# 4th One-day KIAS Workshop, Recent Progress in Condensed Matter Physics, May 25 (Thursday), 2017

This is a one-day condensed matter physics workshop.
The location is Room 8101 on the first floor of the building No.8 at KIAS.
The room is near the KIAS library.

Everyone is welcome to attend the workshop.
Please let us know(ybkim@physics.utoronto.ca, egmoon@kaist.ac.kr) if you have any questions.
The information on the speakers and their talks are as follows:

Organizers: Yong-Baek Kim (KIAS), Eun Gook Moon (KAIST)
Other Core Members:
Sungbin Lee (KAIST)
Ara Go (IBS-PKS)
Gil Young Cho (KIAS)
Bohm-Jung Yang (SNU)
Kun-Woo Kim (KIAS)

10:30-11:15am. Speaker: Gil Young Cho (KIAS).
Title: Symmetric-Gapped Surface States of Fractional Topological Insulators
Abstract: We construct the symmetric-gapped surface states of a fractional topological insulator with electromagnetic $\theta$-angle $\theta_{em} = \frac{\pi}{3}$ and a dynamical $\mathbb{Z}_3$ gauge field. They are the proper generalizations of the T-pfaffian state and pfaffian/anti-semion state, which are the symmetric-gapped terminations of the topological band insulators, and feature the extended periodicity than their counterparts of integer" topological band insulators. We demonstrate that the surface states suffer from the correct anomalies associated with the time-reversal symmetry and charge conservation.
11:15-11:30pm. Break (Coffee Time)

11:30-12:15pm. Speaker: Sangjin Sin (Hangyang University).
Title:Dirac Materials as guides for AdS/CMT
Abstract: The Gauge gravity duality gives us a prescription to calculate transport properties from the nature of AdS black hole. I will postulate it as a principle for the strongly interacting quantum materials and suggest new words in holography dictionary. I will also talk about my efforts to make it more physical by providing experimental evidence for them.

Lunch

2:00-2:45pm. Speaker: Kwon Park (KIAS).
Title: Bilayer Mapping: Guiding Principle for the Fractional Quantum Hall States in the Second Landau Level
Abstract: With very few exceptions, basically all the fractional quantum Hall (FQH) states in the lowest Landau level are well described by the guiding principle known as the composite fermion (CF) theory, where the FQH states of electrons are mapped onto the integer quantum Hall states of weakly interacting CFs. The situation is not so simple in the second Landau level accommodating the famous 5/2 state, which is believed to a paired state of CFs. In this talk, I would like to discuss a novel guiding principle dubbed as bilayer mapping, via which a trial FQH state is constructed as the antisymmetrized projection of the bilayer quantum Hall ground state at a given interlayer distance. The bilayer mapping approach works quite well at filling factor 5/2, where the antisymmetrized product state of two CF seas at quarter filling is shown to have a better overlap with the exact Coulomb ground state than the Moore-Read Pfaffian/anti-Pfaffian state. Similarly, at filling factor 8/3 (7/3), the (particle-hole conjugated) Z4 parafermion state is constructed as the antisymmetrized projection of the Halperin (330) state, which is shown to have a better overlap with exact Coulomb ground state than the Laughlin state.

2:45-3:30pm. Speaker: Yongkwan Kim (KAIST).
Title:Unusual nematic phases in iron based superconductor
Abstract: Recently, the nematic phase, the electronic phase with symmetry lowered down to C2 from C4 rotational symmetry while the crystal symmetry remains under C4 symmetry, became a major playground of iron-based superconductor research by virtue of its exotic nature as well as its possible connection to the superconductivity. However, the microscopic mechanism has not been made, even the driver of the nematic phase is unclear so far.
In this talk, I will present two electronic structure study results by mean of angle resolved photoemission spectroscopy that could give us a new insight on the nematic phase by capturing the unusual behavior of nematic phase. One result contains a separation of orbital order and magnetic order realized in surface-electron-doped BaFe2As2. The other result presents the occupation reversed ferro-orbital order in FeSe bulk crystal where long-range magnetic order does not set in. With these two results, I will discuss the role of orbital degree of freedom on the nematic phase of iron-based superconductor.

3:30-4:00pm. Break (Coffee Time)

4:00-4:45pm. Speaker: Bohm-Jung Yang (SNU).
Title: Unconventional topological phase transition in two-dimensional noncentrosymmetric systems
Abstract: Here I am going to talk about a topological phase transition between a normal insulator and a quantum spin Hall insulator in two-dimensional (2D) systems with time-reversal and two-fold rotation symmetries. Contrary to the case of ordinary time-reversal invariant systems where a direct transition between two insulators is generally predicted, we find that the topological phase transition in systems with an additional two-fold rotation symmetry is mediated by an emergent stable two-dimensional Weyl semimetal phase between two insulators. Here the central role is played by the so-called space-time inversion symmetry, the combination of time-reversal and two-fold rotation symmetries, which guarantees the quantization of the Berry phase around a 2D Weyl point even in the presence of strong spin-orbit coupling. Pair-creation/pair-annihilation of Weyl points accompanying partner exchange between different pairs induces a jump of a 2D $Z_{2}$ topological invariant leading to a topological phase transition. According to our theory, the topological phase transition in HgTe/CdTe quantum well structure is mediated by a stable 2D Weyl semimetal phase since the quantum well, lacking inversion symmetry intrinsically, has two-fold rotation about the growth direction. Namely, the HgTe/CdTe quantum well can show 2D Weyl semimetallic behavior within a small but finite interval in the thickness of HgTe layers between a normal insulator and a quantum spin Hall insulator. We also propose that few-layer black phosphorus under perpendicular electric field is another candidate system to observe the unconventional topological phase transition mechanism accompanied by emerging 2D Weyl semimetal phase protected by space-time inversion symmetry.

4:45-5:30pm. Speaker: Daesu Lee (SNU).
Title: Emergent Physics in Nanoscale Correlated Oxides
Abstract: Emergence of collective phenomena has been a central means for exploring a correlated functional oxide. However, understanding and utilizing such emergence become increasingly challenging, as the dimensionality and size scale are reduced. Collective functional phenomena (such as ferromagnetism and ferroelectricity) tend to be suppressed and even disappear at reduced nanoscale dimensions, e.g., due to a finite-size effect. This remains a long-standing limitation in oxide nanoelectronics. In this talk, I will present my recent findings for overcoming this limitation.

6:00pm- Dinner