Infrared absorption by zone edge lattice vibrations in mixed gallium indium phosphide

The combinations of zone edge phonons of mixed Ga_1?xIn_xP have been studied by means of infrared absorption, these crystals exhibit a one mode behavior for both the zone center and the zone edge phonons.     

Phase transitions in RbCaF3. II: Neutron scattering

The 196 K phase transition of RbCaF₃ has been identified as a phonon condensation at the R point (cubic [111] zone boundary). Above the transition temperature, a ridge of scattering extending from the [110] zone boundary (M point) to the R point was observed, corresponding to a line of soft phonons from M₃ to R₂₅. Below the transition the scattering at M decreases rapidly with temperature, indicating a lattice stabilization which causes the ridge of scattering to disappear.     

Resonant Γ–X–Γ tunneling in GaAs/AlAs/GaAs single barrier heterostructures at zero and elevated magnetic field

We report the observation of resonant tunneling and magneto-tunneling between states of different effective mass derived from zone centre (Γ) and zone edge (X) points of the Brillouin zone in single AlAs barrier diodes. The nature of the X states involved (longitudinal X_zor transverseX_xy ) is deduced from the observed resonances in the conductance versus bias characteristics at zero magnetic field (B). At finite B, the σ–V curves exhibit resonant magneto-tunneling with X_zLandau levels (LL), whilst no evidence of resonances withX_xy LLs is found. Clear observation of both LL index (in-plane momentum) conserving and non-conserving tunneling to X_zallows the transverse effective mass in AlAs to be determined. As a consequence of the different effective masses, momentum-conserving tunneling is inhibited at B = 0, but is restored when high B is applied.     

MRI magic-angle effect in femorotibial cartilages of the red kangaroo

ObjectiveKangaroo knee cartilages are robust tissues that can support knee flexion and endure high levels of compressive stress. This study aimed to develop a detailed understanding of the collagen architecture in kangaroo knee cartilages and thus obtain insights into the biophysical basis of their function.DesignCylindrical/square plugs from femoral and tibial hyaline cartilage and tibial fibrocartilage were excised from the knees of three adult red kangaroos. Multi-slice, multi-echo MR images were acquired at the sample orientations 0° and 55° (“magic angle”) with respect to the static magnetic field. Maps of the transverse relaxation rate constant (R₂) and depth profiles of R₂ and its anisotropic component (R₂^A) were constructed from the data.ResultsThe R₂^A profiles confirmed the classic three-zone organisation of all cartilage samples. Femoral hyaline cartilage possessed a well-developed, thick superficial zone. Tibial hyaline cartilage possessed a very thick radial zone (80% relative thickness) that exhibited large R₂^A values consistent with highly ordered collagen. The R₂^A profile of tibial fibrocartilage exhibited a unique region near the bone (bottom 5–10%) consistent with elevated proteoglycan content (“attachment sub-zone”).ConclusionsOur observations suggest that the well-developed superficial zone of femoral hyaline cartilage is suitable for supporting knee flexion; the thick and well-aligned radial zone of tibial hyaline cartilage is adapted to endure high compressive stress; while the innermost part of the radial zone of tibial fibrocartilage may facilitate anchoring of the collagen fibres to withstand high shear deformation. These findings may inspire new designs for cartilage tissue engineering.     

A note on the deformation induced splitting of degenerate Brillouin zone boundary points of the diamond lattice

Simple group theoretical arguments are used to formulate a necessary condition for the splitting of degenerate Brillouin zone boundary points of a diamond lattice subject to deformation. The influence of [001] and [011] strains on the X₁ level of silicon is examined. The group theoretical arguments also show that the Γ₁₅ (Γ′₂₅) and Γ′₂ (Γ₁) levels in the conduction (valence) band of the first Brillouin zone are inverted and appear in the valence (conduction) band in the next Brillouin zone.     

Multiphonon selection rules for cuprous oxide type crystalline systems

Experimental data on the IR and Raman spectra along with the slow coherent neutron scattering data for Cu₂O were examined for assignments of frequencies to phonon modes not only at the center but those at the boundary of the Brillouin zone. The inconsistency in assignments for the zone center modes and the lack of not only assignments but identifications of the zone boundary phonon required the use of second order Raman and IR spectra as the useful probes. The basis for the interpretation of these spectral data is given in terms of reduction coefficients for ordinary products and symmetrized squares and cubes of representations in the “cuprite” structure O_h⁴-Pn3m. They are presented for the R, X, M and Γ points of the Brillouin zone. They will serve for obtaining selection rules, invariants, and covariants for this structure. Additional tables are given that enable one to identify what pair of phonons correspond to electric dipole, magnetic dipole, electric quadrupole, or Raman transitions, respectively.     

Fundamental reflectivity spectra of Hg1−xMnxTe mixed crystals in the 1.7 to 3.5 eV energy range

The room temperature reflectivity coefficient R(E) for the mixed crystals Hg_1?xMn_xTe (x up to 0.57) in the energy range 1.7 to 3.5 eV was investigated. Two distinct maxima E₁ and E₁ + Δ₁ connected with the transitions in the critical points on the [111] direction of the Brillouin zone for the samples with x up to 0.3 and the more diffused structure of R(E) for the samples with x > 0.3 was observed. A quadratic dependence of E₁ and E₁ + Δ₁ transition energy vs alloy composition with x up to 0.3 was found, with bowing coefficient c = 1.21 ± 0.02 and 1.06 ± 0.02 respectively. The energy variation of an additional shoulder probably connected with the e₁ transitions at L point of the Brillouin zone is also reported.     

Comparative analysis of magnetic properties of the basalts collected in the Red Sea and other rift zones

The magnetic properties of basalts collected from the young rift zone of the Red Sea are studied in detail. Special features revealed in the magnetic behavior of the samples testify to a complex character of the evolution of the Red Sea rift zone. The magnetic characteristics of basalts from the Red Sea rift zone and other rift zones are compared. The reasons for high values of the natural remanent magnetization I _n and Königsberger factor Q _n obtained for the Red Sea basalts are clarified.     

Optical properties and electronic band structure of CdGa2Te4

Optical properties of a defect-chalcopyrite-type semiconductor CdGa₂Te₄ have been studied by optical absorption, spectroscopic ellipsometry (SE), and electroreflectance (ER) measurements. Optical absorption measurements suggest that CdGa₂Te₄ is a direct-gap semiconductor having the band gap of ∼1.36 eV at 300 K. The complex dielectric-function spectra, ε(E)=ε₁(E)+iε₂(E), measured by SE reveal distinct structures at energies of the critical points (CP's) in the Brillouin zone. ER spectrum facilitates the precision determination of the CP parameters (energy position, strength, and broadening). By performing the band-structure calculation, these CP's are successfully assigned to specific points in the Brillouin zone.     

Misalignment sensitivity of solid-state laser resonators with thermal lensing

An analysis of the resonator sensitivity to mirror misalignment in solid-state lasers is presented, considering the rod as a focusing lens with a limited aperture. For each mirror, a misalignment sensitivity factor S_i is defined in terms of resonator parameters. It is shown both theoretically and experimentally that, as a function of the pump power, two stability ranges labeled zone I and II exist, with markedly different S_i values; in particular, near one of the two boundaries of zone II, S_i rapidly diverges. The experiments, performed on a CW Nd:YAG laser, are in good agreement with the theoretical predictions.     

Intermediate dimensional character of charge transfer excitation modes in a two-leg cuprate ladder

Collective charge modes in the cuprate ladder compound Sr₁₄Cu₂₄O₄₁ are studied over the one-dimensional Brillouin zone using resonant inelastic X-ray scattering. Low energy (2-4 eV) spectral weight across the Mott gap is dominated by a broad, dispersive feature containing two distinct peaks that may be interpreted as independent modes. Details of low energy dispersion, intensity distribution across the Brillouin zone and peak composition fall between the characteristic spectra of quasi-1D (e.g. SrCuO₂) and -2D cuprates (e.g. Nd₂CuO₄). We demonstrate that dispersion and splitting between the two observed modes can be understood in a variant of the strong coupling limit (Hubbard-U?t) with a single band Hubbard model.     

The coexistence of zone and self-localized exciton states in vibronic spectra of nitrogen cryocrystals

The shape of bands and their evolution along the vibronic progression of the exciton resonances in the VUV spectra are correlated for nitrogen in its own lattice and in argon matrix. The fact that in solid nitrogen the zone states make a contribution to the formation of a¹II_g ← X¹Σ⁺_g and w¹Δ_u ← X¹Δ⁺_g progression vibrons is supported by the experimental results. The exciton-phonon interaction is treated in terms of the model of coexistence of zone and self-localized states. The role of optical and acoustic modes is discussed and the main parameters of exciton-phonon coupling are estimated.     

On injection of hydrate-forming gas into a gas-saturated snowy agglomerate while transition through the ice melting point

The paper considers the process of injection of hydrate-forming gas (methane) into a snowy agglomerate (ini-tially saturated with methane). The self-similar problem statement demonstrates that if the warm gas (T_e > 0 °C) is injected under a high pressure (p_e ≥ p_*, where the critical values are found from the initial temperature T₀, pressure p₀, volumetric snow saturation S_i0, and permeability of snow) into the filtration zone with phase transition, this produces four characteristic zones: the nearest zone with all snow transformed into hydrate, therefore, the aggregate filled only with gas and hydrate, the two intermediate zones where gas, snow or water and hydrate are in phase equilibrium state, and the distant zone filled only with gas and snow. The obtained analytical and numerical solutions give an analysis of the influence of key input parameters like initial state of the aggregate, gas injection rate, and its temperature, on the structure and the length of four filtration zones.     

Vibrational spectra and elastic,piezoelectric, and magnetoelectric properties of HoFe3(BO3)4 and HoAl3(BO3)4 crystals

Raman spectra of light are obtained for HoFe₃(BO₃)₄ and HoAl₃(BO₃)₄ crystals at various temperatures and are used for determining the frequencies of crystal lattice vibrations at the center of the Brillouin zone. It is also found that the HoFe₃(BO₃)₄ crystal exhibits a phase transition at T _c ≈ 366 K. The magnetoelectric effect in the paramagnetic phase of these compounds is studied experimentally. The lattice vibration frequencies, elastic and piezoelectric moduli, Born dynamic charges, and the high-frequency permittivity are calculated using the density functional method. A peculiar behavior of the transverse acoustic vibration branch is observed in the Γ → Z direction of the Brillouin zone of the HoFe₃(BO₃)₄ crystal. The electric polarization induced by an external field is estimated using the calculated values of piezoelectric moduli and experimental values of magnetostriction.     

Incommensurate pinning mechanism in KCP

An incommensurate pinning mechanism based on the Q-quasimodulated bromine (Br^-) distribution (Q being twice the Fermi wavevector reduced to the first Brillouin zone) is proposed in K₂Pt(CN)₄Br_0.3 3.2H₂O (KCP) which quantitatively accounts for the neutron scattering and far infrared optical spectroscopy data in this material.     

Influence of a small lattice deformation on the superconductive critical temperature of alloys with the Cu3Au-type structure

Lattice dimensions and superconductive critical temperature, T_c, of Ca_1?xSr_xPb₃ alloys have been measured. The Cu₃Au-type cubic structure of CaPb₃ becomes tetragonally distorted near x=0·50. At the same composition a marked increase in T_c is found, which is explained in terms of Brillouin zone effects.     

Angle-resolved photoemission spectra of 2H-NbSe2

Angle-resolved photoemission measurements have been made on the metallic layer compound 2H-NbSe₂. From the results, E vs k_∥ dispersion curves have been obtained along two principal symmetry directions in the Brillouin zone. The experimental energy bands are found to be in good agreement with the results predicted by one-dimensional density of initial states along k_⊥.     

Effects of CO2 dilution on partially premixed CH4-air flames in swirl and bluff body stabilized combustor

The effect of CO₂ dilution on the flame characteristics and pollutant emission of a partially premixed CH₄-air flame in a confined bluff body and swirl influenced flowfield is investigated using optical and laser diagnostic methods. The non-premixed burner produced a converging-diverging flowfield at the burner exit and a lifted flame is produced at all test cases, with an upstream movement of the flame with decreasing global equivalence ratios (?_g). Based on variations in ?_g, two flame stabilization modes – bluff body influenced and swirl stabilized – with a transition mode in-between is observed for the cases with (flame FB) and without dilution (flame FM). The characteristics of the heat release zone are influenced by dilution, with the FB flames being longer and also less intense when compared to FM flames. Pollutant measurement at 30 mm downstream from the combustor exit highlighted the ultra-low NO_x capability of the IIST-GS2 burner. CO₂ dilution leads to a reduction in NO_x emission due to both thermal and chemical effects. For ?_g ≥ 0.7 extreme low levels of CO and unburned hydrocarbons (UHC) are observed for both cases. For ?_g ≤ 0.6 the dramatic increase of both CO and UHC maybe due to the lower flame temperatures and shorter flame zone residence times, respectively.     

First-principles study on the structural and electronic properties of AlNCx nanosheet

The structural and electronic properties of a hydrogen terminated hexagonally AlN nanoribbon with 6 zigzag Al-N chains across the ribbon width (6-ZAlNNR) and the hexagonally bonded hetero-sheets AlNC_x (x=2,4,6) consisting of AlN and graphite strips with zigzag shaped borders have been investigated systemically by using the first-principles. The results show that in 6-ZAlNNR, the states of the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. Introducing the graphite strip C_x and increasing its width lead to the LUCB and HOVB getting closer with each other especially in flat dispersion region around the zone boundary J_y, thus decreasing in the energy gap of the hetero-sheets AlNC₂, AlNC₄ and AlNC₆ successively. Similar to the edge states existing in zigzag edged AlNNR, the flat dispersion border states also exist in the zigzag borders of hexagonally networked hetero-sheets AlNC_x. Unlike the edge states whose charges are localized at one of the edge atoms, the border states are localized at two atoms of the borders with either bonding or antibonding character.