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Nickel/bismuth (Ni/Bi) bilayers have recently attracted attention due to the occurrence of time-reversal symmetry breaking in the superconducting state. Here, we report on the structural, magnetic, and electric characterization of thin film Ni/Bi bilayers with several Bi thicknesses. We observed the formation of a complex layered structure depending on the Bi thickness caused by the inter-diffusion of Bi and Ni which leads to the stabilization of NiBi 3 at the Bi/Ni interface. The superconducting transition temperature and the transition width are highly dependent on the Bi thickness and the layer structure. Magnetoelectric transport measurements in perpendicular and parallel magnetic fields were used to investigate the temperature-dependent upper critical field within the framework of the anisotropic Ginzburg–Landau theory and the Werthamer–Helfand–Hohenberg model. For thicker samples, we observed a conventional behavior, similar to that shown by NiBi 3 bulk samples, including a small Maki parameter (α M = 0), no spin–orbit scattering (λ S O = 0) and nearly isotropic coherence length (γ = ξ ⊥ (0) / ξ ∥ (0) ≈ 1). The values obtained for these properties are close to those characterizing NiBi 3 single crystals. On the other hand, in very thin samples, the Maki parameter increases to about α M = 2.8. In addition, the coherence length becomes anisotropic (γ = 0.32) and spin–orbit scattering (λ S O = 1.2) must be taken into account. Our results unequivocally show that the properties characterizing the superconducting state in the Ni/Bi are strongly dependent on the sample thickness. [ABSTRACT FROM AUTHOR]

This paper serves as a primer on superconductivity, inviting students for further investigation. Although the theory of superconductivity is a many-body quantum theory, here we take a more didactic route based on thermodynamics and symmetry. We briefly survey the more than a century-old field and provide a one-sentence definition of a superconductor. Surprisingly, many textbooks lack such a definition, usually introducing superconductors through their properties rather than by definition. We explain the concept of an order parameter, symmetry, and symmetry breaking. Based on this, we clarify the difference between conventional and unconventional superconductors, which is frequently a confusing topic for newcomers. We provide the reader with a taste of a current research topic in the field of unconventional superconductivity and magnetism. For this, we explain the concept of time-reversal symmetry breaking in condensed matter physics, which is usually associated with a form of magnetism. Here, we show that time-reversal symmetry might be broken in superconductors, leading to magnetic properties due to superconductivity itself. For this purpose, we utilize the method of the Ginzburg-Landau theory for phase transitions. We discuss the field of chiral superconductivity and guide the interested reader to further study. [ABSTRACT FROM AUTHOR]

In the theory of dilute magnetic impurities in superconductors, the effect of all impurity spin-components is expressed via a single magnetic scattering rate Γ m . In a more realistic setting, magnetic impurities are anisotropic. In this case, the spatial randomness of three spin-components of impurities gives rise to generally different scattering rates Γ i (i = 1 , 2 , 3). We explore the effects of anisotropic magnetic impurities on the in-plane critical field in 2D superconductors. We discuss singlet, triplet, and parity-mixed order parameters allowed in systems without the inversion center. Also, the addition of a small amount of magnetic impurities may cause singlet to triplet crossovers. In all cases, different components of impurity spin affect the magnetic field—temperature phase diagram differently. We show that anisotropy of the magnetic impurities can serve as a probe of unconventional triplet or parity-mixed superconductivity. [ABSTRACT FROM AUTHOR]

We develop a model for high-Tc superconductors based on an electronic phase separation where low- and high-density domains are formed. At low temperatures this system may act as a granular superconductor forming an array of Josephson junctions. Cuprates are also known to have low superfluid densities and strong correlation effects. Both characteristics activate a negative Josephson coupling due to frustration that leads to spontaneous currents responsible for the weak ferromagnetic order. This original approach reproduces the observed onset of spontaneous magnetic signal and its dependence on the doping level. [ABSTRACT FROM AUTHOR]