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The conference trip to Hong Kong was quite fruitful. I made new friends in different fields and got to know new places in Hong Kong. The major topic of the conference is topological and correlated matter. The topological phases without external magnetic field has been popularized almost for ten years now, since Charles Kane and Eugene Mele in Pennsylvania University came up with an idea of introducing a spin orbit coupling to graphene to open an energy gap non-trivially to realize quantum spin Hall system1. It is now classified as symmetry protected topological phases in contrast to symmetry enriched topological phases. For example, in the Kane-Mele model the time reversal symmetry protects presence of edge states when interfaced with trivial insulators, or vacuum. Then, people in the community thought of topological classification of gapless materials, and now we know examples in experiments of gapless topological systems such as Weyl semimetal2, nodal ring phase3, etc. On the other hand, there have been progresses beyond the single particle level in classifying topological phases in strongly interacting systems4. This conference is about the topological phases with and without correlation5.

I didn’t know Hong Kong is that humid and hot. Keywords that I had in mind about Hong Kong are shopping and dim sum. Unfortunately, I didn’t have a chance to visit a good shopping district, and dim sum is served only once on Wednesday morning. Yes. Dim sum is not for lunch or dinner in its native country. From the airport of Hong Kong to the place

grc report

글 _ 김건우·고등과학원 물리학부 연구원

1) Check their original papers: PRL 95, 226801, PRL 95, 146802.

2) For Weyl semimetal experiments, http://www.condmatjournalclub.org/?p=2538

3) For example, see Chen et al. Nature communications 6 (2015)

4) For example, see Science 338.6114 (2012): 1604-1606.

5) Here is the link of homepage: https://www.grc.org/programs.aspx?id=16944

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of conference, Hong Kong University of Science and Technology, is about one hour drive by car. Green and Red taxis are available anywhere and anytime. It seems most taxi drivers can’t speak English, and GPS navigator is not equipped. But, it is good that every car is well air-conditioned. One other impression about the city is the shape of apartments. They are mostly very tall, 30 stories at least, and narrow tower shape no matter how old or expensive a building is. I didn’t have much chance to take a look at the city. There were two organized tour, one to the near ocean boat trip to the front sea of HKUST, and the other is to the Victoria peak. And, on Thursday night I went out with friends to Mong Kok, which has lots of night markets like Dongdaemun in Seoul, and took a ferry to cross a river. Night view of Hong Kong was beautiful with colored skyscraper buildings, but it was still hot and humid.

The Gordon research conference was interesting in its style. It takes application to attend, though I don’t think it requires high criteria to be qualified. Any types of recording are prohibited during talks and poster sessions to encourage people to openly discuss unpublished researches they are conducting currently. Ambiance was quite free and

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comfortable to interact with random people. Every meal is served at on-site conference location, HKUST Jockey club institute for advanced study6, and it is the time to meet new people and discuss last presentations and their own research. The conference of this topic takes place every other year, and near the end of the meeting the location in 2017 was discussed and decided to be at the same place, HKUST, though there are some votes for Spain, including me. And, even next vice chairs are directly voted by the participants, very democratic way of running the meeting.

The anomalous quantum Hall effect was hot topic for its successful realization of quantized conductance in a series of experiments7 last a few years. One of the challenges in the community is to understand the high sensitivity on temperature change, which is opposed to the common belief that the quantization of conductance should be very robust up to the temperature of energy gap scale. One explanation of this mystery is the mixing of helical surface states and chiral edge stats. At low temperature, only chiral edge state will contribute to the conductance, while with increasing temperature helical surface states, which are not fully gapped on the side of thin film, will begin to flow and the quantization of conductance will break down. One resolution of the problem will be attaching leads on the side as Xi Dai commented during the conference, but it will be challenging due to the nanometer size of thickness of the film. If one constructed thin film insulator sandwiched by two thick 3d topological insulators, I think putting leads on the side to separate the transport of chiral edge states should be possible. If in the future one can build a device making use of the quantum anomalous Hall effect, the device will suffer much less from the heat generated from the diffusive transport of electrons through circuits. This is actually the first and the last time I heard about a practical application to industrial purposes throughout this conference.

One other interesting topic discussed that I would like to introduce is crystalline symmetries and topological phase transition. As the model proposed by Kane and Mele picks up a topologically non-trivial energy gap opening with the help of the time reversal symmetry, Liang Fu8 came up with the idea of using crystalline symmetry to realize topological insulators. Vidya Madhavan9 gave one of the best talks about experimental realization of the topological crystalline insulator and its verification by means of introducing strain induced by regular edge dislocations in the lattice. The crystalline symmetry will break if a system is strained. Though I think her sample of strained layer is too thin to show a bulk properties to clearly see gapless and well defined the top and bottom surface states, her selective-area-Fourier-transformation measurement done far below Fermi energy clearly shows the shifting of Dirac cone spectrum in accordance with the prediction by strain

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effect. Shuichi Murakami10 talked more about the topological phase transition between band insulator and topological insulator in the absence of the inversion symmetry, and by studying all possible crystalline symmetries he claimed that either Weyl node or nodal ring phase have to appear during the transition. This study is related to a more general question: when two bands touches, will it be avoided or crossing? The answer depends on which irreducible representation each band belongs to. It should be extended to the Floquet band system that I am conducting with Prof. Park at KIAS.

Overall, I am benefited from the trip. I was able to sharpen previous rough idea and got inspiration from the discussions with participants, which probably will lead future collaborations. Plus, I found a few people they are willing to give a talk at KIAS condensed matter journal club & seminar. I also realize that the research fund that is given to every postdoc at KIAS is quite unusual. I appreciate it. Research trip and activities will be much facilitated also with the help of people in the department. Thanks. It has been almost a year since I came to KIAS. Equipped with further motivation and power, I will try to conduct more interesting research in the second half of this year and the following years. By the way, if you are interested and have questions of the topics I mentioned above, please visit my office, 1409, or phone me, ext. 3787.

6) HKUST IAS homepage: http://ias.ust.hk/web/ias/eng/

7) For example, see Ke et al. National Science Review 1.1 (2014): 38-48.

8) For a review of the topic, see arxiv:1501.00531

9) Okada et al. Science 341.6153 (2013): 1496-1499.

10) Okugawa et al. Physical Review B 89.23 (2014): 235315.