Lee, Geunsik

Lee, Geunsik

Lee, Geunsik 이근식
Assistant Professor
Theoretical Chemistry

to predict various properties of materials by using the state-of-the-art simulation tools . to develop advanced software tools for overcoming limitations of existing codes. major achievements : predicting a giant IR absorption in graphene caused by peculiar metallic electrons, unraveling the pairing mechanism of Fe-based superconductors.
Research Summary
Application of conventional modeling methods
First-principle calculations enable us to predict various properties of materials, such as atomic positions, electronic states, and resulting physical and chemical properties. Well-known density functional theory (DFT) has proven a powerful tool. It can describe rather an extended electronic system like solids and also a localized system like molecules with an ad-hoc term U, both of which are roughly categorized to metals and insulators, respectively. However the DFT-based method is not suitable to study strongly correlated materials which are neither metals nor insulators. Recently dynamical mean field theory (DMFT) has been a powerful tool. By combining the three methods, we can calculate or predict a desired property without relying on empirical parameters.

Developing sophisticated simulation tools Software tools are essential for scientists working in various fields such as material science, molecular biology, and bioinformatics. By utilizing these software tools we can understand chemical/physical properties, garner new insights, develop new theories, as well as predict and design new materials exhibiting desirable properties. In the past most tools are oriented to developing material. But nowadays understanding functional properties is demanding, towards next-generation electronic/photonic devices. For this purpose, electron transport calculation based on the non-equilibrium Green's function (NEGF) method became one of popular approaches. Combined with the first-principle DFT method, it possesses a great potential for studying technologically important problems like pn junction, carrier injection. However it suffers from Coulomb interaction. To this end we are developing an advanced simulation package by using real-time quantum Monte Carlo.

기존 모델 방법론 응용
제일원리 계산을 통해 다양한 물성을 예측할 수 있다. 예를 들어 원자구조, 전자상태, 이들로 기인된 물리적/화학적 성질이 있다. DFT 계산 방법이 매우 유용한데, 분자뿐만 아니라 고체와 같이 확장된 시스템에도 널리 응용되고 있다. 또한 강상관 시스템을 기술하기 위해 DMFT 방법도 최근 개발되어 사용되고 있다. 이러한 방법론을 이용하여 파라미터에 의존하지 않고 물질의 특성을 예측할 수 있다.

진보된 시뮬레이션 툴 개발
과거 시뮬레이션 툴들은 주로 격리된 시스템의 성질을 계산하는 것에 국한되어 있다. 하지만 전자/광자 소자와 같은 기능성 물질을 디자인 하기 위해서는 외부와의 상호작용을 기술할 필요가 있다. 이를 위해 NEGF 방법이 유용하게 사용되고 있다. NEGF를 기존의 DFT, DMFT 방법을 결합하여 새로운 시뮬레이션 툴을 개발하고 이를 이용해 물질의 기능성을 계산한다.

Representative Publications
Y. S. Park et. al., ,
"Surface Effect Induced Optical Bandgap Shrinkage in GaN Nanotubes",
Nano Letters 15, 4472 (2015).

Geunsik Lee, Muhammad A. Farhan, Jung Sung Kim, Ji Hoon Shim,
"Anisotropic Dirac electronic structures of AMnBi2 (A = Sr, Ca)",
Phys. Rev. B 87, 245104 (2013).

Geunsik Lee, Hyo Seok Ji, Yeongkwan Kim, Changyoung Kim, Kristjan Haule, Gabriel Kotliar, Bumsung Lee, Seunghyun Khim, Kee Hoon Kim, Kwang S. Kim, Ki-Seok Kim, and Ji Hoon Shim,
"Orbital Selective Fermi Surface Shifts and Mechanism of High Tc Superconductivity in Correlated AFeAs (A=Li, Na)",
Phys. Rev. Lett. 109, 177001 (2012).

Jae Won Yang, Geunsik Lee, Jai Sam Kim, and Kwang S. Kim,
"Gap Opening of Graphene by Dual FeCl3-Acceptor and K-Donor Doping",
J. Phys. Chem. Lett. 2, 2577 (2011).

M. Acik, G. Lee, C. Mattevi, M. Chhowalla, K. Cho, and Y. J. Chabal,
"Unusual infrared-absorption mechanism in thermally reduced graphene oxide",
Nature Mater. 9, 840 (2010).