Investigation of YBa2Cu3O x , Bi(Pb)-HTSC and Nd(123)-Particles in a Running Magnetic Field Near the Superconducting Transition Temperature

A novel system of electromagnetic separation (EMS) employing an alternating magnetic field (RMF) of suspended HTSC-particles in liquid nitrogen is described (Broide in Supercond. Cryoelectron. Summer:33, [1998]; Broide in Superconductor Week 12, No. 21, [1998]; Broide in US Patent 5,919,737, [1999]; Broide et al. in International Conference, Barcelona, Spain, [1999]). The EMS method is realized at temperatures close to those of the HTSC transition by extracting particles of YBaCuO, BiSrCaCuO, BiPbSrCaCuO, and NdBaCuO with optimal physical properties from the powders, concentrating particles with a greater critical current at one side. This process has shown great promise in its ability to produce high quality superconducting materials, being also an effective method of testing and analyzing powders to determine if synthesis adjustments are necessary in order to achieve optimal materials properties. In addition, a correlation between the speed of YBa₂Cu₃O _x particles and the percentage of oxygen (O _x ) was discovered. Since the temperature of the superconducting (SC) transition (T _c ) practically has a linear dependence on the oxygen percentage, the knowledge of the speed of the HTSC-particles during the separation, one may immediately give (within 10 s) the percentage of the oxygen in the chemical structure of the new HTSC materials. The EMS method enables statistical improvement in the uniformity of HTSC powders by decreasing the quantity of defective zones in the separated powders, the number of nonvalid admixtures, and thus, an increase in T _c and I _c (critical current). At the same time, ΔT _c the width of the SC-transition is decreased. Three YBa₂Cu₃O _x thick films have been produced by the paint method on MgO substrates from the source material, the separated high-quality concentrate, and the low-quality remnant (tail). The films exhibit different structural and electrical properties, in particular, different I _c , and ΔT _c . For the film that was made from the high-quality concentrate, ΔT _c is decreased from 5–7 K to 1–2 K. I _c is increased by a factor of 2–5 from the I _c of the source material (before EMS) and T _c is changed from 89 K to 93 K. Thick films, which had been produced from low-quality remnant (tail) do not have SC transition down to 77 K.      

A time-discrete model for dynamic fracture based on crack regularization

We propose a discrete time model for dynamic fracture based on crack regularization. The advantages of our approach are threefold: first, our regularization of the crack set has been rigorously shown to converge to the correct sharp-interface energy Ambrosio and Tortorelli (Comm. Pure Appl. Math., 43(8): 999–1036 (1990); Boll. Un. Mat. Ital. B (7), 6(1):105–123, 1992); second, our condition for crack growth, based on Griffith’s criterion, matches that of quasi-static settings Bourdin (Interfaces Free Bound 9(3): 411–430, 2007) where Griffith originally stated his criterion; third, solutions to our model converge, as the time-step tends to zero, to solutions of the correct continuous time model Larsen (Math Models Methods Appl Sci 20:1021–1048, 2010). Furthermore, in implementing this model, we naturally recover several features, such as the elastic wave speed as an upper bound on crack speed, and crack branching for sufficiently rapid boundary displacements. We conclude by comparing our approach to so-called “phase-field” ones. In particular, we explain why phase-field approaches are good for approximating free boundaries, but not the free discontinuity sets that model fracture.     

Analysing divided wall columns

In chemical processes, beside main products, significant quantities of by-products are often produced due to thermodynamics. Therefore, separation techniques and methods play such an important role, in chemical technologies, as reactor systems. Rectification is one of the most frequently used fluid separation process, which usually involves significant energy consumption. The production of hot and cold energy causes notable environmental load (CO₂ emission, dust pollution, etc.). Hence, from environmental point of view, research on energy saving in separation processes, for instance analysing divided wall columns (DWC), is exceedingly important. This article focuses on how to find more efficient structures with minimal energy consumption. The DWC system is a promising energy-saving alternative for separating multi-component mixtures (Chem Eng Process 38: 549–562, 1999; Chem Eng Process 49: 825–835, 2010). The innovation of this equipment is a wall which divides the inner space of the tower separating the feed and the side stream-product zones. The side stream and the feed stream are prevented from mixing by this wall. The DWC possesses greater efficiency than common column sequences. Conventional columns with side stream can be converted to DWC. In this article, several constructions of DWC are investigated with simulation experiments. The structure of the column is implemented in Aspen Plus in: 2006 Reference Guide, AspenTech Inc., Cambridge, 2006 simulator using the Radfrac unit of the software. The effects of the split ratio, the height and the vertical position of the wall, as the main parameters of DWC, are analysed.     

Numerical study of hydrogen production by the sorption-enhanced steam methane reforming process with online CO<Subscript>2</Subscript> capture as operated in fluidized bed reactors

A three-dimensional (3D) Eulerian two-fluid model with an in-house code was developed to simulate the gas-particle two-phase flow in the fluidized bed reactors. The CO₂ capture with Ca-based sorbents in the steam methane reforming (SMR) process was studied with such model combined with the reaction kinetics. The sorption-enhanced steam methane reforming (SE-SMR) process, i.e., the integration of the process of SMR and the adsorption of CO₂, was carried out in a bubbling fluidized bed reactor. The very high production of hydrogen in SE-SMR was obtained compared with the standard SMR process. The hydrogen molar fraction in gas phase was near the equilibrium. The breakthrough of the sorbent and the variation of the composition in the breakthrough period were studied. The effects of inlet gas superficial velocity and steam-to-carbon ratio (mass ratio of steam to methane in the inlet gas phase) on the reactions were studied. The simulated results are in agreement with the experimental results presented by Johnsen et al. (2006a, Chem Eng Sci 61:1195–1202).     

Optimal pairing computation over families of pairing-friendly elliptic curves

Vercauteren introduced the concept of optimal pairing, which by definition can be computed by using at most (log₂ r)/φ(k) + log₂ k basic Miller iterations, where r is the order of the groups involved and k is the embedding degree Vercauteren (IEEE Trans Inf Theory 56(1):455–461, 2010). Freeman et al. summarized and proposed all of the new constructions of pairing-friendly elliptic curves that currently exist Freeman et al. (J Cryptol 23(2):224–280, 2010). In this paper, we give an optimal pairing for each family of pairing-friendly curves in Freeman et al. (J Cryptol 23(2):224–280, 2010) by taking the Ate or R-ate pairing approach.     

Subsampling weakly dependent time series and application to extremes

This paper provides extensions of the work on subsampling by Bertail et al. in J. Econ. 120:295–326 (2004) for strongly mixing case to weakly dependent case by application of the results of Doukhan and Louhichi in Stoch. Proc. Appl. 84:313–342 (1999). We investigate properties of smooth and rough subsampling estimators for sampling distributions of converging and extreme statistics when the underlying time series is η- or λ-weakly dependent.     

Temperature Dependence of Intermodulation Power in Y1Ba2Cu3O7−δ Films on LaAlO3 Substrates

Measurements of resonators from two groups on the temperature dependence of the third-order intermodulation power of Y₁Ba₂Cu₃O_7−δ films on LaAlO₃ substrates are analyzed, evaluated, and plotted according to very similar definitions. At a fixed circulating power in the resonator both groups found a distinct minimum of the normalized intermodulation power below half the critical temperature in agreement with the theoretical prediction of Dahm and Scalapino [Appl. Phys. Lett. 69, 4248 (1996)] for d-wave superconductors. Also in agreement with theory, for Niobium as a s-wave superconductor, no intermodulation power minimum could be found down to T/T _C≈ 0.2.     

Superconducting Properties of Zn-Doped Cu0.5Tl0.5Ba2Ca2Cu3?yZnyO10?δ Superconductors

A reproducible synthesis and characterization of Zn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y Zn _y O_12−δ (y=0, 0.5, 1.0, 1.5) superconductors at a relatively lower synthesis temperature of 840°C are studied by using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. The X-ray diffraction (XRD) studies of these samples have shown a tetragonal structure in which the c-axis length has been found to decrease with increased Zn doping. The critical temperature and magnitude of diamagnetism have not been significantly affected with the doping of Zn at this synthesis temperature. The magnitude of diamagnetism in the as-prepared undoped samples is decreased, whereas it remains stable (unchanged) in oxygen post-annealed samples. The apical oxygen phonon’s modes of type Tl–O_A–M(2) and Cu(1)–O_A–M(2) {where M=Cu/Zn} and the planar oxygen phonon modes of type M(2)–O_P–M(2) are also softened with the increase of Zn doping. We interpreted the softening of these oxygen related phonon modes linked with the decreased c-axis length, reduced John–Teller distortions and increased mass of Zn (65.38 amu) as compared to that of Cu (63.54 amu) (Kaplan et al., Phys. Rev. B 65, 214509, 2002).     

Monomer recovery of waste plastics by liquid phase decomposition and polymer synthesis

Polycarbonate was decomposed into phenol, bisphenol A and p-isopropenylphenol by the reaction at 130–300 °C in water. The decomposition reaction was accelerated by the addition of Na₂CO₃, and the yield of identified products reached 68% in the reaction at 250 °C for 1 h. By using the decomposed products, the prepolymer of phenol resin was synthesized.     

Templated synthesis of mesoporous aluminas by <Emphasis Type="Italic">graft</Emphasis> copolymer and their CO<Subscript>2</Subscript> adsorption capacities

Mesoporous aluminas were synthesized via a sol–gel process by templating an amphiphilic graft copolymer, PVC–g–POEM, consisting of a poly(vinyl chloride) (PVC) backbone and poly(oxyethylene methacrylate) (POEM) side chains. The mesoporous structures of aluminas with large surface areas were confirmed by X-ray diffraction, transmission electron microscopy, and nitrogen adsorption/desorption analysis. Aluminas synthesized with PVC–g–POEM graft copolymer exhibited higher CO₂ adsorption capacities (0.7 mol CO₂/kg sorbent) than aluminas synthesized without graft copolymer (0.6 mol CO₂/kg sorbent). The adsorption capacity of alumina strongly depends on its structure and calcination temperature; amorphous (400 °C) > γ phase (800 °C) > α phase (1000 °C).     

Nano structures through self-assembly of protected hydrophobic amino acids: encapsulation of rhodamine B dye by proline-based nanovesicles

The development of novel molecules for the creation of nanometer structures with specific properties has been the current interest of this research. We have developed a set of molecules from hydrophobic ω- and α-amino acids by protecting the –NH₂ with Boc (t-butyloxycarbonyl) group and –CO₂H with para-nitroanilide such as BocHN–Xx–CONH–(p–NO₂)·C₆H₄, where Xx is γ-aminobutyric acid (γ-Abu), (l)-isoleucine, α-aminoisobutyric acid, proline, etc. These molecules generate various nanometer structures, such as nanofibrils, nanotubes and nanovesicles, in methanol/water through the self-assembly of bilayers in which the nitro benzene moieties are stacked in the middle and the Boc-protected amino acids parts are packed in the outer surface. The bilayers can be further stacked one over the other through hydrophobic interactions to form multilayer structure, which helps to generate different kinds of nanoscopic structures. The formation of the nanostructures has been facilitated through the participation of various noncovalent interactions, such as hydrophobic interactions, hydrogen bonding and aromatic π-stacking interactions. Fluorescence microscopy and UV studies reveal that the nanovesicles generated from pro-based molecule can encapsulate dye molecules which can be released by addition of acid (at pH 2). These single amino acid based molecules are both easy to synthesize and cost-effective and therefore offer novel scaffolds for the future design of nanoscale structures.     

Effect of Uniaxial Pressure on the Isotope Effect on the Spectrum of Hamiltonians with Electron–Phonon Coupling

We have studied the effect of a change in the equilibrium nearest neighbour distances on the dynamics of charge and ions on a three-site cluster, identified with an O–Cu–O cluster present in high temperature superconductors. We consider a model Hamiltonian that contains an electronic part represented by a single band Hubbard model with onsite electronic correlations and a phononic part consisting of non-interacting Raman and infrared active phonons. The electron–phonon coupling is introduced through the change in interatomic distances generated by Coulomb repulsion between charges at neighbouring sites. For intermediate and strong values of the electron–phonon coupling, this model generates correlated electron–ion motion, i.e., polaron formation. In order to simulate the effect of change in the equilibrium nearest neighbour distances, we assume that the main effect such a change is a variation of the intersite electron hopping probability, t. We, therefore, studied the excitation spectrum of this model and the local lattice distortion in the Cu–O bond length as a function of t. We also studied the effect of a change in t in the polaron tunnelling energy when we use different oxygen isotopic masses, i.e., O¹⁶ and O¹⁸. We find that as function of t, the isotopic shift does not show a monotonic behaviour, as it does as a function of the electron–phonon coupling constant. It exhibits a minimum for values of t for which the electron–phonon coupling generates local lattice distortions with magnitudes similar to those observed experimentally in high-temperature superconductors. This observation could be related with the observed maximum on T _c as a function of the microstrain of the Cu–O bonds (Sanna et al. in Int. J. Mod. Phys. B 14(29–31), 2000; Bianconi et al. in J. Phys.: Condens. Matter 12:10655, 2000; Agrestini et al. in J. Phys. A: Math. Gen. 36:9133, 2003).     

Role of Coexistence of Superconductivity and Paramagnetism in Magnetostriction of High-T c Superconductors

Total magnetostriction in the superconducting state for high T _c superconductors has been separated into critical state and paramagnetic components in terms of a H(x) dependent magnetic flux density. We show that the paramagnetic part is χ(2+χ)〈H(x)²〉, where χ is paramagnetic susceptibility. We have reproduced successfully ΔL/L−H _a curves measured by de la Fuente et al. (Phys. C 244:214, [1995]), in which they clearly observed coexistence of superconductivity and paramagnetism, employing the concepts presented in this work.      

On the Applicability of the Magnetic Susceptibility Models for the Magnetic Superconductor Ru-1212

The applicability of theoretical models for the ac susceptibility measurements of polycrystalline RuSr₂GdCu₂O₈ superconductor has been examined within the temperature range between 8–50 K, ac magnetic field 0.5–25 G, and frequency 20–12500 Hz. In general, a reasonable qualitative agreement between theory and experiment was attained. An evident and detectable asymmetry was observed within the Cole–Cole polar plots with a peak enhancement for both theoretical and experimental data. The modified critical state models are found to generate much better explanation of the ac susceptibility measured data than Bean’s model. For fields above 20 G, the results are agreed roughly with the Bean critical state model, while below 20 G, the Kim–Anderson model is more suitable to account of the magnetic performance. The temperature and field amplitude dependencies of the flux-creep exponent, n, were extracted from the real part of susceptibility, χ′, dependence on frequency. The flux-creep exponent was found to decrease with both temperature and ac field amplitude in accordance to a power-law of the form: n(T,H)=n ₀(H)T ^−s(H). Such dependence might be an indication of a crossover to flux-creep bundles regime.     

Room-Temperature Photomagnetic Effect of Fe<Subscript>3</Subscript>Ga<Subscript>4</Subscript> Grown on GaAs Substrates

Ga–As–Fe composite films prepared by molecular beam epitaxy at 600°C on GaAs(100) substrates with the stacking sequence of [100-nm GaAs/50-nm Fe₃Ga_{2− x} As _x /100-nm GaAs] exhibit the distinct photo-enhanced magnetization at room temperature. Transmission electron microscopy reveals the formation of metamagnetic Fe₃Ga₄ grains on the sample surface. Illumination power dependence of the enhanced magnetization has been carefully compared with the antiferromagnetic-type magnetization–temperature (M–T) curve (Neel temperature of T _N = 340–390 K), from which we have discussed the existence of photon-mode photo-enhanced magnetization of some sort in addition with the enhancement due to the light-induced heating.     

超声协同微球型Bi2O2CO3降解罗丹明B的研究

Bi_2O_2CO_3是一种Bi类半导体催化剂,文章研究了它的超声催化性能。首先,采用水热法制备了微球型的Bi_2O_2CO_3,利用X射线衍射(X-ray Diffraction, XRD)、扫描电子显微镜(Scanning Electron Microscope, SEM)、紫外-可见漫反射光谱对样品的晶体结构、微观形貌、光学特性进行了表征。然后,以罗丹明B(Rh B)作为模型污染物,通过研究超声催化降解罗丹明B来评测Bi_2O_2CO_3的超声催化性能。研究了催化剂的浓度(Ccatalytic)、初始罗丹明B染料的浓度(CRhB)和超声功率(P)等实验因素对超声催化降解效率的影响。得出在Ccatalytic=3 g·L-1,CRhB=10 mg·L-1和P=400 W条件下降解罗丹明B的效率最高,其最高降解效率可以达到91.7%。     

Suhl Instability as a Mechanism of the Catastrophic Relaxation in the Superfluid 3He–B

In order to explain catastrophic relaxation, bulk mechanism based on Suhl instability (J. Phys. Chem. Solids 1, 209, 1957) is studied. It is shown, that at sufficiently low temperatures homogeneous precession of spin becomes unstable in the whole region of tipping angles of spin 0≤β≤π. In comparison with the previous publication of Surovtsev and Fomin (J. Exp. Theor. Phys. Lett. 83, 410, 2006) the leading zero temperature increments for the angles θ ₀≃104°≤β≤π are found. Estimation of the temperature of transition to the unstable state for the angle of 105°, that corresponds to the region of tipping angles in homogeneously precessing domain (HPD), is made.     

Local Lattice Dynamics in the Mg0.5Al0.5B2 Superconductor

We have studied temperature evolution of the local as well as the average crystal structure of MgB₂ and Mg_0.5Al_0.5B₂ using real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction in a wide temperature range of T=10–600 K. The mean square relative displacements (MSRD) of atomic B–B, B–Mg (B–Al) pairs are compared with mean-square displacements (MSD) to calculate atomic correlations. In spite of the enhanced atomic disorder in Mg_0.5Al_0.5B₂, where the boron–boron, and boron–magnesium pair motions are found to be small, we find that the same atomic correlations in MgB₂ assume even slightly lower values and remain nearly constant in a wide temperature range of 0–600 K. This anomalous behavior and its physical interpretation provoke new questions on our understanding to the local lattice dynamics in this material.     

Inelastic neutron scattering from Tl2CaBa2Cu2O8

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl₂CaBa₂Cu₂O₈ (Tl-2122,T _c =107 K) and YBa₂Cu₃O₇ (Y-123,T _c =92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.     

Collapse of <Emphasis Type="Italic">ρ</Emphasis><Subscript><Emphasis Type="Italic">xx</Emphasis></Subscript> Ringlike Structures in 2DEGs Under Tilted Magnetic Fields

In the quantum Hall regime, the longitudinal resistivity ρ _xx plotted as a density–magnetic-field (n _2D –B) diagram displays ringlike structures due to the crossings of two sets of spin split Landau levels from different subbands [see, e.g., Zhang et al., in Phys. Rev. Lett. 95:216801, 2005. For tilted magnetic fields, some of these ringlike structures “shrink” as the tilt angle is increased and fully collapse at θ _c ≈6°. Here we theoretically investigate the topology of these structures via a non-interacting model for the 2DEG. We account for the inter Landau-level coupling induced by the tilted magnetic field via perturbation theory. This coupling results in anticrossings of Landau levels with parallel spins. With the new energy spectrum, we calculate the corresponding n _2D –B diagram of the density of states (DOS) near the Fermi level. We argue that the DOS displays the same topology as ρ _xx in the n _2D –B diagram. For the ring with filling factor ν=4, we find that the anticrossings make it shrink for increasing tilt angles and collapse at a large enough angle. Using effective parameters to fit the θ=0° data, we find a collapsing angle θ _c ≈3.6°. Despite this factor-of-two discrepancy with the experimental data, our model captures the essential mechanism underlying the ring collapse.