Bewegende Hall sonde microscopie van supergeleiders met tegengestelde vortex-vortex interacties

Interaction is explained as one kind of action that occurs as two or more objects have an effect upon one another. While in some cases the interaction is simply monotonic, i.e. purely ``repulsive'' or ``attractive'', in most cases of nature it is a combination of many complex contributions which can even be competitive. The latter, widely exists in nature and results in the main characteristics of many systems. For instance, it is the complex interactions among species of ecosystems that lead to the biodiversity in our beautiful world. In natural sciences, many systems have been found to exhibit complex interactions between different phases. To name a few, we mention the ferrofluids system, reaction-diffusion in chemical mixtures, convection of a fluid with temperature gradient and the superconducting system. All these systems present various modulated patterns due to complex interactions. The study of these systems, especially at microscopic level of the characteristic structures, can provide us a lot of information about the relevant mechanism. It can also help us to better understand the macroscopic properties which may even push forward their applications. Among all these systems, the superconducting system is an ideal candidate since different kinds of superconductors show complex vortex patterns. In type-II superconductors, the Abrikosov vortex lattice with triangular arrangement of vortices forms due to the purely repulsive interactions between them, while in type-I superconductors, vortex interaction becomes long-range repulsive and short-range attractive, leading to the formation of the intermediate state. The interactions between vortices can also be long-range attractive and short-range repulsive as revealed in low-k superconductors and the recently discovered type-1.5 superconductors, both of which display intricate vortex patterns with coexistence of large Meissner area, vortex stripes and clusters. Compared with other systems, it is relatively convenient to manipulate the parameters related to vortex interaction in superconductors, like the magnetic field and temperature. One can also introduce other interactions to the system, e.g. by adding defects to the system as pinning centers, amplifying confinement effect to vortex interaction in mesoscopic and nano superconductors, combining two superconducting condensates with different characteristic lengths (lambda, penetration depth; xi, coherence length) in bilayer structures and so on. The goal of this thesis is to explore, by using scanning Hall probe microscope, the intricate vortex patterns and the related phenomena in a few superconducting systems with different competitive vortex-vortex interactions.Contents Acknowledgements i Preface iii Abstract v Contents ix 1 Introduction 1 1.1 Historical overview . . . . . . . . . . 1 1.2 London equations . . . . . . . . 4 1.3 Ginzburg-Landau theory . . . . . . . . 5 1.3.1 Ginzburg-Landau equations . . . . . 5 1.3.2 Characteristic lengths . . . . . . . 6 1.3.3 Flux quantization . . .. . . . . . 7 1.4 Classification of superconductors . . . .. . . 9 1.4.1 Type-I superconductor . . . . 9 1.4.2 Type-II superconductor . . . . 11 1.4.3 Low-k superconductors . . . . . . . 12 1.4.4 Type-1.5 superconductor . . . . . . 14 1.5 Systems with modulated phases . . . . . . . . 16 2 Experimental techniques 19 2.1 The MBE Varian VT-118 UHV system . . . . . . 19 2.2 Low temperature scanning Hall probe microscopy . . . 20 2.3 AC/DC Magnetometry System . . . . . . . . 25 3 Flux pattern transitions in the intermediate state of a type-I superconducting film . . . . . .29 3.1 Introduction . . . . . . . . 30 3.2 Experimental details . . . . . . . . 32 3.3 Flux patterns of the intermediate state . . .. . . 32 3.4 Flux dynamics in a type-I film . . . . . 35 3.5 Stability of different flux states . . . . .. 41 3.6 Conclusion . . . . . . . . . 42 4 Single quantum vortex in type-I superconductor 45 4.1 Introduction . . . . . . . . . . 46 4.2 Experimental details . . . . . . . . . . 48 4.3 Quantization of flux tubes . . . . . . 48 4.4 Penetration depth of Pb film . . . . . . 50 4.5 Nucleation of single quantum vortex in the IS . . . 52 4.6 Impeded flux penetration . . . .. . . . . . 53 4.7 Vortex-antivortex annihilation . .. . . . . . . 55 4.8 Conclusion . . . . . . . . . . . . . . 57 5 Direct visualization of vortex pattern transition in ZrB12 with Ginzburg-Landau parameter close to the dual point . . . . . .59 5.1 Introduction . . . . . . . . . . . . . . 60 5.2 Sample and experimental details . . . . . . .62 5.3 Results and discussion . . . . . . . . . . 63 5.3.1 Vortex phase diagram . . . . . . . 63 5.3.2 Vortex pattern evolution . . . . . . . . . 66 5.3.3 Statistics of vortex arrangement . . .. . . 68 5.3.4 Vortex cluster formation . . . . . . 70 5.4 Conclusion . .. . . . . . . . . . 71 6 Vortex pinning in Fe-based superconductors 73 6.1 Abstract . . . .. . . . . . . 74 6.2 Introduction . . . . . . . . . . . . . . 74 6.3 Experimental details . . . . . . . . 76 6.4 Strong pinning in Fe-based superconductors . . . . 76 6.4.1 Effective pinning in K0.79Fe1.86Mn0.01Se2 Single Crystal 76 6.4.2 Effective pinning in Ca10(Pt3As8)[(Fe1−xPtx)2As2]5 single crystal . . . . . . . . 80 6.5 Peak effect in Ba0.5K0.5Fe2As2 single crystal . .. . 81 7 Conclusions and outlook 91 List of symbols and abbreviations 95 Bibliography 99 List of publications 119 Curriculum Vitae 123status: publishe