Guided by the belief that Fermi energy EF (equivalently, chemical potential μ) plays a pivotal?role in determining the properties of superconductors (SCs), we have recently derived μ-incorporated Generalized-Bardeen...Guided by the belief that Fermi energy EF (equivalently, chemical potential μ) plays a pivotal?role in determining the properties of superconductors (SCs), we have recently derived μ-incorporated Generalized-Bardeen-Cooper-Schrieffer?equations (GBCSEs) for the gaps (Δs) and critical temperatures (Tcs) of both elemental and composite SCs. The μ-dependent interaction parameters consistent with the values of Δs and Tcs of any of these SCs were shown to lead to expressions for the effective mass of electrons (m*) and their number density (ns), critical velocity (v0), and the critical current density j0 at T = 0 in terms of the following five parameters: Debye temperature, EF, a dimensionless construct y, the specific heat constant, and the gram-atomic volume. We could then fix the value of μ in any SC by appealing to the experimental value of its j0 and calculate the other parameters. This approach was followed for a variety of SCs—elemental, MgB2 and cuprates and, with a more accurate equation to determine y, for Nitrogen Nitride (NbN). Employing the framework given for NbN, we present here a detailed study of Ba0.6K0.4Fe2As2 (BaAs). Some of the main attributes of this SC are: it is characterized by?-wave superconductivity and multiple gaps between 0?-?12 meV;its Tc ~?37 K, but the maximum Tc of SCs in its class can exceed 50 K;EF/kTc = 4.4 (k = Boltzmann constant), and its Tc plotted against a tuning variable has a dome-like structure. After drawing attention to the fact that the?-wave is an inbuilt feature of GBCSEs, we give a quantitative account of its several other features, which include the values of m*, ns, vo, and?coherence length. Finally, we also deal with the issue of the stage BaAs occupies in the BCS-Bose-Einstein Condensation crossover.展开更多
利用熔盐法制备了Ba0.6-y La y K0.4Bi O3和Ba0.6-z Pr z K0.4Bi O32个系列的样品,其中La和Pr的掺杂量y和z分别为0,0.025,0.05,0.1,0.2,0.3,0.4。X射线粉末衍射结果显示,Pr掺杂使得BKBO晶体结构由最初的立方晶系转变成为正交晶系,但是L...利用熔盐法制备了Ba0.6-y La y K0.4Bi O3和Ba0.6-z Pr z K0.4Bi O32个系列的样品,其中La和Pr的掺杂量y和z分别为0,0.025,0.05,0.1,0.2,0.3,0.4。X射线粉末衍射结果显示,Pr掺杂使得BKBO晶体结构由最初的立方晶系转变成为正交晶系,但是La掺杂样品却没有发生此种晶体结构的变化。La掺杂样品Ba0.6-y La y K0.4Bi O3的准晶格参数a p随掺杂量的增大而线性减小,其变化遵循a p=4.28257–0.02469y(0.025≤y≤0.4)的规律。样品磁性测量结果表明,掺杂样品的超导转变温度T c均随着掺杂量的增大而系统性地减小,并且当掺杂量达到0.2时,2个系统中的超导转变均消失。认为掺杂引起的晶体结构扭曲及铋化合价的不均衡导致了BKBO掺杂系统超导电性的变化。展开更多
We investigate the vortex dynamics in two typical hole doped iron based superconductors CaKFe_4As_4(CaK1144) and Ba_(0.6)K_(0.4)Fe_2As_2(BaK122) with similar superconducting transition temperatures. It is found that t...We investigate the vortex dynamics in two typical hole doped iron based superconductors CaKFe_4As_4(CaK1144) and Ba_(0.6)K_(0.4)Fe_2As_2(BaK122) with similar superconducting transition temperatures. It is found that the magnetization hysteresis loop exhibits a clear second peak effect in BaK122 in wide temperature region while it is absent in CaK1144. However, a second peak effect of critical current density versus temperature is observed in CaK1144, which is however absent in BaK122. The different behaviors of second peak effect in BaK122 and CaK1144 may suggest distinct origins of vortex pinning in different systems. Magnetization and its relaxation have also been measured by using dynamical and conventional relaxation methods for both systems. Analysis and comparison of the two distinct systems show that the vortex pinning is stronger and the critical current density is higher in BaK122 system. It is found that the Maley's method can be used and thus the activation energy can be determined in BaK122 by using the time dependent magnetization in wide temperature region, but this is not applicable in CaK1144 system.Finally we present the different regimes with distinct vortex dynamics in the field-temperature diagram for the two systems.展开更多
In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional supercon...In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional superconductors like niobium or lead, the normal state is a Fermi liquid with a well-defined Fermi surface and well-defined quasipartcles along the Fermi surface. Superconductivity is realized in this case by the Fermi surface instability in the superconducting state and the formation and condensation of the electron pairs(Cooper pairing). The high temperature cuprate superconductors, on the other hand, represent another extreme case that superconductivity can be realized in the underdoped region where there is neither well-defined Fermi surface due to the pseudogap formation nor quasiparticles near the antinodal regions in the normal state. Here we report a novel scenario that superconductivity is realized in a system with well-defined Fermi surface but without quasiparticles along the Fermi surface in the normal state.High resolution laser-based angle-resolved photoemission measurements have been performed on an optimally-doped iron-based superconductor(Ba_(0.6)K_(0.4))Fe_2As_2. We find that, while sharp superconducting coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets, no quasiparticle peak is present in the normal state. Its electronic behaviours deviate strongly from a Fermi liquid system. The superconducting gap of such a system exhibits an unusual temperature dependence that it is nearly a constant in the superconducting state and abruptly closes at Tc. These observations have provided a new platform to study unconventional superconductivity in a non-Fermi liquid system.展开更多
The superconducting properties of polycrystalline Sr0.6K0.4Fe2As2 were strongly influenced by Ag doping(Supercond.Sci.Technol.23(2010) 025027).Ag addition is mainly dominated by silver diffusing,so the annealing proce...The superconducting properties of polycrystalline Sr0.6K0.4Fe2As2 were strongly influenced by Ag doping(Supercond.Sci.Technol.23(2010) 025027).Ag addition is mainly dominated by silver diffusing,so the annealing process is one of the essential factors to achieve high quality Ag doped Sr0.6K0.4Fe2As2.In this paper,the optimal annealing conditions were studied for Ag doped Sr0.6K0.4Fe2As2 bulks prepared by a one-step solid reaction method.It is found that the annealing temperature has a strong influence on the superconducting properties,especially on the critical current density Jc.As a result,higher heat treatment temperature(~900℃) is helpful in diffusing Ag and reducing the impurity phase gathered together to improve the grain connectivity.In contrast,low-temperature sintering is counterproductive for Ag doped samples.These results clearly suggest that annealing at ~900℃ is necessary for obtaining high Jc Ag-doped samples.展开更多
文摘Guided by the belief that Fermi energy EF (equivalently, chemical potential μ) plays a pivotal?role in determining the properties of superconductors (SCs), we have recently derived μ-incorporated Generalized-Bardeen-Cooper-Schrieffer?equations (GBCSEs) for the gaps (Δs) and critical temperatures (Tcs) of both elemental and composite SCs. The μ-dependent interaction parameters consistent with the values of Δs and Tcs of any of these SCs were shown to lead to expressions for the effective mass of electrons (m*) and their number density (ns), critical velocity (v0), and the critical current density j0 at T = 0 in terms of the following five parameters: Debye temperature, EF, a dimensionless construct y, the specific heat constant, and the gram-atomic volume. We could then fix the value of μ in any SC by appealing to the experimental value of its j0 and calculate the other parameters. This approach was followed for a variety of SCs—elemental, MgB2 and cuprates and, with a more accurate equation to determine y, for Nitrogen Nitride (NbN). Employing the framework given for NbN, we present here a detailed study of Ba0.6K0.4Fe2As2 (BaAs). Some of the main attributes of this SC are: it is characterized by?-wave superconductivity and multiple gaps between 0?-?12 meV;its Tc ~?37 K, but the maximum Tc of SCs in its class can exceed 50 K;EF/kTc = 4.4 (k = Boltzmann constant), and its Tc plotted against a tuning variable has a dome-like structure. After drawing attention to the fact that the?-wave is an inbuilt feature of GBCSEs, we give a quantitative account of its several other features, which include the values of m*, ns, vo, and?coherence length. Finally, we also deal with the issue of the stage BaAs occupies in the BCS-Bose-Einstein Condensation crossover.
基金National Natural Science Foundation of China(50872116,11004162,11104224)Research Fund for the Doctoral Program of Higher Education of China(20110184120029)+2 种基金Fundamental Research Funds for the Central Universities(SWJTU11BR063,2682013CX004,2682013ZT16,SWJTU11ZT31)National Magnetic Confinement Fusion Science Program(2011GB112001)Program of International S&T Cooperation(2013DFA51050)
文摘利用熔盐法制备了Ba0.6-y La y K0.4Bi O3和Ba0.6-z Pr z K0.4Bi O32个系列的样品,其中La和Pr的掺杂量y和z分别为0,0.025,0.05,0.1,0.2,0.3,0.4。X射线粉末衍射结果显示,Pr掺杂使得BKBO晶体结构由最初的立方晶系转变成为正交晶系,但是La掺杂样品却没有发生此种晶体结构的变化。La掺杂样品Ba0.6-y La y K0.4Bi O3的准晶格参数a p随掺杂量的增大而线性减小,其变化遵循a p=4.28257–0.02469y(0.025≤y≤0.4)的规律。样品磁性测量结果表明,掺杂样品的超导转变温度T c均随着掺杂量的增大而系统性地减小,并且当掺杂量达到0.2时,2个系统中的超导转变均消失。认为掺杂引起的晶体结构扭曲及铋化合价的不均衡导致了BKBO掺杂系统超导电性的变化。
基金supported by the National Key Research and Development Program of China(2016YFA0300401)the National Natural Science Foundation of China(A0402/11534005)
文摘We investigate the vortex dynamics in two typical hole doped iron based superconductors CaKFe_4As_4(CaK1144) and Ba_(0.6)K_(0.4)Fe_2As_2(BaK122) with similar superconducting transition temperatures. It is found that the magnetization hysteresis loop exhibits a clear second peak effect in BaK122 in wide temperature region while it is absent in CaK1144. However, a second peak effect of critical current density versus temperature is observed in CaK1144, which is however absent in BaK122. The different behaviors of second peak effect in BaK122 and CaK1144 may suggest distinct origins of vortex pinning in different systems. Magnetization and its relaxation have also been measured by using dynamical and conventional relaxation methods for both systems. Analysis and comparison of the two distinct systems show that the vortex pinning is stronger and the critical current density is higher in BaK122 system. It is found that the Maley's method can be used and thus the activation energy can be determined in BaK122 by using the time dependent magnetization in wide temperature region, but this is not applicable in CaK1144 system.Finally we present the different regimes with distinct vortex dynamics in the field-temperature diagram for the two systems.
基金supported by the National Key Research and Development Program of China (2016YFA0300300 and 2017YFA0302900)the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB07020300 and XDB25000000)+1 种基金the National Basic Research Program of China (2015CB921000), the National Natural Science Foundation of China (11334010)and the Youth Innovation Promotion Association of CAS (2017013)
文摘In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional superconductors like niobium or lead, the normal state is a Fermi liquid with a well-defined Fermi surface and well-defined quasipartcles along the Fermi surface. Superconductivity is realized in this case by the Fermi surface instability in the superconducting state and the formation and condensation of the electron pairs(Cooper pairing). The high temperature cuprate superconductors, on the other hand, represent another extreme case that superconductivity can be realized in the underdoped region where there is neither well-defined Fermi surface due to the pseudogap formation nor quasiparticles near the antinodal regions in the normal state. Here we report a novel scenario that superconductivity is realized in a system with well-defined Fermi surface but without quasiparticles along the Fermi surface in the normal state.High resolution laser-based angle-resolved photoemission measurements have been performed on an optimally-doped iron-based superconductor(Ba_(0.6)K_(0.4))Fe_2As_2. We find that, while sharp superconducting coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets, no quasiparticle peak is present in the normal state. Its electronic behaviours deviate strongly from a Fermi liquid system. The superconducting gap of such a system exhibits an unusual temperature dependence that it is nearly a constant in the superconducting state and abruptly closes at Tc. These observations have provided a new platform to study unconventional superconductivity in a non-Fermi liquid system.
基金supported by the Beijing Municipal Science and Technology Commission (Grant No. Z09010300820907)the National Natural Science Foundation of China (Grant No. 50802093)the National Basic Research Program of China (Grant No. 2006CB601004)
文摘The superconducting properties of polycrystalline Sr0.6K0.4Fe2As2 were strongly influenced by Ag doping(Supercond.Sci.Technol.23(2010) 025027).Ag addition is mainly dominated by silver diffusing,so the annealing process is one of the essential factors to achieve high quality Ag doped Sr0.6K0.4Fe2As2.In this paper,the optimal annealing conditions were studied for Ag doped Sr0.6K0.4Fe2As2 bulks prepared by a one-step solid reaction method.It is found that the annealing temperature has a strong influence on the superconducting properties,especially on the critical current density Jc.As a result,higher heat treatment temperature(~900℃) is helpful in diffusing Ag and reducing the impurity phase gathered together to improve the grain connectivity.In contrast,low-temperature sintering is counterproductive for Ag doped samples.These results clearly suggest that annealing at ~900℃ is necessary for obtaining high Jc Ag-doped samples.
