Thermal entanglement of two interacting qubits in a static magnetic field

We study systematically the entanglement of a two-qubit Heisenberg XY model in thermal equilibrium in the presence of an external arbitrarily-directed static magnetic field, thereby generalizing our prior work [G. Lagmago Kamta, A.F. Starace, Phys. Rev. Lett. 88, 107901 (2002)]. We show that a magnetic field having a component in the xy-plane containing the spin-spin interaction components produces different entanglement for ferromagnetic (FM) and antiferromagnetic (AFM) couplings. In particular, quantum phase transitions induced by the magnetic field-driven level crossings always occur for the AFM-coupled qubits, but only occur in FM-coupled qubits when the coupling is of Ising type or when the magnetic field has a component perpendicular to the xy-plane. When the magnetic field has a component in the xy-plane, the cut-off temperature above which the entanglement of both the FM- and AFM-coupled qubits vanishes can always be controlled using the magnetic field for any value of the XY coupling anisotropy parameter. Thus, by adjusting the magnetic field, an entangled state of two spins can be produced at any finite temperature. Finally, we find that a higher level of entanglement is achieved when the in-plane component of the magnetic field is parallel to the direction in which the XY exchange coupling is smaller.     

Isolation of sporopollenin-like biopolymer from <Emphasis Type="Italic">Aspergillus niger</Emphasis> and its characterisation

The study sought to isolate asporopollenin-like biopolymer from Aspergillus niger (A. niger) spore. The spore exine (Sp-exine) from A. niger was isolated using four different methods. The highest isolation efficiency (9.32 %) was found for the method based on H₂SO₄ treatment. Optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the isolated Sp-exine had a spherical shape with a spun morphology and was highly uniform in size. The elemental compositions of the bodies of the Sp-exine materials, as well as their surfaces, were observed by means of CHN combustion analysis and SEM with energy-dispersive X-ray spectroscopy (SEM-EDX), respectively. The result demonstrates C, H and O to be the main structural elements in all the Sp-exine samples. Fourier-transform infrared (FTIR) spectroscopy coupled with solid state ¹³C nuclear magnetic resonance (solid state ¹³C NMR) spectroscopy and UV-Vis spectroscopy were used to compare the changes in the functional groups of the Sp-exine samples isolated using the different methods. A thermogravimetric analysis (TGA) study demonstrated all the Sp-exine samples showed high thermal stability. Among all tested methods, the treatment with H₂SO₄ and alcoholic potassium hydroxide exhibited the best results in removing cellular and other contents with minimal drastic effect on the structure and morphology and is proposed as the best method for isolating Sp-exine since it required minimal isolation time and showed good isolation efficiency.     

Separation of rhenium from molybdenum and vanadium by extraction with amyl alcohol

Summary A method is described for the separation of rhenium from molybdenum and vanadium by extraction with amyl alcohol. The sample solution containing Re^VII, Mo^VI, and V^V is boiled with hydrazine sulphate in order to obtain Mo^V and V^IV (Re remains in the heptavalent state). Rhenium is quantitatively extracted into amyl alcohol from this solution, along with very slight amounts of molybdenum and vanadium. After back-extraction, the residual molybdenum is completely removed by extraction of its thiocyanate with amyl acetate. The accompanying vanadium is usually too small to interfere in the thiocyanate method of rhenium estimation. The method is simple, rapid and applicable to all Mo:Re ratios generally met with.

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Zusammenfassung Ein Verfahren zur Abtrennung des Rheniums von Molybdän und Vanadium durch Extraktion mit Amylalkohol wird beschrieben. Rhenium soll als Re^VII, Molybdän als Mo^V und Vanadium als V^IV vorliegen (Mo^VI und V^V werden mit Hydrazinsulfat reduziert). Geringe Mengen Mo und V werden mitextrahiert. Nach Rückextraktion wird Mo vollständig durch Extraktion des Thiocyanats mit Amylacetat entfernt. Die vorhandenen Vanadiummengen sind zu gering, um die nachfolgende Rheniumbestimmung (nach der Thiocyanatmethode) zu stören. Das Verfahren ist einfach und rasch durchführbar und für alle üblichen Mo:Re-Verhältnisse anwendbar.

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Our sincere thanks are due to Prof. S. M. Mukherji, Head of the Chemistry Department, for facilities.     

Oxidative behaviour and relative reactivities of α,β-unsaturated compounds towards bis(dihydrogentellurato)cuprate(III) and -argentate(III) in alkaline medium

The kinetics of oxidation of three alpha beta-unsaturated compounds, CH2CHX (X=CN, CONH2 and CO2-) by bis(dihydrogentellurato)metallate(III) ions ([MIII-(H2TeO6)2]5-, M=Cu or Ag) have been studied in an alkaline medium. The reactions take place according to the rate expression: The k3 values are 2.42*10-2, 1.67*10-2 and 1.10*10-2mol6dm-2s-1 for oxidations of the respective substrate by the copper(III) complex and 1.49*10-2, 1.07*10-2 and 0.80*10-2mol6dm-2s-1, respectively, in the oxidations by silver(III), all at 303K. [CuIII(H2TeO6)2]5- reacts faster than the corresponding reactions with [AgIII(H2TeO6)2]5-. The oxidation rates follow the order: CN CONH2CO2-. The oxidations of the substrates by [CuIII(H2TeO6)2]5- take place by an outer-sphere mechanism, unlike [AgIII-(H2TeO6)2]5-, which reacts by an inner-sphere mechanism. The substrates are oxidized to diols under kinetic control and a tentative reaction mechanism leading to the formation of oxidation products is suggested. The activation parameters of the reactions have been evaluated. This revised version was published online in June 2006 with corrections to the Cover Date.     

Electron transport and magnetic properties of some mixed-valent alkalithiocuprates

Electrical transport properties of some two-dimensional isostructural compounds KCu₄S₃, RbCu₄S₃, and CsCu₄S₃ and a one dimensional conductor Na₃Cu₄S₄ have been investigated. Conductivities have been measured on pelletized materials in the temperature range 300 to 150 K. All of them behave as metallic conductors. The room temperature conductivities of these compounds are KCu₄S₃, 1600 Ω^?1 cm^?1; RbCu₄S₃, 1400 Ω^?1 cm^?1; CsCu₄S₃, 1250 Ω^?1 cm^?1; Na₃Cu₄S₄, 700 Ω^?1 cm^?1. The observed trend in conductivities among the isostructural compounds may be rationalized either on the basis of shortest CuCu distances or on the carrier charge concentrations per unit volume. All of these compounds behave as Pauli paramagnetic materials.     

Anisotropy and magnetic field effects on the entanglement of a two qubit Heisenberg XY chain

We investigate the entanglement of a two-qubit anisotropic Heisenberg XY chain in thermal equilibrium at temperature T in the presence of an external magnetic field B along the z axis. By means of the combined influences of anisotropic interactions and a magnetic field B, one is able to produce entanglement for any finite T, by adjusting the magnetic field strength. This contrasts with the isotropic interaction or the B = 0 cases, for which there is no entanglement above a critical temperature T(c) that is independent of the external B field.     

Formation of single-walled aluminosilicate nanotubes from molecular precursors and curved nanoscale intermediates

We report the identification and elucidation of the mechanistic role of molecular precursors and nanoscale (1-3 nm) intermediates with intrinsic curvature in the formation of single-walled aluminosilicate nanotubes. We characterize the structural and compositional evolution of molecular and nanoscale species over a length scale of 0.1-100 nm by electrospray ionization mass spectrometry, nuclear magnetic resonance spectroscopy ((27)Al liquid-state, (27)Al and (29)Si solid-state MAS), and dynamic light scattering. Together with structural optimization of key experimentally identified species by solvated density functional theory calculations, this study reveals the existence of intermediates with bonding environments, as well as intrinsic curvature, similar to the structure of the final nanotube product. We show that "proto-nanotube-like" intermediates with inherent curvature form in aqueous synthesis solutions immediately after initial hydrolysis of reactants, disappear from the solution upon heating to 95 °C due to condensation accompanied by an abrupt pH decrease, and finally form ordered single-walled aluminosilicate nanotubes. Detailed quantitative analysis of NMR and ESI-MS spectra from the relevant aluminosilicate, aluminate, and silicate solutions reveals the presence of a variety of monomeric and polymeric aluminate and aluminosilicate species (Al(1)Si(x)-Al(13)Si(x)), such as Keggin ions [AlO(4)Al(12)(OH)(24)(H(2)O)(12)](7+) and polynuclear species with a six-membered Al oxide ring unit. Our study also directly reveals the complexation of aluminate and aluminosilicate species with perchlorate species that most likely inhibit the formation of larger condensates or nontubular structures. Integration of all of our results leads to the construction of the first molecular-level mechanism of single-walled metal oxide nanotube formation, incorporating the role of monomeric and polymeric aluminosilicate species as well as larger nanoparticles.