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Register a new userInfluence of the multiband sign changing superconductivity on the vortex cores and on vortex pinning in the new stoichiometric high Tc CaKFe4As4
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We study the superconducting density of states and vortex lattice of single crystals of CaKFe$_4$As$_4$ using a scanning tunneling microscope (STM). This material has a critical temperature of $T_c= 35,$ K, which is one of the highest among stoichiometric iron based superconductors (FeBSC) and is comparable to $T_c$ found near optimal doping in other FeBSC. Using quasi-particle interference we identify the hole sheets around the zone center and find that two superconducting gaps open in these sheets. The scattering centers are small defects that can be localized in the surface topography and just produce quasiparticle interference, without suppressing the superconducting order parameter. This shows that sign inversion is not within hole bands, but between hole and the electron bands. Vortex core bound states show electron-hole asymmetric bound states due to proximity of the top of one of the hole bands to the Fermi level $E_F$. This places CaKFe$_4$As$_4$ in a similar situation as FeSe or related materials, with a superconducting gap $Delta$ just a few times smaller than $E_F$. On the other hand, we also identify locations showing strong suppression of the superconducting order parameter. Their size is of order of the vortex core size and vortices are pinned at these locations, leading to a disordered vortex lattice.
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The lossless current-carrying capacity of a superconductor is limited by its critical current density (Jc). A key to enhance Jc towards real-life applications is engineering defect structures to optimize the pinning landscape. For iron-based superconductors (IBSs) considered as candidate materials for high-field applications, high Jc values have been achieved by various techniques to introduce artificial pinning centres. Here we report extraordinary vortex pinning properties in CaKFe4As4 (CaK1144) arising from the inherent defect structure. Scanning transmission electron microscopy revealed the existence of nanoscale intergrowths of the CaFe2As2 phase, which is unique to CaK1144 formed as a line compound. The Jc properties in CaK1144 are found to be distinct from other IBSs characterized by a significant anisotropy with respect to the magnetic field orientation as well as a remarkable pinning mechanism significantly enhanced with increasing temperature. We propose a comprehensive explanation of the Jc properties based on the unique intergrowths acting as pinning centres.
We present a comprehensive study of the critical current densities and the superconducting vortex phase diagram in the stoichiometric superconductor CaKFe$_4$As$_4$ which has a critical temperature of 35 K. We performed detailed magnetization measurements both of high quality single crystals for different orientations in an applied magnetic field up to 16 T and for a powder sample. We find an extremely large critical current density, Jc, up to 10$^8$ A/cm2 for single crystals when H||(ab) at 5 K, which remains robust in fields up to 16 T, being the largest of any other iron-based superconductor. The critical current density is reduced by a factor 10 in single crystals when H||c at 5 K and significantly suppressed by the presence of grain boundaries in the powder sample. We also observe the presence of the fishtail effect in the magnetic hysteresis loops of single crystals when H||c. The flux pinning force density and the pinning parameters suggest that the large critical current could be linked to the existence of point core and surface pinning. Based on the vortex phase diagram and the large critical current densities, CaKFe$_4$As$_4$ is now established as a potential iron-based superconductor candidate for practical applications.
A scanning superconducting quantum interference device (SQUID) microscope (SSM) is used to study the magnetic imaging of dynamic motion of quantized interlayer vortices induced by the Lorentz force in anisotropic high-Tc La_(1.87)Sr_(0.13)CuO_4 single crystals. It is found that 3 modes of flux motion switch depending on the transport current. By increasing the current a transition from the creep-like behavior of vortices to a steady flow of vortices was observed. Even higher current induced a continuous expansion of vortex-flow area indicating an inhomogeneous distribution of various pinning centers.
The dependence of the vortex penetration and expulsion on the geometry of mesoscopic superconductors is reported. Hall magnetometry measurements were performed on a superconducting Al square and triangle. The stability of the vortex patterns imposed by the sample geometry is discussed. The field-temperature $H-T$ diagram has been reconstructed showing the transitions between states with different vorticity. We have found that the vortex penetration is only weakly affected by the vortex configuration inside the sample while the expulsion is strongly controlled by the stability of the vortex patterns. A qualitative explanation for this observation is given.
We report on far infrared measurements of interplane conductivity for underdoped single-crystal YBa2Cu3Oy in magnetic field and situate these new data within earlier work on two other high-Tc cuprate superconductors, La(2-x)SrxCuO4 and Bi2Sr2CaCu2O(8+d). The three systems have displayed apparently disparate electrodynamic responses in the Josephson vortex state formed when magnetic field H is applied parallel to the CuO2 planes. Specifically, there is discrepancy in the number and field dependence of longitudinal modes observed. We compare and contrast these findings with several models of the electrodynamics in the vortex state and suggest that most differences can be reconciled through considerations of the Josephson vortex lattice ground state as well as the c-axis and in-plane quasiparticle dissipations.
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