Acoustic backscatter measurements from littoral seabeds at shallow grazing angles at 4 and 8 kHz

Direct measurement of acoustic scattering from the seabed at shallow grazing angles and low kilohertz frequencies presents a considerable challenge in littoral waters. Specifically, returns from the air-water interface typically contaminate the signals of interest. To address this issue, DRDC Atlantic has developed a sea-going research system for measuring acoustic scatter from the seabed in shallow-water environs. The system, known as the wideband sonar (WBS), consists of a parametric array transmitter and a superdirective receiver. In this paper, backscatter measurements obtained with the WBS at two sandy, shallow-water sites off North America's Atlantic coast are presented. Data were collected at 4 and 8 kHz at grazing angles from 3 degrees-15 degrees. The backscattering strength is similar at both sites and, below about 8 degrees, it appears to be independent of frequency within the statistical accuracy of the data. The measurements show reasonable agreement with model estimates of backscatter from sandy sediments. A small data set was collected at one of the sites to examine the feasibility of using the WBS to measure the azimuthal variability of acoustic scatter. The data set--although limited--indicates that the parametric array's narrow beamwidth makes the system well-suited to this task.     

Carbohydrate moieties of molluscan Rapana venosa hemocyanin

Using Zn2+ ions as new method, several FUs have been isolated from molluscan Hc Rapana venosa without formation of non-functional proteolytic side products. N-terminal sequences of these fragments in comparison with FUS from other gastropodan Hcs show a very high degree of structural identity. Four Fus, purified from enzyme-treated structural subunits RvH1 and RvH2 (RvH1-a, RvH1-f, RvH2-a and RvH2-e) show identical N-terminal sequences compared to fragments isolated after treatment with Zn2+ ions. However, in some cases trypsin cleaves RvH chains at different positions if compared to the Zn2+ treatment. To analyze the oligosaccharide composition of two FUS from the first structural subunit of Rapana Hc, RvH1-a and RvH1-f, several techniques were applied: capillary electrophoresis, MALDI-MS, ESI-MS in combination with glycosidase digestions. On basis of these results and the determined amino acid sequence two N-linkage sites were identified in the FU RvH1-a, but only one in the FU RvH1-f.     

Masked auditory thresholds in sciaenid fishes: a comparative study

Western Atlantic sciaenids comprise a taxonomically diverse teleost family with significant variations in the relationship between the swim bladder and the otic capsule. In this study, the auditory brainstem response (ABR) was used to test the hypothesis that fishes with different peripheral auditory structures (black drum, Pogonias chromis and Atlantic croaker, Micropogonias undulatus) show differences in frequency selectivity. In a black drum the swim bladder is relatively distant from the otic capsule while the swim bladder in Atlantic croaker possesses anteriorly-directed diverticulas that terminate relatively near the otic capsule. Signals were pure tones in the frequency range, 100 Hz to 1.5 kHz, and thresholds were determined both with and without the presence of simultaneous white noise at two intensity levels (124 dB and 136 dB, re: 1 microPa). At the 124 dB level of white noise background, both the black drum and Atlantic croaker showed similar changes in auditory sensitivity. However, in the presence of the 136 dB white noise masker, black drum showed significantly greater shifts in auditory thresholds between 300 and 600 Hz. The results indicate that the two species differ in frequency selectivity since the Atlantic croaker was less susceptible to auditory threshold shifts, particularly at the higher level of masking. This difference may be linked to peripheral auditory mechanisms.     

Spin excitations in an anisotropic bond-alternating quantum s = 1 chain in a magnetic field: contrast to haldane spin chains

Inelastic neutron scattering experiments on the S = 1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under magnetic fields below and above a critical field Hc at which the energy gap closes. Normal field dependence of Zeeman splitting of the excited triplet modes below Hc has been observed, but the highest mode is unusually small and smears out with increasing field. This can be explained by an interaction with a low-lying two magnon continuum at q(parallel) = pi that is present in dimerized chains but absent in uniform ones. Above Hc, we find only one excited mode, in stark contrast with three massive excitations previously observed in the structurally similar Haldane-gap material NDMAP [A. Zheludev, Phys. Rev. B 68, 134438 (2003)].     

Quantum criticality in an organic magnet

Exchange interactions between S=1/2 sites in piperazinium hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground state. We have determined the field-temperature phase diagram by high field magnetization and neutron scattering experiments. There are two quantum critical points: Hc1=7.5 T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically ordered state while Hc2=37 marks the onset of a fully polarized state. The ordered phase, which we describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum critical regime with short range correlations. A low temperature anomaly in the BEC phase boundary indicates that additional low energy features of the material become important near Hc1.     

The Intramolecular Spin-Spin Interactions in Ruthenium Complexes of Pyrazole Derivatives

The spin-spin coupling can provide useful information for analysing the structure of a system and the extent of non-covalent bonds interactions. In this study, we present the isotropic NMR properties and spin-spin coupling involving ruthenium-ligand (Ru-L) bonds and other spin-spin interactions obtained from DFT calculations. The proton shift which in close proximity with the Ru and Cl (or O) atoms are characterised with lower and higher chemical shift respectively. Though Ru-Cl bond has longer bond length than all other Ru-L bonds, yet its spin-spin coupling is higher than others because of a very high contribution of PSO which is far higher than the contribution from FC terms. In all other Ru-L bonds, FC is the most significant Ramsey terms that define their spin-spin coupling. Both the isotropic and anisotropic shielding of the Hz of the pyrazole is lower than Hc of the cymene and the spin-spin coupling 3J(Hz…Hz) of the pyrazole are less than half of the 3J(Hc…Hc) of the cymene unit in the complexes. There is a little increase in both the 3J(Hc…Hc) and 3J(Hz…Hz) spin-spin coupling in the hydrolysed complexes compare to the non-hydrolysed complexes. The isotropic and anisotropic shielding tensor of Ru atoms increases in magnitude as the complexes get hydrolysed that could be ascribed to a more deshielding chemical environments.     

Effect of pressure on the Ginzburg-Landau parameter kappa=lambda/xi in YB6

Measurements of the transition temperature Tc, the upper critical field Hc2, and the magnetic penetration depth lambda under hydrostatic pressure (up to approximately 9.2 kbar) in the YB6 superconductor were carried out. A pronounced and negative pressure effect (PE) on Tc and Hc2 with dTc/dp=-0.0547(4) K/kbar and micro0dHc2(0)/dp=-4.84(20) mT/kbar, and zero PE on lambda(0) were observed. The PE on the coherence length dxi(0)/dp=0.28(2) nm/kbar was calculated from the measured pressure dependence of Hc2(0). Together with the zero PE on the magnetic penetration depth lambda(0), our results imply that the Ginzburg-Landau parameter kappa(0)=xi(0)/lambda(0) depends on pressure and that pressure "softens" YB6, e.g., moves it to the type-I direction.     

In situ hybridization to chitosan/magnetite nanocomposite induced by the magnetic field

Chitosan/magnetite nanocomposite was synthesized induced by magnetic field via in situ hybridization in ambient condition. Results of XRD patterns and TEM micrographs indicated that magnetite particles with 10–20 nm were dispersed in chitosan homogeneously. An interesting result is that magnetite nanoparticles were assembled to form chain-like structures under the influence of the external magnetic field, which mimics the magnetite chains inside of magnetotatic bacteria. The saturated magnetization (Ms) of nano-magnetite in chitosan was 50.54 emu/g, which is as high as 54% of bulk magnetite. The remanence (Mr) and coercivity (Hc) were 4 emu/g and14.8 Oe, respectively, which indicated that magnetite nanoparticles were superparamagnetic. The key of route is that a pre-precipitated chitosan hydrogel membrane, used as chemical reactor, which controlled the precipitation of chitosan precipitation and in situ transformation of magnetite from the precursor simultaneously in the magnetic field environment.     

The relative effects of particles and turbulence on acoustic scattering from deep-sea hydrothermal vent plumes

Acoustic methods are applied to the investigation and monitoring of a vigorous hydrothermal plume within the Main Endeavor vent field at the Endeavor segment of the Juan de Fuca Ridge. Forward propagation and scattering from suspended particulates using Rayleigh scattering theory is shown to be negligible (log-amplitude variance σ(χ) (2)~10(-7)) compared to turbulence induced by temperature fluctuations (σ(χ) (2)~0.1). The backscattering from turbulence is then quantified using the forward scattering derived turbulence level, which gives a volume backscattering strength of s(V)=6.5 × 10(-8) m(-1). The volume backscattering cross section from particulates can range from s(V)=3.3 × 10(-6) to 7.2 × 10(-10) m(-1) depending on the particle size. These results show that forward scatter acoustic methods in hydrothermal vent applications can be used to quantify turbulence and its effect on backscatter measurements, which can be a dominant factor depending on the particle size and its location within the plume.     

NMR研究超稳Y分子筛水热老化过程中结构与酸性的变化

利用固体核磁共振（NMR）结合探针分子技术,探讨了超稳Y分子筛在水热老化过程中酸中心结构和酸性的变化规律.结果表明,水热老化初期,大量的骨架铝脱出最终形成了非骨架五配位铝,使得丙酮探测到的Brønsted酸（B酸）酸量随着老化时间的增加而减少.而非骨架五配位铝会向表面迁移并形成多聚铝,使得分子筛中Lewis酸（L酸）酸量也随老化时间的增加而减少.在老化过程的前3 h内,酸量的变化幅度较大.整体硅/铝原子比相等的分子筛,其B酸酸量的变化幅度与新鲜剂的晶胞大小呈正比;随着老化时间的继续增加,不同配位的铝物种的含量趋于稳定,B酸和L酸的酸量变化幅度很小.在整个水热老化过程中,超稳Y分子筛中B酸和L酸的协同效应一直存在,由协同效应产生的一种酸性增强的B酸位的稳定性最好.     

Zur plasmachemischen Umsetzung von CO/H2-Gemischen zu leichten Kohlenwasserstoffen in einer Glimmentladung

Mixtures of CO und H₂ can be partially converted into hydrocarbons (Hc), especially methane, by nonthermal activation in a glow discharge. The degree of conversion α CO → Hc and the partial pressure of methane reached values of 12% and 3% resp. The CO-conversion depends on the discharge parameters, the discharge time the energy input resp., and the rate of CO/H₂-mixture. The Hc formed are intermediate products which react further to higher Hc including solid Hc. According to mass spectrometrical analysis of plasma ions a reaction scheme was developed which is considered to be valid for the Hc formation based on an ion as well on neutral particle reactions.     

GaAs(0 0 1) planarization after conventional oxide removal utilising self-governed InAs QD site selection

Native oxide removal on GaAs(0 0 1) wafers under conventional thermal desorption causes severe surface degradation in the form of pitting. Typical surface regeneration requires several hundred nanometres of buffer layer growth. This level of planarization is necessary to fill in the deep pits since Ehrlich-Schwoebel diffusion barriers cause a retardation of layer growth at multiple monolayer step edges. Pits are, however, attractive nucleation sites for quantum dots (QDs), and hence QDs fill the pits via a self-governing phenomenon. In this paper, we show how 1.7 ML of InAs growth on GaAs(0 0 1) immediately after thermal oxide removal aids the healing of the surface and reduces the necessity for thick buffer layer growth.     

Effects of the annealing duration of the ZnO buffer layer on structural and optical properties of ZnO rods grown by a hydrothermal process

In this study, the effects of the annealing duration of a zinc oxide (ZnO) buffer layer on structural and optical properties of ZnO rods grown by a hydrothermal process are discussed. A ZnO buffer layer was deposited on p-type Si (1 1 1) substrates by the metal organic chemical vapor deposition (MOCVD) method. After that, ZnO rods were grown on the ZnO-buffer/Si (1 1 1) substrate by a hydrothermal process. In order to determine the optimum annealing duration of the buffer layer for the growth of ZnO rods, durations ranging from 0.5 to 30 min were tried. The morphology and crystal structure of the ZnO/ZnO-buffer/Si (1 1 1) were measured by field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). The optical properties were investigated by photoluminescence (PL) measurement.     

Chemical reactions of chlorine on a vicinal Si(100) surface studied by ESDIAD

The reaction of Cl₂ on a vicinal Si(100) surface has been studied by ESDIAD. This type of surface possesses two types of sites: pairs of dangling bonds on Si---Si terrace dimers, and single dangling bonds on the 2-atom layer high steps. The reactivity of these two sites is compared. For low coverages the step dangling bonds are identified as the preferred Cl bonding site after 673 K-annealing. Upon higher temperature annealing and SiCl₂(g) desorption, the terrace-site Cl species are depleted more rapidly than the step-site Cl species. Extensive isothermal etching under a continuous Cl₂ flux at 800 K is found to produce a disordered surface structure. Heating to 1123 K causes a reordering of the surface.     

Landau quantization effects in the charge-density-wave system (Per)2M(mnt)2 (where M = Au and Pt)

A finite transfer integral t(a) orthogonal to the conducting chains of a highly one-dimensional metal gives rise to empty and filled bands that simulate an indirect-gap semiconductor upon formation of a charge-density wave (CDW). In contrast to semiconductors such as Ge and Si with band gaps approximately 1 eV, the CDW system possesses an indirect gap with a greatly reduced energy scale, enabling moderate laboratory magnetic fields to have a major effect. The consequent variation of the thermodynamic gap with magnetic field due to Zeeman splitting and Landau quantization enables the electronic band structure parameters (transfer integrals, Fermi velocity) to be determined accurately. These parameters reveal the orbital quantization limit to be reached at approximately 20 T in (Per)2M(mnt)(2) salts, making them highly unlikely candidates for a recently proposed cascade of field-induced CDW states.     

Effects of buffer layer annealing temperature on the structural and optical properties of hydrothermal grown ZnO

ZnO was deposited on bare Si(1 0 0), as-deposited, and annealed ZnO/Si(1 0 0) substrates by hydrothermal synthesis. The effects of a ZnO buffer layer and its thermal annealing on the properties of the ZnO deposited by hydrothermal synthesis were studied. The grain size and root mean square (RMS) roughness values of the ZnO buffer layer increased after thermal annealing of the buffer layer. The effect of buffer layer annealing temperature on the structural and optical properties was investigated by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy. Hydrothermal grown ZnO deposited on ZnO/Si(1 0 0) annealed at 750 °C with the concentration of 0.3 M exhibits the best structural and optical properties.     

Mössbauer spectroscopy as a tool in astrobiology

Two miniaturized Mössbauer spectrometers are part of the Athena instrument package of the NASA Mars Exploration Rovers, Spirit and Opportunity. The primary objectives of their science investigation are to explore two sites on the surface of Mars where water may once have been present, and to assess past environmental conditions at those sites and their suitability for life. Aqueous minerals – jarosite at Meridiani Planum, Opportunity’s landing site, and goethite in the Columbia Hills in Gusev Crater, Spirit’s landing site – were identified by Mössbauer spectroscopy, thus providing in situ proof of water being present at those sites in the past. The formation of jarosite in particular puts strong constraints on environmental conditions during the time of formation and hence on the evaluation of potential habitability. On Earth Mössbauer spectroscopy was used to investigate microbially induced changes in Fe oxidation states and mineralogy at the Loihi deep sea mount, a hydrothermal vent system, which might serve as an analogue for potential habitats in the Martian subsurface and the sub-ice ocean of Jupiter’s icy moon Europa.     

Growth and characterization of A-plane ZnO and ZnCoO based heterostructures

Non polar ZnO and (Zn, Co)O layers were successfully grown on (11̄02) sapphire (R-plane sapphire). The growth process was shown to directly influence the surface morphology as well as the strain state in (112̄0) ZnO (A-plane ZnO). The dominant defect lines seen in photoluminescence were due to basal stacking faults as demonstrated by means of selective photoluminescence and transmission electron microscopy. We present a novel method for growing high quality A-plane ZnO by inserting a (Zn, Co)O thin buffer layer, which strongly reduced the surface roughness. Finally (Zn, Mg)O/ZnO quantum well structures were grown on such a buffer layer. These quantum wells exhibited no intrinsic quantum confined Stark effect. PACS 81.05.Dz; 81.15.Hi; 78.67.Hc; 68.65.Fg     

Quantum size effects on the perpendicular upper critical field in ultrathin lead films

We report the thickness-dependent (in terms of atomic layers) oscillation behavior of the perpendicular upper critical field Hc2perpendicular in the ultrathin lead films at the reduced temperature (t = T/Tc). Distinct oscillations of the normal-state resistivity as a function of film thickness have also been observed. Compared with the Tc oscillation, the Hc2perpendicular shows a considerable large oscillation amplitude and a pi phase shift. The oscillatory mean free path caused by the quantum size effect plays a role in Hc2perpendicular oscillation.     

Disordered states in IPA-Cu(ClxBr1-x)3 induced by bond randomness

The mixtures of two spin-gap compounds IPA-Cu(ClxBr1-x)3 are studied by electron paramagnetic resonance and magnetization processes [M(H)]. From electron paramagnetic resonance spectra, the symmetry of the spin-gap state breaks down, even for x=0.99. From M(H) curves for x=0.95 and 0.92, however, spin gaps survive below mu0Hc1=10+/-1 T, and the M(H) slopes bend at mu0Hc3=40+/-1 T, below the saturation field Hc2. Such a curvature suggests an exotic phase transition: Bose-Einstein condensation of spin triplets occurs at Hc1 相似文献