494 research outputs found
Three dimensional spider-web-like superconducting filamentary paths in single crystals
Since the discovery of high temperature superconductivity in F-doped LaFeAsO,
many new iron based superconductors with different structures have been
fabricated2. The observation of superconductivity at about 32 K in KxFe2-ySe2
with the iso-structure of the FeAs-based 122 superconductors was a surprise and
immediately stimulated the interests because the band structure calculation8
predicted the absence of the hole pocket which was supposed to be necessary for
the theoretical picture of S+- pairing. Soon later, it was found that the
material may separate into the insulating antiferromagnetic K2Fe4Se5 phase and
the superconducting phase. It remains unresolved that how these two phases
coexist and what is the parent phase for superconductivity. In this study we
use different quenching processes to produce the target samples with distinct
microstructures, and apply multiple measuring techniques to reveal a close
relationship between the microstructures and the global appearance of
superconductivity. In addition, we clearly illustrate three dimensional
spider-web-like superconducting filamentary paths, and for the first time
propose that the superconducting phase may originate from a state with one
vacancy in every eight Fe-sites with the root8*root10 parallelogram structure.Comment: 22 pages, 7 figure
Direct Numerical Simulation of the Sedimentation of Solid Particles with Thermal Convection
Dispersed two-phase flows often involve interfacial activities such as chemical reaction and phase change, which couple the fluid dynamics with heat and mass transfer. As a step toward understanding such problems, we numerically simulate the sedimentation of solid bodies in a Newtonian fluid with convection heat transfer. The two-dimensional Navier–Stokes and energy equations are solved at moderate Reynolds numbers by a finite-element method, and the motion of solid particles is tracked using an arbitrary Lagrangian–Eulerian scheme. Results show that thermal convection may fundamentally change the way that particles move and interact. For a single particle settling in a channel, various Grashof-number regimes are identified, where the particle may settle straight down or migrate toward a wall or oscillate laterally. A pair of particles tend to separate if they are colder than the fluid and aggregate if they are hotter. These effects are analysed in terms of the competition between the thermal convection and the external flow relative to the particle. The mechanisms thus revealed have interesting implications for the formation of microstructures in interfacially active two-phase flows
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