We have observed power-law behavior in the current-voltage characteristics of single crystals of Bi2Sr&Ca&Cu208+~and Bii 6Pbo 4Sr2Ca2Cu30 both in zero and in applied magnetic fields. The observed power-law behavior, V~I ' ' near the transition with a characteristic Nelson-Kosterlitz jump in the exponent a( T) at T = T, gives evidence for a Kosterlitz-Thouless (KT) transition in wihch vortexantivortex pairs are dissociated within the superconducting planes. In a magnetic field, the KT transition is suppressed because field-induced vortices induce dissipation and reduce the stability of vortexantivortex pairs. The interaction of vortex pairs with interlayer Aux lines has been discussed.The transport properties of high-T, superconductors have several interesting and important features. Among them, the transport dimensionality and the non-ohmic behavior observed near the superconducting transition temperature need to be explained clearly. It is well known that the high-T, superconductors have twodimensional (2D) properties due to the layered structure of the CuOz superconducting planes which interact weakly with each other. Kosterlitz-Thouless (KT) (Ref. 1) behavior has therefore been expected, and the onset of a resistive state in several high-T, materials is described in terms of a KT transition. The power law of the current-voltage (I V) characte-ristics (V~I ' ') is described in the framework of the KT model near to the mean-field critical temperature T,o.In the KT model of a 2D system the phase transition at a critical temperature TKT is determined by the dissociation of vortex-antivortex pairs (pancake vortices) which interact mutually with a logarithmic potential U(r) =2m. Ektt T ln(r lg), where g is the Ginzburg-Landau (GL) coherence length, and nK=P d/16' k+TA. the stiffness constant of the KT theory. d is the thickness of the fluctuating superconducting sheet and k is the penetration depth in the ab plane. When T"T ( T", the pairs can be broken by an applied current density causing dissipation and a non-ohmic behavior as the temperature is increased. This gives a sudden jump in the exponent a( T) = 1+srK from 3 to 1 at a temperature TzT which gives the 2D nature of the superconductors.The observed power law in the I-V characteristics, suggesting a scaling law, cannot be derived from the standard fluxflow or flux-creep phenomena. The most important characteristic feature of the direct evidence of KT transition is the universal Nelson-Kosterlitz jump in the power-law exponent as a function of temperature. In this paper, we report the observation of the Nelson-Kosterlitz jump in a in zero magnetic field and also the behavior of the exponent a in external applied magnetic fields in single crystals of Bi&Sr&Ca, Cuzos+r (BSCCO) and Bii 6Pbo 4SrzCazCu30"(BPSCCO). We show how an external field plays a crucial role in the dynamics of the KT transition.The BSCCO and BPSCCO crystals used in our studies were grown by a self-flux method. The details of the growth process have been discussed elsewhere.Chemical analysi...
Growth of high-density and aligned ZnO nanorods on ZnO film substrate has been demonstrated using vapor-transport of thermally evaporated Zn metal powders followed by condensation. Morphological studies show that the nanorods grow preferentially from a hexagonal ZnO base with a uniform hexagonal structure following three-dimensional island-like growth mechanism. Structural and spectroscopic properties clearly indicate that the nanorods are relatively good and defect-free in quality. These nanorods have potential for technological implications.
The La(1-x)Sr(x)MnO3 (LSMO) nanoparticles have been synthesized by citric gel process followed by ball milling method. These nanoparticles demonstrated high crystalline quality. Nanoparticle size was further decreased by ball milling technique as observed by the field-emission scanning electron microscopic studies. The ball milled and silica coated LSMO nanoparticles show magnetic transition at about 370 K with a superparamagnetic properties. The ferromagnetic resonance (FMR) spectra analysis of LSMO nanoparticles shows large FMR linewidth due to the surface strain of the nanoparticles. Both magnetization and FMR studies demonstrate that the LSMO nanoparticles are highly anisotropic. The toxicity of the nanoparticles was studied for safe biomedical applications. Measurement of intracellular reactive oxygen species (ROS) and MTT assay results show that LSMO nanoparticles are relatively nontoxic and the toxicity is further reduced by SiO2 coating. These results are very important for applications in the field of biotechnology.
Thick AISI 304L stainless steel plates were welded using the gas metal arc welding process, and through-thickness residual stresses were evaluated by finite element simulation and the deep hole drilling technique. 3D moving heat source-based thermo-mechanical models were implemented to evaluate through-thickness residual stresses. The effects of the weld groove geometries and external restraints on the pattern of through-thickness residual stresses were studied. The maximum magnitude of locked-in residual stresses was recorded beneath the top surface, at a depth of around 6 mm. In comparison to conventional weld groove, the narrow weld groove configuration exhibited a 20–40% reduction in peak residual stresses. A significant rise in residual stresses was observed in constrained welds. The effect of the yield strength of the filler material on the evaluation of the through-thickness residual stress distribution in the course of finite element modeling was illustrated. The evolution of through-thickness residual stresses was also assessed concerning each weld pass.
The electric and Kelvin force probe microscopy were used to investigate the surface potentials on the ZnO seed layer, which shows a remarkable dependence on the annealing temperature. The optimum temperature for the growth of nanorod arrays normal to the surface was found to be at 600 degrees C, which is in the range of right surface potentials and energy measured between 500 degrees C and 700 degrees C. We demonstrated from both electric and Kelvin force probe microscopy studies that surface potential controls the growth of ZnO nanorods, illustrating the fact that this is a promising technique to visualize the control of ZnO nanorod arrays by studying their surface potentials. This study will provide important understanding of growth of other nanostructures.
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