It is well-known that the appearance of almost-empty (shallow) conduction bands in solids strongly affects their superconducting properties. In a shallow band charge carriers are depleted and have nearly zero velocities so that the crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluidity to Bose-Einstein condensation (BEC) is approached. Based on a two-band prototype system with one shallow and one deep band, we demonstrate that the fundamental phase diagram of the superconducting magnetic response changes qualitatively as compared to standard superconductors with only deep bands. The so-called intertype (IT) domain between superconductivity types I and II systematically expands in the phase diagram when passing from the BCS to BEC side: its width is inversely proportional to the squared Cooper-pair radius that shrinks several orders of magnitude through the crossover. We also show that the coupling to a stable condensate of the deep band makes the system rather robust against the otherwise strong superconducting fluctuations. Thus, the BCS-BEC crossover induced by a shallow band pushes standard superconductivity types wide apart so that the IT domain tends to dominate the phase diagram and therefore the magnetic properties of shallow-band superconductors. The crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluidity to Bose-Einstein condensation (BEC) is usually investigated in trapped ultracold fermionic gases with the resonant scattering [1][2][3][4]. However, it was originally proposed for solids with a shallow conduction band whose lower edge is close to the chemical potential [5], see also the review in [6]. Although theoretical studies of the BCS-BEC crossover in superconductors have a long history, its unambiguous experimental evidences have been obtained only recently in FeSe x Te 1−x , where shallow Fermi pockets were proved to play a significant role [7][8][9]. Interest in such superconducting materials is fueled by expectations of a higher critical temperature T c [10,11] and novel multigap/multicondensate coherent phenomena [12] potentially useful for technological applications. Here we demonstrate that the BCS-BEC crossover regime realized in a multiband superconductor can profoundly influence the superconducting magnetic properties so that the relevant phase diagram differs strikingly from the standard one.Earlier investigations of the BCS-BEC crossover in a charged superfluid considered a sharp interchange between types I and II throughout the entire crossover interval [13]. This consideration was based on the GinzburgLandau (GL) theory according to which the types I and II interchange when the GL parameter κ = λ/ξ (λ and ξ are the magnetic and coherence lengths) crosses the critical value κ 0 = 1/ √ 2 [14-16]. However, it is known that the results of the GL theory for the phase diagram of the superconducting magnetic properties are valid only in the limit T → T c . In particular, below T c the intertype (IT) regime is not reduced to the single point κ = κ 0 but occupies a finite t...

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Thin superconducting films are usually regarded as type II superconductors even when they are made of a type I material. The reason is a strong contribution of the stray magnetic field that stabilizes vortices. While very thin films indeed reach this limit, there is a large interval of film thicknesses where the magnetic properties cannot be classified as either of the two conventional superconductivity types. Recent calculations revealed that in this interval the system exhibits spontaneous formation of complex condensate-field patterns that are very sensitive to system parameters, in particular, the temperature and the applied magnetic field. The corresponding superconducting magnetic properties can be attributed to a special regime of the intertype superconductivity whose physical origin lies in the removal of an infinite degeneracy of the self-dual superconducting state at the critical Bogomolnyi point. Here we demonstrate that qualitative characteristics of the intertype superstructures in thin superconducting films are independent of the choice of the in-plane boundary conditions for the order parameter and the magnetic field. arXiv:1812.09621v2 [cond-mat.supr-con]

Small-angle neutron scattering is used in combination with transport measurements to investigate the current-induced effects on the morphology of the intermediate mixed state domains in the inter-type superconductor niobium. We report the robust self-organisation of the vortex lattice domains to elongated parallel stripes perpendicular to the applied current in a steady-state. The experimental results for the formation of the superstructure are supported by theoretical calculations, which highlight important details of the vortex matter evolution. The investigation demonstrates a mechanism of a spontaneous pattern formation that is closely related to the universal physics governing the intermediate mixed state in low-κ superconductors.