铁基氟化物超导体SrFe_(1-x)Co_xAsF(x=0,0.125)声子特性的第一性原理计算研究

采用基于第一性原理的平面波赝势方法,计算了铁基氟化物及其钴掺杂超导体SrFe1-xCoxAsF(x=0,0.125)在四方非磁态与正交条纹反铁磁态下的声子谱(声子色散曲线、声子态密度)及电-声子耦合常数.计算发现:条纹反铁磁相互作用下的自旋-声子耦合效应强于电-声子耦合作用使声子谱的宽度减小;自旋效应使声子的有效质量增加导致条纹反铁磁态下Fe原子与As原子的耦合振动频率减小.另外,掺杂和自旋效应是提高电-声子耦合常数的两个有效方法,但计算所得超导转变温度远小于实验测量值,表明铁基超导电性非简单的电-声子耦合配对机理.     

高压下FCC相金属氢结构稳定性和非谐效应的理论研究

利用第一性原理分子动力学方法研究金属氢体系的非简谐效应, 给出金属氢的声子谱, 讨论金属氢声子谱的温度效应。计算得到氢的同位素氕、氘和氚的FCC相在非零温下的声子谱, 不同温度下的声子谱对比发现零温下3.6 TPa为热力学稳定的临界压强点, 而有限温度下(100 K)临界压强点降到2.8 TPa, 非简谐效应显著地改变了体系的结构稳定性和声子振动性质。     

手性单壁碳纳米管声子谱的计算

本文对不同直径的单壁碳纳米管的声子谱进行了计算。利用碳管的对称性对其晶格动力学矩阵进行了约化,给出全部声子的频率以及振动模对称性。详细分析了布里渊区(点声子的频率与管径的依赖关系,总结出一些规律,并对其进行了分析。     

单壁碳纳米管手性角对声子振动频率的影响

报道了用卷曲法对一系列手性单壁碳纳米管计及卷曲效应声子振动谱的计算结果.基于实际的数值计算，讨论了Brillouin区中心Γ点手性管晶格振动模的分类；得出了其12个非简并的本征频率与手性和管径的关系. 关键词： 单壁碳纳米管 手性管 振动模式     

纤锌矿型量子级联激光器的界面与受限声子比较

利用传输矩阵方法对纤锌矿型量子级联激光器有源区的界面与受限声子进行了研究.计算结果显示:在纵光学频域有一组界面声子与二组受限声子存在,而且最低频率的两个界面声子在一定条件下能转变为受限模式.通过比较界面声子与受限声子的色散及声子势发现两者有很大不同,且计算电声散射速率可以发现由这两种声子引起的散射率是可比拟的.     

纤锌矿型量子级联激光器的界面与受限声子比较(英文)

利用传输矩阵方法对纤锌矿型量子级联激光器有源区的界面与受限声子进行了研究.计算结果显示:在纵光学频域有一组界面声子与二组受限声子存在,而且最低频率的两个界面声子在一定条件下能转变为受限模式.通过比较界面声子与受限声子的色散及声子势发现两者有很大不同,且计算电声散射速率可以发现由这两种声子引起的散射率是可比拟的.     

Ag（110）表面声子谱的分析型EAM模型计算

应用晶格动力学，结合分析型EAM模型，计算了Ag(110)表面声子频率和振动极化方向，通过比较弛豫与未弛豫表面的计算结果，发现弛豫后的结果与He原子散射实验数据符合得很好，表面振动的极化局域特征符合表面的二维对称性，沿方向的S2和S3表面模也和实验结果—致. 关键词： 表面声子谱 弛豫 EAM 极化     

液相外延GaAs_(1-x)P_x混晶的光学声子、等离子体激元和LO声子-等离子体激元耦合模

在40—700cm~(-1)波数和4.2—300K温度范围内研究了不同组份液相外延n型CaAs_(1-x)P_x样品的红外反射谱。对反射谱进行了赝谐振子拟合与K-K关系计算。从而获得了有关描述CaAs_(1-x)P_x样品的光学声子模、等离子体激元、LO声子-等离子体激元耦合模的重要物理参量及红外光学常数信息。基于这些计算结果,提出了双导带谷并计及与X能谷相联系的施主能级模型,用它解释等离子体激元频率和实验观察的三支主要的耦合模的温度变化规律。     

赝势对计算石墨烯声子谱线的第一性原理研究

石墨烯是理论与实验方面研究的热点,而探究其声子谱线结构又为研究力学、热力学等提供基础.本文采用基于密度泛函理论的第一性原理,运用不同的交换关联和赝势方法,计算了石墨烯以及石墨的声子谱线.对比研究发现:在声子谱低频率阶段,不同的赝势计算的结果差别很小;而在声子谱的高频率阶段,不同赝势计算的结果差别显著.相对于GGA交换关联,LDA交换关联计算的高频光学支有所软化,计算结果与实验值更加接近.相对于US赝势方法,PAW赝势方法计算的结果与实验值更加接近.综合比较,PAW-LDA赝势的计算结果与实验值最为接近.     

液相外延GaAs_（1-x）_P_x_混晶的光学声子，等离子体激元和LO声子-等离子激元耦合模

在40--7 0 0 (1/cm)波数和4.2--300 K 温度范围内研究了不同组份液相外延n 型GaAs_（1-x）_P_x__样品的红外反射谱. 对反射谱进行了鹰谐振子拟合与K-K 关系计算.从而获得了有关描述GaAs_（1-x）_P_x__样品的光学声子模、等离子体激元、LO 声子-等离子体激元藕合模的重要物理参量及红外光学常数信息. 基于这些计算结果, 提出了双导带谷并计及与x 能谷相联系的施主能级模型, 用它解释等离子体激元频率和实验观察的三支主要的祸合模的温度变化规律。 关键词：     

Phonon and thermal properties of achiral single wall carbon nanotubes

A detailed theoretical study of the phonon and thermal properties of achiral single wall carbon nanotubes has been carried out using force constant model considering up to third nearest-neighbor interactions. We have calculated the phonon dispersions, density of states, radial breathing modes (RBM) and the specific heats for various zigzag and armchair nanotubes, with radii ranging from 2.8 Å to 11.0 Å. A comparative study of phonon spectrum with measured Raman data reveals that the number of Raman active modes for a tube does not depend on the number of atoms present in the unit cell but on its chirality. Calculated phonon modes at the zone center more or less accurately predicted the Raman active modes. The radial breathing mode is of particular interest as for a specific radius of a nanotube it is found to be independent of its chirality. We have also calculated the variation of RBM and G-band modes for tubes of different radii. RBM shows an inverse dependence on the radius of the tube. Finally, the values of specific heat are calculated for various nanotubes at room temperature and it was found that the specific heat shows an exponential dependence on the diameter of the tube.     

A symmetry-adapted force-constant lattice-dynamical model for single-walled carbon nanotubes

We develop a lattice-dynamical model based on the screw symmetry of single-walled carbon nanotubes that allows for reducing the size of the dynamical matrix to six, for all tube chiralities. The model uses force constants derived from fitting to the phonon dispersion of 2D graphite. We present the calculation procedure in a clear and transparent way, making the model easier to follow. We calculate the phonon dispersions of a number of nanotubes of different chiralities. The splitting of two highest Raman active modes and the radial breathing mode frequency are studied by changing the tube diameter and chirality.     

Phonon structure and dynamics of boron nitride single wall nanotube

We report a detailed study of the phonon properties of hexagonal boron nitride (BN) monolayers as well as nanotubes by using De Launay type of angular force model. The force constants used for calculation of phonon dispersion relations of the nanotube are derived from those for the monolayers of hexagonal by employing force constant method. These force constants have been modified to include the effect of curvature of the tubule. The results are then used to derive the phonon dispersion relations for (10,10) BN nanotubes using ‘zone-folding’ method. Calculated phonon modes are in good agreement with the experimental values obtained so far, for (10,10) armchair BN nanotubes.     

单壁碳纳米管声子谱及比热计算

利用卷曲法计算了有限长单壁碳纳米管的声子色散关系．讨论了单壁碳纳米管的比热随管径、温度的变化趋势．结果表明碳管的比热随温度、管径的增大而增大,并逐渐趋于一恒定数值．根据色散关系的计算结果,给出了有限长（5,5）型单壁碳纳米管的振动模式以及部分振动模式的频率随长度的变化关系． 关键词： 碳纳米管 声子 比热     

Phonon dispersion relations and soft modes of 4? carbon nanotubes

The phonon dispersion relations of three kinds of 4~? carbon nanotubes are calculated by using the density functional perturbation theory. It is found that the frequencies of some phonon modes are very sensitive to the smearing width used in the calculations, and eventually become negative at low electronic temperature. Moreover, two kinds of soft modes are identified for the (5,0) tube which are quite different from those reported previously. Our results suggest that the (5,0) tube remains metallic at very low temperature, instead of the metallic-semiconducting transition claimed before.     

Born-von Kármán force constants of face-centred cubic metals studied by means of phonon frequency sums

Information concerning the Born-von 'Karmán (BvK) force constants of selected cubic metals are obtained from measured phonon frequencies by means of the gradual form of the frequency sum rule. Published model values of the BvK constants are commented on.     

Discontinuity of the surface polaron

Using the Feynman's path integral method, energies and masses are calculated for various types of a surface polaron. It is shown that, when the electron-surface phonon interaction becomes strong, surface optical phonon yields continuous change from free to self-trapped states. On the other hand surface acoustic phonon yields discontinuous change for the deformation type interaction and continuous change for the piezoelectric interaction. All these results reflect the character of the force range of the electron-surface phonon interaction.     

Energy dissipation mechanisms in carbon nanotube oscillators

Energy transfer from the translational degrees of freedom to phonon modes is studied for isolated systems of two coaxial carbon nanotubes, which may serve as a nearly frictionless nano-oscillator. It is found that for oscillators with short nanotubes (less than 30 A) a rocking motion, occurring when the inner tube is pulled about 1/3 out of the outer tube, is responsible for significant phonon energy acquisitions. For oscillators with long nanotubes translational energies are mainly dissipated via a wavy deformation in the outer tube undergoing radial vibrations. Frictional forces between 10(-17) and 10(-14) N per atom are found for various dissipative mechanisms.     

Dynamic properties of graphene

The phonon spectrum of graphene has been studied with the minimum set of the nearest neighbors in the Born-von Kármán model taking into account the electron-electron and electron-phonon interactions. The widths, both natural and owing to interactions with defects, of phonons have been estimated. Symmetry constraints imposed on force constants are taken into account. For symmetry reasons, vibrations with the polarization normal to the plane of the layer are not related to in-plane vibrations. The phonon frequencies at symmetry points and elastic moduli are expressed in terms of force constants.     

Exponential decay lifetimes of excitons in individual single-walled carbon nanotubes

The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode.