Journal of Superconductivity and Novel Magnetism最新文献

Nanocrystalline TiO₂ powders doped with 5 wt% Fe were synthesized by mechanical alloying (MA) with milling times ranging from 0 to 48 h, and their magnetic behavior was systematically investigated. Structural evolution was characterized by X-ray diffraction (XRD) and Rietveld refinement, while magnetic properties were assessed at room temperature using vibrating sample magnetometry (VSM). The structural analysis revealed a progressive anatase-to-rutile transformation with increasing milling time. Metallic Fe reflections, detected at the early stages, diminished as milling progressed, indicating Fe incorporation into the TiO₂ lattice. This structural evolution was accompanied by decreasing crystallite size and increasing microstrain, consistent with enhanced defect generation. Magnetic characterization demonstrated weak ferromagnetism in all samples, with saturation magnetization (Ms) increasing steadily with milling time. The enhancement of Ms is attributed to defect-mediated exchange mechanisms involving Fe substitution at Ti sites and the formation of oxygen vacancies, which act as centers for magnetic interactions. These results underline the strong correlation between structural defects and emergent ferromagnetism in Fe-doped TiO₂ nanostructures, emphasizing the potential of mechanical alloying as an efficient route for engineering dilute magnetic oxides with tunable properties.

We present a brief review of some recent work on the problem of highest achievable temperature of superconducting transition (T_c) in electron – phonon systems. The discovery of record – breaking values of (T_c) in quite a number of hydrides under high pressure was an impressive demonstration of capabilities of electron – phonon mechanism of Cooper pairing. This lead to an increased interest on possible limitations of Eliashberg – McMillan theory as the main theory of superconductivity in a system of electrons and phonons. We shall consider some basic conclusions following from this theory and present some remarks on the limit of very strong electron – phonon coupling. We shall discuss possible limitations on the value of the coupling constant related to possible lattice and specific heat instability and conclude that within the stable metallic phase the effective pairing constant may acquire very large values. We discuss some bounds for (T_c) derived in the strong coupling limit and propose an elementary estimate of an upper limit for (T_c), expressed via combination of fundamental physical constants. Finally we also briefly discuss some pessimistic estimates for (T_c) of metallic hydrogen obtained in “jellium” model

In this work, the nature of vortex pinning has been investigated in overdoped NaFe_0.935Co_0.065As single crystals. We obtain temperature and field dependences of the critical current density from the magnetic hysteresis loops measured for two field orientations B || ab and B || c. The obtained magnetic anisotropy is about γ ≈ 10 near T_c and drops to about γ ≈ 2 at temperatures below 12 K. Using a Dew-Hughes model, we calculate the normalized pinning force and discuss the pinning nature in NaFe_0.935Co_0.065As.

To investigate the proximity and inverse proximity superconductivity (SC) in topological materials (TMs) {such as topological insulators (TIs) and topological crystalline insulators (TCIs)} by the hosting of an interface between TM/SC may be the novel strategy in the understanding of the unexplored Majorana fermions and spin-triplet superconductivity etc. [https://doi.org/10.1103/PhysRevB.81.241310, https://doi.org/10.1103/PhysRevRes.3.033008]. The TMs are known to preserve the non-trivial surface states (SS) which have emerged for exploring the exotic superconducting properties induced by the proximity effect. The observed superconductivity (proximity & inverse proximity) induced due to the mutual competition between the robust SS of TMs and sub energy gap of SCs. This strong competition between the electron–electron interaction (EEI) and cooper pairs correlation lead to proximity (inverse proximity) superconductivity if SC energy gap is dominant over EEI or vice versa (if rapidly mobile Dirac electrons break the phase coherence of cooper pairs at the interface (EEI > SC gap)) as discussed in this article. The subsequent changes discussed in the superconducting proximity with perpendicular and parallel magnetic field (M.F) makes this work very interesting due to the field dependence mechanism. The superconducting onset temperature is found to be (T_c^onset) ~ 4.1 K with the upper critical field (H_c2) ~ 1541 Oersted which is accordance with Ginzburg Landau Theory (GLT) observed in Sb₂Te₃ contacted with s-wave SC indium.

The synthesis, structure, and magnetic properties of Ba₃REB₉O₁₈ (RE = Dy-Yb) compounds were investigated. Rietveld Refinement of the X-ray diffraction (XRD) data confirms that all compounds crystallize in a hexagonal Ba₃YB₉O₁₈-type structure with space group P6₃/m (No. 176) and Pearson symbol hP62. The lattice parameters a and c increase linearly with the increasing ionic radii of the RE³⁺ ions. RE³⁺ ions form two-dimensional triangular layers in the ab plane, which are separated by non-magnetic Ba/B-O polyhedra. The magnetic susceptibility measurement reveals no long-range magnetic order or spin glass behavior in any of the compounds above 2 K. The negative Curie-Weiss temperatures indicate dominant antiferromagnetic (AFM) exchange interactions between the RE³⁺ ions. The combination of triangular lattice and antiferromagnetic interaction suggests this family of compounds is a promising system for researching magnetic frustration.

Topological defects occurring in nonlinear classical field theories are described by a system of second-order differential equations. A breakthrough was made in 1976 by E. B. Bogomoln’yi who demonstrated that in several field theories these equations can be reduced to first-order provided the coupling constants take on particular values. One of the examples involved a string in the Abelian Higgs model which is equivalent to the Abrikosov flux line of the Ginzburg-Landau theory of superconductivity. In a similar vein, in the 1966 textbook Superconductivity of Metals and Alloys P. G. de Gennes explained how to reduce the second-order Ginzburg-Landau equations to first-order at a particular value of the Ginzburg-Landau parameter by a method due to G. Sarma. We analyze the two ways of arriving at the first-order Sarma-Bogomol’nyi equations and conclude that while they both rely on the same operator identity, Sarma’s method is free of the assumption that there is a topological defect. The implication is that Bogomol’nyi equations found in other field theories may be a source of a wider range of solutions beyond topological defects.

In order to develop submerged pumps to transfer liquid hydrogen, high temperature superconducting (HTS) motors to drive them as one of key components are a promising candidate with low electromagnetic loss and large specific power density per unit mass. However, since armature windings wound using HTS wires have a couple of problems such as three-phase unbalanced currents and frequency limitations, HTS wires are used only for rotor windings of the motors and metallic cables have to be applied to their armature windings. Therefore, numerical analyses to evaluate loss properties in multi-strand metal twisted cables for armature windings of HTS motors cooled at liquid hydrogen temperature are carried out by means of a two-dimensional finite element method. On the basis of a clarified physical mechanism of losses generated in the windings arranged within a slot of armature iron core, the obtained numerical results are also reproduced with theoretical expressions of the Joule loss for an alternating transport current and the eddy-current loss for an externally applied AC magnetic field. The influences of losses on the frequency and number of strands in metal twisted cables are investigated quantitatively.

The vortex dynamics for a high-performance BaZrO₃ doped (Y_0.5Gd_0.5)Ba₂Cu₃O_7-δ superconducting films are investigated by a dynamic magnetization-relaxation method. The superconducting films have dense columnar defects with density ~ 1.1×10¹¹ cm⁻². The dynamic relaxation rates exhibit a complex temperature dependence, indicating the presence of multiple magnetic vortex phase transitions and complex vortex-vortex interactions. Based on the experimental data, a vortex phase diagram has been constructed, which shows a large vortex elastic motion region suggesting strong flux pinning and potential applications of the REBCO films. There seems to be a vortex slush region under magnetic fields less than 2 T within the temperature range from 10 to 25 K. The investigation on the vortex pinning mechanisms indicates that δl-pinning takes the key role for the vortex motion in the present REBCO films. Based on the present results, it is believed that introducing strong δT_c-pinning into REBCO superconducting films is the key to further improving their performance at high temperatures and strong magnetic fields.

A new type of magnet called p-wave unconventional magnet is proposed, stimulated by the discovery of altermagnet. We study the tunneling conductance of p-wave unconventional magnet/superconductor junctions by adopting the effective Hamiltonian of p-wave unconventional magnets with time-reversal symmetry breaking, suggested (Hellenes et al. P-wave magnets, http://arxiv.org/abs/2309.016072024). The tunneling conductance shows an asymmetric behavior with respect to bias voltage in the helical p-wave superconductor junctions. It is caused by the missing of helical edge states contributing to the charge conductance owing to the momentum-dependent spin-split feature of the Fermi surface in p-wave unconventional magnets. In chiral d and p-wave superconductor junctions, the resulting spin-resolved tunneling conductance takes a different value for spin sectors due to the time-reversal symmetry breaking in superconductors. Our results qualitatively reproduce the results based on the simplified Hamiltonian (Maeda et al. J. Phys. Soc. Jpn. 93, 114703 2024), where only the odd function of the exchange coupling of p-wave unconventional magnets is taken into account, which gives the shift of the Fermi surface and preserves the time-reversal symmetry similar to the spin-orbit coupling.

In this paper, we investigate the interplay between disorder-induced localization and magneto-optical (MO) effects in one-dimensional magnetophotonic crystals (MPCs) composed of alternating Ce:YIG and GGG layers with randomized thicknesses. Using the transfer matrix method, we analyze the localization length and MO responses across polar, longitudinal, and transverse magnetic geometries for wavelengths inside and outside the photonic band gap (PBG). Our results reveal that disorder strength critically modulates localization: while it enhances transmission at PBG edges (consistent with), it suppresses MO effects in longitudinal/transverse geometries, contrasting with the polar case where nonreciprocal localization emerges for circular polarizations. Notably, in the polar geometry, disorder amplifies Faraday rotation. These findings align with recent studies on disorder-enhanced MO responses while demonstrating the tunability of localization via magnetic and structural parameters. The work provides design principles for disordered MPCs in applications such as optical isolation, sensors, and tuneable filters.