Volume pinning forces were determined for a variety of bulk high-Tc superconductors of the 123-type from magnetization measurements. By means of scaling of the pinning forces, the acting pinning mechanisms in various ...Volume pinning forces were determined for a variety of bulk high-Tc superconductors of the 123-type from magnetization measurements. By means of scaling of the pinning forces, the acting pinning mechanisms in various temperature ranges were identified. The Nd-based superconductors and some YBCO crystals exhibited a dominating pinning of the δTc-type (i.e. , small, superconducting pinning sites). In contrast to this, the addition of insulating 211 particles provided pinning of the δ/-type; providing effective pinning in the entire temperature range acting as a 'background' pinning mechanism for the peak effect. Due to the small coherence lengths of the high-Tc compounds, effective pinning sites are defects or particles of nanometer size relative to ζ3. Integral magnetic measurements of the magnetization as a function of temperature in large applied magnetic fields (up to 7 T) revealed that practically all high-Tc compounds were spatially inhomogeneous, which could be caused by oxygen deficiency (YBCO), solid solutions of Nd/Ba (NdBCO and other light rare earth compounds), intergrowths (Bi-based superconductors), and doping by pair-breaking dopants like Zn, Pr. This implies that the superconducting sample consists of stronger and weaker superconducting areas, coupled together. In large applied fields, this coupling gets broken and the magnetization versus temperature curves revealed more than one superconducting transition. In contrast, irradiation experiments by neutrons, protons, and heavy-ions enabled the artificial introduction of very effective pinning sites into the high-Tc superconductors, thus creating a large variety of different observations using magnetic data. From all these observations, we construct a pinning diagram for bulk high-Tc superconductors explaining many features observed in high-Tc samples.展开更多
As is well known, the relatively low critical current density J_c and the serious flux motion limit a variety of applications of high-temperature superconductors (HTSCs). The introduction
In high Tc YBa2Cu3O7(Y123) superconductors, by substituting Y with some rare earth elements RE, a stress field surrounding these dopants will be generated due to the lattice mismatch. Based on the effective defect mod...In high Tc YBa2Cu3O7(Y123) superconductors, by substituting Y with some rare earth elements RE, a stress field surrounding these dopants will be generated due to the lattice mismatch. Based on the effective defect model, the interaction energy and the maximum elementary interaction force (/? between a fluxoid and a stress center are calculated. It is shown that the calculated values for fp can well interpret the data of the enhancement of measured critical current density j's for the substitution of Y with different rare earth elements. Furthermore, by substituting the Gd with Y in the GdBa2Cu3O7_s thin films, the measured critical current density (js) is increased, while the relaxation rate (S) is clearly lowered, which can be attributed to the presence of extra pinning centers. The temperature dependence of the true critical current density jc(T) and the pinning potential UC(T) were determined from the experiment data for both the pure and the substituted thin films. It is found that the experiment results for the pure film are close to the predictions of -pinning model, while those for the substituted thin film follow the theoretical expressions derived from the single vortex collective pinning model with the small size stress-field pinning centers, which strongly supports the idea that the enhancement of j, by the substitution resulted from the stress induced flux pinning rather than the magnetic pinning.展开更多
Defect control at nanoscale of MgB2 by doping various nanoparticles including Ti, C, nano-diamond, and HOB4, and their roles played to enhance flux pinning force in MgB2 are compared and analyzed. These nanodopants ha...Defect control at nanoscale of MgB2 by doping various nanoparticles including Ti, C, nano-diamond, and HOB4, and their roles played to enhance flux pinning force in MgB2 are compared and analyzed. These nanodopants have different chemical and physical properties, thus bring about different pinning efficiency, especially nanodopants with strong magnetic moment are particularly interesting as pinning centers in MgB2 since magnetic impurities usually have a stronger interaction with magnetic flux line than nonmagnetic impurities and may exert a stronger force to trap the flux lines when they are properly introduced into the superconducting matrix.展开更多
Rare earth(RE) elements and their compounds have been doped into MgB_2 since the discovery of the superconductivity of MgB_2. The doping effects of RE elements and RE compounds are summarized in terms of different dop...Rare earth(RE) elements and their compounds have been doped into MgB_2 since the discovery of the superconductivity of MgB_2. The doping effects of RE elements and RE compounds are summarized in terms of different doping methods and the accompanying variations in the superconducting transition temperature, upper critical field, irreversibility field, and critical current density(Jc). Conclusions are drawn on the doping mechanisms to demonstrate the possibility of improvement of Jcin the future. REBy phase inclusions are observed in MgB_2 as nano-pinning centers with significant enhancement of Jcin MgB_2 doped with Y, La, Dy, Ho, Nd, etc. Furthermore, some REs with larger magnetic moments will provide more efficient flux pinning centers with larger pinning forces, which is different with the doping effects and mechanism of ferromagnetic transition metals in MgB_2. The co-doping effects of RE with other dopants are also reviewed due to the multiple advantages of different doping mechanisms.展开更多
YBa2Cu3O7-x（YBCO） films with co-doping BaTiO3（BTO） and Y2O3 nanostructures were prepared by metal organic deposition using trifluoroacetates（TFA-MOD）. The properties of the BTO/Y2O3co-doped YBCO films with diffe...YBa2Cu3O7-x（YBCO） films with co-doping BaTiO3（BTO） and Y2O3 nanostructures were prepared by metal organic deposition using trifluoroacetates（TFA-MOD）. The properties of the BTO/Y2O3co-doped YBCO films with different excess yttrium have been systematically studied by x-ray diffraction（XRD）, Raman spectra, and scanning electron microscope（SEM）. The optimized content of yttrium excess in the BTO/Y2O3co-doped YBCO films is 10 mol.%, and the critical current density is as high as - 17 mA/cm^2（self-field, 65 K） by the magnetic signal. In addition, the Y2Cu2O5 was formed when the content of yttrium excess increases to 24 mol.%, which may result in the deterioration of the superconducting properties and the microstructure. The unique combination of the different types of nanostructures of BTO and Y2O3 in the doped YBCO films, compared with the pure YBCO films and BTO doped YBCO films, enhances the critical current density（JC） not only at the self-magnetic field, but also in the applied magnetic field.展开更多
High quality GdBa2Cu307-y （Gd123） textured bulks with Nd2BaCuO5 （Nd211） nanoparticle precipitations have been fabricated by a nanoparticle-powders-assisted MTG （melt-textured-growth） technique. The high density ...High quality GdBa2Cu307-y （Gd123） textured bulks with Nd2BaCuO5 （Nd211） nanoparticle precipitations have been fabricated by a nanoparticle-powders-assisted MTG （melt-textured-growth） technique. The high density nanoscale flux pinning sites were introduced into Gd123 by mixing Nd211 nanoparticle powders （about 20-50 nm） with Gd123 nano-precursors before the MTG process. Microstructural analyses reveal that a large number of Nd211 nanoparticles with a size around 50-150 nm were inserted in the Gd123 matrix, forming a kind of superconducting nanocomposites. The critical current density at 77 K is systematically increased and the flux pinning behavior is significantly improved. The scaling behavior of the flux pinning force shows a magnetic field dependent feature with a peak located at hp≈0.4. This may be the fingerprint of melt-textured 123 compounds, which cannot be interpreted by the simple superposition of different types of elementary pinning sources.展开更多
Binary doping effect is studied for the Fe-based superconductors of Nd1-xBaxFeAsO1-2xF2x(x = 0.02, 0.05, 0.1, 0.15, 0.2). The X-ray diffractions show that the c-axis lattice constant decreases monotonously with the do...Binary doping effect is studied for the Fe-based superconductors of Nd1-xBaxFeAsO1-2xF2x(x = 0.02, 0.05, 0.1, 0.15, 0.2). The X-ray diffractions show that the c-axis lattice constant decreases monotonously with the doping content, in contrast to the little change in the a-axis. Temperature dependences of electric resistivity and magnetic susceptibility reveal that the superconductivity for the studied system emerges at x = 0.1, and enhances together with Hc2(0) as the doping content x increases further. In case of x = 0.2, the superconducting critical temperature reaches as high as 50 K, which is the first demonstration of superconductivity with a high fluorine-doping induced by both electron and hole doping in this family. Negative Hall coefficient (RH) indicates that electron-type carriers are dominated in the present samples. The complicated temperature dependence of RH, is believed to arise from a multiband effect together with a complicated scattering, especially at the tem-perature near the TC.展开更多
文摘Volume pinning forces were determined for a variety of bulk high-Tc superconductors of the 123-type from magnetization measurements. By means of scaling of the pinning forces, the acting pinning mechanisms in various temperature ranges were identified. The Nd-based superconductors and some YBCO crystals exhibited a dominating pinning of the δTc-type (i.e. , small, superconducting pinning sites). In contrast to this, the addition of insulating 211 particles provided pinning of the δ/-type; providing effective pinning in the entire temperature range acting as a 'background' pinning mechanism for the peak effect. Due to the small coherence lengths of the high-Tc compounds, effective pinning sites are defects or particles of nanometer size relative to ζ3. Integral magnetic measurements of the magnetization as a function of temperature in large applied magnetic fields (up to 7 T) revealed that practically all high-Tc compounds were spatially inhomogeneous, which could be caused by oxygen deficiency (YBCO), solid solutions of Nd/Ba (NdBCO and other light rare earth compounds), intergrowths (Bi-based superconductors), and doping by pair-breaking dopants like Zn, Pr. This implies that the superconducting sample consists of stronger and weaker superconducting areas, coupled together. In large applied fields, this coupling gets broken and the magnetization versus temperature curves revealed more than one superconducting transition. In contrast, irradiation experiments by neutrons, protons, and heavy-ions enabled the artificial introduction of very effective pinning sites into the high-Tc superconductors, thus creating a large variety of different observations using magnetic data. From all these observations, we construct a pinning diagram for bulk high-Tc superconductors explaining many features observed in high-Tc samples.
文摘As is well known, the relatively low critical current density J_c and the serious flux motion limit a variety of applications of high-temperature superconductors (HTSCs). The introduction
基金Project supported by the Education Foundation of the Chinese National Education Commission and the National Center for R and D on Superconductivity.
文摘In high Tc YBa2Cu3O7(Y123) superconductors, by substituting Y with some rare earth elements RE, a stress field surrounding these dopants will be generated due to the lattice mismatch. Based on the effective defect model, the interaction energy and the maximum elementary interaction force (/? between a fluxoid and a stress center are calculated. It is shown that the calculated values for fp can well interpret the data of the enhancement of measured critical current density j's for the substitution of Y with different rare earth elements. Furthermore, by substituting the Gd with Y in the GdBa2Cu3O7_s thin films, the measured critical current density (js) is increased, while the relaxation rate (S) is clearly lowered, which can be attributed to the presence of extra pinning centers. The temperature dependence of the true critical current density jc(T) and the pinning potential UC(T) were determined from the experiment data for both the pure and the substituted thin films. It is found that the experiment results for the pure film are close to the predictions of -pinning model, while those for the substituted thin film follow the theoretical expressions derived from the single vortex collective pinning model with the small size stress-field pinning centers, which strongly supports the idea that the enhancement of j, by the substitution resulted from the stress induced flux pinning rather than the magnetic pinning.
基金supported Australian Research Council (Nos. DP0559872 and DP0881739)
文摘Defect control at nanoscale of MgB2 by doping various nanoparticles including Ti, C, nano-diamond, and HOB4, and their roles played to enhance flux pinning force in MgB2 are compared and analyzed. These nanodopants have different chemical and physical properties, thus bring about different pinning efficiency, especially nanodopants with strong magnetic moment are particularly interesting as pinning centers in MgB2 since magnetic impurities usually have a stronger interaction with magnetic flux line than nonmagnetic impurities and may exert a stronger force to trap the flux lines when they are properly introduced into the superconducting matrix.
基金Project supported by National Natural Science Foundation of China(51572166)the Shanghai Key Laboratory of High Temperature Superconductors(14DZ2260700)the Program for Professors with Special Appointments at Shanghai Institutions of Higher Learning(Eastern Scholar:TP2014041)
文摘Rare earth(RE) elements and their compounds have been doped into MgB_2 since the discovery of the superconductivity of MgB_2. The doping effects of RE elements and RE compounds are summarized in terms of different doping methods and the accompanying variations in the superconducting transition temperature, upper critical field, irreversibility field, and critical current density(Jc). Conclusions are drawn on the doping mechanisms to demonstrate the possibility of improvement of Jcin the future. REBy phase inclusions are observed in MgB_2 as nano-pinning centers with significant enhancement of Jcin MgB_2 doped with Y, La, Dy, Ho, Nd, etc. Furthermore, some REs with larger magnetic moments will provide more efficient flux pinning centers with larger pinning forces, which is different with the doping effects and mechanism of ferromagnetic transition metals in MgB_2. The co-doping effects of RE with other dopants are also reviewed due to the multiple advantages of different doping mechanisms.
基金Project supported by the National Natural Science Foundation of China(Grant No.51272250)the National Basic Research Program of China(Grant No.2011CBA00105)+1 种基金the National High Technology Research and Development Program of China(Grant No.2014AA032702)the Natural Science Foundation of Beijing,China(Grant No.2152035)
文摘YBa2Cu3O7-x（YBCO） films with co-doping BaTiO3（BTO） and Y2O3 nanostructures were prepared by metal organic deposition using trifluoroacetates（TFA-MOD）. The properties of the BTO/Y2O3co-doped YBCO films with different excess yttrium have been systematically studied by x-ray diffraction（XRD）, Raman spectra, and scanning electron microscope（SEM）. The optimized content of yttrium excess in the BTO/Y2O3co-doped YBCO films is 10 mol.%, and the critical current density is as high as - 17 mA/cm^2（self-field, 65 K） by the magnetic signal. In addition, the Y2Cu2O5 was formed when the content of yttrium excess increases to 24 mol.%, which may result in the deterioration of the superconducting properties and the microstructure. The unique combination of the different types of nanostructures of BTO and Y2O3 in the doped YBCO films, compared with the pure YBCO films and BTO doped YBCO films, enhances the critical current density（JC） not only at the self-magnetic field, but also in the applied magnetic field.
基金supported in part by the National Natural Science Foundation of China (No. 50872116)the Fundamental Research Funds for the Central Universities of China (No. SWJTU09ZT24)+1 种基金the PCSIRT of the Ministry of Education of China (No.IRT0751)the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 200806130023)
文摘High quality GdBa2Cu307-y （Gd123） textured bulks with Nd2BaCuO5 （Nd211） nanoparticle precipitations have been fabricated by a nanoparticle-powders-assisted MTG （melt-textured-growth） technique. The high density nanoscale flux pinning sites were introduced into Gd123 by mixing Nd211 nanoparticle powders （about 20-50 nm） with Gd123 nano-precursors before the MTG process. Microstructural analyses reveal that a large number of Nd211 nanoparticles with a size around 50-150 nm were inserted in the Gd123 matrix, forming a kind of superconducting nanocomposites. The critical current density at 77 K is systematically increased and the flux pinning behavior is significantly improved. The scaling behavior of the flux pinning force shows a magnetic field dependent feature with a peak located at hp≈0.4. This may be the fingerprint of melt-textured 123 compounds, which cannot be interpreted by the simple superposition of different types of elementary pinning sources.
文摘Binary doping effect is studied for the Fe-based superconductors of Nd1-xBaxFeAsO1-2xF2x(x = 0.02, 0.05, 0.1, 0.15, 0.2). The X-ray diffractions show that the c-axis lattice constant decreases monotonously with the doping content, in contrast to the little change in the a-axis. Temperature dependences of electric resistivity and magnetic susceptibility reveal that the superconductivity for the studied system emerges at x = 0.1, and enhances together with Hc2(0) as the doping content x increases further. In case of x = 0.2, the superconducting critical temperature reaches as high as 50 K, which is the first demonstration of superconductivity with a high fluorine-doping induced by both electron and hole doping in this family. Negative Hall coefficient (RH) indicates that electron-type carriers are dominated in the present samples. The complicated temperature dependence of RH, is believed to arise from a multiband effect together with a complicated scattering, especially at the tem-perature near the TC.
