Research
First-principles calculation & Density Functional Theory
Basically, first-principles calculation usually refers to the method which one can obtain the physical properties via Density Functional Theory (DFT). DFT is a computational quantum mechanical modelling which allow us to project the many body Schrödinger equation inside the condensed matter to an equivalent single electronic equation. In the DFT, we use the charge density as variable in our many body problem.Using the density as variable has a strong advantage compare than solve the equation directly, which is almost impossable.Within the First Principle calculation, we can analysis the material and compare the data with the experiment, or more, predict the new material.The calculation material looks kind of like the bridge between the Theory and experiment, and gives the new sight to study the condensed matter.
GW approximation & BSE
Based on the Many-Body Perturbation Theory (MBPT), the first-principles calculations of the ground- and excited-state properties are performed in three main steps:
1. Firstly, we calculate the electronic ground-state (GS) properties within density functional theory (DFT) using a proper mean-field approximation functional (LDA/GGA/PBE, etc.).
2. Secondly, the quasiparticle (QP) self-energy (S) corrections to the DFT eigen-energies (∑ ) are obtained by a one-shot (G0W0)/self-consistent (sc-GW) calculation within a Generalized Plasmon-Pole (GPP) approximation/ full-frequency dependence/ Contour-Deformation method.
3. Finally, based on the quasiparticle eigenvalues, the optical properties including excitonic effects (exciton energies, corresponding electron(e)-hole(h) amplitudes, and exciton wave-functions) can be calculated by solving Bethe-Salpeter Equation (BSE).
The corresponding equations and Feynman diagrams can be represented schematically in the following flowchart:
Wannier Function
Wannier function is an powerful tool to analysis the physical properties involve spatially localized objects such as defect effect, screened effect etc.. . If we only use the bloch basis to describe such the system, it will be causing lots of time. In other words, within the Wannier function, the spending computational time can be effectively reduced.Within the First Principle calculation, we can analysis the material and compare the data with the experiment, or more, predict the new material.The calculation material looks kind of like the bridge between the Theory and experiment, and gives the new sight to study the condensed matter.
Lattice Dynamic & Phonon dispersion
The Atom inside the material is not just freeze at there. Actually, they should be vibrating around the equilibrium point.It means that the atomic position is updating with time. Based on the result form DFT calculation, we can simulate the atom vibration on the computer which we call it phonon dispersion.
So, what can phonon do? Phonon can tell us about the stability of the system,also one can also compute the phonon contribution to specific heat, which could yet be another basic material property of interest. Within the DFPT(Density functional perturbation Theory) method, the phonon at gamma can even give us the Raman tensor, which is a powerful method to identity the structure of the material.
The research result of Tin-Oxide
The Band structure
The Phonon dispersion
Calculated Raman spectrum
The vibration of Tin-Oxide
2D material
Since the Graphene...