Topology and Strong Correlations in Quantum Many-Body Systems

I.    Coordinator:

Chung-Hou Chung,
Dept. of Electrophysics, National Chiao-Tung University
1001 University Rd. HsinChu, 300
Email: chung@mail.nctu.edu.tw

II.   Core Members:

Chung-Hou Chung (NCTU, coordinator) 
Chung-Yu Mou (NTHU) 
Sungkit Yip (Academia Sinica) 
Baruch Rosenstein (NCTU)
Horny-Tay Jeng (NTHU) 
Wei-Feng Tsai (NSYSU)
III.    Major Directions:

The main topics include: Symmetry protected topological insulators (TIs)/superconductors(TSCs), Unconventional superconductivity in correlated materials, Kondo physics and quantum criticality in heavy fermions.
IV.    Activities:

(i). monthly regular meetings where we discuss and exchange new ideas, present briefly what we have done on various topics,
(ii). weekly joint group meetings/discussions/seminars among core members and students/postdocs,
(iii). Private discussions among core members via emails/phones/meetings, and
(iv). Mini-workshops/schools (one mini-workshop in Aug., one joint summer school with AMO TG in Aug., and one mini-workshop in Dec.).
V.   Highlight of Results (2015-2016):
(1) CH Chung and CY Mou study the Kondo lattice system with spin-orbit interactions. The goal is to find a new phase called “topological Kondo semi-metal”.  It is demonstrated that a new Dirac semi-metallic phase emerges at a finite temperature below the Kondo temperature. The resulting Kondo phase is a novel Fermionic critical system at finite temperatures.
“Finite temperature topological transitions and emergence of Dirac semi-metallic phases in a Kondo lattice”, Po-Hao Chou, Liang-Jun Zhai, Chung-Hou Chung, Ting-Kuo Lee, and Chung-Yu Mou, in preparation
* CY Mou, W-F Tsai, and CH Chung, together with visitor from NUS, Singapore Dr, Shin-Ming Huang, former postdoc of CY Mou, have discussed about on the superconductivity for a strongly correlated Kane-Mele TI system. In particular, we found that the topological nature of our discovered "helical" superconductivity, while with TRS broken pairing, is still unclear and thus worth studying further in order to clarify its nature and its corresponding topological phase transition to other phases in the system. After a preliminary study, they realized for the first time a significant duality between SC and TI with FM in the model. By using it, we investigate the complete phase diagram of this superconducting system.
“Duality in topological superconductors and ferromagnetic topological insulators in a honeycomb lattice”, Shin-Ming Huang , Wei-Feng Tsai, Chung-Hou Chung, and Chung-Yu Mou (submitted)
(3) CH Chung, WF Tsai, Prof. Shih-Jye Sun (NKU, Kaohsiung) together with the world leading expert Prof. Fuchun Zhang (Zhejiang U., China) address the interesting quasi-particle excitation called “Majorana fermions” or “Majorana zero modes” in superconductivity of doped 2D correlated layered materials on honeycomb lattice. We discovered pairs of counter-propagating “helical Majorana fermions” on the edges of time-reversal broken d+id’-wave spin-singlet superconductors on doped graphene-based materials.  Our results are surprising as chiral edge states (instead of helical edge states) are expected to emerge in d+id’-wave superconductors. We provide the first example of chiral-helical superconducting phase transition. This work has caught much attention internationally.
Helical Majorana fermions in dx2−y2 + idxy-wave topological superconductivity of dopedcorrelated quantum spin Hall insulators”, Shih-Jye Sun, Chung-Hou Chung, Yung-Yeh Chang, Wei-Feng Tsai, and Fu-Chun Zhang, arXiv:1506.02584
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