principles methods and analytic model analyses, a tantalate monolayer on BaHfO3 is shown to host two-dimensional bands originating from Ta t2g states with strong Rashba spin splittings - up to nearly 10% of the bandwidth - at both the band minima and saddle points due to the maximal breaking of the inversion symmetry. Such 2DEG band structure makes this oxide heterostructure a promising platform for realizing both a topological superconductor which hosts Majorana fermions and the electron correlation physics with strong spin-orbit coupling.

Break

4:00-4:45pm. Speaker: Hyun Yong Lee (SKKU).
Title: Classification of symmetric tensor network states and its application to honeycomb and square lattice
Abstract: Tensor Network States (TNS) represent quantum states in terms of complex networks of contracted tensors capturing the relevant entanglement properties of a many body ground states. Recently, an ambitious new paradigm to classify phases in low-dimensional quantum magnets and construct their explicit wave function has been launched under the theme of TNS. In this talk, a brief introduction to TNS and classification of symmetric TNS's for a given symmetry and topological order will be given. Further, its application to spin-1/2 system on honeycomb lattice to find a featureless quantum state and spin-1 system on square lattice to find a variational ansatz for J1-J2 model.

4:45-5:30pm. Speaker: Kun Woo Kim (KIAS).
Title: Charge and spin shift photocurrent on Dirac surface states
Abstract: The generation of direct photocurrent is of main interest for photovoltaic and optoelectonic applications in condensed matters. Shift current, a nonlinear photoresponse, recently has drawn attention as a dominant player of bulk photovoltaic effect in ferroelectric materials. In Bi2X3 topological insulators with hexagonal warping term, we find that Dirac surface states carries shift currents by linearly polarized light. Plus, spin-shift current is introduced with the time-reversal symmetry breaking perturbation. From a nonlinear photocurrent expression obtained from the Floquet formalism, the charge- and spin-shfit current are successfully interpreted as a correlation between paramagnetic current and diamagnetic current.