CMP-lab - Research

Methods

Custom codes developments
First-principles calculation based on density functional theory
(VASP, Quantum Espresso, SIESTA, AIMS)
Tight-binding modeling (Slater-Koster type or Wannier function type)
Phonons and anharmonic properties from first-principles calculations
(custom code)
Classical and quantum molecular dynamics simulation (VASP, custom code)
Be skilled in various numerical technics (iterative diagonalization, optimization, linear and quadratic programming), script languages and parallel code development

Phase transition of charge density wave

Scientific interest: Charge density wave (CDW) are one or two-dimensional (sometimes three-dimensional with stacking ordering) periodic lattice distortion in crystals. The CDW formation changes the electronic properties of crystal ranging from insulators to superconductors, and has complex phase diagram on temperature, pressure, atom intercalation, and substitutional doping. Recently, exotic quantum phases such as heavy fermion and quantum spin liquid are reported to emerge that intense studies on the origin and structure of CDW are performed.

Approach: Until now, the phase transition of CDW is treated within phenomenological Landau theory. I am investigating the complex phase diagram can be described in a first-principles level, i.e. starting from adiabatic interatomic potential, constructing phase diagram without adjustable parameters. Because incommensurate CDW requires prohibitively large supercell calculation, and ordering process requires thermodynamic simulations, computationally cheap classical potential mimicking first-principles interatomic potential should be constructed. For this, I developed bond-type basis functions, various fitting or learning method, and compatible molecular dynamics and coordinate optimization codes. My approach can not only calculate structural phase transition buy also cover phonon-related phenomena such as thermal transport and anharmonic effects in crystals.

Materials: 2H- or 1T-transition metal dichalcogenides (triangular lattice), RE(rare earth)Pt2Si2 (square lattice), Kagome metals

Molecular film and metal growth on substrates

Scientific interest: Orientation and morphology of self-assembled molecules and atoms and their contact with metallic leads greatly affect their performance in organic electronic devices. The control principles and identifications of them are important topics in high-fidelity device fabrication.

Approach: Strong collaborations with STM experiments are performed. Finite bias STM simulation code, wavefunction tail extrapolation method and Hirshfeld charge-based force analysis, exact Coloumb interaction cutoff technics are developed. Unexpectedly, STM simulation with realistic tip-sample distances (wavefunction tail extrapolation), which requires considerably large computing resources, shows large discrepancy with conventional local density of state simulation. Using those method, we revealed the how the substrate can determine the dimensionality and orientation of organic molecular films. We can separate the intrinsic properties of molecular films from artifacts caused by STM measurement by simulating effect of tip polarization and wavefunction tail effect.

Graphene, 2D materials and heterointerfaces

Scientific interest: Low-dimensional material can be stacked, stitched and interfaced to emerge totally new electronic properties not included in the parent materials. Fundamental electronic properties of atomically sharp interfaces and their response to external conditions are key factors for new devices and operating principles.

Approach: We have studied how the edge and interlayer interaction modifies the electronic properties of graphene, especially for its conductions. Edge conduction states at heterointerface with boron nitride monolayer and unusual topographic inversion in STM image when interfaced with metallic substrate is revealed.

Computational materials design

Scientific interest: Material with exotic or enhanced properties can be computationally suggested by searching materials database, artificial synthesis (element combinatorics) and stability check. Thanks to the increased computational power and open big database, states-of-art technics can treat ternary and quaternary compound.

Approach: From the basic wisdoms of chemistry such as stoichiometry, ionicity, coordination number et al., materials with desired properties are searched with appropriated keywords. Once candidate materials are found, element combinatoric substitution or evolutionary algorithm with high-performance computing are applied to extend the search beyond database. We have suggested new 1D electride in this way and interestingly, its nontrivial topological nature is found. Similar approach is applied to enhancing Tc of quantum anomalous Hall insulator.

Energy materials

Scientific interest: Efficient generation of electricity and its stable and fast storage is central to our civilization. Breakthrough mostly comes from new materials, and theoretical understanding of energy-related process and computational screening are nowadays indispensable part of materials search.

Approach: The energy storage and conversion process occur by changing electronic or ionic degrees of freedoms. Because most energy materials require chemical accuracies for quantitative calculations that we systematically compare various computational approaches beyond DFT and their predictabilities for examples of hydrogen storage and Li ion battery materials. We found the errors from DFT exchange-correlation functional is somewhat large for reasonable experimental design that care must be given for absorption and diffusion energy. Thermodynamic effects on perovskite photovoltaic materials are investigared.

Materials: photovoltaic, hydrogen storage, solid oxide fuel cell, battery

이화여자대학교 자연과학대학 물리학과
EWHA WOMANS UNIVERSITY NATURAL SCIENCES PHYSICS