Phonon density of states of LaFeAsO(1-x)Fx

We have studied the phonon density of states (PDOS) in LaFeAsO(1-x)Fx with inelastic neutron scattering methods. The PDOS of the parent compound (x=0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x approximately 0.1). Good agreement is found between the experimental PDOS and first-principles calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity.     

Phonons and the electronic gap in FeSi

The partial phonon densities of states of iron atoms in the intermetallic compound FeSi have been measured in the temperature range 46–297 K using nuclear resonant inelastic scattering of synchrotron radiation. A significant phonon softening with increasing temperature has been established. The greatest phonon softening for iron atoms is shown to occur in the region of long-wavelength acoustic phonons, for the acoustic branches near the boundary of the Brillouin zone, and for the low-lying weakly dispersive optical branches. The results obtained are analyzed in terms of the views that relate the change in the phonon density of states of iron atoms to the temperature evolution of the electronic density of state for the compound.     

Strong changes in the phonon spectra of 123 superconductors by varying oxygen concentration

By means of inelastic neutron scattering we have studied the phonon densities of states (PDOS) ofY based 123 superconductors with oxygen concentrations varying between O₇ and O₆. We find drastic changes in the PDOS above 40 meV which develop in a systematic manner if we switch from superconducting to semiconducting samples. Model calculations clearly show that this cannot be explained solely by structural changes but is likely to reflect strong differences in the electron-phonon coupling.     

Phonon density of states and compression behavior in iron sulfide under pressure

We report the partial phonon densities of states (DOS) of iron sulfide, a possible component of the rocky planet's core, measured by the 57Fe nuclear resonant inelastic x-ray scattering and calculate the total phonon DOS under pressure. From the phonon DOS, we drive thermodynamic parameters. A comparison of the observed and estimated compressibilities makes it clear that there is a large pure electronic contribution in the observed compressibility in the metallic state. Our results present the observation of thermodynamic parameters of iron sulfide with the low-spin state of an Fe2+ ion at the high density, which is similar to the condition of the Martian core.     

铁基非晶态因瓦合金的声子谱研究

用非弹性中子散射实验方法研究了非晶态Ｆｅ_９０－ｘＣｏ_ｘＺｒ_１０（ｘ＝１０，４０）和Ｆｅ_８０－ｙＣｒ_ｙＰ_１３Ｃ_７（ｙ＝４，８）合金的广义声子谱，在低能区域（?ω≤２０ｍｅＶ）观察到了与因瓦效应相关的声子谱软化现象。初步讨论了这种动力学方面的反常行为。结果表明，声子谱软化可能与在因瓦合金中存在增强的电子－声子相互作用有关。 关键词：     

Engineering photonic density of states using metamaterials

The photonic density of states (PDOS), like its electronic counterpart, is one of the key physical quantities governing a variety of phenomena and hence PDOS manipulation is the route to new photonic devices. The PDOS is conventionally altered by exploiting the resonance within a device such as a microcavity or a bandgap structure like a photonic crystal. Here we show that nanostructured metamaterials with hyperbolic dispersion can dramatically enhance the photonic density of states paving the way for metamaterial-based PDOS engineering.     

Lattice vibration and thermodynamical properties of a single-layer graphene in the presence of vacancy defects

The phonon density of states(PDOS) and the thermodynamical properties including the heat capacity, the free energy,and the entropy of a single-layer graphene with vacancy defects have been studied theoretically. We first analytically derive the general formula of the lattice vibration frequency, and then numerically discuss the effect of the defects on the PDOS. Our results suggest that the vacancy defects will induce the sawtooth-like oscillation of the PDOS and the specific oscillation patterns depend on the concentration and the spatial distribution of the vacancies. In addition, it is verified that the vacancy defects will cause the increase of the heat capacity because of the vacancy-induced low-frequency resonant peak. Moreover, the influences of the vacancies on the free energy and the entropy are investigated.     

Entanglement of electronic states of impurities in a nanocrystal

It is shown that the dynamics of electronic states of impurities in a nanocrystal embedded in a bulk matrix can be described with the help of the model of a single-mode microcavity with quasi-resonant atoms and losses at the mirrors. In the proposed model, the dynamics of impurities is similar to the collective decay of atoms in a common bath. In the case of a nanocrystal, matrix phonons represent this bath. Impurities interact with the localized phonon mode of the nanocrystal owing to the electron-phonon interaction. The localized phonon mode is coupled with bath phonons at the nanocrystal boundary. In the dispersive limit, the standard model of collective relaxation appears, within the framework of which, different types of many-particle entangled electronic states of impurities are described. In the case of exact resonance, similar entangled states also appear, which are discussed for two impurities in a nanocrystal.     

Structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3

Cubic scandium trifluoride (ScF3) has a large negative thermal expansion over a wide range of temperatures. Inelastic neutron scattering experiments were performed to study the temperature dependence of the lattice dynamics of ScF3 from 7 to 750 K. The measured phonon densities of states show a large anharmonic contribution with a thermal stiffening of modes around 25 meV. Phonon calculations with first-principles methods identified the individual modes in the densities of states, and frozen phonon calculations showed that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators. The quartic potential originates from harmonic interatomic forces in the DO9 structure of ScF3, and accounts for phonon stiffening with the temperature and a significant part of the negative thermal expansion.     

Probing the photonic density of states using layer-by-layer self-assembly

The process of spontaneous emission can be dramatically modified by optical microstructures and nanostructures. We have studied the modification of fluorescence dynamics using a variable thickness polymer spacer layer fabricated using layer-by-layer self-assembly with nanometer accuracy. The change in fluorescence lifetime with spacer layer thickness agrees well with theoretical predictions based on the modified photonic density of states (PDOS), and yields consistent values for the fluorophores' intrinsic fluorescence lifetime and quantum yield near a dielectric as well as a plasmonic interface. Based on this observation, we further demonstrate that self-assembled fluorophores can be used to probe the modified PDOS near optical microstructures and nanostructures.     

Dispersion and density of states of phonons and electrons in an α-B<Subscript>12</Subscript> crystal determined from first principles

Using the DFT method, we study the phonon properties of an α-B₁₂ rhombohedral crystal in the basis set of plane waves and its electronic structure in the localized basis set of Gaussians. It follows from the phonon dispersion that the crystal possesses a dynamical stability. The effective Born charges, the oscillator strengths, the transverse–longitudinal splitting, and the dielectric functions of dipole modes are calculated. We show that charge transfer from polar to equatorial atoms takes place in a В₁₂ icosahedron, while В–В bonds have predominantly a covalent character. In the density of states of acoustic modes, we reveal a structure that can manifest itself in the spectra of disordered boron compounds. From the dispersion of electronic bands, the occurrence of an indirect energy gap follows. The overlap of partial densities implies the hybridization of s and p electronic states in boron atoms.     

Local density of states in a disordered chain: A renormalization group approach

We apply renormalization group techiques to evaluate the local density of phonon states for the isotopically (randomly) disordered linear chain. The method is based on a systematic decimation of atoms in the chain. Numerical studies reveal a richly structured spectrum, in reasonable agreement both with numerical simulations and with exact moments results. This is the first analytic alloy approximation which takes into account potential fluctuations of arbitrary range.     

Phonon spectra and the one-phonon and two-phonon densities of states of UO<Subscript>2</Subscript> and PuO<Subscript>2</Subscript>

The vibrational spectra of uranium dioxide UO₂ and plutonium dioxide PuO₂, as well as the one-phonon densities of states and thermal occupation number weighted two-phonon densities of states, have been calculated within the framework of the phenomenological rigid ion model. It has been shown that the acoustic and optical branches of the spectra are predominantly determined by vibrations of the metal and oxygen atoms, respectively, because the atomic masses of the metal and oxygen differ from each other by an order of magnitude. On this basis, the vibrational spectra can be represented in two Brillouin zones, i.e., in the Brillouin zone of the crystal and the Brillouin zone of the oxygen sublattice. In this case, the number of optical branches decreases by a factor of two. The two-phonon densities of states consist of two broad structured peaks. The temperature dependences of the upper peak exhibit a thermal broadening of the phonon lines L01 and L02 in the upper part of the optical branches. The lower peak is responsible for the thermal broadening of the lowest two optical (T02, T01) and acoustic (LA, TA) branches.     

On the curve of the density of phonon states for cubic boron nitride (from the results of photoluminescence measurements)

The fine structure of the phonon wing associated with the zero-phonon line (ZPL) of the BN1 center in the cubic boron nitride is analyzed in comparison with the structure of the phonon wing of the luminescence center at 3.188 eV in diamond, the second-order Raman scattering spectrum, and the theoretically calculated densities of phonon states of the c-BN compound. Taking into account the similarity of the structures of the phonon wings in the spectra of the above centers and the previously made assumptions that the structure of the phonon wing of the center at 3.188 eV is due to the specific features in the density of phonon states of diamond, it is assumed that, in the observed density of phonon states of cubic boron nitride, the critical points are represented by the specific features of the structure of the phonon wing associated with the zero-phonon line (at 3.294 eV) of the BN1 luminescence center. In turn, these latter specific features coincide accurate to within 5–10cm^?1 with the theoretically calculated lattice vibrations of the c-BN compound and the experimental data obtained from the second-order Raman spectra.     

Positive vibrational entropy of chemical ordering in FeV

Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to measure phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The two data sets were combined to give an accurate phonon density of states, and the phonon partial densities of states for V and Fe atoms. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2 ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy by +0.22±0.03 kB/atom, which stabilizes the ordered phase to higher temperatures. First-principles calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites.     

Ab initio calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.     

Model Eliashberg functions for surface states

We present a simplified procedure for the analysis of the phonon-induced lifetimes of surface states. The model includes information about the electron and phonon structure and is thus more reliable than procedures based on phonon Debye models. We apply the model to calculate the lifetime broadening of Cu(1 1 1) and Al(0 0 1) surface states. The obtained Eliashberg functions and lifetimes are in reasonable agreement with previous detailed studies.     

Anisotropic nuclear inelastic scattering of an iron(II) molecular crystal

Nuclear inelastic scattering (NIS) spectra were recorded for a monocrystal of the spin-crossover complex [Fe(tptMetame)] (ClO (4))(2) (tptMetame = 1,1,1-tris([N-(2-pyridylmethyl)-N-methylamino]-methyl)ethane) at T = 30 K (low-spin state) and at room temperature (high-spin state) for different crystal orientations. The high energy resolution (0.65 meV) allowed us to resolve individual molecular vibrations which were unambiguously identified by density functional calculations. From the NIS spectra for the first time the angular-resolved iron-partial density of phonon states (PDOS) was extracted. The PDOS corroborates a vibrational entropy difference as driving force of the spin transition.     

硫在Fe(111)面吸附的密度泛函研究

采用基于密度泛函理论的第一性原理方法,系统研究了不同覆盖度下硫在Fe（111）表面的吸附构型和吸附特性,计算并分析了硫在Fe（111）表面的吸附能、电荷密度、分波态密度、电荷布局、电子局域化函数等数据. 研究结果表明：S在Fe（111）面的H位吸附最稳定,并且吸附能随着覆盖度的增加而增加. 另外,电子态密度、电子局域化函数和布局分析表明Fe、S之间呈较弱的共价键,这种作用力主要是Fe的3d轨道和S的2p轨道杂化所贡献,而随着覆盖度的增加,Fe、S之间的作用力逐渐减弱,这可能是由于S原子之间的排斥力减弱了Fe、S之间的作用. S在Fe（111）、Fe（110）和Fe（100）这三个晶面上吸附情况的对比分析发现,S与Fe（111）表面的相互作用最强,Fe（100）面次之,而Fe（110）面最弱.     

Phonon dispersion curves in ordered and disordered Fe3Al

Comparison is made between experimental phonon dispersion curves for one crystal of composition Fe-22.5% Al in the disordered state (BCC), and two crystals of composition Fe-25% Al, one with B2 order and the other with DO₃ order. The different states of order represent different arrangements of the iron and aluminium atoms on the sites of BCC lattice.