The first-principles molecular-dynamics simulation was performed for liquid Ge at 1253 K by using two kinds of simulation cells: The cubic cell of 64 atoms and the rectangular parallelepiped one of 128 atoms. The rectangular parallelepiped cell of 128 atoms was adopted to obtain the dynamic structure factor of liquid Ge in the small wave number region. The long simulation time was adopted, i.e., 66 ps for the cubic cell and 75 ps for the rectangular parallelepiped one. The present first-principles molecular-dynamics simulation reproduces well the experimental static structure factor and radial distribution function. A broad peak around 100° in the obtained bond angle distribution function implies the existence of the tetrahedral atomic unit in liquid Ge. The self-diffusion coefficient for the rectangular parallelepiped cell is 20% larger than that of the cubic one. The obtained dynamic structure factor agrees well with the experimental one obtained by the inelastic x-ray scattering experiment [Hosokawa et al., Phys. Rev. B 63, 134205 (2001)], which shows the "de Gennes narrowing" of the main peak and the existence of the side peaks. These side peaks represent a longitudinal vibrational motion, which was also supported by the subsidiary peak around 30 ps~(-1) in the spectral density of the velocity autocorrelation function. The gradient of the dispersion relation in the present simulation agrees well with the experimental sound velocity. This "no positive dispersion" accords well with the inelastic x-ray scattering experiment of Hosokawa et al. The reason for this "no positive dispersion" for liquid Ge is discussed in particular concern with its low kinematic viscosity. Though the velocity autocorrelation function itself does not show a cage effect, a microscopic cage effect can be found by the detailed analysis for the trace and environment of the single atomic motion. The atomic movement as a group of 3-5 atoms seems to be present in liquid Ge in addition to individual atomic motions. The covalent bond seems to be also present at least instantaneously in liquid Ge.
展开▼
机译：通过两种模拟单元对液体Ge在1253 K处进行了第一性原理的分子动力学模拟：64个原子的立方单元和128个原子的长方体。采用128个原子的长方体晶胞，在小波数区域获得了液态Ge的动态结构因子。采用了较长的仿真时间，即立方电池为66 ps，长方体电池为75 ps。本第一原理分子动力学模拟很好地再现了实验的静态结构因子和径向分布函数。所获得的键角分布函数中约100°处的宽峰暗示着液体Ge中存在四面体原子单元。矩形平行六面体单元的自扩散系数比立方单元大20％。所获得的动态结构因子与通过非弹性X射线散射实验所获得的实验结构非常吻合[Hosokawa等人，Phys。 Rev. B 63，134205（2001）]，其中显示了主峰的“ de Gennes变窄”和侧峰的存在。这些副峰代表纵向振动运动，速度自相关函数的频谱密度中的30 ps〜（-1）附近的副峰也支持这一现象。在本模拟中，色散关系的梯度与实验声速非常吻合。这种“无正色散”与Hosokawa等人的无弹性X射线散射实验非常吻合。对于液体Ge的这种“无正向分散”的原因特别是由于其运动粘度低而被讨论。尽管速度自相关函数本身没有显示笼效应，但是通过详细分析单个原子运动的轨迹和环境，可以发现微观的笼效应。除单个原子运动外，液态Ge中似乎还存在3-5个原子的原子运动。共价键似乎也至少瞬时存在于液体Ge中。
展开▼
