纤锌矿氮化物量子阱中光学声子模的三元混晶效应

本文先比较了几种常用方法(修正的无规元素等位移模型、虚晶近似和简化相干势近似等)对纤锌矿三元混晶体声子频率的拟合结果,再选用与实验数据接近的拟合方法,结合介电连续和单轴晶体模型导出含纤锌矿三元混晶In_xGa_1－xN和Al_xGa_1－xN单量子阱各类光学声子模的色散关系,进一步分析了声子模随组分的变化. 结果表明,修正的无规元素等位移模型对单模性纤锌矿     

Microscopic theory on the Raman spectra of transition metal dichalcogenides in CDW state

Microscopic mechanisms are clarified for Raman scattering of collective modes, i.e., amplitude and phase modes in the charge density wave (CDW) state of transition metal dichalcogenides. We study three phonon process in triple CDW state and effects of $\text{partial}$ destruction of the Fermi surface due to the phase transition. It has then been understood how both phase and amplitude modes are given Raman activity and how A-E splittings of both modes and the commensurability pinning of the phase mode are related to the three phonon process. We can also explain change of Raman intensity of the originally Raman active A_1g phonon mode at phase transition as interference between paramagnetic and diamagnetic contributions for 2HTaSe₂ as well as other materials.     

Raman spectra of cadmium titanate

Polarized Raman spectra of CdTiO₃ single crystals are recorded for the first time over the frequency range 5 < ν < 1000 cm^?1 at temperatures of 10 to 1200 K. The emphasis was on the low-frequency range, where an anomalous temperature dependence of a few phonon modes was observed. At high temperatures, four phonon modes exhibiting a behavior typical of soft modes were found to exist. These phonon modes are assumed to restore the cubic symmetry of the lattice. Their extrapolated temperature dependences suggest that there exists a sequence of three hypothetical high-temperature phase transitions analogous to those observed in the genuine perovskite CaTiO₃. At temperatures below 78 K, the Raman spectrum exhibits new lines associated with polar distortions of the unit cell. At low frequencies, three lines are observed whose parameters exhibit an anomalous behavior typical of soft modes in a ferroelectric phase. Several different polar states are assumed to exist at low temperatures.     

Vibrational properties of single-wall BC3 nanotubes

The phonon dispersions of single-wall BC₃ nanotubes with any chirality are calculated within a symmetry-based force constant model of the lattice dynamics. Based on the non-symmorphic symmetry group of the BC₃ tubes, the symmetries and number of the Raman- and infrared-active modes at Γ point of the one-dimensional Brillouin zone are given. The neighbor atom-atomic interaction force constants are recalculated by fitting them to the experimental phonon energy-dispersion curves of honeycomb BC₃ sheet. The frequencies of the optically active modes are presented as the function of diameters and chiralities for BC₃ tubes. The obtained phonon density-of-states spectra, phonon dispersion relations, and vibrational patterns of the zone-center phonons are presented and discussed in detail. The calculated frequencies of infrared-active modes are compared with the experimental values reported in the literature. The results provide comprehensive information about the vibrational properties of the BC₃ tubes and shed light on the interpretation of Raman scattering and infrared spectroscopies.     

Impact of anisotropy on polar optical vibration in a wurtzite cylindrical nanowire with ring geometry

The quasi-confined (QC) phonon modes, surface optical (SO) phonon modes and corresponding Fröhlich-like Hamiltonian in a wurtzite cylindrical nanowire with ring geometry are investigated in the framework of the dielectric continuum model and Loudon’s uniaxial crystal model. Numerical calculations are focused on the dispersion relations of the SO phonons and the electron–SO phonon coupling strength. Results show that there are only two branches of SO phonon modes. The dispersions of the two branches of SO phonon modes are obvious when the phonon wave-number k_z or the azimuthal quantum number m is small. Typical degenerating behavior of the SO modes is evidenced due to the anisotropic effect of wurtzite crystal. Moreover, when k_z or m are large enough, the frequencies of the two branches of SO modes converge to a definite limiting frequency in single planar heterostructure. The calculations of the electron–SO phonon coupling strength reveal that the high-frequency SO modes (SO₊) play a more important role in the coupling strength than the low-frequency ones (SO₋). Furthermore, the long-wavelength SO phonons with small m are the main factor contributing to the electron–phonon interaction.     

Polar interface and surface optical vibration spectra in multi-layer wurtzite quantum wires： transfer matrix method

The polar interface optical （IO） and surface optical （SO） phonon modes and the corresponding Froehlich electron phonon-interaction Hamiltonian in a freestanding multi-layer wurtzite cylindrical quantum wire （QWR） are derived and studied by employing the transfer matrix method in the dielectric continuum approximation and Loudon＇s uniaxial crystal model. A numerical calculation of a freestanding wurtzite GaN/AlN QWR is performed. The results reveal that for a relatively large azimuthal quantum number m or wave-number kz in the free z-direction, there exist two branches of IO phonon modes localized at the interface, and only one branch of SO mode localized at the surface in the system. The degenerating behaviours of the IO and SO phonon modes in the wurtzite QWR have also been clearly observed for a small kz or m. The limiting frequency properties of the IO and SO modes for large kz and m have been explained reasonably from the mathematical and physical viewpoints. The calculations of electron-phonon coupling functions show that the high-frequency IO phonon branch and SO mode play a more important role in the electron phonon interaction.     

Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO_1?x (Cr₂O₃) _x . Particularly, in previous literature, the two modes at 514 and 640 cm^?1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)_1?x (Cr₂O₃) _x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr₂O₃. The incorporation of Cr³⁺ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E ₂ (high) peak located at energy 53.90 meV (435 cm^?1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm^?1). A clear picture of the dopant-induced phonon modes along with the B ₁ silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.     

Raman spectroscopy of ZnS nanostructures

We report Raman scattering results of wurtzite ZnS nanowires, nanocombs, and nanobelts. The Raman spectrum obtained from ZnS nanowires exhibits first‐order phonon modes at 272, 284, and 350 cm⁻¹, corresponding to A₁/E₁ transverse optical, E₂ transverse optical, and A₁/E₁ longitudinal optical phonons, respectively. Several multiphonon modes are also observed. The longitudinal optical phonon mode varies in wavenumber for nanocombs and nanobelts, indicating that the residual strain varies during the morphological change from ZnS nanowires to nanocombs and ultimately to nanobelts. Interestingly, a surface optical (SO) phonon mode varies in wavenumber depending on the shape and surface roughness of the ZnS nanostructures. The surface modulation wavelengths of the ZnS nanowires, nanocombs, and nanobelts are estimated using the SO phonon dispersion relations and the observed SO phonon wavenumbers. Copyright © 2012 John Wiley & Sons, Ltd.     

Phonon density of states in nanocrystalline<Superscript>57</Superscript>Fe

The Born-von Karman model is used to calculate phonon density of states (DOS) of nanocrystalline bcc Fe. It is found that there is an anisotropic stiffening in the interatomic force constants and hence there is shrinking in the nearest-neighbour distances in the nanophase. This leads to additional vibrational modes above the bulk phonons near the bottom of the phonon band. It is found that the high energy phonon modes of nanophase Fe are the surface modes. The calculated phonon DOS closely agree with the experimental data except a peak at 37 meV. The calculated phonon dispersion relations are also compared with those of the bulk phonons and anomalous behaviour is discussed in detail. The specific heat in nanophase enhances as compared to bulk phase at low temperatures and the calculated Debye temperature Θ_D agrees with the experimental results. It is predicted that the nanocrystalline Fe may consist of about 14 GPa pressure     

Prediction of superconductivity of Ta2AlC: in situ Raman spectrometry and density functional investigations

In this paper, in situ Raman spectra of Ta₂AlC are measured in the temperature range of 80–500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line‐width of E_2g (ω₃) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon–phonon scatterings. On the other hand, the line‐widths of E_2g (ω₁) and A_1g (ω₄) modes decrease continuously with increasing temperature, which is explained by the electron–phonon couplings of these two phonon modes with the Ta 5d electrons. The electron–phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta₂AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd.     

HfSe2 monolayer stability tuning by strain and charge doping

Strain and charge doping are the effective ways to modulate the electronic and phonon properties of materials. The effects of biaxial tensile strains and charge dopings on the stabilities of HfSe₂ monolayer have been systematically investigated using first-principles methods. Its two-dimensional Young's modulus is only 65.4 N/m, and it is easy to be stretched. When the tensile strain is applied on HfSe₂ monolayer, two of its phonon modes soften with one frequency decreasing to zero at critical strain. Our results show that electron and hole dopings could suppress the softening of phonon modes, and significantly enhance the ideal strength by 28% and 36%, respectively. The calculations for electronic structures and phonon dispersions provide the theoretical references for future nano-device designing.     

Electronic structure and lattice dynamics of CaPd3B studied by first-principles methods

Using first-principles methods, we have studied the electronic structure and lattice dynamics of CaPd₃B and compared them to isostructural MgNi₃C. CaPd₃B possesses less electronic states at the Fermi level, but more phonon modes at low frequencies, than MgNi₃C. According to the phonon density of states, low frequency acoustic modes are dominated by Pd states, corresponding to Ni in MgNi₃C. Furthermore, these Pd modes show soft phonons, which may be significant for second-order phase transitions. Based on the comparison to MgNi₃C, we suggest that the properties of these two compounds may be similar.     

Surface and interface optical phonon modes and electron-phonon interaction in a multi-shell spherical system

Within the framework of the dielectric continuum model, interface optical(IO) and surface optical(SO) phonon modes and the Fr?hlich electron-IO (SO) phonon interaction Hamiltonian in a multi-shell spherical system were derived and studied. Numerical calculation on CdS/HgS/H₂O and CdS/HgS/CdS/H₂O spherical systems have been performed. Results reveal that there are two IO modes and one SO mode for the CdS/HgS/H₂O system, one SO mode and four IO modes whose frequencies approach the IO phonon frequencies of the single CdS/HgS heterostructure with the increasing of the quantum number l for CdS/HgS/CdS/H₂O. It also showed that smaller l and SO phonon compared with IO phonon, have more significant contribution to the electron-IO (SO) phonon interaction. Received 16 October 2001 and Received in final form 23 January 2002 Published online 25 June 2002     

Surface optical Raman modes in GaN nanoribbons

Raman scattering studies were performed in GaN nanoribbons grown along [1 0 0]. These samples were prepared inside Na‐4 mica nanochannels by the ion‐exchange technique and subsequent annealing in NH₃ ambient. Detailed morphological and structural studies including the crystalline orientation were performed by analyzing the vibrational properties in these GaN nanoribbons. Pressure in the embedded structure was calculated from the blue shift of the E₂(high) phonon mode of GaN. Possible red shift of optical phonon modes due to the quantum confinement is also discussed. In addition to the optical phonons allowed by symmetry, two additional Raman peaks were also observed at ∼633 and 678 cm⁻¹ for these nanoribbons. Calculations for the wavenumbers of the surface optical (SO) phonon modes in GaN in Na‐4 mica yielded values close to those of the new Raman modes. The SO phonon modes were calculated in the slab (applicable to belt‐like nanoribbon) mode, as the wavenumber and intensity of these modes depend on the size and the shape of the nanostructures. The effect of surface‐modulation‐assisted electron–SO phonon scattering is suggested to be responsible for the pronounced appearance of SO phonon modes. A scaling factor is also estimated for the interacting surface potential influencing the observed SO Raman scattering intensities. Copyright © 2010 John Wiley & Sons, Ltd.     

Far infrared reflection spectra due to lattice and free carrier in ZnxHg1−xSe

Far infrared reflection spectra of Zn_xHg_1−xSe with x = 0 to 0.4 showed characteristic of two TO-phonon modes behavior and the plasmon-LO phonon coupling in the region from 20 to 500 cm⁻¹ at 5 and 300 K. The composition dependence of TO modes on the frequency is linear for the HgSe-like mode TO₁ and constant for the ZnSe-like mode TO₂. The additional TO mode due to clustering effect is observed at 113 cm⁻¹. The plasmon-LO phonon coupled modes are explained well by our model taken into account the two-mode behavior and the contribution of interband transitions in dynamic dielectric function.     

两耦合半无限超晶格中的局域界面声子-极化激元

采用转移矩阵方法，研究了含结构缺陷层的两耦合半无限超晶格(GaAs/AlAs)中的局域界面声子-极化激元模性质. 研究发现，含不同介电特性的缺陷超晶格结构中的局域界面声子-极化激元模在剩余射线区［ω_TO, ω_LO］的分布情况与数量存在不同，而且反对称模表现出不同的特征. 文中着重研究了缺陷层介电常数与角频率无关的缺陷超晶格，发现该结构中的局域界面声子-极化激元模对组分层的排列顺序与厚度、缺陷层的厚度以及横向波数有着不同程度的依赖.     

Raman scattering in intermediate valent SmB6

We have measured the Raman spectra of intermediate valent (IV) SmB₆ and compared it to LaB₆ and EuB₆. Beside the three high energy Raman active phonons we found additional features in the spectra. Most prominent is a peak at 172 cm^-1 for SmB₆, at 214 cm^-1 for LaB₆ and at ～220 cm^-1 for EuB₆. The spectra are analysed in terms of defect induced phonon excitations yielding a weighted phonon density of states. The softening of the line in IV SmB₆ compared to the other hexaborides is unusual since even in the trivalent MB₆ compounds the Raman active phonon modes normally are at lower frequencies than in semiconducting, divalent samples. This phonon softening is explained in analogy with the phonon anomalies in other IV compounds like Sm_1?xY_xS and TmSe.     

Far Infrared Investigation of Plasmon-LO Phonon Coupling and Surface Polaritons Modes in Donor Doped Cd1-xHgxTe and CdTe Thin Layer on GaAs Substrate

Infrared Fourier transform spectroscopy has been used to investigate phonon and plasmon modes in Cd_1-xHg_xTe and CdTe thin layer on GaAs substrate. Attenuated total reflection (ATR) spectroscopy has been used to excite surface plasmon and phonon polaritons. The plasmon-LO phonon coupling modes in samples are studied by dispersion curve calculation for various carrier concentrations. There exist three branches of coupled modes in dispersion curve. For analysis of the far infrared polarized reflectivity spectra we employ the harmonic oscillator dielectiic function model and the Drude model for free carrier response. We find that the coupling modes dependent to the concentration of free carriers. Furthermore, the experimental data have been used to calculate carrier concentration, composition parameter, mobility of carrier, thickness of layer and gap energy.     

Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al_0.3Ga_0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons.