Journal of Superconductivity and Novel Magnetism最新文献

We show that the electromagnetic proximity effect in superconductor (S) / ferromagnet (F) bilayers favors the formation of magnetic domains in the ferromagnet for the broad range of system parameters. Specifically, for the model of the isolated domain wall separating two domains we determine the energetically favorable domain thicknesses and the magnetic texture. Depending on the F layer thickness the magnetic moments in the neighboring domains are shown to be anti-parallel or noncollinear provided the ferromagnet has easy-plane magnetic anisotropy. Such instability of a homogeneous magnetic state is of particular importance in the design of the superconducting spintronics devices.

We present the study of the Leggett collective oscillations with energy (omega _0) of a superconductor plasma in two-gap superconductors MgB(_2), MgB(_2)+MgO and Mg(_{1-x})Al(_{x})B(_2) with various doping level. We revisit our previous tunneling spectroscopy data to find the (omega _0^2) dependence on (varDelta _{sigma }(0)varDelta _{pi }(0)): (i) observed experimentally, and (ii) simulated using the reduced coupling constants (lambda _{ij}) estimated from the experimental (varDelta _{sigma ,pi }(T)) dependences. The comparison of both results at (T ll T_c) shows the monotonic (omega _0^2(varDelta _{sigma }varDelta _{pi })) behaviour, and the simulated (omega _0) values tending to the Sharapov et al. predictions, those well exceed the experimental (omega _0) values. We conclude on the substantial softening of the Leggett mode energy (omega _0 rightarrow 2varDelta _{pi }(0)) in Mg(_{1-x})Al(_x)B(_2) compounds.

In recent years, a number of studies have predicted the emergence of a nontrivial proximity effect in superconductor/antiferromagnet (S/AF) heterostructures. This effect is of considerable interest for the efficient integration of antiferromagnetic materials into the fields of superconducting spintronics and electronics. A key element of this proximity effect is the Néel triplet correlations, initially predicted for S/AF heterostructures with checkerboard G-type antiferromagnetic ordering. However, various forms of antiferromagnetic ordering exist, and an important open question concerns the generalization of these results to such cases. In this paper, we develop a theory of the proximity effect in S/AF heterostructures with arbitrary two-sublattice antiferromagnetic ordering, aiming to clarify which antiferromagnets are capable of inducing triplet correlations and what structure these correlations may exhibit. We show that, in S/AF heterostructures with collinear compensated antiferromagnets, the dominant superconducting triplet correlations are of the checkerboard Néel type, as originally predicted for G-type antiferromagnets. In contrast, layered Néel triplet correlations—although potentially generated by layered antiferromagnets—are significantly weaker. Consequently, in S/AF heterostructures with layered antiferromagnetic ordering, the proximity-induced triplet correlations may exhibit either a checkerboard Néel or a conventional ferromagnetic structure, depending on the specific antiferromagnet and its orientation relative to the S/AF interface.

A model of the electronic mechanism of charge carrier pairing in real space in the family of bismuthate high-temperature superconducting oxides is presented, based on the experimental data of X-ray absorption spectroscopy using synchrotron radiation and X-ray free-electron laser radiation. It is shown that the decisive role in the implementation of the electron mechanism of pairing in BaBiO({_3}) belongs to the effect of disproportionation of the Bi6s–O2p bond in the form of local pairing of electrons and holes on neighboring octahedral complexes. A negative Anderson potential in bismuthates is assumed to arise from the Coulomb attraction between oppositely charged octahedral complexes

In a two-band system, both conventional sign-preserving (s_{++}) and unconventional sign-changing (s_{pm }) superconducting state may appear at low temperatures. Moreover, they may transform from one to another due to the impurity scattering. To study the details of such a transition here we derive the expression for the Grand thermodynamic potential (Omega) for a two-band model with nonmagnetic impurities considered in a (mathcal {T})-matrix approximation. For the iron-based materials within the multiband Eliashberg theory, we show that the (s_{pm } rightarrow s_{++}) transition in the vicinity of the Born limit is a first order phase transition.

The optical properties of the superconducting K(_{0.8})Fe(_{1.7})(Se(_{0.73})S(_{0.27}))(_2) single crystals with a critical temperature (T_capprox 26) K have been measured in the ab plane in a wide frequency range using both infrared Fourier-transform spectroscopy and spectroscopic ellipsometry at temperatures of 4–300 K. The normal-state reflectance of K(_{0.8})Fe(_{1.7})(Se(_{0.73})S(_{0.27}))(_2) is analyzed using a Drude-Lorentz model with one Drude component. The temperature dependences of the plasma frequency, optical conductivity, scattering rate, and dc resistivity of the Drude contribution in the normal state are presented. In the superconducting state, we observe a signature of the superconducting gap opening at (2Delta (0)=11.8) meV. An abrupt decrease in the low-frequency dielectric permittivity (varepsilon _1(omega )) at (T < T_c) also evidences the formation of the superconducting condensate. The superconducting plasma frequency (omega _{pl,s} = (213pm 5)) cm(^{-1}) and the magnetic penetration depth (lambda =(7.5pm 0.2)) (mu)m are determined.

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.