2019 Fiscal Year Research-status Report
Exploration of the anharmonicity of phonons modes in molecular crystals with temperature-dependent and high-field terahertz spectroscopy and ab initio simulation
| Project/Area Number |
18K05034
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| Research Institution | Kobe University |
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Principal Investigator |
張 峰 神戸大学, 分子フォトサイエンス研究センター, 助手 (10773046)
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| Project Period (FY) |
2018-04-01 – 2021-03-31
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| Keywords | terahertz spectroscopy / MAPbBr3 / solid-state DFT / disorder / ferroelectricity / charge transfer distance / anharmoniic potetial / phonon-phonon coupling |
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| Outline of Annual Research Achievements |
The applicant has completed a sub-project of the THz-mode assignment for an organic perovskite, MAPbBr3, in the last fiscal year. This matter represents one promising system for the fabrication of new generation solar cell materials. Through performing solid-state density functional theory calculations at a high accuracy level, the applicant has satisfactorily reproduced all the observed THz peaks of this sample in a broad frequency range of 5-200 cm-1. Based on the rich structural information revealed in theory, the applicant finally constructed a new model, which features microscopic ferroelectric and macroscopically non-ferroelectric. This new structural model may imply a new solution to interpreting the high power-conversion efficiencies of the organic perovskites. Furthermore, the appliant phenomenologically fitted the the potential surfaces of the first few optical phonon modes with polynomial functions, which may play essential roles in the phase transitions. For most of the phonon modes, the contributions of the first nonlinear terms are always substantial, indicating that the three-phonon coupling may play a role in the phase transitions. The applciant is going to use this piece of information to perform the perturbation calculations using the DFT approaches.
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| Current Status of Research Progress |
Current Status of Research Progress
3: Progress in research has been slightly delayed.
Reason
According to the original design of this research plan, the high-field THz spectroscopic experiments would be performed in Tani group at Fukui University. However, his high-power femtosecond laser lost signal again last spring and has not been fixed till now. The applicant then plans to construct a tilted-pulse-front set-up base on a LiNbO3 single crystal, using the Spitfire Pro (Spectra-Physics) amplifire and a Maitai seeding in Tominaga group. This system produces a femtosecond laser pulse with the power one order of magnitude lower than that of the laster system in Tani Lab. However, this set-up still promises the observation of the nonlinear effects if once the applicant realizes the optimum alignment. Currently, the construction of the home-mode systems is still in progress.
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| Strategy for Future Research Activity |
The applicant will finish the construction of the high-power THz set-up this summer, despite the stagnation by the coronavirus pandemic this spring. He expects to obtain the first signal by this fall and finish all the data analyses by this winter. In terms of the theory calculation, he will continue the sub-project of the AIMD simulations and would be able to get some useful output this summer. The AIMD simulations give only the unharmonic potential surfaces of the phonon modes and do not imply any direct information on the phonon-phonon coupling. In this concern, he will also start the full-scale perturbation calculations based on DFT, which has been included by two software packages of ALAMODE and phono3p, in order to depict the detailed picture of phonon-phonon interactions.
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| Causes of Carryover |
The applicant will continue the construction of the high-field THz spectrometer the next fiscal year. The purchase of optics, translation stages, and a mini-cryostat will consume most of the remaining fund. A small part of the fund will be spend to support the applicant attending the 45th-IRMMWTHz conference, to be held in NewYork on Nov. 8-13, 2020.
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