総説

第一原理計算による有機結晶のテラヘルツ格子振動

Terahertz Phonon in Organic Crystal Studied by the First-Principles Computation

斎藤繁喜・タルガット　インネルバエブ・水関博志・
五十嵐伸昭・野手竜之介・川添良幸
Shigeki SAITO, Talgat INERBAEV, Hiroshi MIZUSEKI, Nobuaki IGARASHI,Ryunosuke NOTE, and Yoshiyuki KAWAZOE

グルコース，ラクトースなどの糖類や，テラヘルツ波発生用の結晶がもつテラヘルツ周波数物性を研究するために，分子モデルや結晶について第一原理計算による振動モード解析を行った。結晶の計算では周辺との相互作用や形状の変化が加わり，計算と実測の結果に良い一致が見られたが，さらに広範囲の分子間モードを特定した。ここではこの低波数振動に対する水素結合の大きな影響や，スペクトル変移の要因について記述する。

First-principles calculations of single molecular vibrations and crystalline phonons of β-D-glucopyranose, lactose, and 4’-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) in the region of terahertz (THz) frequency were performed by using periodic density functional theory calculations. Theoretical frequencies of phonon modes corresponded to central frequencies of experimental absorption bands in THz region, mainly due to intermolecular hydrogen bonds and intramolecular deformations in crystal. Moreover, phonon modes originating from molecular translations or rotations were estimated in a much wide range. This review of theoretical calculations of THz vibrational modes presents a remarkable influence of intermolecular hydrogen-bonded network on THz phonon modes of molecular crystals, and a cause of spectral variations between DAST derivatives by sampling eigenvectors of vibrational modes and force constants.

Keywords:Terahertz wave, Phonon, First principles calculation, Density functional theory, Organic crystal