Upper Critical Field And Flux Pinning Studies In NbN Ultra-thin Films And Iron Based Superconductors

Over the past years,meander nanowires based on ultra thin films show great potential in single photon detection.The superconducting nature of the ultra-thin films or nanowires could affect the device performance greatly.On the other side,high temperature superconductors have great importance due to high T_c and H_c2in high field superconducting magnet applications.A potential application of IBSs is for constructing high field magnets which could be used in future areas such as high energy particle accelerators and next generation fusion reactors.Thus,the study of He⁺ion irradiation effects is of great interest.This dissertation covers my research work on the fabrication and the superconducting properties of thin films and meander nanowire,which are summarized in three parts below:In the first part,High quality Nb N thin films of different thickness were fabricated and the temperature and angular dependence of the upper critical fields have been measured for different field orientations.For ultra-thin samples,we found that the upper critical field in parallel to surface orientation shows bending curvature close to T_c,also confirmed from angular dependence suggesting a two-dimensional?2D?nature of the samples.The temperature dependence could be described by a model based on anisotropic Ginzburg-Landau theory.Interestingly,the results measured in the field perpendicular to film surface orientation show that a similar bending curvature but in a much narrow temperature region close to T_c for ultra-thin samples.The bending curvature is expected to be explained by the suppression of pair-breaking due to local variation of T_c.For thick samples,we found that H_c2 shows maximum value for both parallel and perpendicular orientations.The H_c2 in perpendicular orientation is believed to be due to the columnar structures confirmed by transmission electron microscopy?TEM?formed during the growth of thick films.Further,we have rotated magnetic field in the basal plane and found anisotropic magneto-resistance.Polar plots of the magneto-resistance data show folding symmetry lead to 2D and 3D system.The resistive transitions could be described by the Berezinskii-Kosterlitz-Thouless?BKT?model for ultra thin bridge specimen.The tailed at low temperature in meander nanowire is fitted with phase slip model for quasi one dimensional nanowire.In addition we have also presented current voltage characteristics for given samples.In the second part,IBS thin films were grown,irradiated with He⁺ion and measured electric as well as magnetic properties in VSM installing on PPMS.Ba Fe_1.84Co_0.16As₂ thin films were grown by pulsed laser deposition?PLD?process from Institute of Electrical Engineering CAS.The effect of 600 ke V He⁺ion irradiation on the temperature and magnetic field dependence of the critical current density J_c is investigated.Upon irradiation,the T_c drops slightly from 23 K for the un-irradiated sample to about 20 K for the sample with the highest irradiation level.The results indicate that the irradiation of light element ions He⁺with relatively low energy could enhance the critical current density and the upper critical field remains unchanged.Moreover,the analysis of pinning force dependence on magnetic field shows that the pinning behavior is not changed in the irradiated samples,suggesting more pinning centers of similar nature to those of presented in the un-irradiated samples are introduced by the irradiation process.In the third part,magnetization properties of superconducting KCa₂Fe₄As₄F₂?12442?single crystals have been measured.We have observed the giant flux jump at low temperature 2 K in 12442 type crystals.The jump is large enough to transfer the whole superconducting to normal state.We have seen flux jumps till very high field with relatively decreasing amplitude.The state is stabilized during the magnetic history implied that no any jump occurs.Further the critical current density and flux pinning characteristics are investigated in single crystals.