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.     

Mg2Sn电子结构及热力学性质的第一性原理计算

采用基于第一性原理的赝势平面波方法系统地计算了Mg₂Sn基态的电子结构、弹性常数和热力学性质.计算结果表明Mg₂Sn的禁带宽度为0.1198eV.运用线性响应方法确定了声子色散关系和态密度,得出Mg₂Sn的热力学性质如等容比热和德拜温度.计算Mg₂Sn的热导率并与实验数据相比较. 关键词： 第一性原理 电子结构 弹性常数 热力学性质     

High-pressure phonon dispersion of copper by using the modified analytic embedded atom method

By using the Born-von Krmn theory of lattice dynamics and the modified analytic embedded atom method, we reproduce the experimental results of the phonon dispersion in fcc metal Cu at zero pressure along three high symmetry directions and four off-symmetry directions, and then simulate the phonon dispersion curves of Cu at high pressures of 50, 100, and 150 GPa. The results show that the shapes of dispersion curves at high pressures are very similar to that at zero pressure. All the vibration frequencies of Cu in all vibration branches at high pressures are larger than the results at zero pressure, and increase correspondingly as pressure reaches 50, 100, and 150 GPa sequentially. Moreover, on the basis of phonon dispersion, we calculate the values of specific heat of Cu at different pressures. The prediction of thermodynamic quantities lays a significant foundation for guiding and judging experiments of thermodynamic properties of solids under high pressures.     

High-pressure dynamic,thermodynamic properties,and hardness of CdP_2

Using a pseudopotential plane-waves method,we calculate the phonon dispersion curves,thermodynamic properties,and hardness values of α-CdP_2 and β-CdP_2 under high pressure.From the studies of the phonon property and enthalpy difference curves,we discuss a phase transform from β-CdP_2 to a-CdP_2 in a pressure range between 20 GPa and 25 GPa.Then,the thermodynamic properties,Debye temperatures,and heat capacities are investigated at high pressures.What is more,we employ a semiempirical method to evaluate the pressure effects on the hardness for these two crystals.The results show that the hardness values of both α-CdP_2 and β-CdP_2 increase as pressure is increased.The influence mechanism of the pressure effect on the hardness of CdP_2 is also briefly discussed.     

A first-principle study of the structural and lattice dynamical properties of CaX (X=S,Se, and Te)

We have studied structural, elastic, thermodynamic (Debye temperature and melting temperature), and lattice dynamical (phonon dispersion curves, heat capacity, and entropy) properties of CaX via ab initio calculations within the local density approximations. The results are compared with the available experimental and other theoretical data, and the agreement is, generally, quite good. We also predict the temperature and/or pressure-dependent behaviors of some mechanical, lattice dynamical, and thermodynamic properties for the same compounds.     

Theoretical Calculations for Structural, Elastic and Thermodynamic Properties of γTiAl Under High Pressure

We investigate the structural and elastic properties of γTiAl under high pressures using the norm-conserving pseudopotentials within the local density approximation （LDA） in the frame of density functional theory. The calculated pressure dependence of the elastic constants is in excellent agreement with the experimental results. The elastic constants and anisotropy as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also investigate the thermodynamic properties of γTiAl.     

First-principles investigations on elastic, phonon and thermodynamic properties of SrB6 under pressure

The elastic, phonon and thermodynamic properties of the divalent alkaline-earth hexaboride SrB₆ are investigated by using plane-wave pseudopotential density functional theory method. The calculated structure parameters and bulk modulus are well consistent with the available experiment and theoretical data. The pressure dependences of elastic constants C_ij, bulk modulus B₀, shear modulus G, Young's modulus E and Poisson's ratio σ are also presented. With these elastic parameters, we investigate the mechanical stability and compressibility of SrB₆. For the thermodynamic properties, both phonon and quasi-harmonic Debye model methods are adopted. Through the comparison with experimental and other theoretical results, we found the method of quasi-harmonic Debye model is a little better. Moreover, the phonon dispersion relations are also obtained. It is found that there are two LO/TO splitting around 5 THz and 26 THz, respectively.     

A method to calculate the thermal conductivity of HMX under high pressure

A method based on Debye theory is developed to calculate the thermal conductivity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The phonon–phonon interaction model is built up for solid HMX. The phonon lifetime formula is derived by the phonon–phonon scattering mechanism, and the thermal conductivity tensor is derived by the phonon dispersion model. The thermal conductivities of α/β/δ-HMX are calculated in the temperature range 0–700?K and pressure range of 0–10?GPa. The phonon softening process of HMX is investigated. We have proven that the Debye frequency and thermal conductivity tend to 0 at the phonon softening point. A physical picture of the phonon–phonon interaction, phonon lifetime and phonon softening is built up.     

Electron-Phonon Interaction in an Annular Quantum Dot

The confined longitudinal-optical （LO） phonon and surface-optical （SO） phonon modes of a free-standing annular cylindrical quantum dot are derived within the framework of dielectric continuum approximation. It is found that there exist two types of SO phonon modes： top SO （TSO） mode and side SO（SSO） mode in a cylindrical quantum annulus. Numerical calculation on CdS annulus system has been performed. Results reveal that the two different solutions of SSO mode distribute mainly at the inner or outer surfaces of the annulus. The dispersion relations and the coupling intensions of phonons in a quantum annulus are compared with those in a cylindrical quantum dot. It is found that the dispersion relations of the two different structures are similar, but the coupling intension of the phonon-electron interaction in quantum annulus is larger than that in quantum dot. The Hamiltonians describing the free phonon modes and their interactions with electrons in the system are also derived.     

Influence of a phonon bath in a quantum dot cavity QED system:Dependence of the shape

We present a systematic analysis on the role of the quantum dot （QD） shape in the influence of the phonon bath on the dynamics of a QD cavity QED system. The spectral functions of the phonon bath in three representative QD shapes： spherical, ellipsoidal, and disk, are calculated from the carrier wave functions subjected to the confinement potential provided by the corresponding shape. The obtained spectral functions are used to calculate three main effects brought by the phonon bath, i.e., the coupling renormalization, the off-resonance assisted feeding rate and the pure dephasing rate. It is found that the spectral function of a disk QD has the widest distribution, hence the phonon bath in a disk QD can lead to the smallest renormalization factor, the largest dephasing rate in the short time domains（≤2 ps）, and the oft-resonance assisted feeding can support the widest detuning. Except for the pure dephasing rate in the long time domains, all the influences brought by the phonon bath show serious shape dependence.     

Theory of phonon-modulated electron spin relaxation time based on the projection reduction method

This paper introduces a new method for a formula for electron spin relaxation time of a system of electrons interacting with phonons through phonon-modulated spin-orbit coupling using the projection-reduction method. The phonon absorption and emission processes as well as the photon absorption and emission processes in all electron transition processes can be explained in an organized manner, and the result can be represented in a diagram that can provide intuition for the quantum dynamics of electrons in a solid. The temperature （T） dependence of electron spin relaxation times （T1） in silicon is T1 ∝ T-1.07 at low temperatures and T1 ∝ T-3.3 at high temperatures for acoustic deformation constant Pad = 1.4 × 10^7 eV and optical deformation constant Pod = 4.0 × 10^17 eV/m. This means that electrons are scattered by the acoustic deformation phonons at low temperatures and optical deformation phonons at high temperatures, respectively. The magnetic field （B） dependence of the relaxation times is T1 ∝ B-2.7 at 100 K and T1 ∝ B-2.3 at 150 K, which nearly agree with the result of Yafet, T1 ∝ B-3.0- B -2.5.     

Evolution of the 251 cm-1 temperature in infrared phonon mode with Ba0.6K0.4Fe2As2

The far-infrared optical reflectivity of an optimaUy doped Ba1-xKxFe2As2 （x = 0.4） single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 em-1. This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continu- ously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observa- tions indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Bao.6Ko.4Fe2As2 compound.     

Coupling of two-dimensional atomistic and continuum models for dynamic crack

A concurrent multiscale method of coupling atomistic and continuum models is presented in the two-dimensional system. The atomistic region is governed by molecular dynamics while the continuum region is represented by construct- ing the mass and stiffness matrix dependent on the coarsening of the grids, which ensures that they merge seamlessly. The low-pass phonon filter embedded in the handshaking region is utilized to effectively eliminate the spurious reflection of high-frequency phonons, while keeping the low-frequency phonons transparent. These schemes are demonstrated by numerically calculating the reflection and transmission coefficient, and by the further application of dynamic crack propa- gation subjected to mode-I tensile loading.     

Polar Mixing Optical Phonon Spectra in Wurtzite GaN Cylindrical Quantum Dots： Quantum Size and Dielectric Effects

The interface-optical-propagating （IO-PR） mixing phonon modes of a quasi-zero-dimensional （QOD） wurtzite cylindrical quantum dot （QD） structure are derived and studied by employing the macroscopic dielectric continuum model. The analytical phonon states of IO-PR mixing modes are given. It is found that there are two types of IO-PR mixing phonon modes, i.e. p-IO//z-PR mixing modes and the z-IO//p-PR mixing modes existing in QOD wurtzite QDs. And each IO-PR mixing modes also have symmetrical and antisymmetrieal forms. Via a standard procedure of field quantization, the Frohlich Hamiltonians of electron-（IO-PR） mixing phonons interaction are obtained. Numerical calculations on a wurtzite GaN cylindrical QD are performed. The results reveal that both the radial-direction size and the axial-direction size as well as the dielectric matrix have great influence on the dispersive frequencies of the IO-PR mixing phonon modes. The limiting features of dispersive curves of these phonon modes are discussed in depth. The phonon modes ＂reducing＂ behavior of wurtzite quantum confined systems has been observed obviously in the structures. Moreover, the degenerating behaviors of the IO-PR mixing phonon modes in wurtzite QOD QDs to the IO modes and PR modes in wurtzite Q2D QW and QID QWR systems are analyzed deeply from both of the viewpoints of physics and mathematics.     

Lattice stabilities,mechanical and thermodynamic properties of Al_3Tm and Al_3Lu intermetallics under high pressure from first-principles calculations

The effects of high pressure on lattice stability, mechanical and thermodynamic properties of L1_2 structure Al_3Tm and Al_3Lu are studied by first-principles calculations within the VASP code. The phonon dispersion curves and density of phonon states are calculated by using the PHONONPY code. Our results agree well with the available experimental and theoretical values. The vibrational properties indicate that Al_3Tm and A_3Lu keep their dynamical stabilities in L1_2 structure up to 100 GPa. The elastic properties and Debye temperatures for Al_3Tm and Al_3 Lu increase with the increase of pressure. The mechanical anisotropic properties are discussed by using anisotropic indices AG, AU, AZ, and the threedimensional(3D) curved surface of Young's modulus. The calculated results show that Al_3Tm and Al_3Lu are both isotropic at 0 GPa and anisotropic under high pressure. In the present work, the sound velocities in different directions for Al_3Tm and Al_3Lu are also predicted under high pressure. We also calculate the thermodynamic properties and provide the relationships between thermal parameters and temperature/pressure. These results can provide theoretical support for further experimental work and industrial applications.     

Lattice dynamics and phase diagram of aluminum at high temperatures

The dispersion of phonons in the fcc, hcp, and bcc phases of aluminum is calculated at ultrahigh pressures by the method of small displacements in a supercell. The stability of the phonon subsystem is studied. The thermodynamic characteristics are calculated in the quasi-harmonic approximation, and a phase diagram of aluminum is plotted. As compared to the Debye model, the use of a phonon spectrum calculated in the quasi-harmonic approximation significantly broadens the hcp phase field and strongly shifts the phase boundary between the fcc and bcc phases. The normal isentrope is calculated at megabar pressures. It is shown to intersect the fcc-hcp and hcp-bcc phase boundaries. The sound velocity along the normal isentrope is calculated. It is shown to have a nonmonotonic character.     

First-principles investigations of the structure and physical properties for new TcN crystal structure

Using a newly developed particle swarm optimisation technique on crystal structural prediction, we have predicted an orthorhombic Imm2 structure for TcN crystal, which is energetically much superior to the previously proposed NbO-type and R-3m structures. The new phase is stable against decomposition into the mixture of Tc and N at ambient condition. The stability is confirmed by the subsequent calculations on the phonon dispersion curves and elastic constants. An analysis of the mechanical properties indicates that Imm2-TcN is incompressible and common hard material. The evidence of strong covalent bonding of Tc-N, which plays a leading role to form a hard material, is manifested by the partial densities of state analysis. In addition, the thermodynamic properties, such as Debye temperature, heat capacity, thermal expansion coefficient, and Grüneisen parameter of TcN are investigated by the quasi-harmonic Debye model.     

First-principles investigation on the elastic stability and thermodynamic properties of Ti2SC

Using Vanderbilt-type plane-wave ultrasoft pseudopotentials within the generalized gradient approximation（GGA） in the frame of density functional theory（DFT）,we have investigated the crystal structures,elastic,and thermodynamic properties for Ti2SC under high temperature and high pressure.The calculated pressure dependence of the lattice volume is in excellent agreement with the experimental results.The calculated structural parameter of the Ti atom experienced a subtle increase with applied pressures and the increase suspended under higher pressures.The elastic constants calculations demonstrated that the crystal lattice is still stable up to 200 GPa.Investigations on the elastic properties show that the c axis is stiffer than the a axis,which is consistent with the larger longitudinal elastic constants（C 33,C 11） relative to transverse ones（C 44,C 12,C 13）.Study on Poisson＇s ratio confirmed that the higher ionic or weaker covalent contribution in intra-atomic bonding for Ti2SC should be assumed and the nature of ionic increased with pressure.The ratio（B/G） of bulk（B） and shear（G） moduli as well as B/C 44 demonstrated the brittleness of Ti2SC at ambient conditions and the brittleness decreased with pressure.Moreover,the isothermal and adiabatic bulk moduli displayed opposite temperature dependence under different pressures.Again,we observed that the Debye temperature and Gru篓neisen parameter show weak temperature dependence relative to the thermal expansion coefficient,entropy,and heat capacity,from which the pressure effects are clearly seen.     

Lattice-dynamical functions of solids at very high pressures

Abstract

Vibrational spectra of face-centered cubical (f.c.c.) solids are investigated at the pressure region, where the quantum-statistical model for the electron energy is applicable. Phonon dispersion curves are obtained for different values of the specific volume. The integration of the phonon spectra over the Brillouin zone yields the zero-point vibrational energy, the Debye temperature, the Gruneisen coefficient, mean-square amplitude of zero-point and thermal vibration, the melting temperature. All quantities under consideration are calculated from the universal functions of the reduced volume, using the scaling relations for given atomic number 2.     

Shell Model for Elastic and Thermodynamic Properties of Gallium Nitride with Hexagonal Wurtzite Structure

Shell model molecular dynamic simulation with interatomic pair potential is utilized to investigate the elastic and thermodynamic properties of gallium nitride with hexagonal wurtzite structure （w-GaN） at high pressure. The calculated elastic constants Cij at zero pressure and 300 K agree well with the experimental data and other calculated values. Meanwhile, the dependences of the relative volume V/Vo, elastic constants Cij, entropy S, enthalpy H, and heat capacities Cv and Up on pressure are successfully obtained. From the elastic constants obtained, we also calculate the shear modulus G, bulk modulus B, Young＇s modulus E, Poisson＇s ratio v, Debye temperature ΘD, and shear anisotropic factor Ashear on pressures.