Journal of Superconductivity and Novel Magnetism最新文献

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.

Josephson junctions are key elements in superconducting electronics. The most common type is the overlap (sandwich-type) junction, formed by vertically stacking two superconducting layers. In contrast, planar junctions are fabricated without overlap, at the edge of two superconducting films within a single plane. This geometric distinction has a significant impact on their physical properties. The planar geometry greatly enhances sensitivity to magnetic fields and improves impedance matching for terahertz (THz) devices. Its two-dimensional structure allows for simple and flexible electronic component design, enabling drastic miniaturization. Here I highlight recent advances in the application of planar junctions for novel technologies, including junction-on-cantilever sensors for super-resolution magnetic imaging, vortex-based memory cells, and programmable superconducting diodes. I will also discuss the general requirements, future perspectives, and key challenges in the evolving field of superconducting electronics.

The structural, optical, and magnetic properties of TiO₂ nanoparticles were systematically investigated to explore the origin of room-temperature weak ferromagnetism (RTFM). Structural analysis confirmed the presence of a tetragonal anatase phase with high crystallinity, exhibiting characteristic vibrational modes of pure TiO₂ and surface functional groups associated with Ti–O bonding. Optical studies revealed a indirect bandgap of ~ 3.24 eV, slightly higher than the reported 3.20–3.22 eV for pure anatase, along with defect-related electronic transitions. The morphology indicates uniformly distributed nanocrystallites, and the magnetic measurements demonstrate a weak ferromagnetism at room temperature, which most likely arises from intrinsic defects such as oxygen vacancies. Compared with literature, where anatase TiO₂ is typically diamagnetic and or shows the RTFM only after defect engineering, the observation of weak ferromagnetism in undoped samples highlights the intrinsic role of defects in magnetic ordering. These insights are likely to advance the understanding of defect-driven magnetism and suggest promising applications of anatase TiO₂ nanoparticles in optoelectronics and spintronics.

This study systematically investigates the non-equilibrium magnetic loop responses (DMHBs) of a mixed-spin (1, 7/2) Blume-Capel Ising system (BCIS) on a hexagonal lattice under an sinusoidal magnetic stimuli using the PPM framework (PPM). We explore the effects of the system parameters: exchange interaction parameters (({J}_{sigma S}) and ({J}_{SS})), temperature (T), oscillating magnetic field frequency (ω), crystal field parameter (d), and kinetic rate constants (({k}_{2})). Our findings reveal complex and varied hysteresis loop morphologies, including triple, double, and elliptical loops, for both individual sublattices (({m}^{A}) ​,({m}^{B})​) and total magnetization (({m}^{T})). We analyzed the corresponding coercive fields (CFs) and remanent magnetizations (RMs). The results demonstrate how changes in these parameters significantly influence the DMHBs, leading to characteristics indicative of both hard and soft magnetic materials. Notably, the kinetic rate constant ({k}_{2}) ​ is found to play a crucial role, analogous to wheel speed in rapid solidification processes, affecting the loop area and thus the “hardness” of the simulated magnet. While our theoretical predictions generally align with existing theoretical literature, some interesting discrepancies with experimental observations are noted. These differences may arise from practical limitations in experimental setups, such as the achievable wheel speed in melt spinning, or variations in the compositional concentrations of magnetic alloys. This research provides valuable insights into the non-equilibrium dynamics of molecular-based spin-configured molecular systems and extends the application of the PPM to complex mixed-spin systems.

The Bethe lattice (BL) sites are filled with the diatomic molecules consisting of one spin-1/2 and one spin-3/2 atoms, then they are let to interact with the nearest-neighbor (NN) molecules through bilinear exchange interactions. Additionally, they are affected by both an external field, magnetic field H, and an internal one, the crystal field D. The model is then examined in terms of the exact recursion relations (ERR) by studying the temperature fluctuations in magnetizations associated with each type of spin for various coordination numbers q=3, 4, and 6. The phase diagrams are calculated under ferromagnetic (FM) case on the D-temperature planes with zero H, i.e., spontaneous magnetizations. The effects of H on magnetizations are also displayed. The model displays several critical phenomena which may be new to this model; the existence of the first-order phase transition lines and tricritical points (TCP).