2018 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, 学術研究員 (10773046)
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| Project Period (FY) |
2018-04-01 – 2021-03-31
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| Keywords | THz spectroscopy / high field / high resolution / temperature dependent / phonon modes / anharmonicity |
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| Outline of Annual Research Achievements |
This proposal was originally designed to use THz spectroscopy to study the relationships between the anharmonicity of phonon modes and some fundamental properties of matter such as thermal expansion, heat conduction, and phase transition. The temperature-dependent high-resolution THz spectra of a perovskite material, which is used for fabricating solar cells, have been recorded with high resolution. In the course of temperature rising from 10 K to 300 K, this material undergoes a series of phase transitions. Correspondingly, Some THz peaks exhibit anomalous behaviors at the critical temperature points, implying the strong anharmonicity of these modes. This observation is a strong sign that the anharmonicity of phonon modes is crucial to understanding the microscopic mechanisms of the phase transitions. As the first step of theoretical interpretation, the normal mode simulations under the harmonic approximation have bee carried out with implementing the periodic boundary condition. Rather good agreement between theory and experimentation has been achieved in terms of both frequencies and intensities of THz modes. The characteristic of each simulated THz modes have been quantified with respect to the contributions of the inorganic frame and the organic dipolar to the vibrational energy.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
According to the original design of this proposal, the applicant will spend the first fiscal year recording temperature-dependent THz spectra of the target molecular and material systems and calculating the optical phonon modes under the harmonic approximation. This plan has been completed smoothly. The experimental data of several small molecular crystal and a perovskite material has been collected through the collaboration with Prof. T. Sasaki. High-resolution (200 MHz) and broad-band (0.3-6.0 THz) THz measurements have been achieved, and interesting temperature-dependent behaviors of THz peaks have been observed in almost every target system. THz modes of all the target systems have been reproduced with using the solid-state density functional theory.
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| Strategy for Future Research Activity |
According to the research plan, high-field THz measurements will be performed through the collaboration with Prof. M. Tani. Tani group has achieved the generation of THz pulses with the maximum electric amplitude of 500 KV/cm in this fiscal year. With a continuous improvement, the realization of generating THz pulses with the 1MV/cm amplitude is possible. In parallel, the quasi-harmonic simulations of the optical phonon modes will be performed with using CRYSTAL17 package, and the ab initio molecular dynamic (AIMD) simulations will be performed with using the Quickstep module of the CP2K package. All the calculations will be carried out under the periodic boundary condition. The MD trajectories will be analyzed in TRAVIS program.
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| Causes of Carryover |
The leftover amount of 831,437 yet is planed to be used to purchase LiNbO3 crystals, diffraction gratings, et al. Currently, the high-field THz spectroscopic experiments are planned to be performed in the Lab. of Prof. M. Tani in Fukui University. The applicant plans to construct one THz spectrometer in Kobe; so that he can perform the high-field experiments any time he needs and maintain the setup regularly. Currently, the laser and regular optics are available in Tominaga group. The application in particular needs to purchase the non-linear crystals and gratings for the generation of high-field THz pulses.
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Research Products
(5 results)
