Nanoscience and Engineering in Superconductivity

In type II superconductors, an external magnetic field can partially penetrate into the superconducting phase in the form of magnetic flux lines or vortices. The repulsive interaction between vortices makes them to arrange in a triangular lattice, known as Abrikosov vortex lattice. This periodic vortex distribution is very fragile and can be easily distorted by introducing pinning centers such as local alterations of the superconducting condensate density. The dominant role of the vortex-pinning site interaction not only permits to control the static vortex patterns and to enhance the maximum dissipationless current sustainable by the superconducting material but also allows one to gain control on the dynamics of vortices. Among the ultimate motivations behind the manipulation of the vortex motion are the better performance of superconductor-based devices by reducing the noise in superconducting quantum interference-based systems, development of superconducting terahertz emitters, reversible manipulation of local field distribution through flux lenses, or even providing a way to predefine the optical transmission through the system. In this chapter, we discuss two relevant mechanisms used in most envisaged fluxonics devices, namely the guidance of vortices through predefined paths and the rectification of the average vortex motion. The former can be achieved with any sort of confinement potential such as local depletion of the order parameter or local enhancements of the current density. In contrast, rectification effects result from the lack of inversion symmetry of the pinning landscape which tends to favor the vortex flow in one particular direction. We also discuss a new route for further flexibility and tunability of these fluxonics components by introducing ferromagnetic pinning centers interacting with vortices via their magnetic stray field.

Due to their unique properties, ceramic high-T _c materials offer new perspectives for cryoelectronic applications. However, the 2D-layered structure in combination with the extremely small coherence length imposes extreme demands on the preparation of virtually single crystalline high-T _c films and the handling and manipulation of magnetic flux in devices made from these films. In this chapter, we will give an overview on vortex matter, pinning mechanism, vortex mobility, and vortex manipulation in high-T _c thin films. Vortex manipulation via artificially introduced structures will be demonstrated, and their potential for application in superconducting magnetometer and microwave devices will be sketched.

Many of the cuprate high-temperature superconductors have transition temperatures well above the boiling point of liquid nitrogen and can be operated under technically viable cooling conditions. On the other hand, they have complex and sensitive crystallographic structures that impose severe restrictions for nano-patterning by the established methods. Ion irradiation of these materials offers a unique possibility to create a wide range of different defects and to tailor the electrical and superconducting properties. Depending on the species of ions used during the irradiation, their energy and fluence, nanoscale columnar pinning centres can be created that enhance the critical current, or randomly distributed point defects that change the superconducting properties. The various effects of ion bombardment on the structural and electrical properties of a representative high-temperature superconductor are reviewed with an emphasis on He⁺ irradiation with moderate energy and the prospects discussed to create nanostructures in thin films of these superconductors.

The macroscopic quantum decay phenomena occurring in Josephson structures represent one of the most intriguing issues of the weak coupling superconductivity, due to both fundamental physics implications and stimulating potential applications. In this chapter, the main aspects of the macroscopic quantum tunneling (MQT) are discussed for both low Tc and high Tc Josephson junctions. Particular attention will be dedicated to the latter for which the unconventional character of the order parameter plays a very important role. Both experimental and theoretical aspects are reviewed.

Intrinsic Josephson tunneling takes place between copper-oxide (CuO) planes of several anisotropic high-temperature superconductors, such as Bi₂Sr₂Ca Cu₂O_8 + δ (Bi2212). To study the intrinsic tunneling effects, mesas are usually patterned into the surfaces of Bi2212 single crystals. The mesa structure contains an array of typically 5–30 intrinsic Josephson junctions formed between adjacent superconducting CuO planes. The number of defects that could affect measurements is limited in the small volume of the mesa. Variations in the carrier concentration and introduction of columnar defects by heavy-ion irradiation can be used to control the correlations and pinning of magnetic vortex pancakes. The intrinsic tunneling provides information on the magnetic phase diagram of Abrikosov vortices, allows for direct measurements of the superconducting critical current of the individual CuO planes, permits for the observation of macroscopic quantum tunneling, allows for tunneling spectroscopy of phonons, the superconducting- and pseudo gaps, and shows coherent THz radiation of significant power.

Long Josephson junctions have for some time been considered as a source of THz radiation. Solitons moving coherently in the junctions is a possible source for this radiation. Analytical computations of the bunched state and bunching-inducing methods are reviewed. Experiments showing THz radiation have recently been reported and are discussed at the end of this chapter.

Point-contact spectroscopy offers a unique possibility to study the fundamental superconducting properties. Namely, the superconducting energy gap, its symmetry, multiplicity, the temperature and magnetic field dependence can be addressed usually on the scale of hundreds of nanometers. From the very beginning of the discovery of superconductivity in magnesium diboride, this technique has been applied for the investigation of the two-gap superconductivity in this compound. Very recently discovered superconducting iron pnictides are intensively studied by this technique as well. Here, we shortly review the point contact experiments leading to one of the first experimental evidences of the fact that MgB₂ represents an extraordinary example of the multigap superconductivity. We show that particularly the measurements by point contacts in magnetic fields with the sample in the vortex state provide additional important informations directly in the raw data, thus not depending on a particular model used for fitting. Namely, the direct experimental evidence of the coexistence of two well-distinct superconducting energy gaps up the common transition temperature is shown. The small gap and small superconducting coupling below the BCS value characterize the π band, while the large gap and the strong coupling are found in the hole σ band. Also the carbon and aluminum doped MgB₂ samples have been intensively studied. It is shown that the hole band filling effect leading to a decrease in the density of states due to electron doping by carbon and aluminum is very important. It prevails over the interband scattering introduced by doping MgB₂ by these elements in the investigated doping range. This is the reason why the two gap superconductivity is preserved also in the case when T _c is significantly suppressed. The effect of applied magnetic field on the point-contact spectra is also used to study the intraband scattering processes within the two bands indicating that the carbon doping enhances significantly the scattering inside the π band. This leads to a strong increase in the upper critical magnetic field, particularly at the low temperatures, with importance for practical applications. Recent point contact measurements performed on the iron pnictides also show a presence of multigap superconductivity underlying the multiband character of this new class of the high temperature superconductors.

We show that superconductor–insulator transitions in oxides and FeAs-based high Tc superconducting multilayers may arise due to a charge density wave instability induced by charged impurities and the over-screening of the long-ranged part of the Coulomb interaction, which is enhanced due to decreasing carrier density [1]. When the carrier density is low enough, impurities begin to trap particles and form bound states of clusters of charge carriers, which we call Coulomb bubbles. These bubbles are embedded inside the superconductor and form nuclei of the new insulating state. The growth of a bubble is terminated by the Coulomb force and each of them has a quantized charge and a fluctuating phase. When clusters first appear, they are covered by superfluid liquid due to the proximity effect and invisible. However, when the carrier density decreases the size of bubbles increases and the superconducting proximity inside them vanishes. The insulating state arises via a percolation of these insulating islands, which form a giant percolating cluster that prevents the flow of the electrical supercurrent through the system. We also show the formation of two groups of charge carriers in these compounds associated with free and localized states. The localized component arises due to the Coulomb bubbles. Our results are consistent with the two-component picture for cuprates deducted earlier by Gorkov and Teitelbaum [2] from the analysis of the Hall effect data and ARPES spectra. The Coulomb clusters induce nanoscale superstructures observed in scanning tunneling microscope (STM) experiments [3] and are responsible for the pseudogap [4].

Tunneling spectroscopy played a central role in the experimental verification of the microscopic theory of superconductivity in the classical superconductors. In the case of high-temperature superconductors (HTS), the initial attempts to adopt the same approach were hampered by various problems related to the complexity of these materials. The progresses made in synthesizing high quality samples, and the use of scanning tunneling microscopy/spectroscopy (STM/STS) on these compounds allowed to overcome the main difficulties. In this review, we present some of the experimental highlights obtained with STM/STS techniques over the last decade. Most of the results confirm the fact that this new class of materials differ noticeably from the conventional BCS superconductors, and provide convincing arguments toward the understanding of the microscopic mechanisms at the origin of high-temperature superconductivity.

Scanning tunnelling microscopy and spectroscopy (STM/S) has proved to be a powerful tool to study superconductivity down to atomic level. Vortex lattice studies require characterizing areas of enough size to contain a large number of vortices. On the other hand, it is necessary to combine this capability with high spectroscopic and microscopic resolution. This is a fundamental aspect to measure and detect the subtle changes appearing inside and around a single vortex. We report in this chapter our approach to the use of STM/S, using normal and superconducting tips, to observe the lattice of vortices in several compounds, and the information acquired inside these fascinating entities. The combination of superconducting tips and scanning tunneling spectroscopy, (ST)²S, presents advantages for the study of superconducting samples. It allows to distinguish relevant features of the sample density of states, which manifest itself as small changes in the Josephson coupling between sample and tip condensates, and it has also shown to be very efficient in the study of the ferromagnetic-superconductor transition in the re-entrant superconductor ErRh₄B₄.

We discuss an effect of the electrostatic field on superconductivity near the surface. First, we use the microscopic theory of de Gennes to show that the electric field changes the boundary condition for the Ginzburg–Landau function. Second, the effect of the electric field is evaluated in the vicinity of H _c3, where the boundary condition plays a crucial role. We predict that the field effect on the surface superconductivity leads to a discontinuity of the magnetocapacitance. We estimate that the predicted discontinuity is accessible for experimental tools and materials nowadays. It is shown that the magnitude of this discontinuity can be used to predict the dependence of the critical temperature on the charge carrier density which can be tailored by doping.

In research on superconductor (S)/ferromagnet (F) multilayers, a number of issues are at play simultaneously. In phenomena such as interlayer coupling in S/F/S systems or superconducting spin valve effects in F/S/F systems, questions about the interface transparency, the effect of magnetic domains on the F-side of the interface, or the stray fields produced by the F-layer arise all the time. Investigating bilayers of S/F combinations is useful and necessary to discern the effects which can be produced by the bare interface and one F-layer from those which occur when coupling between S- or F-layers also plays a role. In this chapter, we review insights gained from bilayer studies, using both weak and strong ferromagnets, with particular attention to the interface transparency and the issue of inhomogeneous exchange fields; we also review some of the questions pertaining to the superconducting spin valve effect, again for both weak and strong ferromagnets.

This chapter presents results on transport properties of hybrid structures where the interplay between ferromagnetism and superconductivity plays a central role. In particular, the appearance of so-called odd-frequency pairing in such structures is investigated in detail. The basic physics of superconductivity in such structures is presented, and the quasiclassical theory of Greens functions with appropriate boundary conditions is given. Results for superconductor∣ferromagnet bilayers as well as magnetic Josephson junctions and spin valves are presented. Further phenomena that are studied include transport in the presence of inhomogenous magnetic textures, spin-Josephon effect, and crossed Andreev reflection. We also investigate the possibility of intrinsic coexistence of ferromagnetism and superconductivity, as reported in a series of uranium-based heavy-fermion compounds. The nature of such a coexistence and the resulting superconducting order parameter is discussed along with relevant experimental results. We present a thermodynamic treatment for a model of a ferromagnetic supercondcutor and moreover suggest ways to experimentally determine the pairing symmetry of the superconducting gap, in particular by means of conductance spectroscopy.