Phonons in 3C-, 4H-, and 6H-SiC

Silicon carbide epilayers of cubic (3C) and hexagonal (4H and 6H) polytypes were investigated by Auger electron spectroscopy, high-resolution electron energy-loss spectroscopy and Raman spectroscopy to determine the excitation energies of the optical Fuchs-Kliewer surface phonons and their relation to bulk phonon frequencies. The surfaces were treated in a buffered hydrofluoric acid solution. Loss structures attributed to excitation of Fuchs-Kliewer phonons were clearly resolved. Their energies were found at 115.9 ± 1 meV irrespective of the SiC polytype. The experimental data agree with values calculated from the experimental bulk phonon frequencies and tabulated dielectric constants.     

Two-fluid equations for phonons without momentum

In the customary microscopic derivation of the two-fluid equations of liquid He II explicit use is made of the assumption that an elementary excitation of wave vector k carries a momentum ?k. In this paper it is shown that phonons in a liquid can be defined as carrying momentum ?k (Eulerian phonons) or zero momentum (Lagrangian phonons). A careful analysis—in particular of the concept “velocity of the bearer fluid”—shows that the two-fluid equations turn out to be the same in either case.     

Surface phonons and superstructures; Some applications

Experiments using the technique of low energy electron diffraction (LEED) have shown that the periodic configuration of the atoms on some clean surfaces for a wide range of crystals is different from what is expected from the bulk configuration. A simple lattice dynamical calculation is presented showing that surface superstructures may exist even if interactions near the surface remain unchanged by the “bond-breaking” procedure. A calculation has been made for surface phonons at the (001) surface of some fcc and bcc transition metals. A model consisting of first and second neighbor central interactions together with angle bending forces is used — specific results have been obtained for surface phonons of Pt, Pd, Cu and Ag for fcc metals as well as Fe and W for bcc crystals. The agreement with neutron measurements for the bulk phonons is good. The (001) surface phonons for the above cited transition metals are obtained for the first time to our knowledge. The effect of variations of the interactions near the surface for Pt has been also investigated showing soft surface phonons for surface forces decreased by about 35% to 40% as compared to their bulk values. This model has been extended to study the vibrations of an adsorbed layer namely in the case concerning rare gazes (Ne, Xe,…). A similar attempt showing possible magnetic surface superstructure is also reported.     

Study of surface and bulk acoustic phonon excitations in superlattices using picosecond ultrasonics

We have performed picosecond ultrasonic studies on the surface and bulk acoustic phonons in amorphous Mo/Si superlattices. Localized surface modes within the first, second, and sixth frequency gaps of the zone-folded phonons are observed. A selection rule derived from symmetry considerations provides new understanding of why certain modes are seen and not the others. The excitation strengths and detailed spectral features of these lines are studied, and the results are well explained by an elastic-continuum theory. It is found that the line shapes are significantly modified by the presence of bulk modes near the zone center.     

ZnSe-ZnS应变超晶格的Raman散射

本文报导了Znse—ZnS应变超晶格的Raman光学声子谱.我们观测到,随着应变大小的改变,ZnSe和ZnS的纵向光学声子发生频移.ZnSe层中纵向光学声子可发生较大的蓝移,也可发生较小的红移;ZnS层中的纵向光学声子发生较大的红移.这些现象为“应变场下的光学模理论”所解释.文中还报导了在波数为110cm-1处观测到一很强的散射峰,并把它归结为超晶格表面层单斜Se所引起的散射;在其它地方还观测到非晶态Se、三角Se引起的散射峰.     

Vibrational density of states of hen egg white lysozyme

The resonant Raman scattering in GeSi/Si structures with GeSi quantum dots has been analyzed. These structures were formed at various temperatures in the process of molecular-beam epitaxy. It has been shown that Raman scattering spectra recorded near resonances with the E₀ and E₁ electronic transitions exhibit the lines of Ge optical phonons whose frequencies differ significantly from the corresponding values in bulk germanium. In the structures grown at low temperatures (300–400°C), the phonon frequency decreases with increasing excitation energy. This behavior is attributed to Raman scattering, which is sensitive to the size of quantum dots, and shows that quantum dots are inhomogeneous in size. In the structures grown at a higher temperature (500°C), the opposite dependence of the frequency of Ge phonons on excitation energy is observed. This behavior is attributed to the competitive effect of internal mechanical stresses in quantum dots, the localization of optical photons, and the mixing of Ge and Si atoms in structures with a bimodal size distribution of quantum dots.     

Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering

The temperature dependences (5–300 K) of the Raman spectra of E _2g phonons and optical constants in zinc single crystals are measured in the excitation energy range 1.4–2.54 eV. It is found that phonon damping decreases upon an increase in the wavelength of exciting radiation. The obtained results are compared with the dependence of the phonon width on the excitation energy (the probed wave vector of the excitations under investigation), which are presented for the first time for the transition metal osmium, as well as with the calculated electron-phonon renormalization of damping, taking into account the actual distribution of wave vectors.     

Collective excitations of the polaron-gas

The dispersion of the collective excitations of a polaron-gas is calculated. It is found that the interaction between the L.O. phonons and the plasmons induces a relatively strong wave vector dependence in the ω⁺, ω^- modes. This dispersion is primarily a consequence of the interaction of the L.O. phonons with the resonant plasmon branch in the pair excitation spectrum.     

Lifetime of optical phonons in fs-laser excited bismuth

This paper presents experimental and theoretical results on the temperature-dependent optical response of a single crystal of bismuth to excitation by femtosecond laser pulses. We demonstrate that the measured damping rate of the transient reflectivity oscillations relates to the lifetime of optical phonons. The lifetime is the inverse rate of the decay of optical phonons into two acoustic phonons. This lifetime also indicates the approach to the vibration instability (catastrophe) threshold that manifests the beginning of the disordering of a solid crystal and transition to a liquid state. We observe the red shift of phonon frequency, which increases with the rise of the initial lattice temperature. The red shift is different from the previously observed red shift proportional to the electron temperature, and thus to the excitation laser fluence. The coherent phonon excitation process imprinted into the initial change in the reflectivity and the following reflectivity oscillations allowed us to uncover the temporal phonon history preceding the structural transformation of solid Bi.     

弱耦合多原子半无限晶体中磁极化子的激发能量

近年来国内外对多原子极性晶体中磁极化子性质的研究十分活跃,Zorkani等采用变分法计算了束缚磁极化子的基态能量,Kandemir等采用束缚朗道态讨论了二维大磁极化子的基态和第一激发态能量,国内一些学者采用微扰法和新颖算符法讨论了多原子极性晶体中表面和体磁极化子的性质。采用线性组合算符和幺正变换,研究磁场中多原子半无限极性晶体中电子和光学声子弱耦合相互作用所产生的极化子的第一激发态能量及平均声子数。结果表明:当电子无限接近晶体表面时,磁极化子的基态能量仅为Landau能量;第一激发态能量为Landau基态能量的2倍;平均声子数等于各支与电子耦合的体光学声子数和表面光学声子数之和。而当电子处于晶体深处时,磁极化子的基态能量却为Landau基态能量与各支体光学声子以及表面光学声子分别耦合的能量之和;第一激发态能量仍为Landau基态能量的2倍;平均声子数等于各支与电子耦合的体光学声子数和与所处深度有关的各支体光学声子数之和,而与各支表面光学声子无关。     

Femtosecond dynamics and transport of optically excited electrons in epitaxial Cu films on Si(111)-7×7

Time-resolved two-photon photoemission spectroscopy is used to study ultrafast electron dynamics of epitaxial Cu films grown in situ on a Si(111)-7×7 substrate with 6 to 44 nm thickness. For excitation with femtosecond laser pulses at h=2.35 eV a pronounced increase of the electron relaxation rates is observed with increasing film thickness and even further in comparison to bulk data. This is attributed to an enhanced energy dissipation in thicker films due to transport of excited carriers into the bulk. PACS 73.50.Gr; 73.90.+f; 78.47.+p; 79.60.-i     

Resonant Raman scattering spectra in a ZnCdSe/ZnSe structure with a quantum well and open nanowires

Resonance Raman scattering (RS) spectra of a ZnCdSe/ZnSe sample containing a single quantum well and quantum well-based open nanowires were studied at T=300 K. The longitudinal optical (LO) phonons involved in the formation of the observed spectra of the quantum-well and nanowire regions differ noticeably in energy. The LO phonon energies in the structures under study were calculated taking into account the compositional effect (doping of Cd into ZnSe) and biaxial strain. When excited in the exciton resonance region, RS is shown to occur via free (extended) excitonic states with the involvement of LO phonons of the ZnCdSe strained layer with final wave vectors near the Brillouin zone center. When excited below the excitonic resonance in the ZnCdSe layer, resonance scattering via localized exciton states provides a noticeable contribution to the observed RS lines. Because of the finite size of a localized state, phonons with large wave vectors are involved in these scattering processes. The RS lines produced under excitation in the excitonic region of the thick barrier layers are due to scattering from the ZnSe barrier phonons.     

Time-resolved phonon-sideband spectroscopy

Phonon-assisted recombination in bulk CdS is analyzed by means of time-resolved photoluminescence spectroscopy. The experimental results demonstrate that both excitons and electron–hole plasma contribute to the phonon-sideband emission, depending on the excitation conditions. This observation is supported by a microscopic theory of interacting electrons, phonons, and photons that generalizes previous investigations by including both correlated and uncorrelated luminescence sources.     

Surface temperature dependence of rotational excitation of H(2) scattered from Pd(111)

Classical dynamics simulations are performed to study rotational excitation of H(2) scattered from Pd(111), taking into account energy exchange with surface phonons through a 3D surface oscillator model. We show that dynamic trapping, identified recently in the study of dissociation dynamics, plays a prominent role. The corresponding long interaction time due to several recollisions allows an efficient energy exchange between H(2) molecules and surface phonons. This microscopic mechanism explains the puzzling experimental finding on the role of surface temperature in H(2)(nu = 0,J = 1-->3) excitation.     

Tin- and lead-tunnel junctions as quantitative detector for intense thermal phonon pulses

Pulse-heated constantan films emit thermal phonon pulses into an a-cut sapphire held at a temperature of some Kelvin. The frequencies of these thermal phonons depend on the radiation temperature and can easily be shifted into the THz range. After a path of 6 mm these phonons are detected by observing the momentary current/voltage characteristics of evaporated symmetric tin- or lead-tunnel junctions during irradiation time. These characteristics correspond to characteristics taken in thermal equilibrium at higher substrate temperature which means that a momentary temperature of the electronic system can be deduced. Using a set of model assumptions concerning phonon transmission across interfaces, phonon propagation in the anisotropic sapphire, absorption in the superconducting detector film and reemission of the absorbed power into the substrate, a comparison with theoretically expected diode temperatures within the one-temperature model for the detector material can be made. Using a variety of quite different experimental conditions there is a remarkable and astonishing well agreement between experiment and theory by fitting only two parameters for each phonon polarization and metal:i) the phonon absorption coefficient in the normal state andii) a phonon cutoff-frequency for the detected phonons. From this fit an information about these important, so far unknown parameters is achieved.Supported by Deutsche Forschungsgemeinschaft     

Cyclotron resonance of a polaron in a realistic heterojunction and its pressure effect

The Larsen perturbation method is adopted to study the influence of magnetic fields on polarons in realistic heterojunctions in a quasi-two-dimension approximation. The interaction between an electron and both the bulk longitudinal optical phonons and the two branches of interface optical phonons is taken into account to show the influence of magnetic fields at different ranges on the polaron cyclotron mass due to the coupling of the electron with each branch of phonon modes. The result indicates that not only do the bulk phonons influence the polaron cyclotron mass, but the interface phonons do as well. The pressure effect on the cyclotron mass is also discussed.     

Generation of harmonics and chaos in the dynamics of anharmonic surfaces

Effective equations of motion of a surface atom placed in an anharmonic surface potential are derived for three kinds of substrates (i) continuous dispersionless, (ii) discrete with nearest neighbours harmonic interactions, and (iii) a substrate showing a strong spatial dispersion. The resulting equations of motion are differential equations in the case (i) and Volterra Integro-Differential Equations (VIDE) in the cases (ii) and (iii). Examples of non-linear surface response to phonons coming from the bulk are given for the dispersionless substrate.