Impact of Correlated Disorder on Surface Superconductivity: Revealing the Robustness of the Surface Ordering Effect.

Surface superconductivity, wherein electron pairing occurs at material surfaces or interfaces, has attracted a remarkable amount of attention since its discovery. Recent theoretical predictions have unveiled increased critical temperatures, especially at the surfaces of certain compounds and/or structures. The notion of "surface ordering" has been advanced to elucidate this phenomenon. Employing the framework of self-consistent Bogoliubov-de Gennes equations and a model incorporating correlated disorder, our study demonstrates the persistence of the surface ordering effect in the presence of weak to moderate bulk disorder. Intriguingly, our findings indicate that under moderate disorder conditions the surface critical temperature can be further increased, depending on the intensity and correlation of the disorder.

Effect of Material-Dependent Boundaries on the Interference Induced Enhancement of the Surface Superconductivity Temperature.

Using the tight-binding Bogoliubov-de Gennes formalism, we describe the influence of the surface potential on the superconducting critical temperature at the surface. Surface details are taken into account within the framework of the self-consistent Lang-Kohn effective potential. The regimes of strong and weak coupling of superconducting correlations are considered. Our study reveals that, although the enhancement of the surface critical temperature, originating from the enhancement of the localized correlation due to the constructive interference between quasiparticle bulk orbits, can be sufficiently affected by the surface potential, this influence, nonetheless, strongly depends on the bulk material parameters, such as the effective electron density parameter and Fermi energy, and is likely to be negligible for some materials, in particular for narrow-band metals. Thus, superconducting properties of a surface can be controlled by the surface/interface potential properties, which offer an additional tuning knob for the superconducting state at the surface/interface.

Vortex Interactions and Clustering in Thin Superconductors.

Cluster formation is a focus of interdisciplinary research in both chemistry and physics. Here we discuss the exotic example of this phenomenon in the vortex matter of a thin superconductor. In superconducting films, the clustering takes place because of particular properties of the vortex interactions in the crossover or intertype regime between superconductivity types I and II. These interactions are controlled by the two parameters that are responsible for the crossover, Ginzburg-Landau parameter κ, which specifies the superconducting material of the film, and film thickness d, which controls effects due to stray magnetic fields outside the sample. We demonstrate that their competition gives rise to a complex spatial dependence of the interaction potential between vortices, favoring the formation of chainlike vortex clusters.

Multiband superconductors with degenerate excitation gaps.

There is a tacit assumption that multiband superconductors are essentially the same as multigap superconductors. More precisely, it is usually assumed that the number of excitation gaps in the single-particle energy spectrum of a uniform superconductor (i.e. number of peaks in the density of states of the superconducting electrons) determines the number of contributing bands in the corresponding superconducting model. Here we demonstrate that contrary to this widely accepted viewpoint, the superconducting magnetic properties are sensitive to the number of contributing bands even when the spectral gaps are degenerate and cannot be distinguished. In particular, we find that the crossover between superconductivity types I and II-the intertype regime-is strongly affected by the difference between characteristic lengths of multiple contributing condensates. The reason for this is that condensates with diverse characteristic lengths, when coexisting in one system, interfere constructively or destructively, which results in multi-condensate magnetic phenomena regardless of the presence/absence of the multigap spectrum of a superconducting multiband material.

Giant paramagnetic Meissner effect in multiband superconductors.

Superconductors, ideally diamagnetic when in the Meissner state, can also exhibit paramagnetic behavior due to trapped magnetic flux. In the absence of pinning such paramagnetic response is weak, and ceases with increasing sample thickness. Here we show that in multiband superconductors paramagnetic response can be observed even in slab geometries, and can be far larger than any previous estimate - even multiply larger than the diamagnetic Meissner response for the same applied magnetic field. We link the appearance of this giant paramagnetic response to the broad crossover between conventional Type-I and Type-II superconductors, where Abrikosov vortices interact non-monotonically and multibody effects become important, causing unique flux configurations and their locking in the presence of surfaces.

Angular melting scenarios in binary dusty-plasma Coulomb balls: magic versus normal clusters.

Molecular-dynamic simulations were performed in order to investigate the melting processes of isotropically confined binary systems. We considered two species of particles, which differ by their amount of electric charge. A Lindemann type of criterion was used to determine the angular melting temperature. We demonstrate that the magic-to-normal cluster transition can evolve in two distinct ways, that is, through a structural phase transition of the first order or via a smooth transition where an increase of the shells' width leads to a continuous decreasing mechanical stability of the system. Moreover, for large systems, we demonstrate that the internal cluster exerts a minor effect on the mechanical stability of the external shell. Furthermore, we show that highly symmetric configurations, such as those found for multiple ring structures, have large mechanical stability, i.e., high angular melting temperature.