Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

Finite perturbation theory-density functional theory calculation of the spin-dipolar contribution to NMR spin-spin coupling constants

In this work an implementation of the FPT-DFT approach for calculating the spin-dipolar contribution to NMR spin-spin coupling constants is presented. This method was tested in a set of small molecules, giving results in excellent agreement when comparing them with values taken from the literature, which were obtained with state-of-the-art calculations. To obtain an insight into the relative importance of the spin-dipolar contribution in unsaturated compounds, calculations of J(F,C), J(F,F) and J(F,H) couplings in 1,2-, 1,3-, and 1,4-difluorobenzenes were performed. An important spin-dipolar contribution to ³ J(F, F) and ⁵ J(F,F) was found, suggesting that this term might be important in some cases. When performing DFT calculations the non-singlet instabilities usually found in unsaturated compounds are overcome.     

Orbital contributions to relativistic corrections of the NMR nuclear magnetic shielding tensor originated in scalar field-dependent operators

In this work relativistic corrections to the magnetic shielding constant (σM), which arise from scalar field-dependent operators (both linear and bilinear), are calculated and decomposed into contributions from molecular orbitals (MOs). Numerical results for the magnetic shielding constant of the heavy nucleus X in closed shell atomic ions X^? and HX compounds (X = F, Cl, Br, I) are presented. The relative importance of inner-shell and valence electrons in the definition of each one of these terms is thus assessed and its relation with their sensitivity to changes in a chemical environment is discussed.     

Experimental, Theoretical and Biological Activity Study on the Acyl-Substituted Benzo-18-crown-6, Dibenzo-18-crown-6 and Dibenzo-24-crown-8

Hexadecanoyl, dihexadecanoyl, dioctadecaneoyl, di-10-undecenoyl, and dicis-9-octadecenoyl derivatives of benzo[18]crown-6, dibenzo[18]crown-6 and dibenzo[24]-8 were synthesized by the condensation of carboxylic acids (palmitic, stearic, oleic and undecenoic acid) with benzo and dibenzo crown ethers in the presence of zinc chloride. The extraction equilibrium constants of such macrocyclic ethers with long side chains were estimated using chloroform/water and dichloromethane/water membranes transfer of Na-PAR (4-(2-pyridylazo)-resorcinol mono sodium monohydrate) with UV–Vis spectroscopy. It was found that they were in the range of 10.88–11.71 in dichloromethane and 8.04–11.77 in chloroform. These results actually show that the Na+ binding effect of macrocyclic ethers depends on the type and the length of side chains. The geometrical properties of the molecules were studied employing semi-empirical calculations by simulated annealing technique. The frontier molecular orbital energies and dipole moments were also examined. The biological activity results showed that the synthesized crown ethers have no activity against the studied microorganisms.     

FLUSHING TIME IN THE CIENFUEGOS BAY,CUBA

We use a high‐resolution numerical model to analyze the hydrodynamical exchange processes of the Cienfuegos Bay with the Caribbean Sea. Results show that freshwater inputs and wind are the main factors controlling the water exchange in the bay. Nine simulation case studies with three wind conditions and three freshwater inputs were carried out. The main conclusion is that the Cienfuegos Bay system has a slow exchange rate, with an average flushing time of 39 days during the rainy period and 50 days during the dry one, for the most probable wind direction. Two new temporary scales for Cienfuegos Bay are discussed: the local e‐flushing time and the flushing lag. The results obtained for these new variables validate the slow exchange capacity of the bay.     

Molecular Interactions and Thermodynamic Aspects of the Complexation Reaction between Gentian Violet and Several Cyclodextrins

The complexation process between gentian violet (CV+) and four different cyclodextrins (-, -, -, and HP--CDs) has been investigated under different reaction conditions (pH, solvent, and temperature) by electronic absorption and 1H NMR (NOE and NOESY) spectroscopies. All the binding constants were determined by the direct spectroscopic method. The H and S complexation values have been evaluated and discussed according to the diverse factors that affect the chemical interactions in these systems. A simple association takes place between the secondary hydroxyl or the hydroxypropyl groups of and HP--cycloamyloses, respectively, with the amine group of the gentian violet, while the binding between CV+ and - or -CDs corresponds to a real inclusion. Also, a CV22+ dimmeric species within the -CD cavity was detected in aqueous solution, while two molecules of -CD react with one molecule of gentian violet in DMSO at 294 K. In all the reaction media the -CD forms 1 : 1 complexes, but in the buffered aqueous solution at pH 7.5 the inclusion is deeper than in the other solvents. It is important to point out that the solvophobic effect is the most important binding factor in the complexation of the CV+ with the - and HP--CDs, while the complexes with -, and -cyclodextrins are mainly stabilized by van der Waals interactions between the guest and the host cavity. In all cases, the inclusion orientation is probably determined by the ion-dipole interactions between gentian violet and the solvent.     

Gases Do not adsorb on the interstitial channels of closed-ended single-walled carbon nanotube bundles

We have experimentally determined the binding energies of Xe, CH4, and Ne on samples of closed-ended single-wall nanotube (SWNT) bundles. We find values for these quantities which are larger by approximately 75% on the SWNT samples than the values found for the same adsorbates on planar graphite. We have also determined the monolayer capacity of a SWNT sample using Xe and Ne adsorption. A comparison of all of our results leads us to conclude that none of the gases studied adsorb on the interstitial channels in the SWNT bundles.     

Experimental evidence of dipolar interaction in bilayer nanocomposite magnets

We use magnetic thin film hard/non/soft-magnetic trilayer systems to probe the nature of the hard–soft phase interaction and the role played by dipolar fields in one-dimensional (d) magnetic systems. We have systematically investigated six wedge samples where the thickness of a Cu spacer layer (t _Cu) was gradually changed to create a varying interfacial effect on the interaction between a CoPt hard layer and a Fe soft layer. Magneto-optical Kerr effect was used to obtain the magnetization loops at 28 points on each sample, and the nucleation field (H _N ) as a function of t _Cu was employed to characterize the layer interaction as a function of t _Cu. H _N (t _Cu) show a RKKY oscillatory behavior in addition to a non-negligible dipolar contribution, which had an exponential dependence. The dipolar term, which cannot be always neglected, is affected by the interface roughness and also by the CoPt crystallinity. Therefore, we cannot always consider exchange coupling to be the dominant interaction in one-d hard–soft magnetic bilayer systems, particularly, during magnetic reversal.     

半导体纳米激光

半导体纳米激光的研究是目前纳米技术（或纳米光子学）和半导体激光交汇产生的研究前沿。本文将综述这一领域最近一些最激动人心的进展。我们将集中讨论两种半导体纳米结构的纳米激光：自下而上生长而成的纳米线和自上而下刻蚀制成的纳米柱状结构。本文将综述这些纳米激光器的特殊特征,特别是利用表面等离子效应而成制成的金属{半导体等离子体激光,即目前世界上最小激光器的最新进展。由于这些纳米激光器的微小的尺寸以及对光在空间限制增强,我们必须重新检验半导体激光器中某些熟悉的概念在纳米尺度上的正确性和含义,例如模式增益和光限制因子（CF）的概念。本文将从统一的观点解释光限制因子在电介质和等离子体纳米激光器中的某些似乎反常的行为。在本文的通篇论述中,我们将尝试回答究竟激光器的尺寸能够小到什么程度,或者激光器的尺寸是否存在一个最小极限等基本问题。