Bond-stretching and bond-bending force constant of binary tetrahedral (AB and AB) semiconductors

In this Letter we present the two expressions relating the bond-stretching force constant (α in N/m) and bond-bending force constant (β in N/m) for the A^IIIB^V and A^IIB^VI semiconductors with the product of ionic charges (Z₁Z₂) and nearest neighbor distance d (Å). Interatomic force constants of these compounds exhibit a linear relationship when plotted on a log-log scale against the nearest neighbor distance d (Å), but fall on different straight lines according to the ionic charge product of the compounds. A fairly good agreement has been found between the observed and calculated values of the α and β for binary tetrahedral semiconductors.     

Anomalous dispersion of longitudinal optical phonons in Nd(1.86)Ce(0.14)CuO(4+delta) determined by inelastic x-ray scattering

The phonon dispersions of Nd(1.86)Ce(0.14)CuO(4+delta) along the [xi,0,0] direction have been determined by inelastic x-ray scattering. Compared to the undoped parent compound, the two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are shifted to lower energies. Moreover, an anomalous softening of the bond-stretching band is observed at about q = (0.2,0,0). These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor.     

Structural, elastic, vibrational and electronic properties of amorphous Al2O3 from ab initio calculations

First-principles molecular dynamics calculations of the structural, elastic, vibrational and electronic properties of amorphous Al(2)O(3), in a system consisting of a supercell of 80 atoms, are reported. A detailed analysis of the interatomic correlations allows us to conclude that the short-range order is mainly composed of AlO(4) tetrahedra, but, in contrast with previous results, also an important number of AlO(6) octahedra and AlO(5) units are present. The vibrational density of states presents two frequency bands, related to bond-bending and bond-stretching modes. It also shows other recognizable features present in similar amorphous oxides. We also present the calculation of elastic properties (bulk modulus and shear modulus). The calculated electronic structure of the material, including total and partial electronic density of states, charge distribution, electron localization function and the ionicity for each species, gives evidence of correlation between the ionicity and the coordination for each Al atom.     

聚乙炔孤子的二维局域振动模及其键弯曲势的影响

本文考虑了反式聚乙炔的平面结构,在二维空间内研究了孤子的局域振动模,并在此基础上讨论了键弯曲势对孤子局域模的影响,结果发现:1)晶格在垂链方向的振幅与链向的可比拟;2)当耦合参数λ取定值时,产生新的局域模(与SSH模型比较),且局域模的数目随着键弯曲势参数K的增加而增多,其局域性也随之增强。 关键词：     

Raman scattering under pressure in ZnGa2Se4

The Raman scattering spectra of ZnGa₂Se₄ under pressure were investigated at 300 K up to 18.9 GPa. Two stages were observed in the pressure dependences of Raman bands. Such behavior in accordance with the experimental findings existing in literature and was attributed as arising due to the order–disorder phase transition in the cation sublattice.Using the Harrison–Keating's model of the lattice dynamics modified for the crystals with the tetragonal structure, the bulk modulus B and the mode-Grüneisen parameters Γ_i were determined for the first time. It is shown that a better agreement between the experimental and calculated values of Γ_i is observed, if one takes into consideration different frequency-pressure behavior for the bond-bending and the bond-stretching parameters, which determine the low- (lower than 140 cm⁻¹) and high- (higher than 140 cm⁻¹) frequency phonons, respectively.     

Depth dependence of the anomalous thermal behavior of Cu(110)

We report the results of a mirror electron microscope low energy electron diffraction (MEM-LEED) study of anomalous thermal behavior caused by enhanced thermal vibrational motion on a clean Cu(110) surface. The enhanced vibrational amplitudes cause diffracted intensities to deviate from simple Debye-Waller behavior. These deviations become apparent between 550 K and 600 K; i.e. at about 0.45 of the bulk melting temperature. Because of the finite penetration of low energy electrons, LEED intensities contain information about both surface and subsurface order. Our LEED results are analyzed to extract this information using a kinematical model in which the electron attenuation and the depth and temperature dependent vibrational amplitudes are parametrized. For a large range of model parameters, we conclude that by 850 K large anharmonic vibrations are present in several layers and at least two subsurface layers may have melted.     

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)<Subscript>2</Subscript>FSO<Subscript>3</Subscript> probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)₂FSO₃ along (E ) and perpendicular (E ) to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm^-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. We also report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)₂X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO₃ anion is proposed, in order to explain the anomalous behavior of this new mode.     

q-Dependence of the giant bond-stretching phonon anomaly in the stripe compound La1.48Nd0.4Sr0.12CuO4 measured by IXS

Inelastic X-ray scattering (IXS) was used to study the Cu-O bond-stretching vibrations in the static stripe phase compound La_1.48Nd_0.4Sr_0.12CuO₄. It was found that the intrinsic width in Q-space of the previously reported huge anomalous phonon softening and broadening is approximately 0.08 r.l.u. HWHM. A detailed comparison was also made to inelastic neutron scattering (INS) studies, which indicate a two-peak lineshape (with superimposed broad and narrow peaks) in the vicinity of the anomaly. The high resolution IXS data show that the narrow peak is mostly an artifact of the poor transverse Q-resolution of INS. Otherwise, the agreement between the INS and IXS was excellent.     

On the anomalous <Emphasis Type="Italic">A</Emphasis> dependence of the charge radii of heavy calcium isotopes

The charge radii of calcium isotopes are calculated within the self-consistent theory of finite Fermi systems based on the Fayans energy density functional. The fluctuating contribution of low-energy vibrations, i.e., phonons is taken into account approximately. As a result, an anomalous increase in the charge radii of neutron- rich calcium isotopes observed in a recent experiment has been reproduced.     

Uncovering a pressure-tuned electronic transition in Bi(1.98)Sr(2.06)Y(0.68)Cu(2)O(8+delta) using Raman scattering and x-ray diffraction

We report pressure-tuned Raman and x-ray diffraction data of Bi(1.98.)Sr(2.06)Y(0.68)Cu(2)O(8+delta) revealing a critical pressure at 21 GPa with anomalies in electronic Raman background, electron-phonon coupling lambda, spectral weight transfer, density dependent behavior of phonons and magnons, and a compressibility change in the c axis. For the first time in a cuprate, mobile charge carriers, lattice, and magnetism all show anomalies at a distinct critical pressure in the same experimental setting. Furthermore, the spectral changes suggest that the critical pressure at 21 GPa is related to the critical point at optimal doping.     

Pressure dependence of elastic behaviour and force constants of GaP

The six third order elastic constants of GaP have been obtained from measurements of the effect of hydrostatic and uniaxial pressures on ultrasonic wave velocities. The valence force field model has been used to obtain the bond-bending and stretching force constants. Confirmatory evidence is added to the hypotheses that zinc blende structure crystals undergo the densification transition under pressure when (1) a modified Born stability criterion that $\text{1}\text{2}\text{(C}{}_{11}\text{? C}{}_{12}\text{)}\text{B}$ equals about 0.2 is reached at the phase transition pressure P_t, and (2) the ratio $\text{β}{}_{\mathrm{t}}\text{α}{}_{\mathrm{t}}$ of the second order bond-bending β_t to bond-stretching α_t, force constants is about 0.1. These findings suggest that a macroscopic shear takes place at the transition     

Pressure effect on the space charge in low density polyethylene (LDPE)

Abstract

Electrical conductivity of LDPE films was studied with pressure P (1 to 1500 bar). An anomalous discharge current (ADC) was observed in the transient state following a d.c field application. This anomalous current, depending on these parameters, and the steady state current were attributed to the existence of a space charge in polymer. The space charge phenomena and the conduction mechanisms could be given from experimental results and theoretical calculations.     

Stripe conductivity in La1.775Sr0.225NiO4

We report Raman light-scattering and optical conductivity measurements on a single crystal of La1.775Sr0.225NiO4 which exhibits incommensurate charge-stripe order. The extra phonon peaks induced by stripe order can be understood in terms of the energies of phonons that occur at the charge-order wave vector Q(c). A strong Fano antiresonance for a Ni-O bond-stretching mode provides clear evidence for finite dynamical conductivity within the charge stripes.     

Correlation between local vibrations and metal mass in AlB2‐type transition‐metal diborides

Lattice vibrations have been investigated in TiB₂, ZrB₂ and HfB₂ by temperature‐dependent extended X‐ray absorption fine structure (EXAFS) experiments. Data clearly show that the EXAFS oscillations are characterized by an anomalous behavior of the Debye–Waller factor of the transition‐metal–boron pair, which is suggested to be associated with a superposition of an optical mode corresponding to phonon vibrations induced by the B sublattice and an acoustic mode corresponding to the transition‐metal (TM) sublattice. Data can be interpreted as a decoupling of the metal and boron vibrations observed in these transition‐metal diborides (TMB₂), a mechanism that may be responsible for the significant reduction of the superconducting transition temperature observed in these systems with respect to the parent MgB₂ compound. The vibrational behavior of TM–TM bonds has also been investigated to study the occurrence of anisotropy and anomalies in the lattice vibrational behavior of TM–TM bonds.     

Raman scattering study of anomalous spin, charge, and lattice dynamics in the charge-ordered phase of Bi1-xCaxMnO3 (x > 0.5)

We report an inelastic light scattering study of the effects of charge ordering on the spin, charge, and lattice dynamics of Bi1-xCaxMnO3 (x>0.5). We find that charge ordering results in anomalous phonon behavior, such as the appearance of "activated" modes. More significantly, however, the transition to the charge-ordered phase results in the appearance of a quasielastic scattering response with the symmetry of the spin-chirality operator ( T(1g)); this scattering response is thus indicative of magnetic or chiral spin fluctuations in the antiferromagnetic charge-ordered phase.     

Absence of a spontaneous long-range order in a mixed spin-(1/2, 3/2) Ising model on a decorated square lattice due to anomalous spin frustration driven by a magnetoelastic coupling

The mixed spin-(1/2, 3/2) Ising model on a decorated square lattice, which takes into account lattice vibrations of the spin-3/2 decorating magnetic ions at a quantum-mechanical level under the assumption of a perfect lattice rigidity of the spin-1/2 nodal magnetic ions, is examined via an exact mapping correspondence with the effective spin-1/2 Ising model on a square lattice. Although the considered magnetic structure is in principle unfrustrated due to bipartite nature of a decorated square lattice, the model under investigation may display anomalous spin frustration driven by a magnetoelastic coupling. It turns out that the magnetoelastic coupling is a primary cause for existence of the frustrated antiferromagnetic phases, which exhibit a peculiar coexistence of antiferromagnetic long-range order of the nodal spins with a partial disorder of the decorating spins with possible reentrant critical behavior. Under certain conditions, the anomalous spin frustration caused by the magnetoelastic coupling is responsible for unprecedented absence of spontaneous long-range order in the mixed-spin Ising model composed from half-odd-integer spins only.     

Electronic,optical, and mechanical properties of BN,AlN, and InN with zinc-blende structure under pressure

In this work, the electronic, optical, and mechanical properties of BN, AlN, and InN under the action of pressure are calculated. For each of these compounds, the energy band structure, band gaps(E~L_g, E~Γ_g, E~X_g), refractive index(n),dielectric constants(ε_∞, ε_0), elastic constants(C_11, C_12, C_44), and relevant parameters such as bulk(B_u), shear(S_h), and Young's(Y_0) moduli are studied, and other important parameters such as bond-stretching(α), bond-bending(β) force constant, internal-strain parameter(ζ), effective charges(e~*_T, Z~*), anisotropy factor(I_s), Poisson's ratio(P_o), Cauchy ratio(C_a), the ductility index(μ_D), and linear compressibility(C0_) are also calculated. The effects of pressure on all studied properties are investigated. Our results are in good agreement with the available experimental and theoretical data for BN,AlN, and InN.     

Bond stretching phonon anomalies due to incommensurate charge density wave instabilities in high-Tc cuprates

Phonon anomalies observed in various high T_c cuprates are analyzed theoretically within the Hubbard-Holstein model in the limit of strong local electron correlations and in presence of long-range Coulomb interaction. The phonon self-energy is evaluated by taking into account the charge collective modes that become critical upon doping approaching an instability towards an incommensurate charge density wave (ICDW) driven by electron correlations. The doping dependence of phonon softening features and the highly distinctive phonon self-energy dependence on the wave vector agree with experiments. We discuss relevance of dynamical corrections to the density correlation function to achieve a sizeable bond-stretching phonon softening with a kink-like profile away from the zone boundary.     

Transport anomalies and the role of pseudogap in the 60-K phase of YBa(2)Cu(3)O(7-delta)

We report the result of our accurate measurements of the a- and b-axis resistivity, Hall coefficient, and the a-axis thermopower in untwinned YBa(2)Cu(3)O(y) single crystals in a wide range of doping. It is found that both the a-axis resistivity and the Hall conductivity show anomalous dependences on the oxygen content y in the 60-K phase below the pseudogap temperature T(*). The complete data set enables us to narrow down the possible pictures of the 60-K phase, with which we discuss a peculiar role of the pseudogap in the charge transport.     

Bond-stretching-phonon anomalies in stripe-ordered La1.69Sr0.31NiO4

We report a neutron scattering study of bond-stretching phonons in La1.69Sr0.31NiO4, a doped antiferromagnet in which the added holes order in diagonal stripes at 45 to the Ni-O bonds. For the highest-energy longitudinal optical mode along the bonds, a softening of 20% is observed between the Brillouin zone center and the zone boundary. At 45 to the bonds, a splitting of the same magnitude is found across much of the zone. Surprisingly, the charge-ordering wave vector plays no apparent role in the anomalous dispersions. The implications for related anomalies in the cuprates are discussed.