331 research outputs found
Co Valence by K-Edge X-ray Absorption Spectroscopy, Magnetic Properties, and Structure of Polycrystalline Bulk Zn1 xCoxO
No full textWe report on magnetic properties and a Co K-edge X-ray absorption spectroscopy (XAS) study of polycrystalline bulk Zn1 Co O
( = 0 10 0 15 0 17) samples. All samples show paramagnetic behavior and no ferromagnetism was observed. XAS results showed
that for all samples the valence of Co is 2+. Spin state assessment derived from Currie–Weiss fitting indicated a possible spin state
transition from Co2+ high spin to Co2+ low spin. Calculated e values for samples with = 0 10 and 0 15 indicated an orbital
contribution to the effective magnetic moment of the Co ion
Apparent negative motion of vortex matter due to inhomogeneous pinning
No full textWe investigate the transport of vortices in superconductors with inhomgeneous pinning under a driving force. The inhomogeneity of pinning is simplified as strong-weak pinning regions. It is demonstrated that the interactions between the vortices captured by strong pinning potentials and the vortices in the weak pinning region cause absolute negative motion (ANM) of vortices: The vortices which are climbing toward the high barriers induced by the strong pinning with the help of driving force move toward the opposite direction of the force and back to their equilibrium positions in the weak pinning region as the force decreases or is withdrawn. Our simulations reveal that the hysteresis of ANM is determined by the competition between the speed of the negative motion which depends on the piining inhomogeneity in superconductors and the speed of the driving force. Under the conditions of either larger force scanning rate or higher pinning inhomogeneity, a marked ANM and a larger hysteretic speed-force loop could be observed. This indicates that the time window to observe the ANM should be chosen properly. Moreover, the V-1 characteristics of Ag-sheathed Bi=2223 tapes are measured, and experimental observations are qualitatively in agreement with the simulation
Nonequilibrium dynamics in type-II superconductors with inhomogeneous vortex pinning
No full textWe study numerically the dynamics relating to negative vortex motion in inhomogeneous pinning systems. We show that this dynamical phenomenon results from the internal field effect produced by the growing local barriers with decreasing temperature. We find that the negative motion is characterized
by a peak of negative voltage or resistance in resistance–temperature transport measurements. We also demonstrate that the time window to observe the negative motion is determined by the magnitude of driving force in addition to the temperature scanning rat
Simulation of the phase diagram of magnetic vortices in two-dimensional superconductors: evidence for vortex chain formation
No full textWe study the superconducting vortex states induced by the interplay of long-range Pearl repulsion and short-range intervortex attraction using Langevin dynamics simulations. We show that at low temperatures the vortices form an ordered Abrikosov lattice both in low and high fields. The vortices show distinctive modulated structures at intermediate fields depending on the effective intervortex attraction: ordered vortex chain and kagome-like vortex structures for weak attraction; bubble, stripe and antibubble lattices for strong attraction. Moreover, in the regime of the chain state, the vortices display structural transitions from chain to labyrinthine (or disordered chain) and/or to disordered states depending on the strength of the disorder
Peak effect in the critical current of Type II superconductors with strong magnetic vortex pinning
No full textWe perform 2D Langevin simulations studying the peak effect (PE) of the critical current taking into account the temperature dependence of the competing forces. We observe and report that the PE results from the competition of vortex-vortex interactions and vortex-pin interactions which have different temperature dependencies. The simulations reveal that the PE can take place only for certain pinning strengths, densities of pinning centers, and driving forces, which is in good agreement with experiments. No apparent vortex order-disorder transition is observed across the PE regime. In addition, the PE is a dynamical phenomenon, and thermal fluctuations can speed up the process for the formation of the PE
Thermally activated flux flow in Fe1.06Te0.6Se0.4 single crystal
No full text<b>Highlights</b>\ud
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- Resistivity of Fe<sub>1.06</sub>Te<sub>0.6</sub>Se<sub>0.4</sub> is investigated around superconducting transition region in different magnetic fields.\ud
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- Based on a theoretical prediction of the resistivity and experimental data, thermally activated energy is studied.\ud
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- The study shows that Arrhenius curve slopes are directly related to but not equal to thermally activated energy.\ud
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- Modified thermally activated flux flow model is suggested to use, where the temperature dependence of 2ρ<sub>c</sub>U/T is considered.\ud
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- Modified TAFF method results are in good agreement with very high critical current density values from experimental data.\ud
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<b>Abstract</b>\ud
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Resistivity of Fe<sub>1.06</sub>Te<sub>0.6</sub>Se<sub>0.4</sub> single crystal is investigated around superconducting transition region in different magnetic fields. The thermally activated energy (TAE) is analysed using the Arrhenius relation and modified thermally activated flux flow (TAFF) model. The results indicate that the Arrhenius curve slopes are directly related to but not equal to TAE. Therefore, use of the modified TAFF model is suggested, ρ(T,B) = ρ<sub><font size="small">0f</font></sub> exp(−U/T), where the temperature dependence of the pre-factor ρ<sub>0f</sub> = 2ρ<sub>c</sub>U/T and the nonlinear relation of the TAE should be considered. The modified TAFF method results are in good agreement with the very high critical current density values from the experimental data. It was found that the vortex glass has a narrow region, and it depends weakly on magnetic field. The vortex phase diagram was determined based on the evolution of the vortex-glass transition temperature with magnetic field and the upper critical field
Phase diagram of vortex matter of type-II superconductors
No full textWe propose a model to construct the phase diagram for type-II superconductors through molecular-dynamics simulation. In this model, the Abrikosov vortices interact with long-range repulsion and short-range attraction. We are able to obtain the phase diagrams consisting of the vortex lattice, the intermediate-mixed phase (IMP), and the disordered vortex phase in the B-T, B-?, and B-q planes, where B is induction, ? is the Ginzburg-Landau parameter, and q is the relative strength of the attraction to repulsion interactions between vortices. The IMP is in the form of a superlattice of either vortex bubbles or parallel vortex stripes, agreeing excellently with experimental results that have not yet been explained by existing models.<br/
Suppression of superconductivity with Pr substitution in Nd1-xPrxBaCaCu3O7 system
No full textThe structural, superconducting and magnetic properties of Nd1-xPrxBaCaCu3O7 system with x = 0.0, 0.10, 0.25, 0.35, 0.50, 0.75 and 1.0 have been investigated. X-ray diffraction results reveal that Pr substitutes isostructurally in NdBaCaCu3O7 (Nd:1113) superconductor with complete solubility. The superconducting transition temperature (T-c), measured by ac susceptibility technique, decreases with increasing x. However, suppression of T-c with increasing Pr substitution is less in Nd:1113 superconductor compared to that reported for Nd1-xPrxBa2Cu3O7 system. Interestingly, in the fully Pr substituted compounds of the above series, i.e., in PrBaCaCu3O7 and PrBaxCu3O7, the Pr moments order antiferromagnetically with T-N of 10 and 17 K, respectively. The present results along those reported earlier [V.P.S. Awana, J. Horvat, S.X. Dou, A. Sedky, A.V. Narlikar, J. Magn. Magn. Mater., 182 (1998) L280; V.P.S. Awana, S.X. Dou, S.K. Malik, Rajvir Singh, A.V. Narlikar, D.A. Landinez Tellez, J.M. Ferreira, J. Albino Aguiar, S. Uma, E. Gmelin, W.B. Yelon, J. Magn. Magn. Mater., 187 (1998) 192], clearly suggest that there is a correlation between the T-c suppression due to Pr and the magnetic ordering temperature of the fully substituted Pr moments in these systems. The T-N may be taken to be a measure of the strength of hybridization between the Pr-4f electrons with Cu-O conduction band, and hence a lower T-N may imply a less deleterious effect on superconductivity. (C) 1999 Elsevier Science B.V. All rights reserved.3164167111311
Flux pinning mechanism in BaFe1.9Ni0.1As2 single crystals: Evidence for fluctuation in mean free path induced pinning
No full textThe flux pinning mechanism of BaFe<sub>1.9</sub>Ni<sub>0.1</sub>As<sub>2</sub> superconducting crystals have been investigated systematically by magnetic measurements up to 13 T at various temperatures. The field dependence of the critical current density, J<sub>c</sub>, was analysed within the collective pinning model. A remarkably good agreement between the experimental results and theoretical δl pinning curve is obtained, which indicates that pinning in BaFe<sub>1.9</sub>Ni<sub>0.1</sub>As<sub>2</sub> crystal originates from spatial variation of the mean free path. Moreover, the normalized pinning force density, F<sub>p</sub>, curves versus h = B/B<sub>irr</sub> (B<sub>irr</sub> is the irreversibility field) were scaled using the Dew-Hughes model. Analysis suggests that point pinning alone cannot explain the observed field variation of F<sub>p</sub>
Rechargeable Potassium–Selenium Batteries
No full textFirst published: 06 May 2021Rechargeable potassium–selenium (K–Se) batteries, as an emerging electrochemical energy storage system, has recently captured intensive attention due to the desirable natural abundance and low redox potential of elemental potassium as well as the relatively high electronic conductivity and impressive theoretical volumetric capacity of elemental selenium. Although great progress on cathode materials design and electrochemical performance improvement has been made, K–Se batteries are still confronted with a series of key challenges, including low reactive activity, shuttle effect, volume expansion, potassium dendrite growth, and high chemical activity of potassium metal. The recent advances in rechargeable K–Se batteries are comprehensively summarized with an emphasis on discussing the electrochemical mechanisms and central challenges, presenting the synthesis, properties, and electrochemical performance of selenium-based cathode materials, and extending potential tactics for tackling the key issues and developmental directions for future research.Xiang Long Huang, Zaiping Guo, Shi Xue Dou, Zhiming M. Wan
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