Dr. Yang-hao Chan, IAMS Assistant Research Fellow, received his Ph.D. from the University of Michigan, Ann Arbor under the supervision of Prof. Luming Duan. He held postdoctoral positions with Prof. Mei-Yin Chou in the IAMS and Prof. Steven G. Louie at the University of Berkeley, California before he joined the IAMS in January 2021. His research mainly focuses on excited state calculations from first-principles. We visited Dr. Chan to hear more about his research.
A: What is first-principles calculation, and how can it help us understand materials?
Yang-hao Chan (YHC): A first-principles (or ab initio) calculation means that we compute material properties from basic formulations of quantum mechanics without making assumptions or fitting parameters. Specifically, we compute a material’s properties by solving Schrodinger’s equation using only the atom species and structure model as inputs. Nowadays, the most popular first-principles calculation method is based on Kohn-Sham’s density functional theory (DFT) framework. From first-principles calculations we compute a material’s electronic structure and lattice dynamics, from which we can further obtain its structural stability and structure phase diagram as well as its optical, topological, and electron and thermal transport properties. Many applications such as photovoltaics, charge-transfer materials, catalysts, and even drug research can be studied with DFT.
A: That sounds like a very powerful method. What is the ab initio excited states calculations listed in your profile? How is it different from the DFT you just mentioned?
YHC: Although DFT is the workhorse of first-principles calculations, it only applies to the ground states of materials and thus misses some important properties. For example, the band gap extracted from the DFT band structure is often underestimated and the optical gap is usually not correct. This is because these features are determined by excited state properties. A band gap is defined as the energy required to remove one electron from the ground state and add it to the lowest unoccupied state, and the optical gap is the minimum energy required to create an electron-hole pair. In both cases, we need knowledge of excited states, which are not accessible by DFT calculation. Moreover, electron correlation effects, which are important to materials like high-Tc superconductors, are not considered in DFT. Therefore, many researchers are developing methods beyond DFT.
A: How do people compute excited state properties?
YHC: Several methods, such as dynamical mean field theory and tensor network algorithms, have been applied to studying the excited states properties of materials. In our research, for near-equilibrium systems, we focus on the ab initio many-body perturbation theory (ab-MBPT). We take DFT calculations as a starting point and consider electron-electron or electron-phonon interactions as perturbations to the DFT results. For non-equilibrium systems, we are developing a first-principle time-dependent simulation method, which allows us to study electron dynamics.
A: Could you give us some examples of problems you want to study with the time-dependent simulation methods?
YHC: One interesting problem we want to study is electron or exciton dynamics after light excitation. This concerns photovoltaic effects and thus the working principle of optoelectronic devices. A schematic of carrier dynamics is shown in the figure. First, light will be absorbed and excitons form in the material. Excitons can migrate and transfer to different part of the sample. During migration they can interact with lattice vibrations (phonons), defects, or other electron and holes. They can also recombine to emit photons. A complete description in theory would involve electron–hole, exciton–phonon, exciton–defect, and exciton–photon interactions. We are hoping to understand each type of interaction from ab-MBPT first, before applying the time-dependent simulation method to study the full dynamics. Another example is to simulate time-resolved angle-resolved photoemission spectroscopy (tr-ARPES), which can be performed by Dr. Cheng-Tien Chiang’s group here in the IAMS, from first-principles. Tr-ARPES is a powerful tool for studying ultrafast electron dynamics. Recently, it has been reported that it is possible to observe excited state features by tr-ARPES. We are interested in applying our method to study these features.
A: The calculations seem to require a powerful computer. How do you support your research?
YHC: Indeed, our research relies heavily on powerful computers. Fortunately, at the IAMS we have a powerful local cluster supported by our staff. We have acquired funding from the Director Kuei-Hsien Chen and the Ministry of Science and Technology to renew our computer clusters this year. Moreover, we have strong first-principle calculation groups and senior members with expertise in nanotechnology at the IAMS. Their suggestions and support provide a really wonderful research environment for junior members like me.
Figure 1 is taken from Nature Materials volume 20, pages 728–735 (2021)
