In-situ Hydrostatic Pressure Effect On Flux Pinning And Vortex Dynamics In High Temperature Superconductors

High critical current density?Jc?and small magnetic relaxation are crucial for technological applications of high temperature superconductors.So far,improved Jc has been reported by the pinning of vortices via local structural inhomogeneities in superconductors,which are induced by chemical doping,irradiation,inclusion of non-superconducting secondary phases.It is commonly observed that doping with various kinds of dopants can induce superconductivity and/or increase the flux pinning.However,high doping levels often degrade the superconducting transition temperature?Tc?and Jc.The density of point defects can be significantly enriched by high energy ion treatment,which can lead to significant enhancement of Jc at high fields.The high energy dose,however,often causes Tc to decrease or disappear completely,which is also detrimental to Jc.In our previous work,we have proposed that hydrostatic pressure can enhance both Tc and Jc for various Fe-superconducting systems.Additionally,it has been reported for different superconducting systems that hydrostatic pressure may have other effects,such as 1)shrinking the unit cells and reducing the lattice parameters,leading to the reduction of anisotropy;2)improving grain connectivity,overcoming the grain boundary weak-link problem;3)introducing more pinning centers by increasing defects,causing a further increase in Jc.In this paper,due to their simple layered structure,isostructural FeSe and FeSe0.5Te0.5 are compounds that can provide clues to help us to understand the superconducting mechanism of the superconductors.we extend our study of the effects of hydrostatic pressure on superconductivity and flux pinning in FeSeo.sTeo.s?11 phase?single crystals with or without doping.In addition,we report the first comprehensive study on the significant effect of in-situ hydrostatic pressure on the vortex dynamics in Fei-xCoXSe0.5Te0.5 single crystals by magnetic relaxation.Using the collective creep theory and the extended Maley method,we found that vortex creep rates are significantly suppressed by the in-situ hydrostatic pressure.Most technological applications of high temperature superconductors,however,do not involve polycrystalline crystals or films,but rather so-called coated conductors?CCs?,which are films epitaxially grown on long-length highly-textured buffered metallic tapes.State of the art CCs have the highest critical current densities among superconducting materials and have been incorporated in a variety of in-magnetic-field applications,such as wires for electrical transport,magnets,fault current limiters,transformers,motors,generators,and flywheels.Nevertheless,little is known about the effects of in-situ hydrostatic pressure on these systems.So,we introduce the first systematic study of in-situ hydrostatic pressured induced improvement of critical current density and vortex pinning in state-of-the-art Y?Dy?Ba2Cu30O7-? coated conductors.And,the vortex dynamics are investigated through magnetic relaxation studies at different temperatures,fields,and pressures.Besides,we performed a systematic study of the hydrostatic pressure?HP?effect on the flux pinning and vortex dynamics of MgB2.The fundamental significance of the in-situ pressure induced significant enhancement of flux pinning or Jc is that there is still plenty of room to further improve the supercurrent carrying capability for both Fe-based superconductors and YBCO coated conductors.The following is the importance and systematic study for this work:1.We present our systematic investigations on the critical current density Jc,Hc2,Hirr,the activation energy U0,and the flux pinning mechanism in Fei-xCoxSe0.5Te0.5?x = 0,0.03 and 0.05?single crystals.Remarkably,we observe that the Jc and U0 are significantly enhanced by up to 12 times and 4 times for the 3at.%Co-doped sample,whereas,there is little change in Tc,irreversibility field?Hirr?,and upper critical field?Hc2?.Furthermore,charge-carrier mean free path fluctuation,???pinning,is responsible for the pinning mechanism in Fe1-xCoXSe0.5Teo.5.A comprehensive vortex phase diagram is constructed and analysed for the 3 at.%Co-doped sample.2.We performed a systematic study of the HP effect on the Tc,Jc,Hirr,Hc2,and flux pinning mechanism in un-doped and 3at.%Co-doped FeSe0.5Te0.5 crystals.We found that Tc is increased from 11.5 up to 17 K for HP = 0 to 1.2 GPa.Remarkably,the Jc is significantly enhanced by a factor of 3 to 100 for low and high temperature and field,and the Hi,line is shifted to higher fields by the HP up to 1.2 GPa.Based on the collective pinning model,the???pinning associated with charge-carrier mean free path fluctuation is responsible for the pinning mechanism of Fei-xCoxSe0.5Te0.5 samples with or without pressure.A comprehensive vortex phase diagram in the mixed state is constructed and analysed for the 3at.%Co-doped sample.3.To figure out whether or not pressure can induce extra point defects,we discuss fish effect in the sample with or without pressure.The peak or "fishtail" effect is caused by the matching of the spacing between defects with the vortex lattice spacing for a certain magnetic field?matching field?.We have calculated the intervortex distance d for the cases with or without pressure.The results show the second magnetization peak is present in the Co-doped samples and shifts to high field under pressure.The interyortex distance is known to be dependent on the applied magnetic field:d?-??0/B?0.5,we found that the d decreases from 30 nm?P = 0 GPa?to 15 nm?P = 1.2 GPa?at T = 6 K indicatingthat high pressure can indeed increase point defect density greatly.4.We report the first comprehensive study on the significant effect of in-situ hydrostatic pressure on the vortex dynamics in Fei-xCoxSe0.5Te0.5 single crystals.Using the collective creep theory and the extended Maley method,we found that vortex creep rates are significantly suppressed by the in-situ hydrostatic pressure.In addition,a crossover from elastic to plastic creep is observed.The hydrostatic pressure also induces vortex creep to move from the large bundle to the small bundle region in the vortex phase diagram.Our study indicates that in-situ hydrostatic pressure is very effective for not only significantly increasing the effective pinning energy and the critical current density,but also reducing the size of flux bundles to suppress the decrease in current density from vortex motion.5.We introduce the first systematic study of in-situ hydrostatic pressured induced improvement of critical current density and vortex pinning in state-of-the-art Y?Dy?Ba₂Cu₃O₇-? coated conductors.In addition,the vortex dynamics are investigated through magnetic relaxation studies at different temperatures and fields under the in-situ pressure.Our results show that in-situ hydrostatic pressure greatly enhances the critical current density at high fields and high temperatures.At 80 K and 5 T we observe a ten-fold increase in the critical current density under the pressure of 1.2 GPa,and the irreversibility line is shifted to higher fields without changing the critical temperature.The normalized magnetic relaxation rate shows that vortex creep rates are strongly suppressed due to applied pressure,and the pinning energy is significantly increased based on the collective creep theory.After releasing the pressure,we recover the original superconducting properties,therefore,we speculate that in-situ hydrostatic pressure exerted on the coated conductor enhances the pinning of existing extended-like defects.This is totally different from what has been observed in REBa₂Cu₃O₇-?melt-textured crystals,where the effect of pressure generates point-like defects.