Nickel on lead,magnetically dead or alive?

Two atomic layers of Ni condensed onto Pb films behave, according to anomalous Hall effect measurements, as magnetic dead layers. However, it has been observed that sub-monolayers of Ni lower the superconducting transition temperature of the Pb film. This has lead to the conclusion that the Ni atoms are still very weakly magnetic and that their magnetic scattering causes the reduction of the transition temperature. In the present paper the electron dephasing due to the Ni has been measured by weak localization. The dephasing is smaller by a factor 100 than the T_c-reduction would suggest. This proves that the T_c-reduction in the PbNi films is not due magnetic Ni moments.     

Structure,thermal behavior,and chemical durability of lead–calcium–phosphate glasses

ABSTRACT

Structure and physical properties of 25CaO–xPbO–(75–x)P₂O₅ (0≤x≤35) glasses are investigated in this paper. Substitution of PbO for P₂O₅ in the binary 25CaO–75P₂O₅ glass was found to increase the density and to decrease the molar volume. Fourier transform infrared (FTIR) and Raman spectroscopies show the evolution of the phosphate skeleton when the PbO content increases: Q³ to Q² species (0<x≤25) and Q² phosphate network (x = 25) to short phosphate groups (x > 25) such as (P₄O₁₃^6?) (x = 35). The glass transition temperature first decreases with x, then increases for x values larger than 10%. The evolution of the glass transition temperatures is interpreted from the structural data: the minimum point observed in T_g is attributed to the transition of the ultraphosphate network from the network containing the modifying cations at isolated sites to a network with modifier sub-structure sharing terminal oxygens. At higher PbO content, the large increase in T_g is due to the reticulation of the phosphate network by PbO₄ groups.     

Characterization of bismuth–tin–lead and bismuth–tin–lead–cadmium fusible alloys

Microstructure, electrical, mechanical and thermal properties of quenched bismuth–tin eutectic, Rose (Bi₅₀Sn_22.9Pb_27.1) and Wood’s (Bi₅₀Sn_12.5Pb₂₅Cd_12.5) alloys have been investigated using scanning electron microscopy, X-ray diffraction analysis, the double bridge method, the dynamic resonance method, Vickers hardness measurement and thermal analysis. Wood’s alloy (Bi–Pb–Sn–Cd) has low electrical resistivity and melting point but a high elastic modulus and internal friction when compared with the Rose (Bi–Pb–Sn) alloy. The presence of cadmium in Wood’s alloy decreases its melting point and electrical resistivity with an increase in its elastic modulus, which improves the mechanical properties. Wood’s alloy (Bi–Pb–Sn–Cd) has better properties, which make it useful in various applications such as in protection shields for radiotherapy, locking of mechanical devices and welding at low temperature.     

Crystal structure, morphology, depth profile of elements and mid-infrared optical constants of ‘mild’ lead telluride film

The characterizations of a so-called ‘mild’ PbTe layer thermal-evaporated from an excess of Te (<1 mol.%) evaporable materials are reported. The results reveal that the film obtained is polycrystalline and has a single-phase NaCl-type PbTe crystal structure. It is also demonstrated that the film has a homogeneous surface morphology and a high degree of homogeneous distribution of Te-rich components along the layer. The study of mid-infrared optical constants of a surface-polished film indicates that the influence of surface scattering on optical properties is very small. Received: 3 July 2002 / Accepted: 7 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/6516-9946, E-mail: lbincome@yahoo.com     

Novel photosensitive properties of gadolinium–lead germanate glasses

Glasses of the system xGd₂O₃ · (100 ? x)[7GeO₂ · 3PbO] with 0 ≤ x ≤ 40 mol% were prepared using the melt quenching method. Lead germanate glasses are particularly interesting in the context of the germanate anomaly. In this paper, we investigate changes in the coordination number of germanium in gadolinium–lead germanate glasses using molar volume analysis, density measurements, FTIR and UV–VIS spectroscopy, and density functional theory (DFT). Despite some inconsistencies, the coordination change model remains the currently accepted model for the anomalous behaviour of lead germanate glasses. Based on these experimental results, we propose the following mechanism for the germanate anomaly. (i) The low thermodynamic stability of the [GeO₆] structural unit and the occupation of interstices of larger dimensions (the six-coordinated interstices of the [PbO₆] structural units) in the lead germanate network yield [GeO₅] structural units with higher thermodynamic stability and larger ionic radii. (ii) Not linked to the terminal oxygens of the [GeO₅] structural units and with the formation of smaller network cavities of the lead germanate glass, links are required with [GeO₄] tetrahedra for stabilization, generating the formation of three-membered rings of [GeO₄] tetrahedral structural units.     

Inorganic phosphors in lead–silicate glass for white LEDs

Using spectrophotometry and stationary and kinetic fluorimetry, we have shown that xanthene dye fluorescein forms complexes with polycationic derivative of fullerene in aqueous solutions mainly due to electrostatic interactions. It is found that efficient quenching of singlet excited states of dye occurs in the structure of these complexes due to the transfer of excitation or electron from dye to fullerene. As a result, the photodynamic activity of the newly formed complex is much higher than that of fluorescein and fullerene derivative. This effect makes it possible to predict the formation of new-generation hybrid photodynamic preparations using dyes excited only into a singlet state; as a result, directed searches for these dyes are significantly facilitated.     

Optical transitions in Er^3＋ doped lead germanate glasses

Er^{3+}- and Er^{3+}/Yb^{3+}-doped lead germanate glasses that are suitable for use in fibre lasers and optical amplifiers as well as optical waveguide devices have been fabricated and characterized. The absorption spectra from near-infrared to visible were obtained and the Judd－Ofelt parameters were determined from the absorption band. Intense and broad 1.53μm infrared fluorescence and visible upconversion luminescence were observed under 976 nm diode laser excitation. For 1.53μm emission band, the full widths at half-maximum are 36, 37, 51 nm for GPE, GPYE and GPFE samples, respectively. For frequency upconversion emission, the intense bands centred at around 524, 545, 657nm are due to the {}^4S_{3/2}+{}^2H_{11/2}→{}^4I_{15/2} and {}^4F_{9/2}→{}^4I_{15/2} transitions of Er^{3+} ions. The quadratic dependence of the green and red emissions on excitation power indicates that the two-photon absorption process occurs under the 976nm excitation.     

Surface-modified lead–zirconium-titanate system for solution-processed ferroelectric-gate thin-film transistors

We propose the use of a La₂O₃ (LO) film as the capping layer for improvement of a semiconductor/insulator interface in a solution-processed indium–tin–oxide (ITO) ferroelectric-gate thin-film transistor (FGT) device. It is demonstrated that the LO layer acts as a good barrier film not only for preventing the interdiffusion between the ITO semiconductor and lead–zirconium-titanate (PZT) insulator layers, but also for stabilizing the PZT surface structure. The fabricated FGT device exhibited high I _on/I _off, large M _w, high μ _FE and improved retention time of about 10⁹, 3.5 V, 7.94 cm²?V^?1?s^?1 and 1 day, respectively, which are comparable to or better than those obtained with FGTs fabricated by means of conventional vacuum processes. We also point out that the key origin of the interface improvement is likely due to the incorporation of La into the PZT system, forming a La surface-modified PZT system which is more stable than the pure PZT in terms of Pb volatility and formation of oxygen vacancies.     

Finding tube–lead coupling strength in a single-walled carbon nanotube

For the first time, the electron–electron interaction energy relative to the coupling of a single-walled carbon nanotube to the involved leads is estimated analytically by considering a quantum box transversal to the longitudinal axis of the tube. In addition, the relation of our calculation to creation and annihilation operators associated with the above coupling is discussed.     

Melody lead in piano performance: expressive device or artifact?

As reported in the recent literature on piano performance, an emphasized voice (the melody) tends to be played not only louder than the other voices, but also about 30 ms earlier (melody lead). It remains unclear whether pianists deliberately apply melody lead to separate different voices, or whether it occurs because the melody is played louder (velocity artifact). The velocity artifact explanation implies that pianists initially strike the keys simultaneously; it is only different velocities that make the hammers arrive at different points in time. The measured note onsets in these studies, mostly derived from computer-monitored pianos, represent the hammer-string impact times. In the present study, the finger-key contact times are calculated and analyzed as well. If the velocity artifact hypothesis is correct, the melody lead phenomenon should disappear at the finger-key level. Chopin's Ballade op. 38 (45 measures) and Etude op. 10/3 (21 measures) were performed on a B?sendorfer computer-monitored grand piano by 22 skilled pianists. The hammer-string asynchronies among voices closely resemble the results reported in the literature. However, the melody lead decreases almost to zero at the finger-key level, which supports the velocity artifact hypothesis. In addition to this, expected onset asynchronies are predicted from differences in hammer velocity, if finger-key asynchronies are assumed to be zero. They correlate highly with the observed melody lead.     

Shielding of gamma radiation by hydrated Portland cement–lead pastes

Portland cement mixed with different percentages of granulated lead is studied as shields for gamma radiation. The samples are cured at 100% relative humidity for various time intervals. It is found that cement containing 5% lead has the higher attenuation coefficient value. The results are interpreted by study of the bulk density and the compressive strength of the pastes. The formation of more packed structure of tobermorite by lead addition is studied using the XRD analysis and the SEM micrographs.     

Does synchronization of networks of chaotic maps lead to control?

We consider networks of chaotic maps with different network topologies. In each case, they are coupled in such a way as to generate synchronized chaotic solutions. By using the methods of control of chaos we are controlling a single map into a predetermined trajectory. We analyze the reaction of the network to such a control. Specifically we show that a line of one-dimensional logistic maps that are unidirectionally coupled can be controlled from the first oscillator whereas a ring of diffusively coupled maps cannot be controlled for more than 5 maps. We show that rings with more elements can be controlled if every third map is controlled. The dependence of unidirectionally coupled maps on noise is studied. The noise level leads to a finite synchronization lengths for which maps can be controlled by a single location. A two-dimensional lattice is also studied.     

Spectral studies on Cu ions in sodium–lead borophosphate glasses

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of Cu²⁺ ions in sodium–lead borophosphate glasses doped with different concentrations of Cu²⁺ ions have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in sodium–lead borophosphate glasses are present in octahedral sites with tetragonal distortion. The optical absorption spectra of all the glass samples show a single broad band, which has been assigned to the ²B_1g→²B_2g transition of Cu²⁺ ions. The optical band gap energy (E_opt) and Urbach energy (ΔE) are calculated from their ultraviolet absorption edges. The emission bands observed in the ultraviolet and blue region are attributed to 3d⁹4s→3d¹⁰ triplet transition in Cu⁺ ion. The FT-IR spectra show that the glass system contains BO₃, BO₄ and PO₄ structural units.     

Mössbauer studies and magnetic properties of spinel lead ferrite

In the present communication, we have reported the synthesis of nanocrystalline lead ferrite (PbFe₂O₄) by citrate mediated autocombustion method. X-ray diffraction pattern reveals the single phase formation in cubic (spinel) structure. The particle size and the surface morphology of the samples are characterized by TEM and SEM analysis. Magnetic studies are carried out using vibrating sample magnetometer (VSM) shows a very high coercive field for the material. Mössbauer studies were performed to investigate the local symmetry i.e. Fe is in octahedral/tetrahedral site and the charge states of Fe ions.     

Mössbauer studies of negative-U tin centers in lead chalcogenides

Experimental data on identification of negative-U tin centers in lead chalcogenides and related solid solutions by means of ¹¹⁹Sn Mössbauer spectroscopy are discussed.     

Theoretica Study on defects associated with lead vacancies in PbWO4 crystals

The defects associated with lead vacancies(VPd)in lead tungstate crystals(PbWO4) are investigated by the relativistic self-consistent discrete variational embedded cluster method.We focus on the density of states and the effect of Vpb on surroundings,the results show that the existence of Vpb can diminish the bandwidth of WOr^2- group,however,it can neither produce O^- and Pb^3 ions nor result in absorptions at 350 and 420nm,The charge balance of VPb may be evenly compensated by the surrounding oxygen ions.     

UV–visible,infrared absorption spectra of undoped and TiO2-doped lead phosphate glasses and the effect of gamma irradiation

Undoped and TiO₂-doped lead phosphate glasses were prepared. Ultraviolet (UV)–visible and Fourier transform-infrared (IR) absorption spectra of the prepared samples were measured before and after being subjected to doses of 30 and 60 kGy of gamma irradiation. The parent undoped lead phosphate glass reveals charge transfer UV absorption bands which are attributed to the presence of unavoidable iron impurities contaminated within the raw materials used for the preparation of the glasses and the sharing of divalent lead (Pb²⁺) ions. Experimental spectral data indicate that the doped titanium ions are involved in such glasses in two valences, namely the trivalent and tetravalent states. The predominant trivalent titanium (Ti³⁺) ions are characterized by its purple color and exhibiting two visible absorption bands at about 500–550 and 700–720 nm. The lesser tetravalent titanium (Ti⁴⁺) ions belong to the d⁰ configuration and generally exhibit only an UV absorption band. Spectral data show that gamma irradiation causes noticeable changes in the undoped and TiO₂-doped samples in the UV range while the effects are limited in the visible range. The observed changes in the UV region are attributed to photochemical reactions while TiO₂-doped samples show retardation or shielding toward continuous gamma irradiation together with the sharing of heavy Pb²⁺ ions. IR absorption spectra reveal the vibrations of several phosphate groups including the metaphosphate chains as the main structural building units together with the possible Pb?O vibrations.     

The influence of the growth rate on the eutectic spacings,undercoolings and microhardness of directional solidified bismuth–lead eutectic alloy

Bi–Pb alloy at the eutectic composition was unidirectionally solidified upwards with five different growth rates (V = 7.05–113.09 μm/s) at constant temperature gradient (G = 2.18 K/mm) in a Bridgman type directional growth furnace in order to investigate dependency of eutectic spacing (λ), minimum undercooling (ΔT) and microhardness (HV) on the growth rates (V). The values of λ and HV were measured from the quenched samples and the minimum undercooling (ΔT) were determined from the Jackson–Hunt eutectic theory. The dependency of eutectic spacings, microhardness and undercooling on growth rate was investigated. According to these results it has been found that the value of λ decreases with increasing the value of V and that the values of HV and ΔT increase for a constant G. The values of λ²V, λΔT and ΔTV^?0.5 were determined by using the values of λ, ΔT and V. The results obtained in the present work have been compared with those predicted by the Jackson–Hunt eutectic theory and with similar experimental results.     

Upconversion fluorescence spectroscopy of Er~(3+)-doped lead oxyfluorosilicate glass

Upconversion fluorescence emission of Er3+-doped oxyfluorosilicate glass excited at 975 nm is experimentally investigated. The results reveal that the intense green and red emission, and weak blue emission centered at 525, 543, 655, and 410 nm, respectively. A two-photon upconversion process is assigned to the green and red emission while a three-photon process is responsible for blue upconversion. The possible upconversion mechanisms are discussed based on excited state absorption and energy transfer between excited Er3+ ions. The intense upconversion fluorescence of Er3+-doped lead oxyfluorosilicate glass may be a potentially useful material for developing upconversion optical devices.     

Internal field and self-polarization in sol–gel-derived lead zirconate titanate films

An internal electric field is induced in a sol–gel-derived lead zirconate titanate (PZT) film of thickness 2 m after the rf-sputtering deposition of the top electrode and this is exhibited as a shift of the P–E loop towards the positive side of the electric field axis. The direction of the internal field is towards the bottom electrode. As the discharge power used for the sputtering decreases, the internal field increases. We suggest that the internal field is induced by the high-energy ion bombardment on the film surface during the rf-sputtering process. Due to the occurrence of the internal field, the as-prepared PZT film becomes self-polarized at a high deposition temperature of 150 °C, thus showing a non-zero pyroelectric coefficient at room temperature. As the internal field increases, the observed pyroelectric coefficient increases and has a large value of 230 C/m²K for an as-prepared PZT film with an internal field of 40 kV/cm. PACS 77.22.Ej; 77.70.+a; 77.90.+k