جستجوی مقالات مرتبط با کلیدواژه "superconductivity" در نشریات گروه "فیزیک"

In the context of bottom-up holography, we demonstrate the power of mixed boundary conditions to promote the boundary gauge field to be dynamical. We provide two concrete applications of this idea. First, we consider a holographic dual for a strongly coupled plasma described by dissipative magnetohydrodynamics. Second, we reveal the expected features of the Higgs mechanism in a not counterfeit holographic superconductor.

In this study, a set of self-consistent coupled-integral equations for the local magnetic flux and current distributions in a finite superconducting hollow cylinder under an axial magnetic field has been directly derived by using the Biot-Savart law within the framework of the critical-state model. The equations were first solved numerically in the three-dimensional space before obtaining the hysteresis loops for the Kim and Exponential models. We haveassumed the contribution of the flux penetration from the inner surface of the sample to be higher than that of other surfaces. It is found that the variation in the area of lateral surface changes the magnitude of the magnetic moment of the finite hollow cylinder in the applied magnetic field. The obtained results are in good agreement with calculates. The formalism presented here can be used for an arbitrary shape of the superconducting system in the presence of any magnetic field dependence of the critical current density
 in an external magnetic field of arbitrary direction.

In this research, we aim to clarify the relationship between the structural distortion due to doping and the superconductivity existence in the FeAs4 structure. For this, we have prepared polycrystalline of NdFeAsO0.8F0.2, NdFeAs0.95Sb0.05O0.8F0.2 and Nd0.99Ca0.01FeAsO0.8F0.2 samples by one-step solid state reaction method. The structural and electrical properties of the samples were characterized through the X-ray diffraction pattern and the 4-probe method, respectively. The XRD patterns, refined using the MAUD software and Rietveld’s method, have indicated formation of the tetragonal structure with the space group P4/nmm:2. The variation of the lattice structuralparameters for the NdFeAsO0.8F0.2 doping with Sb and Ca ions were obtained by the Rietveld refinement. The changes of bond lengths and the ⍺, β bond angles have been specified from the corresponding value of ⍺ and β regular FeAs4-tetrahedron with the Sb/As and Ca/Nd doping, where they affect the superconductivity existence. The α and β bond angles increase and decrease with the substitution of Sb/As and Ca/Nd doping, which leads to thecontraction of the FeAs layer, and therefore the lattice parameter “c” decreases. The superconducting transition temperature was reduced from 56 K for NdFeAsO0.8F0.2 sample to 48 K and 46 K for Ca/Nd and Sb/As doping, respectively. It can be concluded from the structural and electrical properties that the superconducting transition temperature decreases with increasing the distortion of FeAs4-tetrahedron from the regular one. So, there is a relation between the structural properties and the superconductivity in the iron based superconductors-1111 regardless of the doping procedures.