Journal of Superconductivity and Novel Magnetism最新文献

The in-situ Cu-7.7Nb microcomposite (MC) wires have been studied at different drawing stages before and after intermediate annealing. It is demonstrated that both deformation and intermediate annealing of the MCs are accompanied by a noticeable change in the Nb/Cu interface density, which affects not only the strength, but also the electrical conductivity of the composite wire. The intermediate annealing temperature affects the conductivity of the wire at all stages of deformation that follow the annealing. The wire has the maximum conductivity after annealing at 800 °C. Based on scanning and transmission electron microscopy data, dependences of the Nb filament thicknesses, their aspect ratios, and interface densities on the composite strain have been constructed. The change of the Nb filaments morphology under intermediate annealing is accompanied with a decrease in the Cu/Nb interface density, the latter being a factor responsible for the increase in composite conductivity under annealing. Conductivity increases with increasing annealing temperature. These differences are preserved throughout the entire deformation process up to the final wire diameter of 0.05 mm.

This study presents a comprehensive first-principles investigation of the structural, elastic, electronic, magnetic, optical, and thermoelectric properties of hexagonal ThX₅ compounds (X = Fe, Ni) using Density Functional Theory (DFT) within the GGA + U framework. The elastic constants and 3D mechanical property visualizations demonstrate strong anisotropy in ThFe₅, making it suitable for directional mechanical applications, while ThNi₅ displays more isotropic behavior, favoring uniform load-bearing roles. In addition, electronic structure analysis, including TDOS, PDOS, and band structures, confirms the metallic and magnetic nature of both materials, with ThFe₅ exhibiting stronger ferromagnetism due to higher magnetic moments and larger spin splitting. These magnetic features significantly influence transport and optical properties. The optical analysis, based on complex dielectric functions, reveals strong direction-dependent optical responses, with plasma frequencies and reflectivity spectra confirming metallic behavior in both compounds. Notably, ThFe₅ shows higher anisotropies in optical conductivity and extinction coefficients than ThNi₅. Furthermore, thermoelectric performance, evaluated using Boltzmann transport theory, exhibits distinct temperature-dependent behavior in the Seebeck coefficient, electrical conductivity, and power factor. ThFe₅ displays a complex spin-dependent thermoelectric response with a temperature-induced carrier-type crossover, while ThNi₅ shows a stable p-type dominance at higher temperatures.

A series of Mn-doped CaKFe₄As₄ samples, CaK(Fe_1 − xMn_x)₄As₄ with x values of 0, 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05, are synthesized using two distinct routes: conventional synthesis process at ambient pressure (CSP), and high gas-pressure and high-temperature synthesis (HP-HTS) method. Comprehensive characterizations are performed on these samples to investigate their superconducting properties. This study examines the effects of Mn substitution at Fe sites in the FeAs layer on the superconducting properties of the CaKFe₄As₄ (1144) material. The HP-HTS process improves the microstructure and phase purity of the parent sample (x = 0), resulting in an enhanced superconducting transition temperature (T_c). In contrast, Mn doping via the CSP method in CaKFe₄As₄ reduces the sample quality and superconducting performance. Notably, the high-pressure synthesis method leads to an increase in the T_c by 3 to 7 K, particularly at low Mn concentrations. While the critical current density (J_c) of the parent sample (x = 0) shows a significant enhancement under the applied magnetic fields, J_c decreases for Mn-doped CaKFe₄As₄ bulks. These results demonstrate that high-pressure synthesis is an effective approach to improve the superconducting properties of Mn-doped 1144 compounds.

The SrFe_{12 − 2x}(CoSn)_xO₁₉ (x = 0.1 and 0.3) hexaferrites and their [y(SrFe_{12 − 2x}Co_xSn_xO₁₉)+(1 − y)PLA] (y = 0.0–1.0) composites were synthesized via solid-state reaction assisted by high-energy milling and melt-compression. X-ray diffraction (XRD) and Rietveld refinement confirmed the magnetoplumbite structure with minor α-Fe₂O₃ and CoFe₂O₄ phases (≤ 14 wt%) at x = 0.3. Lattice parameters expanded slightly (a = 5.884 to 5.892 Å; c = 23.06 to 23.09 Å), accompanied by crystallite growth (τ ≈ 57.5→61.3 nm) and microstrain increase (ε = 0.09→0.12%). Saturation magnetization (Ms) varied nonlinearly with doping, decreasing to 216 × 10³ A·m⁻¹ at x = 0.1, then rising to 259 × 10³ A·m⁻¹ for x = 0.3, while coercivity (Hc) dropped from 204 × 10³ to 65 × 10³ A·m⁻¹. The effective anisotropy constant (K_eff) ranged 2.3–3.2 × 10⁵ J·m⁻³ and the anisotropy field (Ha) 1.46–1.67 × 10⁶ A·m⁻¹. For SrMCoSn/PLA composites, XRD revealed coexistence of amorphous PLA and hexaferrite phases, with ferrimagnetic response emerging from y ≥ 0.1. Mₛ increased from 3.4 × 10³ to 59.7 × 10³ A·m⁻¹ as y rose to 0.6, whereas Hc remained nearly constant (~ 2 × 10⁵ A·m⁻¹) up to y = 0.5, decreasing thereafter. The linear rise of Ha (2.45 to 2.60 × 10⁷ A·m⁻¹) with ferrite fraction highlights the interfacial magnetic coupling within the polymer matrix. These results demonstrate tunable structural and magnetic properties in Co-Sn-doped SrM/PLA composites, enabling their application in magnetically responsive and multifunctional materials.

This study reports the synthesis of pure and transition metal doped at the Ni-site of LNO perovskites using solid-state reaction route. Various characterization techniques such as X-ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HR-TEM), Raman spectroscopy, Vibrating Sample Magnetometer (VSM), dielectric spectroscopy and ferroelectric (P-E loop) tests are employed to explore the influence of transition metal ions, i.e. Cr, Fe and Mn on the Ni-site of pure LNO samples. The structure is reported to be distorted rhombohedral phase, with a better phase purity for Fe-LNO comparatively. The average crystallite sizes calculated for pure LNO, Cr-LNO, Fe-LNO and Mn-LNO are 6.59 nm, 10.96 nm, 9.15 nm, and 12.67 nm respectively. HR-TEM reveals that the corresponding grain sizes are approximately 230 nm, 210 nm, 240 nm and 280 nm. Raman spectra exhibit five active vibrational modes, and noticeable blue shift in the doped samples, attributing to the compressive lattice strain. Magnetic characterization by VSM demonstrates weak ferromagnetism in Cr-doped LNO, soft magnetic behavior in Fe -doped LNO, and antiferromagnetic characteristics in Mn-doped LNO. Dielectric measurements show an increase and decrease in dielectric constant and loss for Mn and Fe doped LNOs compared to pure and Cr doped LNO. Dielectric properties with frequency show a slight relaxor behavior. The P-E hysteresis loops however are unsaturated, lossy and lack spontaneous polarization, indicating no evidence for ferroelectric behavior in all the samples.

We present brief review of theoretical studies of thermopower and Hall effect in the Hubbard model within the DMFT approximation for correlated metal and Mott insulator (considered as prototype cuprate superconductor) for different concentrations of current carriers. Analysis is performed within standard DMFT approximation. For Mott insulator we consider the typical case of partial filling of the lower Hubbard band (hole doping). We calculate the dependence of the Hall coefficient and thermopower on doping level and determine the critical concentration of carriers corresponding to sign change of these quantities. A significant temperature dependence of the Hall coefficient and an anomalous dependence of thermopower on temperature (significantly different from linear typical for the usual metals) is obtained. The role of disorder scattering is analyzed on qualitative level. We also perform a comparison of our theoretical results with some known experiments on doping dependence of Hall number in the normal state of YBCO and Nd-LSCO, demonstrating rather satisfactory agreement of theory and experiment. Violation of electron-hole symmetry leads to the appearance of the relatively large interval of band-fillings (close to the half-filling) where thermopower and Hall effects have different signs. We propose a certain scheme allowing to determine the number of carriers from ARPES data and perform semi-quantitative estimate of both thermopower and Hall coefficient using the usual DFT calculations of electronic spectrum.

In this paper we report our calculations regarding the heat absorbed by type-I superconducting needles with cross-sections of different geometric shapes, all having the same area. We used standard London theory to calculate the magnetic field inside these needles, allowing us to examine the magnetocaloric effect produced when the needles are subjected to a magnetic field difference. The cross-section geometries studied here include circular, square, triangular, heptagonal, hexagonal and ellipsoidal shapes. We also studied how small cylinders of different superconductors placed inside these needles influence the magnetocaloric effect of the overall system. Our main finding reveals a hierarchy in the ability to absorb heat based on the geometry of the straight cross-section. Notably, we discovered that there exists a temperature where above it, this hierarchy is inverted.

We study the effect of out-of-plane Zeeman field on zero-energy vortex- and corner-localized excitations in a two-dimensional second-order topological superconductor. It is shown analytically and numerically that zero-energy vortex modes are robust against the Zeeman-field effect as long as superconductivity is not destroyed by the field and a superconducting bulk gap is open. On the other hand, Majorana Kramers pairs existing at each corner of a square lattice at zero field and protected by time-reversal symmetry are gapped, in general, by the field. Despite that, we also found the specific model parameters supporting pairs of Majorana corner modes with zero energy in the presence of a magnetic field. In this parametric regime, the corner modes can coexist with zero-energy vortex modes in the presence of a vortex in the model. The precursor for coexisting zero-energy vortex and corner modes under the Zeeman field is the gapless bulk spectrum with Dirac cones in the normal (nonsuperconducting) state.

The study focuses on analyzing the critical current angular dependencies of coated conductor samples that are sliced in both longitudinal and transverse directions of the tape. It is evident that in both cases, beside the dependence on magnetic field direction there is a distinct dependence of the critical current value on the direction of the Lorentz force that acting on the vortex matter. Asymmetry of the peaks is detected in the longitudinal sample, whereas this phenomenon is notably absent in the transverse sample. This is found to consist with the prevalent ab-plane tilt around the axis that coincides with the tape direction. An interpretation of this asymmetry is proposed that naturally follows from the empirically noted duality of the critical current anisotropy with respect to both the magnetic field and the Lorentz force directions. This analysis establishes, for the first time, a direct link between the ab-plane inclination angle and pinning characteristics, marking an advance in elucidating the structure–property relationship.

Almost all the properties of cuprate high-temperature superconductors are no less mysterious than their superconductivity. From the very beginning of their study, it was noted that a two-component system of charge carriers is suitable for their description, which, however, had to be introduced phenomenologically. Recently it has been shown that ground and low-excited states of a system with strong long-range electron-phonon interaction and cuprates-like dispersion at carrier densities characteristic of cuprate superconductors are a two-liquid system of charge carriers comprising Bose-liquid of large bipolarons and Fermi-liquid of delocalized carriers. The phase diagram of such a system was demonstrated to coincide with that observed in cuprates. The temperature of the superconducting transition in them was shown to increase with the number of conducting layers in the unit cell, like in cuprates, due to the change in the spectrum of elementary excitations of the bipolaron liquid. Here we calculate temperature and doping behavior of the electronic specific heat and Hall resistance of such systems and compare it with that observed in cuprates. The low-temperature electronic specific heat obtained demonstrates giant increase at increasing doping, like that observed in cuprates at the overdoping. The increase is related with gradual decay of bipolarons at increasing temperature or doping. We also show that this decay may be responsible for the decrease in Hall resistance with increasing temperature observed in cuprates.