[en] Epitaxial films of LiTi₂O₄ on single crystalline substrates of MgAl₂O₄, MgO, and SrTiO₃ provide model systems to systematically explore the effects of lattice strain and microstructural disorder on the superconducting state. Lattice strain that affects bandwidth gives rise to variations in the superconducting and normal state properties. Microstructural disorder, such as antiphase boundaries that give rise to Ti network disorder, reduces the critical temperature, and Ti network disorder combined with Mg interdiffusion lead to a much more dramatic effect on the superconducting state. Surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness

[en] Highlights: • We study the magnetic impurity effects in iron-based superconductors. • We study the quasiparticle interference effect in two impurities system. • The magnetic scattering play a primary role in the appearance of the C₂ symmetry. - Abstract: We consider the effect of magnetic impurities, modeled by classical spins, in iron-based superconductors. By using a self-consistent Bogoliubov–de Gennes formalism, we show that one magnetic impurity scattering exhibits pronounced two fold symmetry (C₂) in iron-based superconductors. We also study the effects of quantum interference in impurity structures consisting of two magnetic impurities. We show that the quantum interference leads to characteristic signatures of twofold symmetry not only in the local density of states (LDOS), but also in the spatial form of the superconducting order parameter. We suggest that these results deserve more attention in the analysis of C₂ symmetry found in scanning tunneling microscopy experiments for different iron-based superconductors

[en] Highlights: • Concave triangle-like relation is shown between T_c/N(atom) and electronegativity. • Maximum T_c (or T_c/N(atom)) is obtained at around the threshold electronegativity. • T_c value can be empirically estimated by the linear equation of N(ν)r(eff)³. • Enhancement of sp²d²-hybridization is the important factor to attain higher T_c value. - Abstract: It is tried to empirically understand the superconducting critical temperature T_c of various materials (24 elements, 286 A_nB (n = 1, 2, 3) compounds, 34 Fe- and 49 Cu-based compounds) by the effective pseudopotential radius, r(eff), and the effective orbital electronegativity, χ(eff)(=[N(v)/r(eff)]^1/2). By giving the sets of values of r(eff) and the number of effective electron, N(v), for 65 elements under the assumption that both the hybridization state and N(v) can be assigned to 65 elements in advance by considering their electronic characters, the T_c/N(atom)–χ(eff) and T_c–N(v)r(eff)³ relations are examined, where N(atom) is the number of atom in compounds. It is found that a convex triangle-like relation is obtained between T_c/N(atom) and χ(eff) and the maximum of T_c/N(atom) is observed at around the threshold χ(eff) corresponding to metal–semiconductor transition. The cuprates and Fe-compounds with the χ(eff) closer to the threshold value show the higher T_c value. Applying the linear relation between T_c and N(v)r(eff)³ empirically obtained for the elements to the compounds, it is indicated that about two-thirds of A_nB compounds as well as the Fe- and Cu-based compounds are well placed along the linear relation. These results allow us to estimate the T_c value in compound materials empirically based on the effective pseudopotential radius determined by the assumed hybridization

[en] Highlights: • MgB₂ thin film can be fabricated on single crystal Ag(1 1 1) face. • As the thickness of Ag layer increases, the T_c and J_c of MgB₂ decreases. When the Ag layer is 20 nm thick, the reduction of T_c is about 2 K. • Single crystal Ag layer is better than polycrystalline Ag layer in MgB₂ deposition. • May enable development of MgB₂ SNS junctions with single crystal Ag in the future. - Abstract: We have recently developed a method to deposit MgB₂ thin films on single crystal Ag layers, and found that thin Ag layers reduced the the superconductivity of MgB₂ not much, which may be a possible material to fabricate MgB₂ SNS Josephson junctions. The single crystal Ag layers were deposited on (0 0 0 1) SiC substrates, and then the MgB₂ thin films were deposited on the Ag layers. All the measurements included the XRD, M–T and M–H curves. The transition temperature of MgB₂ films decreases from 39 K to 37 K, while the thickness of Ag layers grows from 0 to 20 nm, and the critical current density at 4.2 K decreases from 1.76 MA/cm² to 1.24 MA/cm², too

[en] Highlights: • We calculate Meissner ac susceptibility of square film as a function of Pearl length. • New technique is developed for determining London penetration depth of square films. • The technique does not require special instrumentation and complex calculations. • Intrinsic and applied superconducting properties may be studied for the same film. - Abstract: A technique is developed for determining the London penetration depth by ac susceptibility measurements of a square film. It is based on the numerical calculation of the Meissner susceptibility of square superconducting films as a function of the Pearl length and the critical-state susceptibility of thin disks previously derived analytically. With this technique, intrinsic superconducting properties may be studied for each film together with critical-current density. The application of the technique to an actual YBa₂Cu₃O_7−δ film allows us to obtain its temperature dependent penetration depth and depairing current density

[en] Highlights: • We provide an explanation to the asymmetric tunneling in cuprate superconductors. • We reproduce qualitatively main features of the differential tunneling spectrum. • The asymmetric tunneling is a consequence due to the presence the pseudogap. • The symmetric tunneling spectrum recovers in the heavily overdoped regime. - Abstract: Within the framework of the kinetic energy driven superconducting mechanism, the doping and temperature dependence of the asymmetric tunneling in cuprate superconductors is studied by considering the interplay between the superconducting gap and normal-state pseudogap. It is shown that the asymmetry of the tunneling spectrum in the underdoped regime weakens with increasing doping, and then the symmetric tunneling spectrum recovers in the heavily overdoped regime. The theory also shows that the asymmetric tunneling is a natural consequence due to the presence of the normal-state pseudogap

[en] Highlights: • The guidance force was decayed by the application of the AC external magnetic field. • The guidance force was higher for the bulk with bigger radius. • The guidance force decay rates of the bulks were approximately equal despite of the different radius in the maglev vehicle system. - Abstract: Bulk superconductors had great potential for various engineering applications, especially in a high-temperature superconducting (HTS) maglev vehicle system. In such a system, the HTS bulks were always exposed to AC external magnetic field, which was generated by the inhomogeneous surface magnetic field of the NdFeB guideway. In our previous work, it was observed that the guidance force of the YBCO bulk over the NdFeB guideway used in the HTS maglev vehicle system was decayed by the application of the AC external magnetic field. In this paper, we investigated the influence of the radius of the cylinder HTS bulk exposed to an AC magnetic field perturbation on the guidance force in the maglev vehicle system. From the results, it was found that the guidance force was stronger for the bulk with bigger radius and the guidance force decay rates of the bulks were approximately equal despite of the different radius in the maglev vehicle system. Therefore, in order to obtain higher guidance force in the maglev vehicle system, we could use the cylinder HTS bulks with the bigger radius

[en] Highlights: • We study tunneling conductance in a gapped graphene-based normal metal/d-wave superconductor junction in virtual regime. • The evanescent type of Andreev reflections occurs above the critical electron incident angle. • In evanescent wave regime, pure imaginary wavevector of hole-like quasiparticle and tunneling conductance is obtained. • Conductance is enhanced due to virtual Andreev reflection comparing with the same s-wave superconductor junction. • Conductance in virtual regime is more sensitive to the superconductor gap characterized by orbital rotated angle, α. - Abstract: In this paper, we investigate the current resulted from virtual Andreev process, passing through a N/S gapped graphene-based junction, where superconductivity in the S-region is induced by depositing unconventional d-wave superconductor. It is shown that the evanescent type of Andreev reflections occurs above the critical electron incident angle, and needs to take into account its effect when calculating the tunneling conductance in the both conventional and unconventional superconductor junctions. In the evanescent wave regime, the new pure imaginary wavevector of hole-like quasiparticle is obtained, and consequently, the tunneling conductance formalism is modified. We show that due to the nature of pair interaction in d-wave superconductivity, considering the virtual Andreev process affects significantly the behavior of charge carriers in the structure. In particular, the conductance in the virtual regime is more sensitive to the superconductor gap characterized by orbital rotated angle, so that with increasing α the width of zero conductance versus bias energy decreases and above α=0.1π the current in junction is supplied just by virtual Andreev process

[en] Highlights: • We calculated the local density of states of (1 1 0) surface states of an YBCO superconductor. • In this material, the zero-bias conductance peak can split due to chain-plane coupling. • In this system, the lattice was modeled by the tight binding approximation. - Abstract: The electronic density of states (DOS) of (1 1 0) surface states in an YBCO high temperature superconductor were theoretically studied within a two-band approach for a semi-infinite-thickness film. As the conducting chain and plane layers in the material exhibit the superconductivity, the dispersions of those layers were taken into account, and modeled by the tight-binding bands under consideration to the second next nearest neighbor state. The impact of the chain-plane coupling strength was investigated in the calculation. The model parameters were extracted from the photoemission experiments. For comparison, the superconducting order parameter was modeled either d_x²-y² or d_x²-y²+Δ_s, where Δ_s is the s-wave component. It was found that the zero-bias conductance peak of the plane layer in the case of d_x²-y²+Δ_s order parameter can immediately split even if there is no coupling strength between the bands. However, in another case, this peak splits into two peaks with the presence of the coupling strength. Particularly, the splitting of the peak strongly depends on the size of the coupling strength

[en] Highlights: • The effects of malic acid addition on the levitation force properties of bulk MgB₂ has been first time investigated and reported. • The malic acid adding has a positive impact on the levitation properties. • 4 wt% and 6 wt% malic acid added samples exhibited a higher vertical and lateral force than pure sample. - Abstract: The effects of malic acid addition (from 0 to 15 wt% of the total MgB₂) on the levitation force properties of bulk MgB₂ have been investigated. All samples were prepared from magnesium powder, amorphous boron powder, malic acid (C₄H₆O₅) and toluene (C₇H₈) by using two-step solid state reaction method. Vertical and lateral levitation force measurements that are under both zero-field-cooled (ZFC) and field-cooled (FC) regimes were carried out at different temperatures of 24, 28 and 32 K for samples with various adding level. It was found that the reasonable malic acid adding has a positive impact on the levitation properties. At 24 K and 28 K, the 4 wt% and 6 wt% malic acid added samples exhibits a higher levitation force than pure sample. In the case of the optimally additive 4 wt% sample, the maximum levitation force corresponds to 18.60 N, whereas the pure sample shows 16.95 N at 24 K for ZFC regime. In this study the enhancing effect of malic acid adding on the levitation force properties of MgB₂ has been first time investigated and reported