Specific heat is a powerful tool to investigate the physical properties of condensed materials.Superconducting state is achieved through the condensation of paired electrons,namely,the Cooper pairs.The condensed Coope...Specific heat is a powerful tool to investigate the physical properties of condensed materials.Superconducting state is achieved through the condensation of paired electrons,namely,the Cooper pairs.The condensed Cooper pairs have lower entropy compared with that of electrons in normal metal,thus specific heat is very useful in detecting the low lying quasiparticle excitations of the superconducting condensate and the pairing symmetry of the superconducting gap.In this brief overview,we will give an introduction to the specific heat investigation of the physical properties of superconductors.We show the data obtained in cuprate and iron based superconductors to reveal the pairing symmetry of the order parameter.展开更多
Iron based superconducting wires(IBSCs)produced by the Powder in Tube(PIT)method rely on the use of silver sheaths as chemical buffer between the outer metal and the superconducting core.The adoption of silver entails...Iron based superconducting wires(IBSCs)produced by the Powder in Tube(PIT)method rely on the use of silver sheaths as chemical buffer between the outer metal and the superconducting core.The adoption of silver entails however some limitations,such as the viable temperature range when coupled with copper,and the incompatibility with calcium-based IBSCs already at 600℃,driving the research towards other wires architecture.Taking inspiration from the low temperature superconductors field,we decided to evaluate the adoption of tantalum as diffusion barrier in a layered Cu/Ta architecture,choosing a Ca/K-1144 IBSC as case study considering the high reactivity issues already reported in the case of silver sheaths for this compound.Squared wires were produced through a groove rolling lamination process coupled with a thermal treatment at 800℃.The microstructural analyses show the absence of interdiffusion between the different parts of the wire,and the magnetic characterization shows performance in line with similar polycrystalline manufacts,with margin of enhancement to be pursued via the optimization of the mechanical process and other experimental variables.The reported results suggest thus the effectiveness of tantalum as diffusion barrier for Ca/K-1144 PIT wires.展开更多
Iron‐based superconductors(IBSCs)are a class of material under investigation for the development of superconducting wires in the low‐temperature‐high magnetic fields power application.Among the various families of ...Iron‐based superconductors(IBSCs)are a class of material under investigation for the development of superconducting wires in the low‐temperature‐high magnetic fields power application.Among the various families of IBSCs,the 1144 CaKFe_(4)As_(4) compound is a promising material able to achieve outstanding superconducting properties with a cheap and simple chemical composition.Oxidation,in these compounds,is considered an obstacle for high intergranular critical current density,J_(c,GB).A study devoted to the evaluation of oxidation phenomena and their effects on the superconducting properties is thus needed in order to fully understand the involved mechanisms.From the evaluation of polycrystalline samples obtained by a mechanochemically assisted synthesis route,a degradation of the critical temperature and critical currents has been observed concurrently with oxygen accumulation at grain boundaries in open porosities.However,the crystalline structure at an atomic level seems not affected,as well as intragranular superconducting properties assessed by means of calorimetric methods.These results suggest that loss of superconducting properties in Ca/K‐1144 compounds following oxidation is significantly associated with the worsening of grain connectivity.展开更多
We grew a series of NaFe_(1-x)V_(x) As(0 ≤ x ≤ 0.03) single crystals and performed the measurements of resistance,magnetic susceptibility, and specific heat to study the superconducting phase diagram by doping V int...We grew a series of NaFe_(1-x)V_(x) As(0 ≤ x ≤ 0.03) single crystals and performed the measurements of resistance,magnetic susceptibility, and specific heat to study the superconducting phase diagram by doping V into Na Fe As. Both the structural and the spin-density-wave(SDW) transitions are slightly suppressed by V-doping. While superconducting transition temperature is enhanced to the maximum value of 15 K when the optimal doping level x = 0.007 and then is suppressed rapidly with further V-doping, displaying a small superconducting dome. Our results suggest that V-impurities should act as strong magnetic scattering centers which cause the sharp suppression of superconductivity in NaFe_(1-x)V_(x) As.展开更多
The vortex pinning determining the current carrying capacity of a superconductor is an important property to the applications of superconducting materials.For layered superconductors,the vortex pinning can be enhanced...The vortex pinning determining the current carrying capacity of a superconductor is an important property to the applications of superconducting materials.For layered superconductors,the vortex pinning can be enhanced by a strong interlayer interaction in accompany with a suppression of superconducting anisotropy,which remains to be investigated in iron based superconductors(FeSCs)with the layered structure.Here,based on the transport and magnetic torque measurements,we experimentally investigate the vortex pinning in two bilayer FeSCs,CaKFe_(4)As_(4)(Fe1144)and KCa_(2)Fe_(4)As4F_(2)(Fe12442),and compare their superconducting anisotropyγ.While the anisotropyγ≈3 for Fe1144 is much smaller thanγ≈15 in Fe12442 around Tc,a higher flux pinning energy as evidenced by a higher critical current density is found in Fe1144,as compared with the case of Fe12442.In combination with the literature data of Ba_(0.72)K_(0.28)Fe2As_(2) and Nd Fe As_(O0.82)F_(0.18),we reveal an anti-correlation between the pinning energy and the superconducting anisotropy in these Fe SCs.Our results thus suggest that the interlayer interaction can not be neglected when considering the vortex pinning in Fe SCs.展开更多
The second magnetization peak(SMP)effect has been observed widely in many type‐Ⅱsuperconductors,but the reason remains elusive.This effect manifests an enhanced critical current density with magnetic field and shoul...The second magnetization peak(SMP)effect has been observed widely in many type‐Ⅱsuperconductors,but the reason remains elusive.This effect manifests an enhanced critical current density with magnetic field and should be very useful for applications.By measuring the magnetization of optimally doped Ba_(0.6)K_(0.4)Fe_(2)As_(2)single crystals with different thickness,it is found the SMP effect exists in thick samples and gradually becomes invisible when the sample thickness is reduced to the scale of micrometer.Detailed inves-tigation on the vortex dynamics on samples with different thickness clearly show that there is a common behav-ior of vortex dynamics in the low field region,which may be characterized by the Bragg glass like elastic vortex motion.This feature holds on in the whole field region for the thin samples,while it turns into the SMP effect for thicker samples when the field is increased.The results suggest that the SMP effect may be induced by the entanglement of the vortex system,and the absence of the SMP effect in thin samples is attributed to the cutoff of the entangled vortex length along c‐axis.展开更多
文摘Specific heat is a powerful tool to investigate the physical properties of condensed materials.Superconducting state is achieved through the condensation of paired electrons,namely,the Cooper pairs.The condensed Cooper pairs have lower entropy compared with that of electrons in normal metal,thus specific heat is very useful in detecting the low lying quasiparticle excitations of the superconducting condensate and the pairing symmetry of the superconducting gap.In this brief overview,we will give an introduction to the specific heat investigation of the physical properties of superconductors.We show the data obtained in cuprate and iron based superconductors to reveal the pairing symmetry of the order parameter.
基金funded by the European Union via the Euratom Research and Training Programme(Grant Agreement No 101052200-EUROfusion).
文摘Iron based superconducting wires(IBSCs)produced by the Powder in Tube(PIT)method rely on the use of silver sheaths as chemical buffer between the outer metal and the superconducting core.The adoption of silver entails however some limitations,such as the viable temperature range when coupled with copper,and the incompatibility with calcium-based IBSCs already at 600℃,driving the research towards other wires architecture.Taking inspiration from the low temperature superconductors field,we decided to evaluate the adoption of tantalum as diffusion barrier in a layered Cu/Ta architecture,choosing a Ca/K-1144 IBSC as case study considering the high reactivity issues already reported in the case of silver sheaths for this compound.Squared wires were produced through a groove rolling lamination process coupled with a thermal treatment at 800℃.The microstructural analyses show the absence of interdiffusion between the different parts of the wire,and the magnetic characterization shows performance in line with similar polycrystalline manufacts,with margin of enhancement to be pursued via the optimization of the mechanical process and other experimental variables.The reported results suggest thus the effectiveness of tantalum as diffusion barrier for Ca/K-1144 PIT wires.
基金Part of the work was carried out in the framework of the Cooperative Research And Development Agreement CRADA FRA‐2022‐0041Northwestern University’s NUANCE Center,which has received support from the SHyNE Resource(NSF ECCS‐2025633)the IIN,and Northwestern's MRSEC program(NSF DMR‐1720139).
文摘Iron‐based superconductors(IBSCs)are a class of material under investigation for the development of superconducting wires in the low‐temperature‐high magnetic fields power application.Among the various families of IBSCs,the 1144 CaKFe_(4)As_(4) compound is a promising material able to achieve outstanding superconducting properties with a cheap and simple chemical composition.Oxidation,in these compounds,is considered an obstacle for high intergranular critical current density,J_(c,GB).A study devoted to the evaluation of oxidation phenomena and their effects on the superconducting properties is thus needed in order to fully understand the involved mechanisms.From the evaluation of polycrystalline samples obtained by a mechanochemically assisted synthesis route,a degradation of the critical temperature and critical currents has been observed concurrently with oxygen accumulation at grain boundaries in open porosities.However,the crystalline structure at an atomic level seems not affected,as well as intragranular superconducting properties assessed by means of calorimetric methods.These results suggest that loss of superconducting properties in Ca/K‐1144 compounds following oxidation is significantly associated with the worsening of grain connectivity.
基金Project supported by the National Natural Science Foundation of Chinathe National Key Research and Development Program of China。
文摘We grew a series of NaFe_(1-x)V_(x) As(0 ≤ x ≤ 0.03) single crystals and performed the measurements of resistance,magnetic susceptibility, and specific heat to study the superconducting phase diagram by doping V into Na Fe As. Both the structural and the spin-density-wave(SDW) transitions are slightly suppressed by V-doping. While superconducting transition temperature is enhanced to the maximum value of 15 K when the optimal doping level x = 0.007 and then is suppressed rapidly with further V-doping, displaying a small superconducting dome. Our results suggest that V-impurities should act as strong magnetic scattering centers which cause the sharp suppression of superconductivity in NaFe_(1-x)V_(x) As.
基金Project supported by the National Natural Science Foundation of China(Grant No.11574338)the National Natural Science Foundation of China–China Academy of Engineering Physics NSAF Joint Fund(Grant No.U1530402)+4 种基金supported by the Superconducting Electronics Facility(SELF)of Shanghai Institute of Microsystem and Information Technology.The work at IOPCAS was supported by the National Key Research and Development Program of China(Grant No.2018YFA0704200)the National Natural Science Foundation of China(Grant Nos.11822411 and 11961160699)the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(CAS)(Grant No.XDB25000000)the Youth Innovation Promotion Association of CAS(Grant No.2016004)。
文摘The vortex pinning determining the current carrying capacity of a superconductor is an important property to the applications of superconducting materials.For layered superconductors,the vortex pinning can be enhanced by a strong interlayer interaction in accompany with a suppression of superconducting anisotropy,which remains to be investigated in iron based superconductors(FeSCs)with the layered structure.Here,based on the transport and magnetic torque measurements,we experimentally investigate the vortex pinning in two bilayer FeSCs,CaKFe_(4)As_(4)(Fe1144)and KCa_(2)Fe_(4)As4F_(2)(Fe12442),and compare their superconducting anisotropyγ.While the anisotropyγ≈3 for Fe1144 is much smaller thanγ≈15 in Fe12442 around Tc,a higher flux pinning energy as evidenced by a higher critical current density is found in Fe1144,as compared with the case of Fe12442.In combination with the literature data of Ba_(0.72)K_(0.28)Fe2As_(2) and Nd Fe As_(O0.82)F_(0.18),we reveal an anti-correlation between the pinning energy and the superconducting anisotropy in these Fe SCs.Our results thus suggest that the interlayer interaction can not be neglected when considering the vortex pinning in Fe SCs.
基金supported by the National Natural Science Foundation of China(Grants No.A0402/11927809,No.A0402/11534005)National Key R and D Program of China(Grant No.2022YFA1403201)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB25000000).
文摘The second magnetization peak(SMP)effect has been observed widely in many type‐Ⅱsuperconductors,but the reason remains elusive.This effect manifests an enhanced critical current density with magnetic field and should be very useful for applications.By measuring the magnetization of optimally doped Ba_(0.6)K_(0.4)Fe_(2)As_(2)single crystals with different thickness,it is found the SMP effect exists in thick samples and gradually becomes invisible when the sample thickness is reduced to the scale of micrometer.Detailed inves-tigation on the vortex dynamics on samples with different thickness clearly show that there is a common behav-ior of vortex dynamics in the low field region,which may be characterized by the Bragg glass like elastic vortex motion.This feature holds on in the whole field region for the thin samples,while it turns into the SMP effect for thicker samples when the field is increased.The results suggest that the SMP effect may be induced by the entanglement of the vortex system,and the absence of the SMP effect in thin samples is attributed to the cutoff of the entangled vortex length along c‐axis.
